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Bone Mineral Density of Adolescents as Affected by Calcium Intake through Milk and Milk Products
PII : S0958-6946(98)00120-4
Int. Dairy Journal 8 (1998) 759—764 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0958-6946/99/$ — see front matter
Bone Mineral Density of Adolescents as Affected by Calcium Intake through Milk and Milk Products E. Renner a*, M. Hermes a1 and H. Stracke b ?Dairy Science Section, Justus Liebig University Giessen, Bismarckstrasse 16, D-35390 Giessen, Germany @Medical Department of Internal Medicine, Justus Liebig University Giessen, Rodthohl 6, D-35392 Giessen, Germany (Received 10 December 1997; accepted 10 October 1998) ABSTRACT Elevated levels of milk and milk products were incorporated into the diet of 15—16 yr old adolescents with subaverage bone mineral density in order to obtain a daily calcium intake of about 1200 mg. The bone mineral density was significantly improved by about 50% when compared with control subjects without such a dietary intervention (#0.053 vs #0.036 g cm\). The rate of increase, however, was significantly lower for girls than for boys (also by about 50%: #0.019 vs #0.038 g cm\), possibly due to the more advanced bodily development of girls at this age. As the rate of increase of the bone mineral density is significantly decreasing for adolescents of this age group (20—35% in boys 16 yr of age when compared with 15 yr old ones, and 40—50% in girls), it can be predicted that such a dietary intervention starting at 15 yr of age will lead to an average level of the bone mineral values after 3—4 yr and even to an optimum level after 6—7 yr in boys with an originally subaverage level. However, such improvement is no longer feasible for girls at this age. The concentration of osteocalcin and parathyroid hormone as well as the activity of alkaline phosphatase in blood serum indicated that the higher than proportional increase of bone mineral density in the intervention group can be attributed to a decreased bone turnover which was caused by an increased intake of calcium through milk and dairy products. 1999 Elsevier Science Ltd. All rights reserved Keywords: bone mineral density; bone mineral content; adolescents; dietary intervention; calcium intake; milk and dairy products; prophylaxis to osteoporosis
INTRODUCTION
METHODS
Our previous studies on the relationship between calcium intake in different periods of life and the bone mineral content as well as the incidence of osteoporosis in elderly people (Renner et al., 1991; Stracke et al., 1993) have shown that the bone mineral content and bone mineral density of young adults are directly related to the calcium intake through milk and milk products in childhood and adolescence. In addition, calcium intake through these food products at this early stage of life had been significantly lower in osteoporotic patients than in age-matched control persons without bone disease. In later periods of life, there were no significant differences between the calcium intakes of both groups. Therefore, it was concluded that an adequate calcium intake through milk and milk products, particularly in childhood and adolescence, is an essential factor for obtaining a maximum bone mass (peak adult bone mass) and for the prevention of osteoporosis. Based on these results, a new study was conducted to examine whether it is possible to improve the bone status in 15—16 yr old teenagers with bone mineral values below average by increased consumption of calcium-rich foods such as milk and milk products.
Subjects 190 adolescents (113 females and 77 males) were included in the study. Initially, their bone mineral status and some clinical parameters were examined and their calcium intake through milk and milk products was determined by using a food frequency questionnaire combined with a 24 h recall. Based on these results, the following four groups were formed: E an intervention group (bone mineral values below average), E a control group (bone mineral values below average, no dietary intervention), E a medium group (average bone mineral values, no dietary intervention: a second control group), E an ‘optimum group’ (bone mineral values above average, no dietary intervention: a third control group). One year after the initial measurements were carried out, a second examination was conducted in order to determine the changes which had taken place in the meantime. In total, 129 adolescents took part in this follow-up investigation (76 females and 53 males). Reasons for the reduced numbers of participants included lack of interest
*Corresponding author. Present address: Abbott GmbH, Max Planck Ring 2, D65205, Wiesbaden-Delkenheim, Germany. 759
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of some subjects, moving away or staying abroad, or dropping out of the intervention group. Bone density measurement Bone mineral content and density were measured on the proximal radius of the non-dominant forearm by single-photon absorptiometry (Osteometer, 1991) with I as radioactive source and a very low radiation exposure. This method is considered to be a suitable procedure for epidemiological investigations in healthy persons (Schaafsma et al., 1987) and has a high measurement accuracy (Mazess, 1984; Dambacher and Ru¨eggsegger, 1985). As the participants of our study were adolescents, only two measurements per subject in a peripheral area of the body had been permitted by the German authorities for radiation protection. The same method had been used in our previous study (Stracke et al., 1993), so that the data of both studies can be compared. As the trends indicated by measurements of bone mineral content (g cm\) and bone mineral density (g cm\) are very similar, mainly the data of the bone mineral density are presented here.
Physical activity As physical activity is considered to be an essential factor which contributes to the formation of bone mass (Pollitzer and Anderson, 1989), we tried to establish a measure for the intensity of physical activity by the participants of the study, based on a questionnaire about the frequency with which individual sporting activities were conducted. A weighting factor was used for the various kinds of sport according to their specific energy expenditure and this was multiplied by a frequency factor to yield an activity point score for each kind of sporting activity and—by summarizing these individual point scores—the total score for the physical activity. Statistical evaluation The Statistical Package for the Social Sciences for Microcomputers (SPSS/PC#) in the version 6.0 was used for the statistical evaluation of the data by several statistical methods, including t-test, analysis of variance, multiple range-test, regression, s-test.
