Salt intak assessment by questionnaire and urinary sodium excretion

NUTRITION RESEARCH, Vol. 7, pp. 557-568, 1987 0271-5317/87 $3.00 + .00 Printed in the USA. Copyright (c) 1987 Pergamon Journals Ltd. All rights reserved.

SALT INTAKE ASSESSMENTBY QUESTIONNAIRE AND URINARY SODIUMEXCRETION R. Shepherd Ph.D., and C.A. Farleigh B.Sc. AFRC Institute of Food Research, Colney Lane, Norwich NR4 7UA, United Kingdom.

ABSTRACT A questionnaire designed to assess salt intake was distributed to a sample of 113 healthy female subjects. The questionnaire was validated with 36 subjects collecting complete 24-h urine samples (analysed for sodium) for 7 days, along with using pre-weighed table and cooking salt pots. Intake was well predicted for table salt use (r=0.67), cooking use (r=0.57), nondiscretionary intake (r=0.65), and total salt intake (r=0.61). The test-retest r e l i a b i l i t y for the estimate of total intake was good (r=0.75). Measuredtotal salt intake for the 36 subjects was found to be 7.4g NaCl/day, with only 11% from discretionary salt use. Mean questionnaire estimates agreed closely with the measured values. Keywords: Sodium, urine, questionnaire INTRODUCTION There is current interest in salt in the diet, and there have been suggestions that i t should be reduced (1,2). However, estimates of normal intake are not generally very good. It is possible to estimate intake from the average production and sale of salt, from food consumption data (e.g. the National Food Survey (3)), from dietary intake studies (e.g. 24-h recalls), or from urinary excretion of sodium. Fregly and Fregly (4) have reviewed American data from these sources and concluded that, whilst the different methods give somewhat different values, the general estimate of average intake is in excess of 10g NaCl/day. However, there are d i f f i c u l t i e s in generalising from data obtained in one country to another country where intake may be very different. In considering whether salt intake in the UK should be reduced, COMA(5) gave the estimate of average daily intake to be between 7 and 109 NaCl, with no reference for the source of this estimate. NACNE(2) quoted an average intake of 12g NaCl/day. This was derived from work by Bull and Buss (6), who calculated average intake from the National Food Survey (3) in conjunction with McCance and Widdowson's The Composition of Foods (7). Whilst this method gives an estimate for average intake derived from foods, there are d i f f i c u l t i e s in accounting for discretionary salt use. This is also a problem with 24-h recall or weighed intake methods. One method which gives a measure of total intake is urinary excretion of sodium, which although variable will give good estimation of intake when assessed over several days (8). Generally this does not allow for separate estimation of intake from different sources, but two methods have been used to achieve this. Shepherd et al (9) used pre-weighed table and cooking salt pots over the period of u r i n e ~ e c t i o n , in order to get an estimate of intake 557

