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Uses of recommended intakes
Uses of recommended intakes The purpose of dietary recommendations
J.C. Waterlow
The definition of recommended (RDVRDAI in intakes/allowances official reports implies that the RDI represents the minimum amount of nutrient which should be a consumed by each individual. The author argues that this definition makes it impossible to apply the recommendations to groups, either for planning diets or for assessing the adequacy of intakes. He proposes an alternative and more useful definition of the RDI as the head average intake per recommended for a group. J.C. Waterlow Nutrition
at
is Professor the
Human
Nutrition,
Hygiene
and
Keppel
Street,
of Human
Department London
Tropical London
of
School
of
Medicine, WClE
7HT,
Definitions
UK. This article is based on a paper given at a meeting of the Societe de Nutrition et Dieteti&e de Langue Francaise held in Tours in Mav 1978 in honour of the late Professor ‘Jean Tremolieres, and published in French in the Cahiers de Nutrition et Dibtbtique. I am grateful to the editors of the journal for permission to reproduce the article in English.
’ ‘Round table on comparison of dietary recommendations in different European countries’, in N. Zollner, G. Wolfram and Ch. Keller, eds, Proceedings of Second European Nutrition Conference, Munich 1976 Karger, Basel, 1977, pp 210-279.
0306-9192/79/020107-08
$02.00
Many national and international bodies have devoted a great deal of time to reviewing our knowledge of nutrient requirements and making recommendations on intakes and allowances. The US National Academy of Sciences has issued eight reports on this subject since 1943, and a great many other countries, including the UK, have published their own recommendations.’ The Food and Agriculture Organization (FAO) and the World Health Organization (WHO) have jointly produced a large number of reports on requirements for energy and various nutrients. z In spite of this volume of literature there seems to be some confusion about how the figures should be used; there is very little information about the extent to which they are in fact useful and it is clear that they are sometimes misused. Some of these matters have been discussed by Hegs&d3 The purpose of this article is to go one step further in trying to define the dilflculties. The discussion will be based primarily on the report from the UK’s Department of Health and Social Security (DHSS),4 and the report from the Food and Nutrition Board of the US National Academy of Sciences (NAS).5
The title of the DHSS report is Recommended Intakes of Nutrients6 for the United Kingdom (RDI); that of the NAS report is Recommended Dietary Allowances (RDA). According to the definitions given, the meanings of RDI and RDA are the same. In the DHSS report, ‘Recommended intakes for nutrients are defined as the amounts sufficient or more than sufficient for the nutritional needs of practically all healthy persons in a population’. According to the NAS report, ‘Recommended dietary allowances are the levels of intake of essential nutrients considered . . . to be adequate to meet the known nutritional needs of practically all healthy persons’. These definitions are the same, almost word for word, as the US definition equates a recommended allowance with a level of intake. In plain English, however, there is a difference between an intake and an allowance.
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Uses of recommended intakes
Figure
1. Recommended
nutrients
(DHSS,
intakes
for
1969 definitions).
L Figure
2.
Recommended
intakes
for
energy.
z Summarized in R. Passmore, B.M. Nicol, M. Narayana Rao, G.H. Beaton and E.M. Handbook on Human DeMaeyer, Nutritional Requirements, World Health Organization Monograph Series No 61, WHO, Geneva, 1974. Hegsted. ‘Dietary standards’, 3 D.M. the American Dietetic Journal of Association, Vol66, 1975, pp 13-20; and ‘On dietary standards’, Nutrition Reviews, Vol 36, 1978, pp 33-36. 4 Department of Health and Social Recommended Intakes of Security, Nutrients for the United Kingdom, Reports on Public Health and Medical Subjects No 120, HMSO, London, 1969. Sciences, 5 National Academy of Recommended Dietary Allowances. 8th revised edition, National Academy of Sciences, Washington DC, 1974. 6 In official writings in the UK it is technically incorrect to considered describe energy as a nutrient. This distinction seems not to be made in the USA. 7 In French, apport. * Hegsted, ‘On dietary standards’, op cit. Ref 3. 9 NAS. op cit. Ref 5. p 3. lo DHSS, op cit. Ref 4. para 9. I1 Passmore et al, op cit. Ref 2. I2 Hegsted, ‘On dietary standards’, op cit. Ref 3. and Protein Energy l3 FAO/WHO, Joint Report of a Requirements, FAOWHO Ad Hoc Expert Committee, FAO Nutrition Meetings Report Series No 52, FAO, Rome 1973.
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‘Intake’ refers to what is eaten, ‘allowance’ to what is provided;’ not exactly the same thing. The distinction is only important when it comes to application of the recommendations. What is important is to examine precisely what these definitions mean. The natural meaning is that each individual in a group should get at least the amount recommended, and that this amount will be adequate for all or almost all members of the group. Hegsted* calls it the ‘minimum no risk level of intake’. It might be appropriate to call a recommendation with this meaning an ‘individual recommendation’, as illustrated diagrammatically in Figure 1. The situation with energy (calorie intake) is quite different. Here all authorities agree that the RDI or RDA for a group should be equal to the average requirement of the group, no greater and no less. The reasons for this are well known and need not be further considered here. A recommendation of this type could be called an ‘average recommendation’, as illustrated in Figure 2. The key question is whether the authors of the DHSS and NAS reports really intended their recommendations for nutrients other than energy to be of the type called ‘individual recommendations’. It is probable that they did, since in both reports a clear distinction is made between the recommendations for energy and those for other nutrients. Furthermore, the NAS report says categorically: RDA are neither estimates of the amounts of nutrients needed per capita (sic) in the national or local food supply, nor even in the food purchased . . . RDA (except for energy) are estimated to exceed the requirements of most individuals, and thereby ensure that the needs of nearly all are met.9
Likewise, the UK report says, ‘The recommended intakes, which are judged to be sufficient for practically all individual members of a population, must of necessity be in excess of the requirements of most of them’.‘O The meaning of these statements, and of the definitions given for RDI and RDA, is that the recommendations are what are termed here individual recommendations: no individual should get less than the amount stated (Figure 1). This meaning leads to great difficulties when it comes to applying them.