Bone specific laboratory parameters
RESULTS
In addition to the bone mineral status, several laboratory parameters related to the bone metabolism were analyzed. The serum concentrations of the follicule stimulating hormone (FSH), the luteinizing hormone (LH) and the parathyroid hormone (PTH) were measured by using radioimmunoassay (RIA). The test sets for FSH and LH came from Diagnostic Products Corporation (DPC, 1993), the test set for PTH from Nichols Institute Diagnostica (Nichols Institute, 1994). Osteocalcin in serum was determined by a RIA procedure (No. 15065) of Incstar Corporation, MN, USA. The analysis of serum calcium which is based on the o-cresolphthaleine complex method was conducted by using a test set of Boehringer Mannheim (No. 1125621). Test sets of Boehringer Mannheim were also used for the analysis of the serum activity of alkaline phosphatase (No. 1040669) and for the determination of inorganic phosphate (No. 1040898) which is based on the molybdate reaction. The determination of the pyridine derivates, pyridinoline and deoxypyridinoline in urine, was based on competitive enzyme immunoassays with test sets of DPC Biermann GmbH, Bad Nauheim (Nos. MB003 and MB005).
At the end of the 1 yr experimental period, all groups of adolescents showed a significant increase of the bone mineral density (P40.001) which indicates the anabolic bone metabolism status at this age (Table 1). In some subjects, however, a stagnation could be observed, more frequently in girls than in boys. The increase of bone mineral density was more pronounced in the intervention groups, confirming that an improved calcium supply was able to substantially raise subnormal bone mineral values. The difference from the other groups was statistically highly significant (P40.01). In all groups, boys showed a greater increase of bone mineral density than girls (P40.001). There was a large variability in the increase of bone mineral density between the individual subjects of the intervention group: the measured values ranged between 0.041 and 0.065 g cm\ for the boys and between 0.011 and 0.068 g cm\ for girls. At the time of the first measurement, there was no significant difference in bone mineral density between boys and girls, but 1 yr later male adolescents had significantly higher values than females (P40.001). When the study was started, most of the subjects were between 15 and 16 yr of age. Therefore, it could be examined, whether the age change (although only by 1 yr) has an effect on the changes of the bone mineral density during the 1 yr investigation period. As it can be seen from Table 2, the rate of increase of the bone mineral values was lower in the 16 yr old adolescents compared with the 15 yr old subjects: by about 40% in the nonintervention groups, by about 20% in the male and up to 50% in the female intervention group. However, the absolute increase of the bone mineral density in the 16 yr old girls of the intervention group was significantly higher than in the other groups. An effect of the physical activity on the bone mineral values could not be observed in the present study; no significant correlation could be found between the activity point score and the bone mineral density and bone
Dietary intervention The subjects in the intervention group received, for one year and free of charge, enough milk and milk products which supplied 1000 mg calcium per day. They could choose the products according to their individual preferences from a wide selection: low-fat milk, milk drinks (four varieties), low-fat yoghurt, fruit yoghurt (several varieties), low-fat quarg, fruit quarg (four varieties), Camembert, and Butter Cheese. They were given the exact quantity of the preferred products to supply the required amount of calcium and they were advised on how the products could be best integrated into the daily diet.
761
Bone mineral density of adolescents
Table 1. Development of Bone Mineral Density in Male and Female Adolescents between Two Measurements During One Year Experimental Period (See Text for Statistical Significance) Group
Bone mineral density (g cm\) Males n
Intervention Control Medium Optimum
12 11 15 15
Measurement 1
2
0.389 0.376 0.454 0.509
0.441 0.412 0.491 0.544
Females Difference
0.053 0.036 0.037 0.035
mineral content, neither in the male nor in the female adolescents. A similar result was also reported from a study conducted by van den Bergh et al. (1995) with 1379 boys and girls aged 7—11 yr, where no linear association was found between physical activity and bone mineral content. However, it is difficult to obtain an objective measure for the physical activity; if, therefore, on the basis of our methodical approach a relationship between physical activity and bone mineral density could not be seen, it does not mean that such a relationship actually does not exist. In general, it is argued that physical activity has an effect on the bone due to the increased mechanical load while, on the other hand, immobility leads to a reduction of bone mass due to the reduced load of the skeleton (Stewart et al., 1982). A significant negative correlation between the concentration of the parathyroid hormone (PTH) in the serum and the bone mineral density was found in boys (r"0.44, P40.001), but only to a lesser extent in girls. This result can be explained by the fact that PTH is essentially involved in calcium homeostasis: a low serum calcium level induces a raised PTH concentration which induces a release of calcium from the bone (Lo¨ffler et al., 1995). During the whole study, the male and female optimum groups had the lowest average PTH concentration (24.0 vs 34.2 pg mL\, and 27.3 vs 30.8 pg mL\). A negative correlation was found for the 1 yr test period between the changes of PTH level and the increase of bone mineral density in boys (r"!0.30, P40.05); from the resulting regression coefficient it can be concluded that the improved bone mineral density in the male intervention group can be attributed to the 20% decrease in the PTH level. Somewhat surprising are the results related to the changes of the osteocalcin concentration and the activity of alkaline phosphatase during the 1 yr test period. Serum concentration of osteocalcin is considered to be a measure for the build-up of bone mass and the activity of alkaline phosphatase indicates the extent of new formation of bone (Hyldstrup et al., 1988). Abrams et al. (1997) observed in their investigations with boys and girls 9—14 yr old that calcium retention (balance) correlated positively with serum alkaline phosphatase activity. Therefore, it seems to be inconsistent that, in both cases, our values were reduced in spite of the increased bone mineral density, the trend most notably expressed in the intervention groups: osteocalcin !3.1 vs !0.1 ng mL\; alkaline phosphatase !151 vs !84 U L\ in boys and !69 vs
n
12 14 28 22
Measurement 1
2
0.376 0.409 0.460 0.500
0.409 0.427 0.474 0.516
Difference
0.033 0.018 0.014 0.016
!15 U L\ in girls, respectively. However, Johnston et al. (1992) also observed a significantly reduced osteocalcin concentration in children at a simultaneously improved bone mineral density after a 3 yr period of increased calcium intake. These results, particularly the greater decrease rate of both parameters in the intervention group, could be caused by the increased calcium intake, thereby the synthesis of PTH is diminished which implies a reduced assault on bone substance together with a reduced bone build-up rate. As, nevertheless, the highest net gain of bone mass can be seen in this group, it can be supposed that the reduced build-up of bone mass is overcompensated by an even greater reduction in decomposition of bone mass. A positive relationship was found between the body mass index (BMI), a criterion relating the body weight to the body height, and the bone mineral content in female adolescents. This may be explained by the fact that girls with a higher BMI consume more food and, therefore, usually also more calcium, or that a greater BMI imposes a greater burden on the skeletal system which induces a better bone mineralisation. But, as it becomes evident from Fig. 1, strongly reduced bone mineral values are observed particularly at a BMI of less than 20, which is characterized as underweight, in most of the subjects. In Germany, an exaggerated slimming endeavour can be seen very often particularly in female adolescents of this age group, so that this result has to be considered as a very unfavourable development with regard to an effective prophylaxis towards the incidence of osteoporosis.
