Net Calcium Absorption Research Articles

Correction: Quantification of Dialytic Removal and Extracellular Calcium Mass Balance during a Weekly Cycle of Hemodialysis.

[This corrects the article DOI: 10.1371/journal.pone.0153285.].

Hypocalciuria as a Predictor of Reduced Intestinal Calcium Absorption.

Impaired intestinal calcium absorption contributes to osteoporosis, but its measurement is limited to research settings. We hypothesized that 24-hour urine calcium (24HUC) values could diagnose low fractional calcium absorption (FCA). We performed a post hoc analysis of clinical trial data to determine whether 24HUC predicted low FCA compared with the gold standard dual calcium isotope method. Two hundred thirty postmenopausal women <75 years old without osteoporosis underwent 445 FCA measurements using calcium isotopes (8 mg of oral 44Ca, 3 mg of intravenous 42Ca) and a 24-hour inpatient urine collection at 0 and 12 months. We determined subjects’ total calcium intake via review of food diaries and supplements. Net calcium absorption (NCA) was total calcium intake × FCA. NCA and 24HUC values demonstrated a positive correlation (r = 0.34; 95% confidence interval, 0.25 to 0.42; P < 0.001). We calculated sensitivity, specificity, positive (PPV) and negative predictive value (NPV) for the ability of 24HUC thresholds to predict calcium malabsorption. When low calcium absorption was defined as <120 mg/d, a 24HUC value <150 mg demonstrated 65% sensitivity, 67% specificity, 31% PPV, and 89% NPV. When calcium malabsorption was defined as <100 mg/d, a 24HUC value <150 mg demonstrated 72% sensitivity, 65% specificity, 22% PPV, and 94% NPV. A 24HUC value <150 mg demonstrated a high NPV for calcium malabsorption. We suggest that 24HUC levels can exclude calcium malabsorption in postmenopausal women.

Quantification of Dialytic Removal and Extracellular Calcium Mass Balance during a Weekly Cycle of Hemodialysis.

ObjectivesThe removal of calcium during hemodialysis with low calcium concentration in dialysis fluid is generally slow, and the net absorption of calcium from dialysis fluid is often reported. The details of the calcium transport process during dialysis and calcium mass balance in the extracellular fluid, however, have not been fully studied.MethodsWeekly cycle of three dialysis sessions with interdialytic breaks of 2-2-3 days was monitored in 25 stable patients on maintenance hemodialysis with calcium concentration in dialysis fluid of 1.35 mmol/L. Total and ionic calcium were frequently measured in blood and dialysate. The volume of fluid compartments was measured by bioimpedance.ResultsWeekly dialytic removal of 12.79 ± 8.71 mmol calcium was found in 17 patients, whereas 9.48 ± 8.07 mmol calcium was absorbed per week from dialysis fluid in 8 patients. Ionic calcium was generally absorbed from dialysis fluid, whereas complexed calcium (the difference of total and ionic calcium in dialysis fluid) was removed from the body. The concentration of total calcium in plasma increased slightly during dialysis. The mass of total and ionic calcium in extracellular fluid decreased during dialysis in patients with the dialytic removal of calcium from the body and did not change in patients with the absorption of calcium from dialysis fluid.ConclusionsWe conclude that about one third of patients on dialysis with calcium 1.35 mmol/L in dialysis fluid may absorb calcium from dialysis fluid and therefore individual prescriptions of calcium concentration in dialysis fluid should be considered for such patients.

A Grape-Enriched Diet Increases Bone Calcium Retention and Cortical Bone Properties in Ovariectomized Rats1–3