558

R. SHEPHERDand C.A. FARLEIGH

from these sources. An alternative method has been to use salt with a lithium marker which allows estimation of the ingestion of both table and cooking salt from the excretion of lithium (10). Both of these methods have given similar estimates of discretionary salt use (9,10). The use of urine collections has two drawbacks, the f i r s t is that i t may alter habitual intake in the same way as do other intrusive dietary methods (11). There are also problems of compliance, and of practicability where a large number of subjects are to be assessed (e.g. in epidemiological studies). For these reasons various questionnaires have been devised for assessing nutrient intake (e.g. 12,13), and some have either included, or been specifically designed to measure, salt intake (14-20). Some of these have shown no prediction of intake assessed by urinary excretion (17), whilst others have shown good prediction (19,20). Questionnaire studies of salt intake have been reviewed elsewhere (20). The present study was designed to validate a modification of the questionnaire developed by Shepherd et al (20), and to use this to estimate intake with a larger number of female subjects. METHOD Questionnaire Design The questionnaire was a modification of one validated previously (20); the details of the questions and the scoring procedures are given elsewhere (20). The questions relevant to salt intake are shown in Figs. 1 and 2. In addition to these questions the questionnaire contained some which were irrelevant to salt intake and i t was t i t l e d 'Survey of Food Intake' in order to disguise i t s real purpose. The main change from the earlier version was the replacement of the original lOOmm response lines with lO-category scales. Table salt was estimated using the same question as previously (question 14), with subjects ticking whether they added salt at the table to a l i s t of nine foods (See Fig. 1). The responses to this question were then modified by the reported frequency of consumption of these foods, and the sum of responses over the nine foods was calculated (Q14FR). Subjects could also write in any other foods they added salt to, and the number of extra items was also included in the analysis (Q14EX). There were questions on adding salt to food at the table before tasting i t (Q3), and on the importance of salt for the enjoyment of food (QT) (see Fig. 2). Intake of salt present in foods (non-discretionary intake) was assessed from ratings of the frequency of consumption of a l i s t of 14 types of foods with relatively high salt content (question 15). The frequencies were rated on a 10-category scale from 'less than once a year' to 'three times a day or more', with all of the categories labelled (see Fig. 2). In addition to breakfast cereals the foods were bread, margarine or salted butter, cheese (except cottage cheese), bacon, sausages, meat pies (along with pate, luncheon meat), Oxo (along with Marmite, Bovril in foods or drinks), tinned vegetables (including baked beans), crisps, salted peanuts, soups (packet or tinned), shellfish (eg. prawns, cockles, shrimps), pickles and sauces (including salad cream). Estimates of salt intake from these foods were then calculated as previously (20), based on estimates of average portion sizes (21), and the composition of the foods (7). The contribution from milk was assessed with a separate question on the number of pints consumed per week (question 1) (see Fig, 2). The estimate of salt intake from all non-discretionary sources was calculated from these two questions (QIANDQ15). The only question changed substantially from the previous questionnaire was the one on cooking salt use, which was found to predict use only poorly.

ASSESSMENT OF SALT INTAKE

14)

559

Do you usually add salt to the following foods at the table?

Yes

(i)

Chips

(ii)

Boiled eggs

(iii)

Eggs

(iv)

Tomatoes

(v)

Boiled potatoes

(vl)

Baked potatoes

(vii)

Boiled rice

(viii)

Spaghetti or other pasta

(ix}

Porridge

(x)

Other foods

No

Not eaten

(other than boiled)

-

please specify

FIG. 1 Question on addition of table salt to foods.

The new question asked for an estimate of the number of teaspoons of salt per person added to vegetables cooked in the home (question 9) which is shown in Fig. 2. The response on this scale was then modified by the reported frequency of eating 'cooked vegetables', 'boiled potatoes', ' r i c e ' and 'spaghetti'; the total gave the value Q9FR. Questionnaire Distribution Questionnaires were distributed to 113 female subjects who were attending a General Practioners' Surgery in the Norwich area. These subjects were healthy and attending for routine purposes. Their ages ranged from 20 to 56 years.

560

R. SHEPHERDand C.A. FARLEIGH

1]

HOW many p i n t s of milk do you d r i n k in an average week? 0

I

1 -

I

2

I

3,

I

q

I

5

I

6

I

7

8

I

I

More

I

I

3) DO yOU add salt to any foor on y o u r plate before t a s t i n g it? Never

l

AIw.ays

I

7)

]

I

I

]

I

I

I

I

t

How important is salt to y o u r enjoyment of flood ?

Not Important

I

Very important

I ,1 .,I

I

t ,.I

]

I

I

I

9} C o u l d y o u estimate t h e n u m b e r of teaspoons o f salt ( p e r person~ added to vegetables d u r i n g Cooking at home? Q.

i

~

1

,

1

I

1}

I

2

I

2t

I

3

I

[

15) HOw often have you eaten the following types of food

Less Less Once than then a once a I)~ce a month year month

2 o r 3 Dnce times a week month ,

389

over

2 o~" 3 times a week

4

1

More

I

t h e past few months?

q or 5 Once times a a day ;w~k

Twice 3 or a more day times a day

(i) Breakfast cereeis (except p u f f e d Wheat, S u g a r Puffs, Shredded Wheat and

Muesli)

FIG. 2 Questions on adding table and cooking salt to foods, and an example from question 15 on the frequency of consumption of highsalt foods.