Fig. 1. Relationship between the body mass index and the bone mineral content of female adolescents (r"0.51, P40.001).
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In male adolescents, this relationship exists only to a much smaller extent.
DISCUSSION Achievement of the genetically determined peak bone mass in the young adult is a prerequisite for the prophylaxis of osteoporosis. A sufficient supply of calcium during childhood and adolescence is considered as one of the most important factors for building up maximum bone mass. Therefore, the principal aspect of this study was the question whether it is still possible to use a dietary intervention to achieve a substantial improvement in adolescents with an unfavourable bone mineral status. As a substantial result of the study, it can be emphasized that such a dietary intervention based on an improved calcium intake through milk and milk products was able to produce higher rates of increase in the bone mineral density and bone mineral content when compared to the control groups without the dietary intervention. The study design comprised a 1 yr intervention period. Therefore, the question arises, to what extent it is possible to improve the bone status in adolescents with under-average bone mineral values to reach medium or even optimum bone values by the improved calcium supply. Such an evaluation could be attempted by examining the varying rates of increase in the bone mineral values of the 15 and 16 yr old adolescents, respectively, for the prediction of the development in the following years, based on the following mathematical model y "y #* )F L L\ L\ where y , y , y , y "bone mineral density (g cm\) at L year 0, 1, 2, year n; * "difference in bone mineral L\ density between years (n!1) and (n!2) * * "y !y , * "y !y , F" * Example (male intervention group): y "0.380 (g cm\), y "0.440 (g cm\), y "0.489 (g cm\) 0.049 * "0.060, * "0.049, F" "0.817 0.060 y "0.489#0.049 ) 0.817"0.529 (g cm\) If, in adolescents with below-average bone mineral values, an improved calcium supply will start at the age of 15 by the dietary intervention described, not only a substantial improvement of the bone status may be expected in comparison with the control group without dietary intervention, but it also seems possible that the bone mineral values of the medium group may be achieved after 3—4 yr (i.e. at the age of 18—19), and the values of the optimum group might be achievable after 6—7 yr (i.e. at the age 21—22), as seen in Fig. 2 which shows the predicted development in the following years. Such a peak bone mass in young adults would represent an essential prophylactic measure towards the incidence of osteoporosis in later periods of life.
Fig. 2. Predicted development of bone mineral density in male adolescents starting at the age of 15 yr ("year 0) (groups: O"optimum, M"medium, I"dietary intervention, C"control).
Table 2. Effect of Age at the Beginning of the Study on the Increase of the Bone Mineral Density (BMD) Group
Gender
Age (years)
Increase of BMD (g cm\)
Significance
Intervention
Boys
15 16 15 16 15 16 15 16
0.060 0.049 0.040 0.019 0.040 0.026 0.020 0.012
P40.05
Girls Other groups
Boys Girls
P40.01 P40.001 P40.001
The situation for girls was clearly different from that of boys, as the rate of increase in the bone mineral density of female adolescents showed a more substantial decline at this period of age (see Table 2). As it can be seen from Fig. 3 it, therefore, does not appear possible in girls of this age with below-average bone mineral values to obtain a medium or even optimum level of bone density, even by applying a dietary intervention which provides sufficient calcium supply through milk and dairy products. A limited improvement was observed, when compared with the control group with suboptimal bone mineral values but without dietary intervention. An interpretation of these results might be that boys of this age group are still in the phase of a significant build-up of bone mass while girls already approach a metabolic situation where the intensity of anabolic and catabolic processes is not very different so that a substantial net gain of bone mass can no longer be achieved. Similarly, Gunnes (1994) found that, up to the age of about 16 yr, bone mineral density of the distal forearm is higher in girls than in boys, while in older subjects the opposite results were found; he concludes that the build-up of bone mass may be almost finished in female adolescents of this age, and their bone mineral density may be comparable to that of adult women.
Bone mineral density of adolescents
763
1200—1500 mg day\ for adolescents and young adults. Jackman et al. (1997) observed that the intake of 1300 mg calcium per day was the smallest intake that allowed some adolescent females to achieve 100% of maximal calcium retention; it is suggested that calcium retention plateaus at a certain calcium intake although it continues to increase even at intakes of more than 2 g day\. However, Albertson et al. (1997) found from their survey that 77% of the adolescent females consumed less than twothirds of the recommended dietary allowances of calcium and they concluded that calcium intakes have declined with age, which appears to be related to a decline in fluid milk consumption.