Recommendations and requirements Recommendations and requirements are clearly not the same thing, but the distinction is sometimes overlooked. The FAO/WHO Handbook on Human Nutritional Requirements” summarizes all the previous reports in a table headed ‘Recommended intakes of nutrients’, but gives no table of requirements. Hegstedlz has made the same point. Requirement is a physiological concept, meaning the amount of a nutrient needed to maintain health. The transition from requirement to recommendation seems to be determined by two factors: the and the need for individual variability of requirements, recommendations to be practical within a social context. The classic case, illustrating both points, is that of protein. Variability
The coefficient of variation of individual protein requirements has been estimated at 15%.13If requirements are normally distributed, an intake
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Usesof recommended intakes
equal to the mean +_30% should cover the needs of 97.5% of the individuals in a population. At this intake the number of people who remain at risk (2.5%) is small enough to be acceptable from a public health point of view. This is the justification for the term ‘safe level of intake’ adopted by FAO/WHO. l4 In principle, unless other factors intervene, the recommended intake or allowance would be set at this level. The term ‘safe’ has the advantage of introducing the idea of risk, and hence of probability. Everyone accepts that a person who has an intake lower than the safe level is not necessarily deficient, but there is a probability that he/she may be deficient, which becomes greater the lower the intake. However, some authorities consider it unwise to promote the idea that people who are perfectly healthy may theoretically be in an unsafe situation, and prefer the term ‘group requirement’. Practicality
The second consideration which has to be taken into account in making recommendations is that they should be practicrl. Therefore they must not be out of line with established dietary patterns and eating habits. In the UK and the USA the habitual protein intake of almost everyone is well above the ‘safe level’, so that to recommend this level as an intake to be aimed for would be unrealistic. In the UK, the recommendation is that the protein intake should be not less than 10% of the total energy intake, which is a purely pragmatic approach. In the NAS report the recommended allowance of protein is the same as the FAO/WHO safe level, eg 54 g per day for an adult man. The report admits that this is quite unrealistic in the North American context and says: ‘RDA should not be used as justifications for reducing habitual intakes of nutrients’. An allowance is thus recommended, and then it is advised that the recommendation should be ignored! If practical considerations are taken into account, as they obviously must be, it is perfectly reasonable that recommendations should differ from one country to another, even though estimates of human requirements may be the same in the different countries. The range of individual variation in requirements has not been established with confidence for other nutrients, apart from energy. In general recommendations are based on a mixture of clinical, epidemiological and experimental evidence. Very often a margin of safety is added with the aim of producing an alleged optimum nutritional state. Different sets of recommendations differ in their criteria of what is adequate. For example, for vitamin C the NAS recommends a daily allowance for an adult of 45 mg, in order to produce tissue saturation. The DHSS Committee did not consider that tissue saturation was important. Their recommended intake of 30 mg/day was based on the fact that 10 mg is enough to prevent and cure deficiency; and then a ‘generous margin of safety’ was added. In the case of vitamin A the NAS report considers that ‘500-600 mcg of retinol . . . is a minimum requirement for adults to maintain an adequate blood concentration and to prevent all deficiency symptoms’. However, the report goes on to recommend a daily allowance in excess of this, because surveys in the USA and Canada had shown that ‘significant fractions of the populations had low reserves’. The assumption is made that in some way it is important for
l4 Ibid.
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Uses of recommended intakes
health to have good ‘stores’ of vitamin A; as is shown below, this leads to a very unsatisfactory situation. Everyone recognizes that the criteria on which requirements and recommendations are based are in many cases rather speculative. The purpose of this article is not to criticize the derivation of these criteria but to discuss the difficulties encountered in applying them.
Uses of recommendations The main uses are of two kinds: for prescription or planning, and for diagnosis. A third use, which is becoming increasingly important, is for the description of foods on packages and labels.
I5Eg vitamin C in the winter in northern vitamin A in some tropical Europe, countries, thiamin where the staple is rice, etc. l6 Except under special circumstances, such as in the formulation of baby foods.
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Prescriptive use The NAS report includes among appropriate uses of RDAs, ‘planning and procuring food supplies of population groups’ and ‘guidelines in establishing policy for health and welfare groups’. The UK report merely says that ‘The recommendations may be used as guides for caterers and dietitians when planning diets for groups’. In both cases the operative word is groups. It is likely that those who are planning food supplies and diets take account of the recommendations only in a rough and ready way. Suppose that the planner is concerned with a school feeding programme which is designed to supply half the child’s daily recommended intake/allowance. It seems probable that the first step will be to determine the average energy requirement per head, ie the amount of food, with the usual allowance for wastage and other factors. The second step will probably be to decide how to provide this food in terms of a diet which fulfils certain criteria: it should be acceptable, and conform more or less to the eating pattern of the population; it would have to be within whatever economic limits are laid down; finally, it should be ‘balanced’, containing a mixture of the Because our knowledge of main categories of foodstuffs. requirements, both quantitative and qualitative, is very limited, the reports emphasize the importance of a diet based on as wide a variety of foods as possible. Perhaps in some cases there will be a third step in the planning process: a check on a few nutrients which are likely to be in short supply in the dietary pattern of the community.15 All this is quite straightforward, although it seems inevitable that the hypothetical planning process as described above will leave a good part of our scientific knowledge unused. For example, one wonders whether people will really ever plan diets to make sure that they provide 15 mg of zinc per day (as recommended by the NAS).i6 The real difficulty is that the planning process does not fit with the definition of RDAs or RDIs as individual recommendations. If, on the other hand, food is provided according to the average amount needed per head by the group - and this seems the only realistic way of providing it - some will eat more and some less. The only way of ensuring that every individual receives the recommended amount of every nutrient would be to measure the food intake of the smallest eaters, and then devise a diet with a high enough concentration of all other nutrients for the recommendations to be met. This is the kind of thing which may have to be done for special groups with low energy intakes, such as those on slimming diets or the elderly, but for genera1
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Figure nutrients
3.