Grapes and their associated phytochemicals have been investigated for beneficial effects on cardiovascular health, cancer prevention, and other chronic diseases, but the effect of grape consumption on bone health has not been fully determined. We previously found short-term benefits of grape products on reducing bone turnover in ovariectomized rats. The objective of this study was to determine the long-term benefits of a grape-enriched diet on bone in ovariectomized rats. Rats were ovariectomized at 3 mo of age and were administered a single dose of (45)Ca to prelabel bones at 4 mo of age. After a 1-mo equilibration period, baseline urinary (45)Ca excretion was determined. Rats (n = 22/group) were then randomly assigned to a modified AIN93M diet containing 25% freeze-dried grape powder or to a control diet for 8 wk. Urinary (45)Ca excretion was monitored throughout the study to determine changes in bone (45)Ca retention. Calcium balance was assessed after 1 and 8 wk of consuming the experimental diets, and a calcium kinetic study was performed at 8 wk. After 8 wk, femurs were collected for micro-computed tomographic imaging, 3-point bending, and reference point indentation. Rats fed the grape-enriched diet had 44% greater net bone calcium retention than did rats fed the control diet. There were no differences in calcium balance due to diet at either week 1 or week 8, but there was a significant increase in net calcium absorption (10.6%) and retention (5.7%) from week 1 to week 8 in the grape-enriched diet group only. Grape-enriched diet-fed rats had 3% greater cortical thickness and 11% greater breaking strength. There were no differences in femur bone mineral density, trabecular microarchitecture, or reference point indentation variables due to diet. This study of ovariectomized rats indicates that the consumption of grape products may improve calcium utilization and suppress bone turnover, resulting in improvements in bone quality.

Differences in gastrointestinal calcium absorption after the ingestion of calcium-free phosphate binders

Both calcium-containing and noncalcium-containing phosphate binders can increase gastrointestinal calcium absorption. Previously, we observed that lanthanum carbonate administration to rats with renal failure is not associated with increased calciuria. Additionally, lanthanum carbonate treatment in dialysis patients has been associated with a less pronounced initial decrease in serum parathyroid hormone compared with other phosphate binders. For 8 days, male Wistar rats received a diet supplemented with 2% lanthanum carbonate, 2% sevelamer, 2% calcium carbonate, or 2% cellulose. Calciuria was found to be increased in animals with normal renal function treated with sevelamer or calcium carbonate but not with lanthanum carbonate. In animals with renal failure, cumulative calcium excretion showed similar results. In rats with normal renal function, serum ionized calcium levels were increased after 2 days of treatment with sevelamer, while calcium carbonate showed a smaller increase. Lanthanum carbonate did not induce differences. In animals with renal failure, no differences were found between sevelamer-treated, calcium carbonate-treated, and control groups. Lanthanum carbonate, however, induced lower ionized calcium levels within 2 days of treatment. These results were confirmed in normal human volunteers, who showed lower net calcium absorption after a single dose of lanthanum carbonate compared with sevelamer carbonate. In conclusion, these two noncalcium-containing phosphate-binding agents showed a differential effect on gastrointestinal calcium absorption. These findings may help to improve the management of calcium balance in patients with renal failure, including concomitant use of vitamin D.

Factors Associated with Calcium Absorption in Postmenopausal Women: A Post Hoc Analysis of Dual-Isotope Studies

Reduced calcium absorption is a risk factor for osteoporosis. This study examined factors associated with fractional calcium absorption (FCA) and net calcium absorption in postmenopausal women in a post hoc analysis of three completed dual-isotope studies. Data were analyzed from 50 postmenopausal women undergoing 121 inpatient research visits in three studies evaluating changes in FCA related to correction of vitamin D insufficiency (n=19), use of proton pump inhibitors (n=21), and use of aromatase inhibitors to treat breast cancer (n=10). Net calcium absorption was the product of FCA and total calcium intake in milligrams per day. Variables included subjects' age, race, body mass index, serum calcium, creatinine, parathyroid hormone, 1,25-dihydroxyvitamin D, 25-hydroxyvitamin D, and habitual intake of kilocalories, protein, fat, carbohydrate, fiber, calcium, iron, magnesium, oxalate, phosphorus, potassium, and vitamin D based on outpatient diet diaries. In multivariate models, subjects' age, dietary intake of kilocalories, carbohydrates, fat, fiber, calcium, and potassium were significant predictors of FCA. In multiple variable models predicting net calcium absorption, dietary intake of kilocalories, fat, fiber, calcium, potassium, and serum 1,25-dihydroxyvitamin D were significant. The square of the correlation between actual and predicted values (an approximation of R2) was 0.748 for FCA and 0.726 for net calcium absorption. Similar to other studies, this study found that age, 1,25-dihydroxyvitamin D, and dietary calcium and fat were associated with calcium absorption. Dietary intake of kilocalories, carbohydrates, and potassium were new factors that were significantly associated with FCA and net calcium absorption. In summary, the study suggests that several dietary habits play a role in calcium absorption, beyond vitamin D and calcium.

