Michelle Braun

Vitamin D status and calcium metabolism in adolescent black and white girls on a range of controlled calcium intakes.

BACKGROUND There are limited data in adolescents on racial differences in relationships between dietary calcium intake, absorption, and retention and serum levels of calcium-regulating hormones. OBJECTIVES The aim of this study was to investigate these relationships cross-sectionally in American White and Black adolescent girls. METHODS Calcium balance studies were conducted in 105 girls, aged 11-15 yr, on daily calcium intakes ranging from 760-2195 mg for 3-wk controlled feeding periods; 158 observations from 52 Black and 53 White girls were analyzed. RESULTS Black girls had lower serum 25-hydroxyvitamin D [25(OH)D], higher serum 1,25-dihydroxyvitamin D, and higher calcium absorption and retention than White girls. Calcium intake and race, but not serum 25(OH)D, predicted net calcium absorption and retention with Black girls absorbing calcium more efficiently at low calcium intakes than White girls. The relationship between serum 25(OH)D and serum PTH was negative only in White girls. Calcium intake, race, and postmenarcheal age explained 21% of the variation in calcium retention, and serum 25(OH)D did not contribute further to the variance. CONCLUSIONS These results suggest that serum 25(OH)D does not contribute to the racial differences in calcium absorption and retention during puberty.

Calcium retention in adolescent boys on a range of controlled calcium intakes

BACKGROUND National calcium requirements in the United States for boys are based on data from girls. On average, boys develop larger skeletons than do girls, yet it is unknown whether the additional skeletal accretion in boys requires additional dietary calcium intake. OBJECTIVE The objective was to determine calcium retention in adolescent boys in response to a range of controlled intakes and to compare the intake needed for maximal retention in boys with that needed in adolescent girls studied under the same conditions. DESIGN Thirty-one boys aged 12-15 y participated in 3-wk metabolic balance studies testing a range (700-2100 mg/d) of calcium intakes in a crossover study design with a 2-wk washout period. Calcium intake was varied by using a beverage fortified with calcium citrate malate. After a 1-wk equilibration period, calcium retention was calculated as dietary calcium intake minus the calcium excreted in the feces and urine over the following 2 wk. The dietary intake at which maximal calcium retention occurred was determined by using a nonlinear regression model. The results in boys were compared with those obtained in 35 adolescent girls previously studied under the same protocol. RESULTS Maximal calcium retention in boys was achieved at an intake of 1140 mg/d. Calcium retention was higher (by 171 +/- 38 mg/d) in boys than in girls at all calcium intakes studied. CONCLUSION The higher calcium retention in boys than in girls was attained through higher net calcium absorption and lower urinary excretion than in girls.

Calcium requirements and metabolism in Chinese-American boys and girls

Calcium requirements of North American adolescents were set at 1300 mg/day based on data from white girls. Calcium requirements for Asian‐American adolescents have not been studied. Using metabolic balance protocols and a range in calcium intakes, skeletal calcium retention was determined in Chinese‐American adolescents. A sample of 29 adolescents, 15 boys aged 12 to 15 years and 14 girls aged 11 to 15 years, was studied twice on paired calcium intakes ranging between 629 to 1835 mg/day using a randomized‐order crossover design. Calcium absorption and bone turnover rates using double‐stable calcium isotope kinetic analysis on two calcium intakes per subject were measured and compared in boys and girls. Girls and boys had low habitual mean calcium intakes of 648 and 666 mg/day, respectively, and low mean serum 25‐hydroxyvitamin D concentrations of 19.1 and 22.2 ng/mL, respectively. True fractional calcium absorption varied inversely with calcium load. Boys had significantly higher bone turnover rate than girls at the same calcium intake. Calcium retention increased with calcium intake; calcium intakes to achieve maximal calcium retention were 1100 mg/day in boys and 970 mg/day in girls. Recommendations for calcium requirements should be lowered for Chinese‐American adolescents.

Predictors of calcium retention in adolescent boys

CONTEXT The relationship between calcium (Ca) intake and Ca retention in adolescent boys was recently reported. OBJECTIVE This study evaluated the influence of Ca intake, serum hormone levels, biomarkers of bone metabolism, habitual physical activity, habitual Ca intake, and physical fitness on Ca retention in the same sample. DESIGN This study was a randomized, cross-over design that consisted of two 3-wk metabolic balance periods. SETTING The study took place on a university campus as a summer camp. PATIENTS OR OTHER PARTICIPANTS A total of 31 American white boys (13-15 yr) participated in the study. INTERVENTIONS Each subject consumed a controlled diet with one of five high-low Ca intake pairs that ranged from 670-2003 mg/d, which was manipulated utilizing a fortified beverage. MAIN OUTCOME MEASURES Ca retention was determined by Ca intake minus urinary and fecal Ca excretion during each balance period. RESULTS Ca intake explained 21.7% of the variability in Ca retention, and serum IGF-I concentration explained an additional 11.5%. Other serum hormone levels did not significantly add to the model. Biomarkers of bone metabolism, habitual physical activity, habitual Ca intake, and physical fitness were not significant predictors of Ca retention in adolescent boys. CONCLUSIONS IGF-I, a regulator of growth during puberty, is an important predictor of Ca retention in adolescent boys. However, dietary Ca intake is an even greater predictor of Ca retention during this period of growth.