Recommended (proposed
intakes
for
definition).
application it would be an extremely wasteful and unpractical procedure. It may be concluded that for purposes of prescription, recommendations must apply to groups and not to individuals; and that in planning the provision of nutrients, the recommendations in practice must represent the average amount to beprovidedper head. This concept is illustrated in Figure 3. The RDI or RDA for a nutrient, defined in this way, is an average recommendation, like that for energy. However, there is an important difference: the recommendation for energy is equated to the average requirement, while that for other nutrients is set at a level higher than the average requirement, in order to cover individual variations and to provide a margin of safety (compare Figures 2 and 3). However, it is apparent from Figure 3 that if the average intake is equal to that recommended, we can never be sure that a few individuals may not be getting less than their requirements. Therefore Hegsted’s proposal” to call the RDA the ‘no risk level’ represents an unattainable ideal. It follows from this discussion that the original definition of RDAs and RDIs as individual requirements (see Figure 1) is inconsistent with the usage. Therefore, the definition needs to be changed. Use in diagnosis It is common practice to compare the intakes observed in dietary surveys with RDI or RDA. In both the NAS and the DHSS reports the point is made that an intake below that recommended does not mean deficiency, but a risk of deficiency - a risk which must increase the further the actual intake falls below the recommended one. The presence of deficiency can only be proved by clinical and/or biochemical examination. Surveys are made on groups of individuals. Warnings are given in the reports against expressing the results of such surveys as average intakes, although this is often done. In the UK the National Food Survey presents quarterly reports of the average amounts of nutrients purchased by a representative sample of the population, expressed as percentages of the amounts recommended. Figures of this kind are of value for showing trends, but for little else from a nutritional point of view. The survey is, however, designed to provide running information about food moving into consumption, rather than information about nutritional state. The median intake would be better than the mean for purposes of assessment, because it reduces the effect of extreme results in either direction. It is more useful to provide some information about the number of people whose intake is below that recommended. It would be a simple matter to grade the intakes (eg 60-SO%, 80-100% of the RDI), and attention could then be concentrated on those subjects with intakes at the lower levels. A practical example
I7 Hegsted. ‘On dietary standards’, op tit, Ref 3. ‘*Nutrition Canada, Vol 1, Governmen Printer, Ottawa, 1973.
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These principles are obvious enough. How do they work out in practice? A good example is provided by the large scale survey, Nutrition Canada.1n In the summary volume of the report the results are presented as the percentage of subjects whose intake falls in each of three categories, defined as follows: Inadequate intakes are
consideredto
be below the desirable amount of a nutrient.
111
Uses of recommended intakes Table 1. Proportion
of subjects receiving ‘less than adequate’
intakes of four vitamins.
% of subjects receiving less than adequate Vitamin %
Source: Government
Nutrition Canada, Vol Printer, Ottawa, 1973.
I,
Vitamin 56
A
Males, age (years): 10-19 20-39 40-64 65+
29 27 30 46
Females, age (years): IO-19 20-39 40-64 65+
48
ia
47 55 54
21 17 13
‘Adequate’
daily intake:
Thiamin 96
C
28
14 14 16 17
30 mg
759jLg
intake Riboflavin %
42 40 41
22 32 30 41
38 48
32 42
50 53
48 48
0.8 mg
1.1 mg
Table 2. Proportion of subjects receiving ‘less than adequate’ intakes of vitamins A o; Ca compared with those at moderate or high risk of deficiency by biochemical criteria.
Vitamin
a For ‘less Table I.
than
adequate
intake’
see
b
Biochemical criteria of risk, for vitamin A (all ages..): < 30 pg/lOO ml plasma: for vitamin C (O-l 9 years): < 0.6 mg/lOO ml plasma. and (20+ years):<0.4 mg/lOO ml plasma. Source: Nutrition Canada, Vol Government Printer, Ottawa, 1973.
1,
Children, 04 5-9
%at biochemical risk
9.5
23.0 15.0
% with less than adequate intake
C %at biochemical risk
age (years) :
Males, age (years)
10-19 20-39 40-64 65+
Vitamin
A
% with less than adequate intake
la.0
10.5 15.5
20.0 17.0
14.0 14.0 16.0 17.0
25.0 25.0 30.0 33.0
: 29.0 27.0 30.0 46.0
0.6 0.6 0.4 0.6
Less than adequate intakes are those above the minimum requirement but below the desirable amount of a nutrient. Adequate intakes are those providing a desirable margin of safety in meeting the body’s needs of a nutrient.
