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Original Research |
Department of Foods and Nutrition (Y.-C.L., L.D.M., C.M.W., D.T.), Purdue University, West Lafayette, Indiana
Department of Health, Kinesiology and Leisure Studies (R.M.L.), Purdue University, West Lafayette, Indiana
Department of Statistics (G.P.M.), Purdue University, West Lafayette, Indiana
Address reprint requests to: Dorothy Teegarden, Ph.D., Department of Foods and Nutrition, Stone Hall 1264, Purdue University, West Lafayette IN 47907. E-mail: Teegarden{at}CFS.Purdue.edu
| ABSTRACT |
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Design: Two year prospective non-concurrent analysis of the effect of calcium intake on changes in body composition during a two year exercise intervention.
Subjects: 54 normal weight young women, 18 to 31 years of age.
Measures of Outcome: Mean intakes of nutrients of interest were determined from three-day diet records completed at baseline and every six months for two years. The change in total body weight and body composition (assessed by dual x-ray absorptiometry) from baseline to two years was also determined.
Results: Total calcium/kilocalories and vitamin A together predicted (negatively and positively, respectively) changes in body weight (R2 = 0.19) and body fat (R2 = 0.27). Further, there was an interaction of calcium and energy intake in predicting changes in body weight, such that, only at lower energy intakes, calcium intake (not adjusted for energy) predicted changes in body weight.
Conclusions: Regardless of exercise group assignment, calcium adjusted for energy intake had a negative relationship and vitamin A intake a positive relationship with two year changes in total body weight and body fat in young women aged 18 to 31 years. Thus, subjects with high calcium intake, corrected by total energy intake, and lower vitamin A intake gained less weight and body fat over two years in this randomized exercise intervention trial.
Key words: body composition, diet, premenopausal women, calcium, milk, dairy
| INTRODUCTION |
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The effect of a variety of nutrients, particularly the micronutrients, on changes in weight has been extensively studied. Though the results remain somewhat controversial, the evidence suggests that high carbohydrate diets improve weight maintenance [6]. The effect of micronutrients, particularly calcium, on weight regulation has not been well studied. In fact, avoidance of dairy products is common in weight conscious individuals since they are viewed as high fat products. However, several studies in which prepubertal to premenopausal subjects were supplemented with calcium or dairy products showed either no change or no difference in changes in body weight compared to control groups [710].
No prospective or intervention study has investigated the effects of calcium intake on changes in body composition in young women. The current study investigates prospectively, over two years, the relationships of micronutrient intakes, including total dietary calcium, dairy and non-dairy calcium, to body weight and body composition in young women aged 18 to 31 years.
| MATERIALS AND METHODS |
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Exclusion Criteria
Exclusion criteria included chronic intake of medication which interferes with calcium metabolism, irregular menses or a history of high blood pressure, heart disease or diabetes. Subjects were also excluded if they were greater than 20% overweight or 15% underweight. None of the participants had participated in more than two hours a week of exercise in the year prior to entry into the study.
Dietary Intake and biochemical parameters
Dietary intake was assessed by a three-day diet record, including intake of mineral and vitamin supplements, at baseline and every six months for up to 30 months. Dietary records were analyzed using Computrition (Chatsworth, CA). Based on both repeated measures and t tests from baseline to each time point, there were no differences in nutrient intake between baseline and 6, 12, 18 or 24 months. Thus a mean was calculated for each nutrient for each subject. As a result, 2, 11, 31 and 10 subjects had 9, 12, 15 and 18 days of records which were used to establish their mean dietary intakes. Using repeated measures, there were also no differences in nutrient intakes within or between, over time, in the exercisers or the non-exercisers. Daily nutrient intakes are shown in Table 1. There were outliers for iron, thiamin, riboflavin, vitamin B12, folic acid, retinol equivalents and vitamin C. Values for these individual nutrients were excluded from their respective analyses; thus, numbers ranged from 52 to 54 in Table 1. Dairy calcium was determined from the calcium content (Computrition) of the dairy foods only. Non-dairy calcium was calculated as the difference between the total calcium intake and the dairy calcium intake. Ratios were determined using the mean of the variables for each individual.