Fig. 3. Predicted development of bone mineral density in female adolescents starting at the age of 15 yr ("year 0) (groups: O"optimum, M"medium, I"dietary intervention, C"control).
Thus in girls with subaverage bone mineral values, dietary intervention providing sufficient calcium supply should probably start already several years earlier to aim at an improvement of the situation. This is supported by a study of Matkovic et al. (1994) showing that girls build up a substantial portion of bone mass already at the age of 11—15 yr. This period was designated as the ‘window of opportunity’ by Jackman et al. (1997) indicating that sufficient calcium should be applied to females aged 12—15 yr, because bone growth occurs almost exclusively during adolescence, or during the years following the onset of puberty for women. According to the National Institutes of Health (1994) this might be due to the fact, that calcium accumulation in bone during preadolescence is between 140 and 165 mg day\ and may be as high as 400—500 mg day\ in the pubertal period. In a study with girls aged 12.2 yr (SD 0.3) conducted by Cadogan et al. (1997), the intervention group consumed, on average, an additional 300 mL of milk a day; this increased milk consumption significantly increased bone mineral density and bone mineral content. Weaver et al. (1996) found in investigations with girls aged 11—14 yr that, based on a calcium intake of 1332 mg day\, girls of this age would require 2 yr to reach the total bone calcium of the young adults (women aged 19—30 yr). Martin et al. (1997) measured the bone mineral content and estimated the calcium accretion in children to provide insight into dietary calcium requirements during growth; they observed that peak velocity of bone mineral accretion occured later in boys than in girls (13.3 vs 11.4 yr). On the basis of an overview of several studies with adolescent girls at an entry age of about 11 yr, Kerstetter and Insogna (1995) described a positive effect of dairy products on bone mineral density when used as a source of supplemental calcium in adolescent girls of this age. In our study, the calcium intake through milk and dairy products was raised, on average, to 1100 mg day\ (girls) and 1200 mg day\ (boys) in the intervention groups. Taking into consideration an additional calcium intake of about 200—300 mg day\ from other dietary sources, it can be concluded that the calcium intake of these adolescents met the currently recommended optimal calcium intake which, for example, is estimated by the National Institutes of Health (1994) to be
CONCLUSIONS This study appears to indicate that it is justified to conduct a screening of the bone mineral status of adolescents as it seems possible to design a dietary intervention leading to improvement of the status. For this purpose, dietary habits need to be changed leading to an increased calcium intake, e.g. by an increased consumption of milk and dairy products. Although a relationship between the calculated point score of the physical activity and the bone mineral density could not be observed in the present study, there is general agreement that emphasis should be placed also on sufficient physical activity which is considered to be another important prophylactic factor towards the incidence of osteoporosis.
ACKNOWLEDGEMENTS We gratefully acknowledge the financial support of the Bavarian Ministry of Food, Agriculture and Forestry in Munich which enabled us to conduct this research project.
REFERENCES Abrams, S. A., Grusak, M. A., Stuff, J. and O’Brien, K. O. (1997) Calcium and magnesium balance in 9—14 yr old children. American Journal of Clinical Nutrition 66, 1172—1177. Albertson, A. M., Tobelmann, R. C. and Marquart, L. (1997) Estimated dietary calcium intake and food sources for adolescent females: 1980—1992. Journal of Adolescent Health 20, 20—26. Cadogan, J., Eastell, R., Jones, N. and Barker, M. E. (1997) Milk intake and bone mineral acquisition in adolescent girls: randomised, controlled intervention trial. British Medical Journal 315, 1255—1260. Dambacher, M. A. and Ru¨eggsegger, P. (1985) Nichtinvasive Untersuchungsmethoden bei Osteoporosen. ¹herapeutische ºmschau 42, 339—350. DPC (1993) FSH-RIA and LH-RIA. Testbeschreibung. Diagnostic Products Corporation, Hermann Biermann GmbH, Bad Nauheim, Germany. Gunnes, M. (1994) Bone mineral density in the cortical and trabecular distal forearm in healthy children and adolescents. Acta Paediatrica 83, 463—464. Hyldstrup, L., Clemmensen, I., Jensen, B. A. and Transbo¨l, I. (1988) Noninvasive evaluation of bone formation: measurements of serum alkaline phosphatase, whole body retention of diphosphonate and serum osteocalcin in metabolic bone disorders and thyroid disease. Scandinavian Journal of Clinical and ¸aboratory Investigations 48, 611—619.
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Jackmann, L. A., Millane, S. S., Martin, B. R., Wood, O. B., McCabe, G. P., Peacock, M. and Weaver, C. M. (1997) Calcium retention in relation to calcium intake and postmenarcheal age in adolescent females. American Journal of Clinical Nutrition 66, 327—333. Johnston, C. C., Miller, J. Z., Slemenda, C. W., Reister, T. K., Hui, S. and Christian, J. C. (1992) Calcium supplementation and increases in bone mineral density in children. New England Journal of Medicine 327, 82—87. Kerstetter, J. E. and Insogna, K. (1995) Do dairy products improve bone density in aedolescent girls? Nutrition Reviews 53, 328—332. Lo¨ffler, T., Arndt, T., Wisser, H., Knoll, E., Kiess, W., Ho¨flich, A., Vogt, W., Bindlingmaier, F. and Kleesiek, K. (1995) Endokrines System. In ¸ehrbuch der Klinischen Chemie und Pathochemie. 3. neubearbeitete Auflage. Hrsg. H. Greiling, and A. M. Gressner. S. 947-1063. Stuttgart, New York: Schattauer. Martin, A. D., Bailey, D. A., McKay, H. A. and Whiting, S. (1997) Bone mineral and calcium accretion during puberty. American Journal of Clinical Nutrition 66, 611—615. Matkovic, V., Jelic, D., Wardlaw, G. M., Ilich, J., Goel, P. K., Wright, J. K., Andon, M. B., Smith, K. T. and Heaney, R. P. (1994) Timing of peak bone mass in Caucasian females and its implications for the prevention of osteoporosis. Journal of Clinical Investigation 93, 799—808. Mazess, R. B. (1984) Advances in single- and dual-photon absorptiometry. In Osteoporosis, eds C. Christiansen, C. D. Arnaud, B. E. C. Nordin, A. M. Parfitt, W. A. Peck and B. L. Rigg. Proceedings of the International Symposium on Osteoporosis, pp. 57—63. Aalborg: Stiftsbogtrykkeri. National Institutes of Health (1994) Optimal calcium intake. NIH Consens Statement Online 6—8 June 1994. 12(4), 1—31.