l9 See Ref 3
112
These definitions are not very clear (eg what is the difference between ‘minimum’ and ‘desirable’?), but they can be taken as referring to zones of risk. The cut-off points between the zones are given in tables. The report does not make any explicit use of figures for RDI or RDA, but in general the cut-off point between ‘less than adequate’ and ‘adequate’ corresponds with recommended intakes or allowances in the UK or USA. Examples of the results are presented in Tables 1 and 2. It is apparent that, in adults, 25-50% of the intakes of vitamin A, thiamin and riboflavin, and up to 20% of the intakes of vitamin C are considered to be less than adequate. Information is not given about the extent to which the inadequate intakes of the different vitamins are found in the same subjects. This high prevalence of less than adequate intakes seems a remarkable situation for a developed country, but it is certainly not unique. Similar results have been found for some vitamins in some age and/or sex groups in surveys in the UK (unpublished). How are such findings to be interpreted? The first possibility is that the estimates of intake are incorrect, since, as Hegstedi9 has pointed out, there are many sources of error. The Canadian results were obtained by 24 hour recall, but in the UK intakes have been measured
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Uses of recommended intakes
by the more accurate method of weighing over several days. If the intakes are accepted, the possibility then has to be considered that some subjects really were deficient. The next step, therefore, is to look at the clinical and biochemical picture. In the Canadian survey there were no clinical signs of vitamin A or riboflavin deficiency in any subgroup. The prevalence of signs indicating ‘high risk’ of vitamin C deficiency ranged from 0 to 0.8% in the various age groups. The prevalence of signs of thiamin deficiency was similar in subjects below 65 years old.20 A comparison between biochemical assessment and nutrient intakes is shown in Table 2 for vitamins A and C. Results for thiamin and riboflavin are not included because the method used (urinary excretion of the vitamin) is probably an index only of recent intake. It is apparent from the table that for vitamin A there is absolutely no agreement between the two methods of assessment, so that in one of them the criteria for the cut-off points must be erroneous. For vitamin C the results given by the two methods fit better. It should be noted that these are comparisons of averages; no information is given about the extent to which intake and biochemical levels are correlated in individuals. This is not the place to discuss the interpretation of biochemical findings in so-called ‘subclinical’ deficiency states. The only point to be made is that even if we do have information which relates intake and biochemical level, the biochemistry adds nothing unless independent criteria (clinical, experimental) have been established for interpreting it. If vitamin A is taken as an example, can it be considered a satisfactory situation if 50% of subjects are receiving less than the recommended intake? If, as in the original definition, the recommendation requires that each individual should have a certain minimum intake of vitamin A, then the recommendation is clearly not being fulfilled. It may be said that this does not matter, since there were no clinical or biochemical signs of deficiency, but it seems to make nonsense of an official recommendation if it is immaterial whether or not it is met. The logical conclusion would be that the recommendation is unrealistically high, and has no practical value. If, on the other hand, one accepts that the recommendation is intended to specify the average intake for a group, then it can be said that in this group the result was operationally satisfactory; the mean intake was close to that recommended, and there were no individuals with signs of deficiency. On the basis of this experience the planner could use the recommended figure with some expectation that it would be adequate for the group as a whole. The recommendation may still be unnecessarily high. This will become apparent if other groups are found whose average intake is lower, but who still have no signs of deficiency.
Diagnosis of deficiency 2oAbove 65 the prevalence increased to 5-7%. The criteria of a ‘high risk’ of thiamin deficiency were any two of three signs: bilateral absence of knee jerks; bilateral pretibial pitting or oedema; loss of vibratory sense at the ankle. These are rather non-specific signs.
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There is obviously no certainty that, even where the average intake is equal to that recommended, no individuals will be deficient. This will depend on the range of intakes and the range of individual requirements. If the intake of each individual were known, and the mean and standard deviation of individual requirements (assumed to be
113
Uses of recommended intakes
Figure
4. Computation
subjects be
whose
less
than
requirements subjects
of number
of
(I) are likely to
their
(0)
individual
when
have an intake
recommended a
intakes
50%
of
less than the
amount.
SD units = standard
deviation
units.
It is assumed: (a) that requirements are normally distributed; fb) that the recommended intake (R) is equal to the mean requirement +2 standard deviations; and (c) that 50% of intakes are CR. with the distribution shown in the histogram. In the worst case, all the intakes in each column will fall on the left hand border of the column: in the best case they will all fall on the right hand border. Thus for the worst case, column by column:
% of subjects
Probability that I < 0
5 9 15 21
0.50 0.31 0.16 0.07
Total
46 of subjects for whom it is likely that I < Q 2.5 2.8 2.4 1.5 9.2%
A similar computation for the best case gives 4.5% of subjects whose intake is likely to be less than their requirement.
21There
is a parallel with reference weights for children, which are (or should be) used for comparative purposes rather than as ideals or targets.
114
normally distributed), it would be possible to estimate the number of subjects whose intake was actually below their requirement. An example of such a calculation is shown in Figure 4. The outcome of this calculation is that, on the basis of the assumptions made, the number of people who are deficient (defined as having an intake less than their individual requirement) is likely to be less by almost an order of magnitude than the number of people at risk (defined as those having an intake less than that recommended). Since estimates of requirements are in most cases rather uncertain, and knowledge of the range of individual variation is almost nonexistent (except for energy and protein), it would be absurd to push such calculations too far. They do, however, seem to fit in with general experience, that where the mean or median intake of a nutrient is close to the recommended intake/allowance, although a large proportion of subjects will have less than the amount recommended, there is unlikely to be any overt deficiency. Perhaps one might say pragmatically that so long as the median intake is greater than the RDA, the situation is acceptable for the purpose of public health. It would be important, however, to have a system for monitoring the state of those whose intakes were at the lower end of the scale. If this interpretation is accepted, as in practice many authorities seem to do in evaluating the results of their surveys, it quite clearly does not fit the original definition of RDI and RDA for nutrients other than energy. If that definition were abandoned, and the one proposed here used instead, it is likely that more use would be made of dietary surveys for testing the validity of recommendations. At present the aim of such surveys, if the results are used at all, is to determine whether a significant number of people are supposed to be at risk. There is very little feedback, so that opportunities for revising and refining the recommendations are lost.
Conclusion The definition of recommended intakes or allowances, as given in the DHSS and NAS reports, appears to be unusable both for the purpose of prescription and planning and for that of diagnosis. It is clearly a great mistake if official recommendations are made which cannot be, and which are not expected to be, fulfilled. It is therefore suggested that the word recommended should no longer be used, and that the figures used should be called reference amounts of a nutrient. This term could be applied both for prescription and for diagnosis. The word ‘reference’ would be defined as the average amount of a nutrient which, on the basis of current knowledge and experience, should be provided or consumed per person, to meet the needs of a group. If an individual consumed less than the reference amount he would, as now, be considered potentially at risk, but there would be no implication that a government recommendation was not being fulfilled. This would only be the case if a group received less than the reference amount. ‘Group requirement’ might also be a suitable term; however ‘reference’ has the useful implication that the figures are not intended to be absolutes, but are used mainly for comparing groups or monitoring trends.*’ This article ends, as it began, with a discussion of nomenclature. This is important; confusion over names leads to confusion of ideas.