Evidence in Favor of a Severely Impaired Net Intestinal Calcium Absorption in Patients with (Early-Stage) Chronic Kidney Disease

Introduction: Calcium and phosphorus are essential to many vital physiological processes. Little is known about the net and fractional intestinal absorption of calcium and phosphorus in patients with chronic kidney disease (CKD) and their clinical and hormonal determinants. Methods: Blood and 24-hour urine samples were collected in 20 healthy volunteers (HV) and 72 stable CKD stage 1–4 patients and analyzed for parameters of mineral metabolism including calcidiol, calcitriol, and parathyroid hormone (PTH). Dietary intake was assessed by dietary history. Results: The 24-hour urinary calcium excretion, as opposed to the phosphorus excretion, showed a stepwise decrease across CKD stages (median of 219, 84, 40, and 22 mg/day in HV and patients with CKD stages 1–2, 3 and 4, respectively). Younger age, high serum calcitriol, and high estimated GFR were associated with a high 24-hour urinary calcium excretion. High serum calcitriol levels and dietary phosphorus intake were associated with a high 24-hour urinary phosphorus excretion. The fractional intestinal calcium absorption, as estimated by the urinary-to-ingested calcium ratio, decreased across CKD stages. Conclusions: The 24-hour urinary excretion of calcium, as opposed to phosphorus, is markedly decreased in CKD, even in early-stage disease. This is partly explained by low calcitriol levels and older age. Assuming a neutral calcium balance at the time of urine collection, we infer that net intestinal calcium absorption may be severely impaired in CKD.

Prebiotics Enhance Magnesium Absorption and Inulin‐based Fibers Exert Chronic Effects on Calcium Utilization in a Postmenopausal Rodent Model

Age-related changes in calcium metabolism play a role in the development of osteoporosis. A 4-wk feeding study was conducted in 5-mo-old ovariectomized (OVX) Sprague-Dawley rats to assess the effect of various dietary fibers on mineral metabolism and bone health parameters. There were 6 treatment groups: sham-Control, OVX-Control, OVX rats receiving daily estradiol (E₂) injections, and OVX rats receiving an AIN-93M diet supplement with either an inulin-based fiber (Synergy1® or Fruitafit HD®) or a novel fiber (polydextrose) at 5% wt. of diet. Calcium and magnesium metabolic balances were performed after early (3 d) and late exposure (4 wk) to dietary treatments. Rats receiving polydextrose had significantly higher net calcium absorption efficiency and retention than all control groups and a trend (P≤ 0.10) for higher calcium absorption when compared to inulin-based fibers after early exposure but the advantage did not persist over long-term exposure. The inulin-based fibers had positive chronic effects on calcium metabolism that were related to changes in the gut, that is, production of short chain fatty acids and higher cecal wall weights. All fibers improved magnesium absorption and retention in early and late metabolic balances; effects on magnesium metabolism were more pronounced than for calcium. Steady growth in US middle-aged and elderly populations has led to higher incidences of several chronic diseases including osteoporosis, a bone disease that primarily affects postmenopausal women. Recent research suggests that certain dietary fibers (prebiotics) enhance mineral absorption and may impart bone health benefits. This work examines the impact of prebiotic supplementation on mineral metabolism and bone health using a postmenopausal rat model. Study findings will aid future investigations in ascertaining the factors related to potential bone health benefits of prebiotic which will aid in developing an effective prebiotics food product/supplement that will address the bone health needs of consumers.