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Statistical Analysis
Means, standard deviations and correlations were computed for all variables. Regression methods were used to relate body composition measures to dietary variables. The experimental variable for the primary analysis was exercise. Thus, the influence and interaction of exercise were carefully examined. For the secondary analysis, the explanatory variables were dietary intakes of selected nutrients, and the response variables were changes in weight, fat and lean mass. All computations were performed using SAS statistical software [11]. Results were considered significant when p < 0.05.
| RESULTS |
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| DISCUSSION |
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The effect of calcium was specific to dairy calcium, as total calcium and dairy calcium, both adjusted for energy intake, predicted percent change in body weight and body fat, but non-dairy calcium, adjusted for energy, did not predict any changes in body composition. In addition, the percent changes in body weight were likely due to percent change in body fat, as none of the calcium variables predicted percent change in lean mass.
A number of hypotheses could be invoked to explain why dairy calcium, not non-dairy calcium, adjusted for energy intake, negatively predicts changes in body fat. The bioavailability of calcium is generally not greater from dairy sources compared with non-dairy sources and thus does not explain the difference. It is possible that another component of dairy products which was not analyzed in this study could be the factor which influences body weight. For example, conjugated linoleic acid (CLA) is a component of milk products which may impact body weight and adiposity [1213]. Mice fed CLA supplemented diets had reduced body weight [12], body fat [13] and increased lean mass [13] compared to controls. However, studies by Zemel, et al., indicate that nonfat dairy products, which would not contain CLA, prevent increases in genetically obese mice [14], suggesting that the effect of dairy products is not due to CLA content.
Though the results of this study suggest that the impact of calcium on changes in body weight and body fat are due to dairy calcium, there are several reasons total calcium, including non-dairy calcium, may also be important. First, the range for the non-dairy calcium is quite narrow (2.5 to 460 mg/day). This range may not be wide enough to be able to predict changes in body weight or body fat. In addition, it is possible that the effect of total calcium or dairy calcium is dependent on achieving a threshold level, and intake of dairy products are necessary to reach this threshold level. Thus, the impact of total calcium intake on changes in weight warrants further exploration.
Another possibility is that, if total dietary calcium increases, another nutrient decreases. In this case calcium intake would negatively correlate with the other nutrient. In this study, none of the nutrient intakes examined were negatively associated with total calcium intake or dairy calcium intake. In addition, no other nutrient adjusted for total energy intake significantly predicted change in body weight or body fat. Thus, the impact of calcium adjusted for energy intake on body weight and body fat was specific to this nutrient.
The proposed mechanism for the effect of calcium intake on changes in weight and fat mass would support that exercisers would have increased lipolysis; thus, the effect may be greater in the exercisers. The lack of influence of exercise, however, is not surprising, as these sedentary women assigned to the exercise group increased their exercise on average by only two hours/week. In addition, the exercise protocol of weight lifting and only a few minutes at a time of rope jumping may not influence changes in fat mass. Thus, a more rigorous aerobic exercise protocol may well lead to increased changes in the exercise group.
In addition, the impact of calcium on changes in body weight and body fat had an interesting interaction with energy intake. Total calcium and dairy calcium had a negative impact on changes in body weight and body fat only in women with energy intakes below the mean noted in this study. Calcium and dairy calcium had no impact on changes in body weight and body fat in women whose energy intakes were greater than the mean intake, but energy intake itself predicted changes in body weight and body fat at these higher energy intakes. It is possible that higher energy intakes overwhelm the impact of calcium on changes in body composition. These results have implications in weight reduction, since high calcium intakes may improve the effectiveness of energy-restricted diets or plans. This approach will be an interesting area to explore.
Several studies in animal models support that higher calcium intakes can reduce weight. Bursey, et al. showed that high calcium diets (2%) lowered body weight in both lean and fatty Zucker rats [15]. Higher intakes of calcium and sodium reduced body fat increases in spontaneously hypertensive rats [16]. In addition, recent data by Zemel, et al., demonstrated that mice which overexpress the agouti gene, and thus are genetically obese, have less of an increase in both weight and specifically fat mass when fed diets high in calcium or non-fat dairy products [14].
The effect of calcium intake on fat mass was examined in humans epidemiologically in the NHANES III data set [14]. After controlling for energy intake, a strong inverse relationship was noted between calcium intake and body fat in both women and men, with the highest quartile of calcium intake having a relative risk of 0.16, compared to 1.0 for the lowest intake in women. These results strongly support that calcium intake may effect fat mass in humans as well as in animal models.