FOOD
POLICY
May 1979
J.C. Waterlow
The definition of recommended (RDVRDAI in intakes/allowances official reports implies that the RDI represents the minimum amount of nutrient which should be a consumed by each individual. The author argues that this definition makes it impossible to apply the recommendations to groups, either for planning diets or for assessing the adequacy of intakes. He proposes an alternative and more useful definition of the RDI as the head average intake per recommended for a group. J.C. Waterlow Nutrition
at
is Professor the
Human
Nutrition,
Hygiene
and
Keppel
Street,
of Human
Department London
Tropical London
of
School
of
Medicine, WClE
7HT,
Definitions
UK. This article is based on a paper given at a meeting of the Societe de Nutrition et Dieteti&e de Langue Francaise held in Tours in Mav 1978 in honour of the late Professor ‘Jean Tremolieres, and published in French in the Cahiers de Nutrition et Dibtbtique. I am grateful to the editors of the journal for permission to reproduce the article in English.
’ ‘Round table on comparison of dietary recommendations in different European countries’, in N. Zollner, G. Wolfram and Ch. Keller, eds, Proceedings of Second European Nutrition Conference, Munich 1976 Karger, Basel, 1977, pp 210-279.
0306-9192/79/020107-08
$02.00
Many national and international bodies have devoted a great deal of time to reviewing our knowledge of nutrient requirements and making recommendations on intakes and allowances. The US National Academy of Sciences has issued eight reports on this subject since 1943, and a great many other countries, including the UK, have published their own recommendations.’ The Food and Agriculture Organization (FAO) and the World Health Organization (WHO) have jointly produced a large number of reports on requirements for energy and various nutrients. z In spite of this volume of literature there seems to be some confusion about how the figures should be used; there is very little information about the extent to which they are in fact useful and it is clear that they are sometimes misused. Some of these matters have been discussed by Hegs&d3 The purpose of this article is to go one step further in trying to define the dilflculties. The discussion will be based primarily on the report from the UK’s Department of Health and Social Security (DHSS),4 and the report from the Food and Nutrition Board of the US National Academy of Sciences (NAS).5
The title of the DHSS report is Recommended Intakes of Nutrients6 for the United Kingdom (RDI); that of the NAS report is Recommended Dietary Allowances (RDA). According to the definitions given, the meanings of RDI and RDA are the same. In the DHSS report, ‘Recommended intakes for nutrients are defined as the amounts sufficient or more than sufficient for the nutritional needs of practically all healthy persons in a population’. According to the NAS report, ‘Recommended dietary allowances are the levels of intake of essential nutrients considered . . . to be adequate to meet the known nutritional needs of practically all healthy persons’. These definitions are the same, almost word for word, as the US definition equates a recommended allowance with a level of intake. In plain English, however, there is a difference between an intake and an allowance.
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107
Uses of recommended intakes
Figure
1. Recommended
nutrients
(DHSS,
intakes
for
1969 definitions).
L Figure
2.
Recommended
intakes
for
energy.
z Summarized in R. Passmore, B.M. Nicol, M. Narayana Rao, G.H. Beaton and E.M. Handbook on Human DeMaeyer, Nutritional Requirements, World Health Organization Monograph Series No 61, WHO, Geneva, 1974. Hegsted. ‘Dietary standards’, 3 D.M. the American Dietetic Journal of Association, Vol66, 1975, pp 13-20; and ‘On dietary standards’, Nutrition Reviews, Vol 36, 1978, pp 33-36. 4 Department of Health and Social Recommended Intakes of Security, Nutrients for the United Kingdom, Reports on Public Health and Medical Subjects No 120, HMSO, London, 1969. Sciences, 5 National Academy of Recommended Dietary Allowances. 8th revised edition, National Academy of Sciences, Washington DC, 1974. 6 In official writings in the UK it is technically incorrect to considered describe energy as a nutrient. This distinction seems not to be made in the USA. 7 In French, apport. * Hegsted, ‘On dietary standards’, op cit. Ref 3. 9 NAS. op cit. Ref 5. p 3. lo DHSS, op cit. Ref 4. para 9. I1 Passmore et al, op cit. Ref 2. I2 Hegsted, ‘On dietary standards’, op cit. Ref 3. and Protein Energy l3 FAO/WHO, Joint Report of a Requirements, FAOWHO Ad Hoc Expert Committee, FAO Nutrition Meetings Report Series No 52, FAO, Rome 1973.
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‘Intake’ refers to what is eaten, ‘allowance’ to what is provided;’ not exactly the same thing. The distinction is only important when it comes to application of the recommendations. What is important is to examine precisely what these definitions mean. The natural meaning is that each individual in a group should get at least the amount recommended, and that this amount will be adequate for all or almost all members of the group. Hegsted* calls it the ‘minimum no risk level of intake’. It might be appropriate to call a recommendation with this meaning an ‘individual recommendation’, as illustrated diagrammatically in Figure 1. The situation with energy (calorie intake) is quite different. Here all authorities agree that the RDI or RDA for a group should be equal to the average requirement of the group, no greater and no less. The reasons for this are well known and need not be further considered here. A recommendation of this type could be called an ‘average recommendation’, as illustrated in Figure 2. The key question is whether the authors of the DHSS and NAS reports really intended their recommendations for nutrients other than energy to be of the type called ‘individual recommendations’. It is probable that they did, since in both reports a clear distinction is made between the recommendations for energy and those for other nutrients. Furthermore, the NAS report says categorically: RDA are neither estimates of the amounts of nutrients needed per capita (sic) in the national or local food supply, nor even in the food purchased . . . RDA (except for energy) are estimated to exceed the requirements of most individuals, and thereby ensure that the needs of nearly all are met.9
Likewise, the UK report says, ‘The recommended intakes, which are judged to be sufficient for practically all individual members of a population, must of necessity be in excess of the requirements of most of them’.‘O The meaning of these statements, and of the definitions given for RDI and RDA, is that the recommendations are what are termed here individual recommendations: no individual should get less than the amount stated (Figure 1). This meaning leads to great difficulties when it comes to applying them.