Galactooligosaccharides Improve Mineral Absorption and Bone Properties in Growing Rats through Gut Fermentation

Galactooligosaccharides (GOS), prebiotic nondigestible oligosaccharides derived from lactose, have the potential for improving mineral balance and bone properties. This study examined the dose-response effect of GOS supplementation on calcium and magnesium absorption, mineral retention, bone properties, and gut microbiota in growing rats. Seventy-five 4-week-old male Sprague-Dawley rats were randomized into one of five treatment groups (n = 15/group) and fed a diet containing 0, 2, 4, 6, or 8% GOS by weight for 8 weeks. Dietary GOS significantly decreased cecal pH and increased cecal wall weight and content weight in a dose-dependent manner (p < 0.0001). Fingerprint patterns of the 16S rRNA gene PCR-DGGE from fecal DNA indicated the variance of bacterial community structure, which was primarily explained by GOS treatments (p = 0.0001). Quantitative PCR of the samples revealed an increase in the relative proportion of bifidobacteria with GOS (p = 0.0001). Net calcium absorption was increased in a dose-response manner (p < 0.01) with GOS supplementation. Dietary GOS also increased (p < 0.02) net magnesium absorption, femur ⁴⁵Ca uptake, calcium and magnesium retention, and femur and tibia breaking strength. Distal femur total and trabecular volumetric bone mineral density (vBMD) and area and proximal tibia vBMD increased (p < 0.02) with GOS supplementation. Trabecular-rich bones, that is, those that rapidly turn over, were most benefited. Regression modeling showed that GOS benefited calcium and magnesium utilization and vBMD through decreased cecal pH, increased cecal wall and content weight, and increased proportion of bifidobacteria.

Calcium intake increases risk of prostate cancer among Singapore Chinese.

Consumption of dairy products, the primary source of calcium in Western diets, has been found to be positively associated with prostate cancer. In an Asian diet, nondairy foods are the major contributors of calcium. Thus, a study of dietary calcium and prostate cancer in Asians can better inform on whether calcium, as opposed to other dairy components, is responsible for the dairy foods-prostate cancer association. We examined calcium intake and prostate cancer risk among 27,293 men in the Singapore Chinese Health Study that was established between 1993 and 1998. As of December 31, 2007, 298 incident prostate cancer cases had been diagnosed among the cohort members. Diet was assessed at baseline with a validated 165-item food-frequency questionnaire. It is hypothesized that there is greater net absorption of calcium in smaller individuals. Therefore, the calcium-prostate cancer association was also assessed in stratified analyses by median body mass index. Vegetables were the largest contributor of daily calcium intake in the study population. Overall, we observed a modest, statistically nonsignificant 25% increase in prostate cancer risk for the 4th (median = 659 mg/d) versus 1st (median = 211 mg/d) quartiles of calcium intake after adjustment for potential confounders. The association became considerably stronger and achieved statistical significance (hazard ratio, 2.03; 95% confidence interval, 1.23-3.34; P for trend = 0.01) for men with a below median body mass index (22.9 kg/m(2)). Dietary calcium might be a risk factor for prostate cancer even at relatively low intake.

Setting Dietary Reference Intakes with the use of bioavailability data: calcium

The determination of Dietary Reference Intakes (DRIs) for calcium, especially in children, has relied in significant part on the evaluation of the relation between calcium intake and calcium absorption and retention. At present, most of these studies are conducted with the use of dual-tracer stable isotope, although mass balance or other isotope methods are still used occasionally. Studies carried out to evaluate DRI values need to be conducted under the most controlled conditions possible. However, the achievement of such conditions can be difficult, especially in studies in small children, because strict, long-term dietary monitoring and sample collections are not well tolerated. Other dietary factors, which include vitamin D status and the presence of enhancers and inhibitors of calcium absorption, may have to be considered. However, for most healthy populations who do not have very low calcium intakes or serum 25-hydroxyvitamin D concentrations, other dietary factors will not be major determinants of the net calcium absorption or retention that will be used for the establishment of DRI values. Ultimately, DRI values must be chosen based on an attempt to achieve some targeted value for calcium absorption/retention or to maximize, within constraints, the overall calcium absorbed and retained. In children, it is important to use data obtained at the age and pubertal status being evaluated rather than to interpolate from data performed in other age groups.