Interestingly, calcium metabolism has been implicated in obesity and metabolic disorders of obesity, including diabetes and hypertension [1719]. Intracellular calcium levels are elevated in adipocytes of obese patients [19]. In addition, it is known that low calcium diets can increase levels of parathyroid hormone [20], and likely 1,25 dihydroxyvitamin D levels would be parallel these changes. Parathyroid hormone [21] and 1,25 dihydroxyvitamin D [14] have been shown to act at adipocytes to increase levels of intracellular calcium. The agouti gene protein product can increase lipogenesis, by inducing increases in the expression and activity of fatty acid synthase and decreasing lipolysis in adipocytes [22,23]. These effects are dependent on intracellular calcium levels; thus, an increase in intracellular calcium, as a result of decreased dietary calcium, increases lipogenesis and decreases lipolysis. Therefore, it is possible that decreased dietary calcium intake may lead to increased calcium levels in cells, which in turn may lead to an increase in lipogenesis and increased body fat potentially by enhancing effects of intracellular calcium levels, acting as a second messenger, on protein activity or gene expression.
Unfortunately, the published results of calcium intervention trials in this age group [710] cannot be compared to the results of the current study for several reasons. First, changes in weight and body fat may be specific to dairy products, not calcium supplementation alone; thus, only dairy supplementation studies may apply. Second, none of the intervention trials considered an interaction with caloric intake in investigating changes in body composition in calcium or dairy product supplemented groups. Thus, none of the published results are comparable with the current study results.
The results demonstrating a positive impact of vitamin A on changes in body weight, body fat and lean mass are perplexing. One study investigating the relationship of serum
-tocopherol and retinol to insulinemia in 17 obese children demonstrated a positive correlation of plasma retinol to body weight, but not body fat [24]. In the current study, vitamin A intake did not correlate with total energy intake (r = 0.13), fat intake (r = 0.01), protein intake (r = 0.11) or carbohydrate intake (r = 0.25). Vitamin A intake correlated with other nutrients found in fruits, vegetables and grain sources, such as fiber, vitamin C, B vitamins and zinc as well as non-dairy calcium. Some of these food sources may interfere with calcium bioavailability; however, no interaction was noted between calcium intake and vitamin A in predicting changes in body weight or body fat. Thus, the observed relationship between vitamin A intake and changes in body weight and body fat warrants further investigation.
Similarly, the negative impact of cholesterol adjusted for energy on change in lean mass is perplexing. The intakes of cholesterol by these subjects were relatively low (176 ± 49 mg/day). Cholesterol intake correlated with all the micronutrients as well as phosphorus and sodium, but cholesterol adjusted for energy intake correlated negatively with iron intake, adjusted and unadjusted for energy intake, and vitamin B12 intake adjusted for energy intake. Thus, the source of cholesterol for most of these women is not likely to be meat, but may be eggs and cheese. This does not explain the negative association of cholesterol adjusted for energy on lean mass, and further investigation is warranted.
The results of the regression analysis suggest that calcium intake and vitamin A can have a substantial impact on changes in body weight and body fat. For example, an individual with an intake of 1600 kcal/day, a calcium intake of 600 mg/day and 4000 IU/day vitamin A would have a change of -0.75 kg in body weight (5.18 + (0.0004*4000) + (-16.09*(600/1600))) and -0.61 kg in body fat (2.9 + (0.0005*4000) + (-14.68*(600/1600))) over two years. If calcium intakes were increased to 1000 mg/day, their body weight and body fat changes would be -3.28 kg and -4.28, respectively, compared with the above example. Alternatively, if their vitamin A intake increased to 10000 IU/day in either of the above examples, then the changes would be 3.15 and -0.88 kg body weight, and 2.40 and -1.27 kg in body fat in two years, respectively. Thus, to lose weight and body fat, the results of this study suggest intakes of 1600 kcal, 4000 IU vitamin A and 1000 mg/day of calcium. If subjects followed the recommendations for calcium (1000 m/day), vitamin A (5000 IU/day) intakes and average energy intake of 1958 kcal (determined in NHANES III for 16 to 19 year olds) they would lose weight (-1.04 kg) and body fat (-2.10 kg) in two years instead of experiencing the typical increases noted in this age group.
In summary, in this exercise intervention study, higher calcium intakes were associated with weight loss, specifically fat mass, in these young women, independent of exercise group assignment. Both the diet records and the body composition methodology represent the best available assessment techniques for these factors, supporting the results of the current study. In addition, the length of the study (two years) and relatively large number of women (n = 54) lend credence to results, suggesting a relationship between dietary calcium intake and the prevention of increased adiposity in this age group. Therefore, current recommendations to the public which emphasize increased calcium intake to promote bone health may also contribute to weight maintenance in young women.
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| ACKNOWLEDGMENTS |
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Received January 7, 2000. Accepted August 31, 2000.
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