Recommendations and requirements Recommendations and requirements are clearly not the same thing, but the distinction is sometimes overlooked. The FAO/WHO Handbook on Human Nutritional Requirements” summarizes all the previous reports in a table headed ‘Recommended intakes of nutrients’, but gives no table of requirements. Hegstedlz has made the same point. Requirement is a physiological concept, meaning the amount of a nutrient needed to maintain health. The transition from requirement to recommendation seems to be determined by two factors: the and the need for individual variability of requirements, recommendations to be practical within a social context. The classic case, illustrating both points, is that of protein. Variability
The coefficient of variation of individual protein requirements has been estimated at 15%.13If requirements are normally distributed, an intake
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Usesof recommended intakes
equal to the mean +_30% should cover the needs of 97.5% of the individuals in a population. At this intake the number of people who remain at risk (2.5%) is small enough to be acceptable from a public health point of view. This is the justification for the term ‘safe level of intake’ adopted by FAO/WHO. l4 In principle, unless other factors intervene, the recommended intake or allowance would be set at this level. The term ‘safe’ has the advantage of introducing the idea of risk, and hence of probability. Everyone accepts that a person who has an intake lower than the safe level is not necessarily deficient, but there is a probability that he/she may be deficient, which becomes greater the lower the intake. However, some authorities consider it unwise to promote the idea that people who are perfectly healthy may theoretically be in an unsafe situation, and prefer the term ‘group requirement’. Practicality
The second consideration which has to be taken into account in making recommendations is that they should be practicrl. Therefore they must not be out of line with established dietary patterns and eating habits. In the UK and the USA the habitual protein intake of almost everyone is well above the ‘safe level’, so that to recommend this level as an intake to be aimed for would be unrealistic. In the UK, the recommendation is that the protein intake should be not less than 10% of the total energy intake, which is a purely pragmatic approach. In the NAS report the recommended allowance of protein is the same as the FAO/WHO safe level, eg 54 g per day for an adult man. The report admits that this is quite unrealistic in the North American context and says: ‘RDA should not be used as justifications for reducing habitual intakes of nutrients’. An allowance is thus recommended, and then it is advised that the recommendation should be ignored! If practical considerations are taken into account, as they obviously must be, it is perfectly reasonable that recommendations should differ from one country to another, even though estimates of human requirements may be the same in the different countries. The range of individual variation in requirements has not been established with confidence for other nutrients, apart from energy. In general recommendations are based on a mixture of clinical, epidemiological and experimental evidence. Very often a margin of safety is added with the aim of producing an alleged optimum nutritional state. Different sets of recommendations differ in their criteria of what is adequate. For example, for vitamin C the NAS recommends a daily allowance for an adult of 45 mg, in order to produce tissue saturation. The DHSS Committee did not consider that tissue saturation was important. Their recommended intake of 30 mg/day was based on the fact that 10 mg is enough to prevent and cure deficiency; and then a ‘generous margin of safety’ was added. In the case of vitamin A the NAS report considers that ‘500-600 mcg of retinol . . . is a minimum requirement for adults to maintain an adequate blood concentration and to prevent all deficiency symptoms’. However, the report goes on to recommend a daily allowance in excess of this, because surveys in the USA and Canada had shown that ‘significant fractions of the populations had low reserves’. The assumption is made that in some way it is important for
l4 Ibid.
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Uses of recommended intakes
health to have good ‘stores’ of vitamin A; as is shown below, this leads to a very unsatisfactory situation. Everyone recognizes that the criteria on which requirements and recommendations are based are in many cases rather speculative. The purpose of this article is not to criticize the derivation of these criteria but to discuss the difficulties encountered in applying them.
Uses of recommendations The main uses are of two kinds: for prescription or planning, and for diagnosis. A third use, which is becoming increasingly important, is for the description of foods on packages and labels.
I5Eg vitamin C in the winter in northern vitamin A in some tropical Europe, countries, thiamin where the staple is rice, etc. l6 Except under special circumstances, such as in the formulation of baby foods.
110
Prescriptive use The NAS report includes among appropriate uses of RDAs, ‘planning and procuring food supplies of population groups’ and ‘guidelines in establishing policy for health and welfare groups’. The UK report merely says that ‘The recommendations may be used as guides for caterers and dietitians when planning diets for groups’. In both cases the operative word is groups. It is likely that those who are planning food supplies and diets take account of the recommendations only in a rough and ready way. Suppose that the planner is concerned with a school feeding programme which is designed to supply half the child’s daily recommended intake/allowance. It seems probable that the first step will be to determine the average energy requirement per head, ie the amount of food, with the usual allowance for wastage and other factors. The second step will probably be to decide how to provide this food in terms of a diet which fulfils certain criteria: it should be acceptable, and conform more or less to the eating pattern of the population; it would have to be within whatever economic limits are laid down; finally, it should be ‘balanced’, containing a mixture of the Because our knowledge of main categories of foodstuffs. requirements, both quantitative and qualitative, is very limited, the reports emphasize the importance of a diet based on as wide a variety of foods as possible. Perhaps in some cases there will be a third step in the planning process: a check on a few nutrients which are likely to be in short supply in the dietary pattern of the community.15 All this is quite straightforward, although it seems inevitable that the hypothetical planning process as described above will leave a good part of our scientific knowledge unused. For example, one wonders whether people will really ever plan diets to make sure that they provide 15 mg of zinc per day (as recommended by the NAS).i6 The real difficulty is that the planning process does not fit with the definition of RDAs or RDIs as individual recommendations. If, on the other hand, food is provided according to the average amount needed per head by the group - and this seems the only realistic way of providing it - some will eat more and some less. The only way of ensuring that every individual receives the recommended amount of every nutrient would be to measure the food intake of the smallest eaters, and then devise a diet with a high enough concentration of all other nutrients for the recommendations to be met. This is the kind of thing which may have to be done for special groups with low energy intakes, such as those on slimming diets or the elderly, but for genera1
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Uses of recommended intakes
Figure nutrients
3.