Nichols Institute (1994) Intakt-PTH. Testbeschreibung. Nichols Institute Diagnostic GmbH, Bad Nauheim, Germany. Osteometer AS (1991) Osteometer Diagnostic Test 100. Operators manual and user guide. Roedovre, Denmark. Pollitzer, W. S. and Anderson, J. J. B. (1989) Ethnic and genetic differences in bone mass: a review with a hereditary vs environmental perspective. American Journal of Clinical Nutrition 50, 1244—1259. Renner, E., Knie, G., Schatz, H., Stracke, H., Weber, K., Minne, H. W. and Leidig, G. (1991) On the incidence of osteoporosis in relation to the calcium intake with milk and milk products. International Dairy Journal 1, 77—82. Schaafsma, G., van Beresteyn, E. C. H. and Raymakers, J. A. (1987) Nutritional aspects of osteoporosis. ¼orld Review of Nutrition and Dietetics 49, 121—159. Stewart, A. F., Adler, M., Byers, C. M., Segre, G. V. and Broadus, A. E. (1982) Calcium homeostasis in immobilization: an example of resorptive hypercalciuria. New England Journal of Medicine 306, 1136—1139. Stracke, H., Renner, E., Knie, G., Leidig, G., Minne, H. and Federlin, K. (1993) Osteoporosis and bone metabolic parameters in dependence upon calcium intake through milk and milk products. European Journal of Clinical Nutrition 47, 617—622. van den Bergh, M. F. Q., de Man, S. A., Witteman, J. C. M., Hofman, A., Trouerbach, W. T. and Grobbee, D. E. (1995) Physical activity, calcium intake and bone mineral content in children in the Netherlands. Journal of Epidemiology and Community Health 49, 299—304. Weaver, C. M., Peacock, M., Martin, B. R., Plawecki, K. L. and McCabe, G. P. (1996) Calcium retention estimated from indicators of skeletal status in adolescent girls and young women. American Journal of Clinical Nutrition 64, 67—70.
Int. Dairy Journal 8 (1998) 759—764 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0958-6946/99/$ — see front matter
Bone Mineral Density of Adolescents as Affected by Calcium Intake through Milk and Milk Products E. Renner a*, M. Hermes a1 and H. Stracke b ?Dairy Science Section, Justus Liebig University Giessen, Bismarckstrasse 16, D-35390 Giessen, Germany @Medical Department of Internal Medicine, Justus Liebig University Giessen, Rodthohl 6, D-35392 Giessen, Germany (Received 10 December 1997; accepted 10 October 1998) ABSTRACT Elevated levels of milk and milk products were incorporated into the diet of 15—16 yr old adolescents with subaverage bone mineral density in order to obtain a daily calcium intake of about 1200 mg. The bone mineral density was significantly improved by about 50% when compared with control subjects without such a dietary intervention (#0.053 vs #0.036 g cm\). The rate of increase, however, was significantly lower for girls than for boys (also by about 50%: #0.019 vs #0.038 g cm\), possibly due to the more advanced bodily development of girls at this age. As the rate of increase of the bone mineral density is significantly decreasing for adolescents of this age group (20—35% in boys 16 yr of age when compared with 15 yr old ones, and 40—50% in girls), it can be predicted that such a dietary intervention starting at 15 yr of age will lead to an average level of the bone mineral values after 3—4 yr and even to an optimum level after 6—7 yr in boys with an originally subaverage level. However, such improvement is no longer feasible for girls at this age. The concentration of osteocalcin and parathyroid hormone as well as the activity of alkaline phosphatase in blood serum indicated that the higher than proportional increase of bone mineral density in the intervention group can be attributed to a decreased bone turnover which was caused by an increased intake of calcium through milk and dairy products. 1999 Elsevier Science Ltd. All rights reserved Keywords: bone mineral density; bone mineral content; adolescents; dietary intervention; calcium intake; milk and dairy products; prophylaxis to osteoporosis
INTRODUCTION
METHODS
Our previous studies on the relationship between calcium intake in different periods of life and the bone mineral content as well as the incidence of osteoporosis in elderly people (Renner et al., 1991; Stracke et al., 1993) have shown that the bone mineral content and bone mineral density of young adults are directly related to the calcium intake through milk and milk products in childhood and adolescence. In addition, calcium intake through these food products at this early stage of life had been significantly lower in osteoporotic patients than in age-matched control persons without bone disease. In later periods of life, there were no significant differences between the calcium intakes of both groups. Therefore, it was concluded that an adequate calcium intake through milk and milk products, particularly in childhood and adolescence, is an essential factor for obtaining a maximum bone mass (peak adult bone mass) and for the prevention of osteoporosis. Based on these results, a new study was conducted to examine whether it is possible to improve the bone status in 15—16 yr old teenagers with bone mineral values below average by increased consumption of calcium-rich foods such as milk and milk products.