Validation A group of thirty-six subjects was selected from the 113 who had completed questionnaires. These collected complete 24-h urine samples over a period of 7 consecutive days, and used pre-weighed table and cooking salt pots over the same period. The urine was analysed for sodium content, which was then used as an estimate of total salt intake (8). Table salt use was assessed from the difference in weight of the table salt pot from the start to

ASSESSMENT OF SALT INTAKE

561

the end of the week. The cooking salt use was adjusted for the number of people cooked for, and an estimate of the proportion ingested. The subjects recorded foods eaten over the 7 days, whether cooking salt was added to each food, and i f so then for how many people the food was cooked. T h i s allowed the amount consumed by an individual to be estimated. This was further reduced to account for the amount of cooking salt thrown away. A recent study using a lithium marker technique revealed that only 30 per cent on average was ingested (10); this estimate was therefore used in the present study. Nondiscretionary salt intake was calculated as the difference between total intake and table and cooking salt use. The same t h i r t y - s i x subjects also completed the questionnaire for a second time, approximately two months after the f i r s t occasion. The scores were calculated in exactly the same way in order to assess the test-retest r e l i a b i l i t y of the questionnaire. Data Analy s i ~ The associations between the questionnaire estimates and measured intake were assessed u s i n g regression analysis. G i v e n that several tests of s t a t i s t i c a l significance have been performed, i t needs to be borne in mind that the probability levels quoted are for single tests. With multiple tests the probability levels would be s l i g h t l y raised but this would not substantially change the conclusions drawn. The relationships between the questionnaire items were assessed using principal components analysis. This was calculated using the correlation matrix in order to normalize the responses on the different variables, which could vary over different ranges. RESULTS Pr i nci pal _C~onent s Analys i s In order to test how well the questionnaire items were related to each other, a principal components analysis was calculated from the responses to the questions relating to salt intake. The f i r s t two cmnponents were used, and these accounted for 42% and 18% respectively of the variance in the responses. The loadings for the questions on these components are shown in Table 1. A loading close to 1 means that the component represents the original variable very well, whereas a loading close to zero means that the component is unrelated to the original variable. The variables with high loadings on one component are related together and may be taken as measuring a single underlying factor. The f i r s t component was found to be related to table salt use and to the general enjoyment of food with s a l t . The second component had a high loading for the questions on non-discretionary salt from foods, and a moderately high loading for the cooking salt question. Validation For the t h i r t y - s i x subjects whose intake was measured, simple regressions were calculated between the measured intake from table use, cooking use and non-discretionary sources, and the questionnaire items expected to predict intake from these sources. The correlation coefficients and the coefficients from the regression equations are shown in Table 2. From the regression equations an estimate of total intake was calculated as the sum of intake from the three sources. The correlation of this estimate with measured total intake is also shown in Table 2. I t can be seen that salt intake from a l l three sources and total intake are well predicted from the questionnaire responses.

562

R. SHEPHERD and C.A. FARLEIGH

TABLE 1 Loadings on the First Two Components from the Principal Components Analysis

(n=113). Question

Component i

component2

General Q7

0.65

0.32

Table salt Q3 QI4FR Q14EX

0.86 0.78 0.62

0.12 0,32 -0.04

Cooking salt Q9FR

0.19

0.49

Non-discretionaFy QIANDQ15

0.07

0.76

TABLE 2 Correlations and Regression Coefficients from the Regressions of Salt Intake Against the Appropriate Questionnaire Items (n=36), Variable

r

Regression Constant

Regression Coefficient

Table use vs QI4FR Cooking use vs QgFR Non-discretionary vs QIANDQ15 Total intake vs Total estimate