Recommended (proposed
intakes
for
definition).
application it would be an extremely wasteful and unpractical procedure. It may be concluded that for purposes of prescription, recommendations must apply to groups and not to individuals; and that in planning the provision of nutrients, the recommendations in practice must represent the average amount to beprovidedper head. This concept is illustrated in Figure 3. The RDI or RDA for a nutrient, defined in this way, is an average recommendation, like that for energy. However, there is an important difference: the recommendation for energy is equated to the average requirement, while that for other nutrients is set at a level higher than the average requirement, in order to cover individual variations and to provide a margin of safety (compare Figures 2 and 3). However, it is apparent from Figure 3 that if the average intake is equal to that recommended, we can never be sure that a few individuals may not be getting less than their requirements. Therefore Hegsted’s proposal” to call the RDA the ‘no risk level’ represents an unattainable ideal. It follows from this discussion that the original definition of RDAs and RDIs as individual requirements (see Figure 1) is inconsistent with the usage. Therefore, the definition needs to be changed. Use in diagnosis It is common practice to compare the intakes observed in dietary surveys with RDI or RDA. In both the NAS and the DHSS reports the point is made that an intake below that recommended does not mean deficiency, but a risk of deficiency - a risk which must increase the further the actual intake falls below the recommended one. The presence of deficiency can only be proved by clinical and/or biochemical examination. Surveys are made on groups of individuals. Warnings are given in the reports against expressing the results of such surveys as average intakes, although this is often done. In the UK the National Food Survey presents quarterly reports of the average amounts of nutrients purchased by a representative sample of the population, expressed as percentages of the amounts recommended. Figures of this kind are of value for showing trends, but for little else from a nutritional point of view. The survey is, however, designed to provide running information about food moving into consumption, rather than information about nutritional state. The median intake would be better than the mean for purposes of assessment, because it reduces the effect of extreme results in either direction. It is more useful to provide some information about the number of people whose intake is below that recommended. It would be a simple matter to grade the intakes (eg 60-SO%, 80-100% of the RDI), and attention could then be concentrated on those subjects with intakes at the lower levels. A practical example
I7 Hegsted. ‘On dietary standards’, op tit, Ref 3. ‘*Nutrition Canada, Vol 1, Governmen Printer, Ottawa, 1973.
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These principles are obvious enough. How do they work out in practice? A good example is provided by the large scale survey, Nutrition Canada.1n In the summary volume of the report the results are presented as the percentage of subjects whose intake falls in each of three categories, defined as follows: Inadequate intakes are
consideredto
be below the desirable amount of a nutrient.
111
Uses of recommended intakes Table 1. Proportion
of subjects receiving ‘less than adequate’
intakes of four vitamins.
% of subjects receiving less than adequate Vitamin %
Source: Government
Nutrition Canada, Vol Printer, Ottawa, 1973.
I,
Vitamin 56
A
Males, age (years): 10-19 20-39 40-64 65+
29 27 30 46
Females, age (years): IO-19 20-39 40-64 65+
48
ia
47 55 54
21 17 13
‘Adequate’
daily intake:
Thiamin 96
C
28
14 14 16 17
30 mg
759jLg
intake Riboflavin %
42 40 41
22 32 30 41
38 48
32 42
50 53
48 48
0.8 mg
1.1 mg
Table 2. Proportion of subjects receiving ‘less than adequate’ intakes of vitamins A o; Ca compared with those at moderate or high risk of deficiency by biochemical criteria.
Vitamin
a For ‘less Table I.
than
adequate
intake’
see
b
Biochemical criteria of risk, for vitamin A (all ages..): < 30 pg/lOO ml plasma: for vitamin C (O-l 9 years): < 0.6 mg/lOO ml plasma. and (20+ years):<0.4 mg/lOO ml plasma. Source: Nutrition Canada, Vol Government Printer, Ottawa, 1973.
1,
Children, 04 5-9
%at biochemical risk
9.5
23.0 15.0
% with less than adequate intake
C %at biochemical risk
age (years) :
Males, age (years)
10-19 20-39 40-64 65+
Vitamin
A
% with less than adequate intake
la.0
10.5 15.5
20.0 17.0
14.0 14.0 16.0 17.0
25.0 25.0 30.0 33.0
: 29.0 27.0 30.0 46.0
0.6 0.6 0.4 0.6
Less than adequate intakes are those above the minimum requirement but below the desirable amount of a nutrient. Adequate intakes are those providing a desirable margin of safety in meeting the body’s needs of a nutrient.