Difructose anhydride III promotes iron absorption in the rat large intestine

Objectives We examined the intestinal segment responsible for and the mechanisms involved in the non-digestible disaccharide difructose anhydride III (DFAIII)-mediated enhancement of iron absorption. Methods Net iron and calcium absorption (percentage) were observed in male Sprague-Dawley rats fed an AIN93G-based diet with or without DFAIII (30 g/kg of diet) for 2 wk after cecectomy or laparotomy (experiment 1). The absorption rates of calcium and iron from ligated jejunal loops (experiment 2) or ligated cecal sacs (experiment 3) in anesthetized rats fed a control or DFA diet were observed with or without the addition of DFAIII to the instilled mucosal fluid. Levels of the major iron transporter divalent metal transporter-1 (DMT-1) mRNA per 28S rRNA were evaluated by real-time polymerase chain reaction in the intestinal mucosa. Results Net absorption (percentage) of iron and calcium was higher in the DFA group than in the DFA-free group in laparotomized rats. The enhancement of iron absorption was almost and calcium absorption was partly abolished by cecectomy. The absorption rate of calcium, but not of iron, from the jejunal loops was increased by the addition of DFAIII (100 mmol/L). Iron absorption was clearly higher in the ligated cecal sacs of the DFAIII-fed group than in the DFA-free group with or without the mucosal addition of DFAIII. DMT-1 mRNA level per 28S rRNA was maintained in the cecal mucosa through large increases in cecal size due to DFAIII feeding. Conclusion The DFAIII-induced increases in iron absorption are the result of increased cecal iron absorptive capacity through expansion of the cecal mucosa maintaining DMT-1 mRNA expression.

Vitamin D Status and Calcium Metabolism in Adolescent Black and White Girls on a Range of Controlled Calcium Intakes

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. The aim of this study was to investigate these relationships cross-sectionally in American White and Black adolescent girls. 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. 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. These results suggest that serum 25(OH)D does not contribute to the racial differences in calcium absorption and retention during puberty.

Calcium balance during calcitriol and paricalcitol administration in healthy humans

Secondary hyperparathyroidism in hemodialysis patients requires optimal correction of vitamin D deficiency with active vitamin D and analogues. It has been postulated that new vitamin D analogues, i.e. paricalcitol, efficiently suppress parathyroid hormone serum levels (PTH), but do not increase intestinal calcium absorption as much as calcitriol. The effects of calcitriol and paricalcitol on calcium balance can best be characterized under standardized conditions in healthy individuals with normal renal function, because the urinary calcium excretion at steady state corresponds to the net calcium absorption in the gut. In a randomized, double-blind, placebo-controlled, 3-way crossover Phase I study in 13 healthy individuals we investigated the changes compared to placebo in PTH and urinary calcium excretion during 6-day treatment periods with paricalcitol (1.5 microg/day) and calcitriol (0.5 microg/day). 24-hour urinary calcium excretion was stable during 6 days of placebo administration. Neither paricalcitol nor calcitriol significantly changed calcium excretion. Urinary creatinine, magnesium and phosphate excretion also remained unchanged over the study periods irrespective of the treatment. However, calcitriol was shown to be effective in reducing iPTH levels during 6 days of treatment (mean reduction 4.03+/-0.69 pmol/l), whereas paricalcitol had no effect. Using a dosing ratio of 1:3 for calcitriol:paricalcitol, i.e. the same conversion factor used previously in studies on hemodialysis patients, only calcitriol was able to reduce iPTH levels in healthy individuals. Low-dose calcitriol reduced iPTH levels without raising calcium absorption and without including any hypercalcemia.

Vitamin D Endocrine Physiology

Quantitative aspects of vitamin D(3) endocrine physiology are briefly reviewed, together with the supporting evidence. Net calcium absorption of at least 200 mg/d is required to offset typical obligatory losses and thereby to protect the skeleton. The ability of the body to regulate intestinal calcium absorption is optimal at serum 25-hydroxyvitamin D(3) concentrations >80 nM. Vitamin D(3) inputs from all sources required to sustain such a level amount to 3600-4200 IU/d. Daily oral intakes as high as 10,000 IU are safe.