Subjects 190 adolescents (113 females and 77 males) were included in the study. Initially, their bone mineral status and some clinical parameters were examined and their calcium intake through milk and milk products was determined by using a food frequency questionnaire combined with a 24 h recall. Based on these results, the following four groups were formed: E an intervention group (bone mineral values below average), E a control group (bone mineral values below average, no dietary intervention), E a medium group (average bone mineral values, no dietary intervention: a second control group), E an ‘optimum group’ (bone mineral values above average, no dietary intervention: a third control group). One year after the initial measurements were carried out, a second examination was conducted in order to determine the changes which had taken place in the meantime. In total, 129 adolescents took part in this follow-up investigation (76 females and 53 males). Reasons for the reduced numbers of participants included lack of interest
*Corresponding author. Present address: Abbott GmbH, Max Planck Ring 2, D65205, Wiesbaden-Delkenheim, Germany. 759
760
E. Renner et al.
of some subjects, moving away or staying abroad, or dropping out of the intervention group. Bone density measurement Bone mineral content and density were measured on the proximal radius of the non-dominant forearm by single-photon absorptiometry (Osteometer, 1991) with I as radioactive source and a very low radiation exposure. This method is considered to be a suitable procedure for epidemiological investigations in healthy persons (Schaafsma et al., 1987) and has a high measurement accuracy (Mazess, 1984; Dambacher and Ru¨eggsegger, 1985). As the participants of our study were adolescents, only two measurements per subject in a peripheral area of the body had been permitted by the German authorities for radiation protection. The same method had been used in our previous study (Stracke et al., 1993), so that the data of both studies can be compared. As the trends indicated by measurements of bone mineral content (g cm\) and bone mineral density (g cm\) are very similar, mainly the data of the bone mineral density are presented here.
Physical activity As physical activity is considered to be an essential factor which contributes to the formation of bone mass (Pollitzer and Anderson, 1989), we tried to establish a measure for the intensity of physical activity by the participants of the study, based on a questionnaire about the frequency with which individual sporting activities were conducted. A weighting factor was used for the various kinds of sport according to their specific energy expenditure and this was multiplied by a frequency factor to yield an activity point score for each kind of sporting activity and—by summarizing these individual point scores—the total score for the physical activity. Statistical evaluation The Statistical Package for the Social Sciences for Microcomputers (SPSS/PC#) in the version 6.0 was used for the statistical evaluation of the data by several statistical methods, including t-test, analysis of variance, multiple range-test, regression, s-test.
Bone specific laboratory parameters
RESULTS
In addition to the bone mineral status, several laboratory parameters related to the bone metabolism were analyzed. The serum concentrations of the follicule stimulating hormone (FSH), the luteinizing hormone (LH) and the parathyroid hormone (PTH) were measured by using radioimmunoassay (RIA). The test sets for FSH and LH came from Diagnostic Products Corporation (DPC, 1993), the test set for PTH from Nichols Institute Diagnostica (Nichols Institute, 1994). Osteocalcin in serum was determined by a RIA procedure (No. 15065) of Incstar Corporation, MN, USA. The analysis of serum calcium which is based on the o-cresolphthaleine complex method was conducted by using a test set of Boehringer Mannheim (No. 1125621). Test sets of Boehringer Mannheim were also used for the analysis of the serum activity of alkaline phosphatase (No. 1040669) and for the determination of inorganic phosphate (No. 1040898) which is based on the molybdate reaction. The determination of the pyridine derivates, pyridinoline and deoxypyridinoline in urine, was based on competitive enzyme immunoassays with test sets of DPC Biermann GmbH, Bad Nauheim (Nos. MB003 and MB005).
At the end of the 1 yr experimental period, all groups of adolescents showed a significant increase of the bone mineral density (P40.001) which indicates the anabolic bone metabolism status at this age (Table 1). In some subjects, however, a stagnation could be observed, more frequently in girls than in boys. The increase of bone mineral density was more pronounced in the intervention groups, confirming that an improved calcium supply was able to substantially raise subnormal bone mineral values. The difference from the other groups was statistically highly significant (P40.01). In all groups, boys showed a greater increase of bone mineral density than girls (P40.001). There was a large variability in the increase of bone mineral density between the individual subjects of the intervention group: the measured values ranged between 0.041 and 0.065 g cm\ for the boys and between 0.011 and 0.068 g cm\ for girls. At the time of the first measurement, there was no significant difference in bone mineral density between boys and girls, but 1 yr later male adolescents had significantly higher values than females (P40.001). When the study was started, most of the subjects were between 15 and 16 yr of age. Therefore, it could be examined, whether the age change (although only by 1 yr) has an effect on the changes of the bone mineral density during the 1 yr investigation period. As it can be seen from Table 2, the rate of increase of the bone mineral values was lower in the 16 yr old adolescents compared with the 15 yr old subjects: by about 40% in the nonintervention groups, by about 20% in the male and up to 50% in the female intervention group. However, the absolute increase of the bone mineral density in the 16 yr old girls of the intervention group was significantly higher than in the other groups. An effect of the physical activity on the bone mineral values could not be observed in the present study; no significant correlation could be found between the activity point score and the bone mineral density and bone
Dietary intervention The subjects in the intervention group received, for one year and free of charge, enough milk and milk products which supplied 1000 mg calcium per day. They could choose the products according to their individual preferences from a wide selection: low-fat milk, milk drinks (four varieties), low-fat yoghurt, fruit yoghurt (several varieties), low-fat quarg, fruit quarg (four varieties), Camembert, and Butter Cheese. They were given the exact quantity of the preferred products to supply the required amount of calcium and they were advised on how the products could be best integrated into the daily diet.