0.67***

0.044

0.55

0.57***

0.18

0.062

0.65***

2.90

2.24

0.61"**

0,99

0.87

*** p
ASSESSMENT OF SALT INTAKE

563

TABLE3 Correlations Between the Scores on the Principal Components and Intake from Different Sources (n=36). Source

Component 1

Table salt Cooking salt Non-discretionary Total salt intake

0.64*** 0.32 0.05 0.22

Component-20.54*** 0.44** 0.47** 0.56***

** p
Test-retest correlation

Q3 Q7 Q14FR Q14EX QIANDQ15

0.72*** 0.69*** 0.65*** 0.75*** 0.33* 0.73***

Principal Components Component 1 Component 2

0.71"** 0.72***

Q9FR

Total salt intake Estimate from separate sources Estimate from principal component scores * p
***

0.75*** 0.72***

p
Distribution of Intakes The means and standard deviations of the measured salt intake from different sources are shown in Table 5. Using the regression equations shown in Table 2, the intake of salt from different sources was calculated for the original 113 subjects. The means and standard deviations for these

564

R. SHEPHERDand C.A. FARLEIGH

calculations are also shown in Table 5. The results are similar to those measured on the 36 subjects, with a mean intake of 7.4g NaCl/day. TABLE 5 Means and Standard Deviations of S a ~ k e from Different Sources, Measured for 36 Female Subjects and Estimated by Questionnaire for 113 Female Subjects. Measured ]nta-ke (n:36)

Questionnaire (n=113)

Source (g NaCl/day) Table salt Cooking salt Non-discretionary salt Total salt intake

(g NaCl/day)

0.5 0.3 6.6

0,5 0.9 1.8

0.5 0.4 6.4

0,4 0.1 1,3

7.4

2.0

7.4

1,3

DISCUSSION As found previously with an earlier version of this questionnaire (20), prediction of intake from different sources was v e r y successful. In particular the prediction of cooking salt use was an improvmnent on that obtained with the previous version of the questionnaire. The r e l i a b i l i t i e s for the individual questions and for the estimates of total intake were around 0.7, with the exception of the number of extra items added to the l i s t of salted foods. The distribution of responses on this question was very skewed (with most subjects not writing in any extra items) and so the use of a parametric correlation may have been inappropriate. For the other questions the r e l i a b i l i t i e s were similar to those found with similar types of questionnaire (19,20), and means that the responses on this questionnaire are reasonably stable. Completion of the questionnaire at an interval of two months might have reduced these correlations due to differences in the consumption of seasonal foods. The results of the principal components analysis of the questionnaire responses did not reveal a single underlying component related to salt intake from all sources. Rather i t showed intake from table salt use to be separable from that for cooking use and non-discretionary intake. Taking this twocomponent solution and calculating a multiple regression on total intake gave relatively good prediction of intake, the non-discretionary component being the major predictor. When the questionnaire was used with the larger group of subjects both the means and standard deviations of intake were similar to those measured using urinary sodium excretion. Hencethis questionnaire would be useful as a method for assessing intake of salt where the use of urinary sodium excretion would prove impracticable. It should be noted, however, that the regression coefficients in Table 2 show that when the total intake is calculated from the sum of the questionnaire estimates of table, cooking and non-discretionary use, this does not give the best estimate of measured total intake. Ideally the regression constant should be zero and the regression coefficient should be equal to 1. The observed relationship shows the questionnaire slightly underestimates the low intakes and overestimates the high intakes.

