Methodological considerations in measuring human calcium absorption: relevance to study the effects of inulin-type fructans

Daily Intake of a Functional Synbiotic Yogurt Increases Calcium Absorption in Young Adult Women

BACKGROUND

Foods containing both prebiotics and probiotics (synbiotics) might enhance calcium bioavailability.

OBJECTIVES

We investigated the acute effect in young adult women on calcium absorption of consuming (185 mL) a synbiotic yogurt (SYN) containing inulin (4 g) and Lactobacillus rhamnosus GG (>1 × 107 CFU/mL) compared with a control yogurt (CON).

METHODS

Adult normal-weight women (25.0 ± 3.5 y, n = 30) participated in a 3-wk crossover study testing daily consumption of SYN compared with CON. Habitual dietary intake, bone mineral density (BMD), calcium biomarkers, and serum 25-hydroxyvitamin D were measured at baseline. Calcium absorption was tested after each phase of the study using a 42Ca oral tracer. Cumulative tracer recovery was measured in 0-4-h,  0-24-h, and 0-36-h urine pools collected postdosing. The SYN/CON tracer ratio from the timed urine pools was the primary outcome. A beneficial response to SYN was defined as 0-36-h SYN/CON tracer ratio >1.

RESULTS

Net 42Ca recovered increased over time in each of the SYN and CON urine pools postdosing (Friedman, P < 0.001), with a trend for higher 42Ca recovery in the 0-36-h urine pool postdosing in the SYN (1.14%) compared with the CON (0.90%) study (Wilcoxon, P = 0.07). For CON, the majority of total tracer was recovered in the 0-24-h pool (86%), whereas for SYN only 50% of total tracer was recovered in the 0-24-h pool (Friedman, P = 0.001). The SYN/CON tracer ratio in the 0-36-h pool (1.24) was >1 (Wilcoxon, P = 0.015). About two-thirds (n = 19) of women studied responded to the SYN treatment. Responders had higher vegetable intake (P = 0.03), tended to have higher potassium and calcium intakes (P ≤ 0.08), and had higher total body BMD (P = 0.09), than nonresponders.

CONCLUSIONS

Short-term daily consumption of a synbiotic yogurt enhanced calcium absorption relative to a control yogurt in adult women. Given the observed time delays in tracer recovery, enhancement of calcium absorption likely occurred in the large intestine.The study was registered at clinicaltrials.gov (study registration ID: NCT03420716).

Effects of casein phosphopeptides on calcium absorption and metabolism bioactivity in vitro and in vivo

Casein phosphopeptides (CPPs) are a series of peptides containing serine phosphate, which prevents calcium precipitation in the small intestine, so that it can be absorbed. Calcium metabolism studies have been typically carried out using traditional metabolic balance tests. Non-radioactive stable isotopes have rarely been used to examine the in vivo absorption and metabolism of calcium in animal models due to their high cost and the difficulty in their accurate detection. However, they have recently attracted intense research attention. In this study, we developed InertSep ME-1 using a chelating resin to effectively separate 42Ca from spectral-interference in the process of ICP-MS detection. The method effectively removes interfering ions like potassium and sodium and greatly improves the accurate detection of the calcium ion isotope. We also investigated the absorption, distribution, and metabolism of different CPPs through both in vitro cell experiments and in vivo animal experiments. The results indicate that CPPs strongly promote calcium absorption, especially the P5 active monomer component. The results of our in vivo studies show that the calcium isotope can be absorbed from the small intestine into the blood. Then, one part is transported to various organs through tissue fluids while another part is excreted into the urine through the kidneys. In general, our results reveal that CPPs promote the absorption of calcium significantly and positively affect calcium metabolism.

Inulin: Properties, health benefits and food applications

Inulin is a water soluble storage polysaccharide and belongs to a group of non-digestible carbohydrates called fructans. Inulin has attained the GRAS status in USA and is extensively available in about 36,000 species of plants, amongst, chicory roots are considered as the richest source of inulin. Commonly, inulin is used as a prebiotic, fat replacer, sugar replacer, texture modifier and for the development of functional foods in order to improve health due to its beneficial role in gastric health. This review provides a deep insight about its production, physicochemical properties, role in combating various kinds of metabolic and diet related diseases and utilization as a functional ingredient in novel product development.

Free 25(OH)D and Calcium Absorption, PTH, and Markers of Bone Turnover

CONTEXT

It has been proposed that serum free 25-hydroxyvitamin D [25(OH)D] may better reflect vitamin D action than total 25(OH)D. An ELISA for serum free 25(OH)D has recently become available, permitting direct assay.