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Bone mineral density of adolescents
Table 1. Development of Bone Mineral Density in Male and Female Adolescents between Two Measurements During One Year Experimental Period (See Text for Statistical Significance) Group
Bone mineral density (g cm\) Males n
Intervention Control Medium Optimum
12 11 15 15
Measurement 1
2
0.389 0.376 0.454 0.509
0.441 0.412 0.491 0.544
Females Difference
0.053 0.036 0.037 0.035
mineral content, neither in the male nor in the female adolescents. A similar result was also reported from a study conducted by van den Bergh et al. (1995) with 1379 boys and girls aged 7—11 yr, where no linear association was found between physical activity and bone mineral content. However, it is difficult to obtain an objective measure for the physical activity; if, therefore, on the basis of our methodical approach a relationship between physical activity and bone mineral density could not be seen, it does not mean that such a relationship actually does not exist. In general, it is argued that physical activity has an effect on the bone due to the increased mechanical load while, on the other hand, immobility leads to a reduction of bone mass due to the reduced load of the skeleton (Stewart et al., 1982). A significant negative correlation between the concentration of the parathyroid hormone (PTH) in the serum and the bone mineral density was found in boys (r"0.44, P40.001), but only to a lesser extent in girls. This result can be explained by the fact that PTH is essentially involved in calcium homeostasis: a low serum calcium level induces a raised PTH concentration which induces a release of calcium from the bone (Lo¨ffler et al., 1995). During the whole study, the male and female optimum groups had the lowest average PTH concentration (24.0 vs 34.2 pg mL\, and 27.3 vs 30.8 pg mL\). A negative correlation was found for the 1 yr test period between the changes of PTH level and the increase of bone mineral density in boys (r"!0.30, P40.05); from the resulting regression coefficient it can be concluded that the improved bone mineral density in the male intervention group can be attributed to the 20% decrease in the PTH level. Somewhat surprising are the results related to the changes of the osteocalcin concentration and the activity of alkaline phosphatase during the 1 yr test period. Serum concentration of osteocalcin is considered to be a measure for the build-up of bone mass and the activity of alkaline phosphatase indicates the extent of new formation of bone (Hyldstrup et al., 1988). Abrams et al. (1997) observed in their investigations with boys and girls 9—14 yr old that calcium retention (balance) correlated positively with serum alkaline phosphatase activity. Therefore, it seems to be inconsistent that, in both cases, our values were reduced in spite of the increased bone mineral density, the trend most notably expressed in the intervention groups: osteocalcin !3.1 vs !0.1 ng mL\; alkaline phosphatase !151 vs !84 U L\ in boys and !69 vs
n
12 14 28 22
Measurement 1
2
0.376 0.409 0.460 0.500
0.409 0.427 0.474 0.516
Difference
0.033 0.018 0.014 0.016
!15 U L\ in girls, respectively. However, Johnston et al. (1992) also observed a significantly reduced osteocalcin concentration in children at a simultaneously improved bone mineral density after a 3 yr period of increased calcium intake. These results, particularly the greater decrease rate of both parameters in the intervention group, could be caused by the increased calcium intake, thereby the synthesis of PTH is diminished which implies a reduced assault on bone substance together with a reduced bone build-up rate. As, nevertheless, the highest net gain of bone mass can be seen in this group, it can be supposed that the reduced build-up of bone mass is overcompensated by an even greater reduction in decomposition of bone mass. A positive relationship was found between the body mass index (BMI), a criterion relating the body weight to the body height, and the bone mineral content in female adolescents. This may be explained by the fact that girls with a higher BMI consume more food and, therefore, usually also more calcium, or that a greater BMI imposes a greater burden on the skeletal system which induces a better bone mineralisation. But, as it becomes evident from Fig. 1, strongly reduced bone mineral values are observed particularly at a BMI of less than 20, which is characterized as underweight, in most of the subjects. In Germany, an exaggerated slimming endeavour can be seen very often particularly in female adolescents of this age group, so that this result has to be considered as a very unfavourable development with regard to an effective prophylaxis towards the incidence of osteoporosis.
Fig. 1. Relationship between the body mass index and the bone mineral content of female adolescents (r"0.51, P40.001).
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In male adolescents, this relationship exists only to a much smaller extent.
DISCUSSION Achievement of the genetically determined peak bone mass in the young adult is a prerequisite for the prophylaxis of osteoporosis. A sufficient supply of calcium during childhood and adolescence is considered as one of the most important factors for building up maximum bone mass. Therefore, the principal aspect of this study was the question whether it is still possible to use a dietary intervention to achieve a substantial improvement in adolescents with an unfavourable bone mineral status. As a substantial result of the study, it can be emphasized that such a dietary intervention based on an improved calcium intake through milk and milk products was able to produce higher rates of increase in the bone mineral density and bone mineral content when compared to the control groups without the dietary intervention. The study design comprised a 1 yr intervention period. Therefore, the question arises, to what extent it is possible to improve the bone status in adolescents with under-average bone mineral values to reach medium or even optimum bone values by the improved calcium supply. Such an evaluation could be attempted by examining the varying rates of increase in the bone mineral values of the 15 and 16 yr old adolescents, respectively, for the prediction of the development in the following years, based on the following mathematical model y "y #* )F L L\ L\ where y , y , y , y "bone mineral density (g cm\) at L year 0, 1, 2, year n; * "difference in bone mineral L\ density between years (n!1) and (n!2) * * "y !y , * "y !y , F" * Example (male intervention group): y "0.380 (g cm\), y "0.440 (g cm\), y "0.489 (g cm\) 0.049 * "0.060, * "0.049, F" "0.817 0.060 y "0.489#0.049 ) 0.817"0.529 (g cm\) If, in adolescents with below-average bone mineral values, an improved calcium supply will start at the age of 15 by the dietary intervention described, not only a substantial improvement of the bone status may be expected in comparison with the control group without dietary intervention, but it also seems possible that the bone mineral values of the medium group may be achieved after 3—4 yr (i.e. at the age of 18—19), and the values of the optimum group might be achievable after 6—7 yr (i.e. at the age 21—22), as seen in Fig. 2 which shows the predicted development in the following years. Such a peak bone mass in young adults would represent an essential prophylactic measure towards the incidence of osteoporosis in later periods of life.