ASSESSMENT OF SALT INTAKE

565

Previous UK estimates of intake have been rather variable. Druce (22) estimated intake to be in the range 10.8 to 12.2g NaCl/day using several different methods of estimation, but where he included males and females separately (with food recording for 24h) he found average daily intake for the females to be 7.4g N a C l . This presumably did not take into account the discretionary salt added to foods. Bull and Buss (6) using data from the National Food Survey calculated 6.6g NaCl/day to be the average intake from foods as bought and consumed in the home. The value for table and cooking salt intake is more d i f f i c u l t since all that is bought will not be ingested, but the value bought is 3.2g NaCl/day. I f all this were ingested i t would take the overall intake up to 9.8g NaCl/day, but Bull and Buss (6) used a different estimate for the amount added in cooking (although not at the table) to arrive at an estimate for total intake of 8.1g NaCl. The details of this estimate of cooking salt are not clear from the published abstract. In t h e i r proposals for dietary change, NACNE (2) arrived at a figure of 12g NaCl/day from Marmot's estimate (23), which took Bull and Buss's 8.1g estimate and added an arbitrary 20% for food consu,ned outside the home and an arbitrary 2g for table salt use. Since the National Food Survey does not include foods eaten outside the home, nor does i t account adequately for waste (24), this procedure is unlikely to yield a good esti~nate of average intake. There have been several studies using urinary sodium excretion to estimate average intake. Many of these have been designed to investigate differences between normotensive and hypertensive subjects, or to test the effects of salt reduction in the diet, rather than to try to estimate normal intake. The results of these studies are summarised in Table 6. The present finding of an average intake of 7.4g NaCl is somewhat lower than some of these estimates. However, taking into account that this is for females only, i t is in agreement with some of the more recent findings. TABLE 6 Results of Previous Studies Measuring Salt Intake in UK Populations.

.............................. Phear (25) Miall {26) Bing et al (27) Thomas et al (28) Beevers et al (29) Parfrey et al (30) Parfrey~ (31) Sanchez-Castillo et al

H~y_pertensives 9.1 7.2 9.1 8.8 I0.6 8.4 I0 .i 12.1a

(10T

Shepherd et al (9) Williams and Bingham {32)

Normotensives

Sex

9.0 8.0 8.9 9.2 11.5 10.1 10.1 11.3 8.9

Male Female Mixed Mixed Male Female Mixed Male Mixed

8.1 10.0 7.5

Mixed Male Female

a subjects with hypertensive parents

than

The proportion of salt derived from discretionary sources is also lower is sometimes considered (22). U s i n g data on salt sales and salt

566

R. SHEPHERDand C.A. FARLEIGH

consumption from the National Food Survey, Druce estimated discretionary intake to be 27% of total intake (22). However, in a study using a lithium marker to determine the proportion of discretionary salt ingested, discretionary intake accounted for only 12% of the total (10). In a study using pre-weighed salt pots, discretionary intake was found to account for 16% of the total intake (9), The present finding of 11% of intake derived from discretionary salt is in line with these findings. With this low proportion of intake deriving from discretionary salt use, only relatively small reductions in average intake may be effected by restricting discretionary use. Larger reductions in intake would require either the choice of foods containing lower amounts of sail or the reduction of salt added to foods before purchase. Salt intake may vary between individuals due to many other factors including taste preferences, attitudes, personality and the intake of water and other nutrients (8,9,33). In order to understand the role of salt intake in disease and to make reco~nmendations about i t s reduction, i t is necessary to understand the roles played by these factors and to assess average intake in the population. The questionnaire described in this paper offers one method for assessing normal intake. ACKNOWLEDGEMENTS The authors would like to thank Dr. G.M. Clayton for his help in recruiting the subjects, and to thank the subjects who took part in the study. REFERENCES io

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2.

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3.

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4.

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5.

Committee on Medical Aspects of Food Policy. Disease. London: HMSO, 1984.

6.

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7.

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8.

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Diet and Cardiovascular

ASSESSMENT OF SALT INTAKE

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10.

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11.

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12.

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13.

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17.

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19.

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31.

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33.

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and blood pressure in

Preferences, attitudes and personality as Hum. Nutr: Appl. Nutr. 1986; 40A: 195-208.

Accepted for publication March 16, 1987.