OBJECTIVE

To determine whether serum free 25(OH)D provides additional information in relation to calcium absorption and other biomarkers of vitamin D action compared to total serum 25(OH)D.

SETTING

Ambulatory research setting in a teaching hospital.

OUTCOME

Serum free 25(OH)D measured in a previously performed study of varied doses of vitamin D3 (placebo and 800, 2000, and 4000 IU) on calcium absorption, PTH, procollagen type 1 N-terminal propeptide, and C-terminal telopeptides of type I collagen. Free 25(OH)D was measured by ELISA. Calcium absorption was measured at baseline and at 10 weeks using stable dual calcium isotopes.

RESULTS

Seventy-one subjects completed this randomized, placebo-controlled trial. Baseline group mean free and total 25(OH)D varied from 4.7 ± 1.8 to 5.4 ± 1.5 pg/mL, and from 23.7 ± 5.9 to 25.9 ± 6.1 ng/mL, respectively. Participants assigned to the 4000-IU dose arm achieved free 25(OH)D levels of 10.4 pg/mL and total 25(OH)D levels of 40.4 ng/mL. Total and free 25(OH)D were highly correlated at baseline and after increasing vitamin D dosing (r = 0.80 and 0.85, respectively). Free 25(OH)D closely reflected changes in total 25(OH)D. PTH was similarly correlated at baseline and follow-up with total and free 25(OH)D. Serum C-terminal telopeptides of type I collagen had a moderate positive correlation with total and free 25(OH)D at follow-up. The serum 1,25-dihydroxyvitamin D change increased significantly with the change in 25(OH)D but not with the change in free 25(OH)D.

CONCLUSION

There was no advantage from measuring free over total 25(OH)D in assessing the response of calcium absorption, PTH, and markers of bone turnover to vitamin D. Free 25(OH)D responded to increasing doses of vitamin D in a similar fashion to total 25(OH)D.

Treatment of Vitamin D Insufficiency in Postmenopausal Women: A Randomized Clinical Trial

IMPORTANCE

Experts debate optimal 25-hydroxyvitamin D (25[OH]D) levels for musculoskeletal health.

OBJECTIVE

To compare the effects of placebo, low-dose cholecalciferol, and high-dose cholecalciferol on 1-year changes in total fractional calcium absorption, bone mineral density, Timed Up and Go and five sit-to-stand tests, and muscle mass in postmenopausal women with vitamin D insufficiency.

DESIGN, SETTING, AND PARTICIPANTS

This randomized, double-blind, placebo-controlled clinical trial was conducted at a single center in Madison, Wisconsin, from May 1, 2010, through July 31, 2013, and the final visit was completed on August 8, 2014. A total of 230 postmenopausal women 75 years or younger with baseline 25(OH)D levels of 14 through 27 ng/mL and no osteoporosis were studied.

INTERVENTIONS

Three arms included daily white and twice monthly yellow placebo (n=76), daily 800 IU vitamin D3 and twice monthly yellow placebo (n=75), and daily white placebo and twice monthly 50,000 IU vitamin D3 (n=79). The high-dose vitamin D regimen achieved and maintained 25(OH)D levels≥30 ng/mL.

MAIN OUTCOMES AND MEASURES

Outcome measures were 1-year change in total fractional calcium absorption using 2 stable isotopes, bone mineral density and muscle mass using dual energy x-ray absorptiometry, Timed Up and Go and five sit-to-stand tests, functional status (Health Assessment Questionnaire), and physical activity (Physical Activity Scale for the Elderly), with Benjamini-Hochberg correction of P values to control for the false discovery rate.

RESULTS

After baseline absorption was controlled for, calcium absorption increased 1% (10 mg/d) in the high-dose arm but decreased 2% in the low-dose arm (P = .005 vs high-dose arm) and 1.3% in the placebo arm (P = .03 vs high-dose arm). We found no between-arm changes in spine, mean total-hip, mean femoral neck, or total-body bone mineral density, trabecular bone score, muscle mass, and Timed Up and Go or five sit-to-stand test scores. Likewise, we found no between-arm differences for numbers of falls, number of fallers, physical activity, or functional status.

CONCLUSIONS AND RELEVANCE

High-dose cholecalciferol therapy increased calcium absorption, but the effect was small and did not translate into beneficial effects on bone mineral density, muscle function, muscle mass, or falls. We found no data to support experts' recommendations to maintain serum 25(OH)D levels of 30 ng/mL or higher in postmenopausal women. Instead, we found that low- and high-dose cholecalciferol were equivalent to placebo in their effects on bone and muscle outcomes in this cohort of postmenopausal women with 25(OH)D levels less than 30 ng/mL.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00933244.