Fig. 2. Predicted development of bone mineral density in male adolescents starting at the age of 15 yr ("year 0) (groups: O"optimum, M"medium, I"dietary intervention, C"control).
Table 2. Effect of Age at the Beginning of the Study on the Increase of the Bone Mineral Density (BMD) Group
Gender
Age (years)
Increase of BMD (g cm\)
Significance
Intervention
Boys
15 16 15 16 15 16 15 16
0.060 0.049 0.040 0.019 0.040 0.026 0.020 0.012
P40.05
Girls Other groups
Boys Girls
P40.01 P40.001 P40.001
The situation for girls was clearly different from that of boys, as the rate of increase in the bone mineral density of female adolescents showed a more substantial decline at this period of age (see Table 2). As it can be seen from Fig. 3 it, therefore, does not appear possible in girls of this age with below-average bone mineral values to obtain a medium or even optimum level of bone density, even by applying a dietary intervention which provides sufficient calcium supply through milk and dairy products. A limited improvement was observed, when compared with the control group with suboptimal bone mineral values but without dietary intervention. An interpretation of these results might be that boys of this age group are still in the phase of a significant build-up of bone mass while girls already approach a metabolic situation where the intensity of anabolic and catabolic processes is not very different so that a substantial net gain of bone mass can no longer be achieved. Similarly, Gunnes (1994) found that, up to the age of about 16 yr, bone mineral density of the distal forearm is higher in girls than in boys, while in older subjects the opposite results were found; he concludes that the build-up of bone mass may be almost finished in female adolescents of this age, and their bone mineral density may be comparable to that of adult women.
Bone mineral density of adolescents
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1200—1500 mg day\ for adolescents and young adults. Jackman et al. (1997) observed that the intake of 1300 mg calcium per day was the smallest intake that allowed some adolescent females to achieve 100% of maximal calcium retention; it is suggested that calcium retention plateaus at a certain calcium intake although it continues to increase even at intakes of more than 2 g day\. However, Albertson et al. (1997) found from their survey that 77% of the adolescent females consumed less than twothirds of the recommended dietary allowances of calcium and they concluded that calcium intakes have declined with age, which appears to be related to a decline in fluid milk consumption.
Fig. 3. Predicted development of bone mineral density in female adolescents starting at the age of 15 yr ("year 0) (groups: O"optimum, M"medium, I"dietary intervention, C"control).
Thus in girls with subaverage bone mineral values, dietary intervention providing sufficient calcium supply should probably start already several years earlier to aim at an improvement of the situation. This is supported by a study of Matkovic et al. (1994) showing that girls build up a substantial portion of bone mass already at the age of 11—15 yr. This period was designated as the ‘window of opportunity’ by Jackman et al. (1997) indicating that sufficient calcium should be applied to females aged 12—15 yr, because bone growth occurs almost exclusively during adolescence, or during the years following the onset of puberty for women. According to the National Institutes of Health (1994) this might be due to the fact, that calcium accumulation in bone during preadolescence is between 140 and 165 mg day\ and may be as high as 400—500 mg day\ in the pubertal period. In a study with girls aged 12.2 yr (SD 0.3) conducted by Cadogan et al. (1997), the intervention group consumed, on average, an additional 300 mL of milk a day; this increased milk consumption significantly increased bone mineral density and bone mineral content. Weaver et al. (1996) found in investigations with girls aged 11—14 yr that, based on a calcium intake of 1332 mg day\, girls of this age would require 2 yr to reach the total bone calcium of the young adults (women aged 19—30 yr). Martin et al. (1997) measured the bone mineral content and estimated the calcium accretion in children to provide insight into dietary calcium requirements during growth; they observed that peak velocity of bone mineral accretion occured later in boys than in girls (13.3 vs 11.4 yr). On the basis of an overview of several studies with adolescent girls at an entry age of about 11 yr, Kerstetter and Insogna (1995) described a positive effect of dairy products on bone mineral density when used as a source of supplemental calcium in adolescent girls of this age. In our study, the calcium intake through milk and dairy products was raised, on average, to 1100 mg day\ (girls) and 1200 mg day\ (boys) in the intervention groups. Taking into consideration an additional calcium intake of about 200—300 mg day\ from other dietary sources, it can be concluded that the calcium intake of these adolescents met the currently recommended optimal calcium intake which, for example, is estimated by the National Institutes of Health (1994) to be
CONCLUSIONS This study appears to indicate that it is justified to conduct a screening of the bone mineral status of adolescents as it seems possible to design a dietary intervention leading to improvement of the status. For this purpose, dietary habits need to be changed leading to an increased calcium intake, e.g. by an increased consumption of milk and dairy products. Although a relationship between the calculated point score of the physical activity and the bone mineral density could not be observed in the present study, there is general agreement that emphasis should be placed also on sufficient physical activity which is considered to be another important prophylactic factor towards the incidence of osteoporosis.
ACKNOWLEDGEMENTS We gratefully acknowledge the financial support of the Bavarian Ministry of Food, Agriculture and Forestry in Munich which enabled us to conduct this research project.
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