Fructo-oligosaccharides and calcium absorption and retention in adolescent girls.

Background: Several studies have shown a positive effect of fructo-oligosaccharides on calcium absorption and retention in animals and humans. Effects of levels of these pre-biotics that can be functionally incorporated into manufactured foods, have not been studied in controlled feeding studies. Objectives: This study was designed to evaluate the effect of 9 g/d of fructo-oligosaccharides as part of a controlled diet on calcium absorption and retention in adolescent girls. Design: Fourteen healthy adolescent girls aged 11–13 y were studied in a metabolic setting for two 3-week periods separated by a 2-week washout period. In a randomized, double-blinded, crossover design, the teens received a diet containing either 9 g/d oligofructose-enriched inulin in a calcium-fortified cereal or the control cereal with no inulin. Both diets contained ∼1500 mg calcium daily. Calcium retention was determined on the third week of each period. On day 14 of the diet period, fractional calcium absorption was determined from the enrichment of 44Ca in 4-day urine collections. Results: Calcium absorption (67 ± 3 vs. 66 ± 3%) and retention (409 ± 394 vs. 464 ± 241 mg/d) were not significantly different when diets contained 9 g/d oligofructose-enriched inulin or not in a calcium-fortified cereal. Conclusions: Daily consumption of cereal containing a combination of short- and long-chain fructo-oligosaccharides as part of a controlled diet did not benefit calcium absorption or retention in adolescent girls. Lack of response to the prebiotic in this cohort may relate to their already high calcium absorption efficiency.

Calcium, dairy products, and energy balance in overweight adolescents: a controlled trial

BACKGROUND Dairy product and calcium consumption have been associated with modifying body fat and body weight in children and adults. OBJECTIVE In overweight adolescent boys and girls, we aimed to determine the effect of the doubling of habitual calcium intake to the recommended intake from dairy or calcium carbonate on energy balance and purported mechanisms including fecal fat excretion, macronutrient use, and parathyroid hormone suppression. DESIGN Twenty-five girls with a mean (±SD) BMI (in kg/m(2)) of 33 ± 5 and 17 boys with a BMI of 28 ± 5, aged 12-15 y, participated in two 3-wk controlled feeding sessions that used a crossover design in random order as a summer research camp. In one session, 756 mg Ca/d was consumed; in the other session, an additional 650 mg Ca/d was provided as dairy or calcium carbonate supplements that were matched to the control in macronutrient content. Total energy and macronutrient intakes were controlled and were the same for the 2 sessions for each subject. Primary outcome measures were energy balance, fecal fat excretion, lipid oxidation, and postprandial energy expenditure. RESULTS There were no effects of quantity or source of calcium on energy or fat balance, despite calcium-induced increases (P <0.01) in postprandial serum parathyroid hormone suppression. CONCLUSION These data lend little evidence to support the proposed mechanisms for the relation between an increase in calcium intake from calcium carbonate or dairy and weight loss or weight maintenance in children. This trial was registered at clinicaltrials.gov as NCT00592137.

Racial differences in potassium homeostasis in response to differences in dietary sodium in girls.

BACKGROUND Racial differences in the renal disposition of potassium may be related to mechanisms for the greater susceptibility to hypertension in blacks than in whites. OBJECTIVE Our objective was to study the racial differences in the renin-angiotensin-aldosterone system and in potassium balance in black and white girls consuming a controlled diet that was low in potassium with 2 amounts of sodium intake (low compared with high). DESIGN The studies reported here were performed in 40 black and 28 white girls, aged 11-15 y, under highly controlled metabolic conditions. The studies comprised 2 sessions of 20-d metabolic balance sessions, at 2 amounts of dietary sodium intake (58 and 170 mmol . L(-1) . d(-1)), in a crossover design and with a constant dietary potassium intake of 50 mmol . L(-1) . d(-1). Repeated-measures analysis of variance was used to test for racial differences in potassium output and retention by sodium intakes. RESULTS Thirty black and 20 white girls completed the study. Urinary potassium excretion was lower in blacks than in whites, regardless of sodium intake (P < 0.05), with no differences in fecal or sweat potassium excretion. Cumulative potassium retention was significantly higher in blacks while consuming the low sodium diet. Plasma aldosterone concentrations after upright posture were significantly lower in blacks than in whites but were similar when supine, as were urinary aldosterone excretion rates. On week 3, blood pressure, body weight, urinary volume, creatinine, and serum sodium and potassium were similar. CONCLUSION The well-known racial difference in urinary potassium excretion appears to be at least in part due to greater renal retention of potassium in black girls.