Vitamin D supplementation increases calcium absorption without a threshold effect

BACKGROUND

The maximal calcium absorption in response to vitamin D has been proposed as a biomarker for vitamin D sufficiency.

OBJECTIVE

The objective was to determine whether there is a threshold beyond which increasing doses of vitamin D, or concentrations of serum 25-hydroxyvitamin D [25(OH)D], no longer increase calcium absorption.

DESIGN

This was a placebo-controlled, dose-response, randomized, double-blind study of the effect of vitamin D on calcium absorption in healthy postmenopausal women. Seventy-six healthy postmenopausal women were randomly assigned to placebo or 800 IU (20 μg), 2000 IU (50 μg), or 4000 IU (100 μg) vitamin D₃ for 8 wk. The technique of dual isotopes of stable calcium was used with a calcium carrier to measure calcium absorption at baseline and after 8 wk.

RESULTS

Seventy-one women with a mean ± SD age of 58.8 ± 4.9 y completed the study. The mean calcium intake was 1142 ± 509 mg/d and serum 25(OH)D was 63 ± 14 nmol/L at baseline. A statistically significant linear trend of an increase in calcium absorption adjusted for age and body mass index with increasing vitamin D₃ dose or serum 25(OH)D concentration was observed. A 6.7% absolute increase in calcium absorption was found in the highest vitamin D₃ group (100 μg). No evidence of nonlinearity was observed in the dose-response curve.

CONCLUSIONS

No evidence of a threshold of calcium absorption was found with a serum 25(OH)D range from 40 to 130 nmol/L. Calcium absorption in this range is not a useful biomarker to determine nutritional recommendations for 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 R(2)) 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.

Evaluation of an inexpensive calcium absorption index in healthy older men and women

OBJECTIVE

Calcium absorption is an important determinant of calcium retention and bone metabolism. However, most methods of measuring calcium absorption, including the well-established dual stable isotope method, are costly and cumbersome to implement. We evaluated whether an oral calcium tolerance test (OCTT), which involves measuring calcium excretion in a fasting 2-h urine collection and two 2-h collections following an oral calcium dose, may be a useful index of calcium absorption in older adults consuming a fixed calcium intake of 30 mmol/day.

DESIGN

After a 10-day metabolic diet containing 30 mmol/day of calcium, subjects had calcium absorption measured using the dual stable isotope method and the OCTT.

PARTICIPANTS

Eleven healthy subjects aged 54-74 years.

MEASUREMENTS

Fractional calcium absorption (FCA), calcium excretion in a fasting 2-h urine collection and two 2-h collections in response to a 10-mmol calcium dose (total intake 30 mmol/day).

RESULTS

Calcium excretion from several combinations of the urine collections was examined in relation to FCA. The most predictive of FCA was calcium excretion 4 h following the calcium dose. This measure was significantly correlated with FCA (r = 0.735, P = 0.010), fitting 54% of the variability in FCA.

CONCLUSION

Urinary calcium excretion during the 4 h after a 10-mmol calcium dose is a useful index of calcium absorption among older adults consuming recommended calcium intakes. This test is inexpensive, easy to implement and potentially useful in large clinical studies.

A simple single serum method to measure fractional calcium absorption using dual stable isotopes

The dual stable isotope method with a timed 24-h urine collection is the gold standard approach to measure fractional calcium absorption. However, the need to collect urine for 24 h makes this technique time-consuming and laborious. Our study sought to determine whether a dual isotope method using a single serum sample obtained 4 h after administration of the initial isotope provides a useful approach to measure fractional calcium absorption. Following a metabolic diet with a fixed calcium intake of 30 mmol/day for 10 days, nineteen healthy subjects age 54-74 were given a test meal with an oral isotope ((44)Ca) followed 2 h later by an intravenous isotope ((42)Ca). Once the oral isotope was administered, urine was collected for 24 h, and a serum sample was obtained after 4 h. The ratio of the oral to intravenous isotopes was measured in the urine and serum by mass spectroscopy. Fractional calcium absorption was 16.2 ± 7.7% by the 4-h single serum method versus 18.5 ± 7.5% by the 24-h urine method. There was a small mean difference between the urine and serum methods of 2.33% with a confidence interval -3.97 to 8.60%. The two methods showed a strong linear association (r = 0.912, p<0.001). Use of dual stable isotopes with a 4-h single serum method gives fractional calcium absorption values that are 12.5% lower than with the 24-h urine method; however, it rank orders subjects accurately thus making it a useful alternative method in clinical research applications.

Vitamin D insufficiency: disease or no disease?