l9 See Ref 3
112
These definitions are not very clear (eg what is the difference between ‘minimum’ and ‘desirable’?), but they can be taken as referring to zones of risk. The cut-off points between the zones are given in tables. The report does not make any explicit use of figures for RDI or RDA, but in general the cut-off point between ‘less than adequate’ and ‘adequate’ corresponds with recommended intakes or allowances in the UK or USA. Examples of the results are presented in Tables 1 and 2. It is apparent that, in adults, 25-50% of the intakes of vitamin A, thiamin and riboflavin, and up to 20% of the intakes of vitamin C are considered to be less than adequate. Information is not given about the extent to which the inadequate intakes of the different vitamins are found in the same subjects. This high prevalence of less than adequate intakes seems a remarkable situation for a developed country, but it is certainly not unique. Similar results have been found for some vitamins in some age and/or sex groups in surveys in the UK (unpublished). How are such findings to be interpreted? The first possibility is that the estimates of intake are incorrect, since, as Hegstedi9 has pointed out, there are many sources of error. The Canadian results were obtained by 24 hour recall, but in the UK intakes have been measured
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Uses of recommended intakes
by the more accurate method of weighing over several days. If the intakes are accepted, the possibility then has to be considered that some subjects really were deficient. The next step, therefore, is to look at the clinical and biochemical picture. In the Canadian survey there were no clinical signs of vitamin A or riboflavin deficiency in any subgroup. The prevalence of signs indicating ‘high risk’ of vitamin C deficiency ranged from 0 to 0.8% in the various age groups. The prevalence of signs of thiamin deficiency was similar in subjects below 65 years old.20 A comparison between biochemical assessment and nutrient intakes is shown in Table 2 for vitamins A and C. Results for thiamin and riboflavin are not included because the method used (urinary excretion of the vitamin) is probably an index only of recent intake. It is apparent from the table that for vitamin A there is absolutely no agreement between the two methods of assessment, so that in one of them the criteria for the cut-off points must be erroneous. For vitamin C the results given by the two methods fit better. It should be noted that these are comparisons of averages; no information is given about the extent to which intake and biochemical levels are correlated in individuals. This is not the place to discuss the interpretation of biochemical findings in so-called ‘subclinical’ deficiency states. The only point to be made is that even if we do have information which relates intake and biochemical level, the biochemistry adds nothing unless independent criteria (clinical, experimental) have been established for interpreting it. If vitamin A is taken as an example, can it be considered a satisfactory situation if 50% of subjects are receiving less than the recommended intake? If, as in the original definition, the recommendation requires that each individual should have a certain minimum intake of vitamin A, then the recommendation is clearly not being fulfilled. It may be said that this does not matter, since there were no clinical or biochemical signs of deficiency, but it seems to make nonsense of an official recommendation if it is immaterial whether or not it is met. The logical conclusion would be that the recommendation is unrealistically high, and has no practical value. If, on the other hand, one accepts that the recommendation is intended to specify the average intake for a group, then it can be said that in this group the result was operationally satisfactory; the mean intake was close to that recommended, and there were no individuals with signs of deficiency. On the basis of this experience the planner could use the recommended figure with some expectation that it would be adequate for the group as a whole. The recommendation may still be unnecessarily high. This will become apparent if other groups are found whose average intake is lower, but who still have no signs of deficiency.
Diagnosis of deficiency 2oAbove 65 the prevalence increased to 5-7%. The criteria of a ‘high risk’ of thiamin deficiency were any two of three signs: bilateral absence of knee jerks; bilateral pretibial pitting or oedema; loss of vibratory sense at the ankle. These are rather non-specific signs.
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May 1979
There is obviously no certainty that, even where the average intake is equal to that recommended, no individuals will be deficient. This will depend on the range of intakes and the range of individual requirements. If the intake of each individual were known, and the mean and standard deviation of individual requirements (assumed to be
113
Uses of recommended intakes
Figure
4. Computation
subjects be
whose
less
than
requirements subjects
of number
of
(I) are likely to
their
(0)
individual
when
have an intake
recommended a
intakes
50%
of
less than the
amount.
SD units = standard
deviation
units.
It is assumed: (a) that requirements are normally distributed; fb) that the recommended intake (R) is equal to the mean requirement +2 standard deviations; and (c) that 50% of intakes are CR. with the distribution shown in the histogram. In the worst case, all the intakes in each column will fall on the left hand border of the column: in the best case they will all fall on the right hand border. Thus for the worst case, column by column:
% of subjects
Probability that I < 0
5 9 15 21
0.50 0.31 0.16 0.07
Total
46 of subjects for whom it is likely that I < Q 2.5 2.8 2.4 1.5 9.2%
A similar computation for the best case gives 4.5% of subjects whose intake is likely to be less than their requirement.
21There
is a parallel with reference weights for children, which are (or should be) used for comparative purposes rather than as ideals or targets.
114
normally distributed), it would be possible to estimate the number of subjects whose intake was actually below their requirement. An example of such a calculation is shown in Figure 4. The outcome of this calculation is that, on the basis of the assumptions made, the number of people who are deficient (defined as having an intake less than their individual requirement) is likely to be less by almost an order of magnitude than the number of people at risk (defined as those having an intake less than that recommended). Since estimates of requirements are in most cases rather uncertain, and knowledge of the range of individual variation is almost nonexistent (except for energy and protein), it would be absurd to push such calculations too far. They do, however, seem to fit in with general experience, that where the mean or median intake of a nutrient is close to the recommended intake/allowance, although a large proportion of subjects will have less than the amount recommended, there is unlikely to be any overt deficiency. Perhaps one might say pragmatically that so long as the median intake is greater than the RDA, the situation is acceptable for the purpose of public health. It would be important, however, to have a system for monitoring the state of those whose intakes were at the lower end of the scale. If this interpretation is accepted, as in practice many authorities seem to do in evaluating the results of their surveys, it quite clearly does not fit the original definition of RDI and RDA for nutrients other than energy. If that definition were abandoned, and the one proposed here used instead, it is likely that more use would be made of dietary surveys for testing the validity of recommendations. At present the aim of such surveys, if the results are used at all, is to determine whether a significant number of people are supposed to be at risk. There is very little feedback, so that opportunities for revising and refining the recommendations are lost.
Conclusion The definition of recommended intakes or allowances, as given in the DHSS and NAS reports, appears to be unusable both for the purpose of prescription and planning and for that of diagnosis. It is clearly a great mistake if official recommendations are made which cannot be, and which are not expected to be, fulfilled. It is therefore suggested that the word recommended should no longer be used, and that the figures used should be called reference amounts of a nutrient. This term could be applied both for prescription and for diagnosis. The word ‘reference’ would be defined as the average amount of a nutrient which, on the basis of current knowledge and experience, should be provided or consumed per person, to meet the needs of a group. If an individual consumed less than the reference amount he would, as now, be considered potentially at risk, but there would be no implication that a government recommendation was not being fulfilled. This would only be the case if a group received less than the reference amount. ‘Group requirement’ might also be a suitable term; however ‘reference’ has the useful implication that the figures are not intended to be absolutes, but are used mainly for comparing groups or monitoring trends.*’ This article ends, as it began, with a discussion of nomenclature. This is important; confusion over names leads to confusion of ideas.
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May 1979