Magnesium retention from metabolic-balance studies in female adolescents: impact of race, dietary salt, and calcium

BACKGROUND Previously, we showed that black girls retained more calcium than white girls did and that salt loading negatively affected calcium retention. Racial differences likely exist in other bone minerals also, such as magnesium, in response to salt loading during growth. OBJECTIVE We studied racial differences in magnesium metabolism in response to dietary sodium and calcium during rapid bone growth. DESIGN Twenty-seven white and 40 black girls (11-15 y old) were studied for 3 wk while they consumed low-sodium (1.3 g/d) and high-sodium (3.8 g/d) diets by using a randomized-order, crossover metabolic study with 3 dietary calcium intakes; the magnesium dietary intake was fixed at 230 mg/d. Urine and feces were collected during each 3-wk period in 24-h pools and analyzed for magnesium. A mixed-model ANOVA was used to determine the effect of race and dietary sodium with calcium intake as a covariate. RESULTS Salt loading or calcium intake had no significant effect on urinary magnesium excretion. Blacks excreted significantly less urinary magnesium (mean ± SD: 83.8 ± 25.6 mg/d) than did whites (94.9 ± 27.3 mg/d; P < 0.05). No effects were observed in fecal magnesium excretion. Magnesium retention was higher with the low-sodium diet (50.1 ± 44.0 mg/d) than with the high-sodium diet (39.3 ± 49.8 mg/d) (P < 0.05), with no effects of race or calcium intake. Salt loading had no effect on biomarkers. Whites had higher 25-hydroxyvitamin D and insulin-like growth factor binding protein 3 but lower 1,25-dihydroxyvitamin D and parathyroid hormone concentrations. CONCLUSIONS Blacks excreted less urinary magnesium than did whites. Magnesium retention was similar between races but higher with the low-sodium diet. Kinetic studies are needed to fully explain magnesium homeostasis. This trial was registered at clinicaltrials.gov as NCT01564238.

Obesity Augments Calcium-Induced Increases in Skeletal Calcium Retention in Adolescents

CONTEXT Overweight adolescents have low bone mineral content for weight and are at increased risk for fractures. OBJECTIVE The aim was to determine whether overweight and obesity influence the positive relationship between dietary calcium intake and skeletal calcium retention in adolescents. DESIGN Analysis of pooled data from calcium balance studies in adolescents. SETTING Participants each underwent a 3-wk calcium balance study in a controlled environment. PARTICIPANTS Participants included 280 White, Black, and Asian boys (n = 73) and girls (n = 207) ages 10-16 yr. MAIN OUTCOME MEASURE The relationship among body mass index (BMI), calcium intake, and calcium retention was modeled using linear regression. RESULTS Calcium intake, BMI, sex, race, and age explained 27.9% of the variation in calcium retention. At low calcium intakes, there was no effect of BMI on skeletal calcium retention, but at higher calcium intakes, BMI increased skeletal calcium retention. CONCLUSIONS Greater gains in calcium retention occur with increases in calcium intake in adolescents with higher BMI compared with those with lower BMI. Additional studies are needed to investigate whether increasing calcium intake reduces the increased risk of fracture associated with overweight and obesity in adolescents.

A Call to Evaluate the Impact of Calcium-Fortified Foods and Beverages

C alcium, one of the most studied nutrients, has been associated with proposed and existing health and structure/function claims. But do the claims apply equally to the calcium that is added to fortify foods? This is a call to better evaluate the contributions of fortified foods and supplements to calcium intakes. Many clinical trials have been conducted showing the benefit of increasing dietary calcium at different life stages on bone mineral content and bone mineral density (BMD). Higher BMD has been associated with reduction in osteoporotic fractures. For example, a meta-analysis of osteoporotic fractures in postmenopausal women estimated an odds ratio of 0.96 (95% CI 0.93Y0.99) per 300 mg calcium increase per day. Americans are aware of the connection between calcium and osteoporosis, yet seem reluctant to change dietary habits in order to increase calcium intake as shown by a recent dietary intake survey. Clearly, increasing awareness of the variety of sources of calcium available is needed.