Vitamin D insufficiency (VDI) is widely reported. In patients with normal PTH, the diagnosis rests on increases in fractional calcium absorption (FCA) when 25(OH)D increases above 30 ng/ml. However, estimates of increased FCA after correction of VDI vary dramatically, depending on study methods. We used a dual stable calcium isotope to clarify the impact of vitamin D repletion on FCA in postmenopausal women with VDI. We hypothesized that FCA would increase with vitamin D repletion. We studied postmenopausal women with VDI [25(OH)D = 16-24 ng/ml] and an estimated calcium intake <or=1100 mg daily. Exclusion criteria included hypercalcemia, hypercalciuria, renal insufficiency, nephrolithiasis, gastrointestinal disorders, osteomalacia, prior adult fragility fracture, baseline T-score < -3.0, and use of medications known to interfere with vitamin D or calcium metabolism. Each woman underwent inpatient FCA studies before and after correction of VDI. We used ergocalciferol 50,000 IU/d for 15 days to achieve vitamin D repletion. During each study, women consumed their typical diet. They ingested (44)Ca orally with breakfast and received (42)Ca intravenously. We collected urine for 24 h and measured its calcium isotope content by mass spectrometry. Eighteen women completed the study; all but two had normal PTH. During the first and second FCA studies, their mean 25(OH)D level was 22 +/- 4 and 64 +/- 21 ng/ml, respectively (p < 0.001). Subjects' average FCA was 24 +/- 7% initially and 27 +/- 6% after vitamin D repletion (p = 0.04). Thus, FCA increased by 3 +/- 1% with correction of VDI. Postmenopausal women with VDI experience small FCA increments with vitamin D therapy. In existing literature, this small change in FCA does not associate with lower fracture rates or consistently higher bone mass. Future studies should ascertain whether small FCA increments favorably affect the skeleton.

Inulin-type fructans: functional food ingredients

A food (ingredient) is regarded as functional if it is satisfactorily demonstrated to affect beneficially 1 or more target functions in the body beyond adequate nutritional effects. The term inulin-type fructans covers all beta(2<--1) linear fructans including native inulin (DP 2-60, DP(av) = 12), oligofructose (DP 2-8, DP(av) = 4), and inulin HP (DP 10-60, DP(av) = 25) as well as Synergy 1, a specific combination of oligofructose and inulin HP. Inulin-type fructans resist digestion and function as dietary fiber improving bowel habits. But, unlike most dietary fibers, their colonic fermentation is selective, thus causing significant changes in the composition of the gut microflora with increased and reduced numbers of potentially health-promoting bacteria and potentially harmful species, respectively. Both oligofructose and inulin act in this way and thus are prebiotic: they also induce changes in the colonic epithelium and in miscellaneous colonic functions. In particular, the claim "inulin-type fructans enhance calcium and magnesium absorption" is scientifically substantiated, and the most active product is oligofructose-enriched inulin (Synergy 1). A series of studies furthermore demonstrate that inulin-type fructans modulate the secretion of gastrointestinal peptides involved in appetite regulation as well as lipid metabolism. Moreover, a large number of animal studies and preliminary human data show that inulin-type fructans reduce the risk of colon carcinogenesis and improve the management of inflammatory bowel diseases. Inulin-type fructans are thus functional food ingredients that are eligible for enhanced function claims, but, as more human data become available, risk reduction claims will become scientifically substantiated.

Use of stable isotopes to evaluate the functional effects of nutrients

PURPOSE OF REVIEW

Lifestyle, particularly dietary habits, plays a major role in the increasing prevalence of obesity, type 2 diabetes and atherosclerosis. Understanding how diet and specific foods can modify important functions of the body, in a beneficial or detrimental way, is therefore important. This review presents recent advances in the use of stable isotopes to investigate how nutrients can influence pathways of glucose, lipids and protein metabolism, and also intestinal absorption, body composition and fat mass turnover.

RECENT FINDINGS

Recent developments have focused mainly on the measurement of intratissular metabolic pathways, particularly in lipids and proteins. Advances in the field of cell proliferation rates, body composition and energy expenditure have also been made. Many of these developments rely on the use of deuterated water to trace multiple metabolic pathways.

SUMMARY

Stable isotopes can now be used to measure the kinetics and oxidation rate of circulating metabolites and also body composition, colonic fermentation, intestinal absorption, and important intratissular metabolic pathways, such as gluconeogenesis, lipogenesis, cholesterol synthesis, the turnover rate of triglycerides, individual protein synthesis and cell proliferation. Deuterated water has emerged as a powerful and versatile tool, allowing the simultaneous investigation of several aspects of protein, lipid and glucose metabolism.