Exercise and Calcium Supplementation

Exercise-induced sweat calcium losses have been reported as substantial in male athletes. The first aim of the study was to quantify the increase in 24-h total dermal calcium losses and the net changes in calcium retention in active sportswomen after a 1-h strenuous exercise session. A second aim was to determine the effectiveness of calcium supplementation to offset any calcium loss. Twenty-six premenopausal sportswomen completed three 8-d intervention phases in a randomized-order, crossover design. The three phases were placebo+no exercise (control), placebo+exercise, and 400 mg of calcium as calcium carbonate (TUMS Ultra) twice daily+exercise. The supervised exercise was 1 h.d(-1) cycling at 65-70% of heart rate reserve. A controlled diet of approximately 450 mg.d(-1) of calcium and 24-h pooled urine and fecal collections allowed determination of calcium balance on days 5-8 of each phase. Twenty-four-hour dermal collections were made at the end of each phase using a whole-body washdown procedure. Exercise increased (P<0.05) dermal calcium losses (means+/-SD, 92+/-49 vs 79+/-31 mg.d(-1) in the nonexercise intervention period), which was no longer significant (P=0.14) when calcium supplementation was provided (83+/-49 mg.d(-1)). Higher (P<0.01) urinary calcium excretion during calcium supplementation is suggestive of higher net calcium absorption. Exercise did not affect urinary calcium excretion indicating lack of compensation for dermal losses. Net calcium retention was positive only during the exercise+calcium supplementation intervention period. Calcium supplementation can correct for negative calcium balance attributable to low calcium dietary intake and additional dermal losses from exercise.

Racial differences in skeletal calcium retention in adolescent girls with varied controlled calcium intakes

Background:Higher bone mass in blacks than in whites has been related to greater calcium utilization efficiency. Dietary calcium requirements for maximal skeletal calcium accretion during puberty may differ between the races.Objective:This study compared the relation between calcium intake and calcium retention in black and white adolescent girls.Design:A range of controlled calcium intakes (760–1981 mg Ca/d) were used in 3-wk controlled balance studies. Some subjects were studied more than once; a total of 182 observations from 55 black girls and 66 white girls were analyzed.Results:Blacks had 185 ± 32 mg/d greater mean skeletal calcium retention than did whites (P< 0.0001) at all calcium intakes as a result of significantly greater net calcium absorption (P< 0.001) and lower calcium excretion (P< 0.0001).Conclusions:Dietary calcium requirements did not differ with race. Higher calcium retention at all calcium intakes during adolescence may underlie the higher bone mineral content of adult blacks than of adult whites.

Calcium retention in adolescent boys on a range of controlled calcium intakes1–3

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. 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. 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. 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. The higher calcium retention in boys than in girls was attained through higher net calcium absorption and lower urinary excretion than in girls.

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.

Combined intervention of dietary soybean proteins and swim training: effects on bone metabolism in ovariectomized rats

Soybean proteins, a rich source of isoflavones, taken immediately after an ovariectomy prevent bone loss in rats. Exercise-induced stimuli are essential for bone growth. Few studies exist about the combined effects of swim training and soybean protein supplementation on bone metabolism. So, the purpose of this study was to investigate, in 48 female Sprague-Dawley rats (12 weeks old) the effects of an 8-week swim-training regimen (1 h/day, 5 days/week) and dietary soybean proteins (200 g/kg diet) on bone metabolism. Rats were randomly assigned to four groups: (1) ovariectomized fed with a semisynthetic control diet; (2) ovariectomized fed with a soybean protein-enriched semisynthetic diet; (3) ovariectomized trained to exercise and fed with control diet; (4) ovariectomized trained to exercise and fed with a soybean protein diet. Following the treatment period, body weight gain was identical in the four groups. Soybean protein supplementation increased bone calcium content, and reduced plasma osteocalcin values, without significant modification of calcium balance and net calcium absorption. Swim training enhanced plasma and bone calcium content and calcium balance and net calcium absorption. It did not modify either plasma osteocalcin values or urinary deoxypyridinoline excretion. Both exercise and soybean protein intake increased plasma on bone calcium without modifying net calcium absorption or bone markers. In conclusion, we demonstrated, in ovariectomized rats, that swimming exercise and dietary supplementation with soy proteins do not have synergistic effects on calcium metabolism and bone markers.