Factors influencing the measurement of bioavailability, taking calcium as a model

For non-metabolizable supplemental nutrients, bioavailability is effectively equivalent to absorbability. Methods for measuring absorbability (balance, pharmacokinetic, tracer, urine increment, evoked physiological responses, and in vitro) are briefly characterized and their utility compared. When intrinsic labeling of a source is possible, tracer methods are generally the most accurate and precise, as well as often the least expensive. Factors influencing the measured end points of the various methods are described briefly. These include source factors such as pharmaceutic formulation, subject factors such as mucosal mass and the need status of the absorbing subject, and co-ingested factors such as other foods or food constituents. Extensive experience has shown that absorbability is difficult to predict from knowledge of the chemistry of the source, or even from the results of in vitro testing. Hence direct measurement of absorbability is essential to assure regulators and the general public that the source delivers what it promises.

Calcium Needs of the Elderly to Reduce Fracture Risk

Contemporary calcium intakes in the industrialized nations are substantially lower than those to which human physiology is adapted by evolution. As a result, compensatory adjustment is required lifelong. This adjustment consists of high levels of parathyroid activity, leading to parathyroid hyperplasia, high circulating levels of 1,25(OH)2D and high bone turnover. The capacity of these compensatory mechanisms to provide sufficient calcium to offset daily losses from the body declines with age; hence, increasingly the body tears down bone to access its calcium. As a result, the calcium requirement for skeletal maintenance is said to rise with age. Supplemented intakes to a total in the range of 32.5-42.5 mmol (1300-1700 mg)/day have been shown to arrest age-related bone loss and to reduce fracture risk in individuals 65 and older and intakes of 60 mmol (2400 mg), to restore the setting of the parathyroid glands to young adult values. Intakes at such levels also minimize the expression of other disorders such as colon cancer, hypertension and obesity, all of which, while multifactorial, have a calcium deficiency component. Milk, mainly because of constructive interactions among its several key nutrients, is probably the most nutritionally and cost effective way of meeting the calcium requirement in the elderly.

Calcium intake and body weight

Five clinical studies of calcium intake, designed with a primary skeletal end point, were reevaluated to explore associations between calcium intake and body weight. All subjects were women, clustered in three main age groups: 3rd, 5th, and 8th decades. Total sample size was 780. Four of the studies were observational; two were cross-sectional, in which body mass index was regressed against entry level calcium intake; and two were longitudinal, in which change in weight over time was regressed against calcium intake. One study was a double-blind, placebo-controlled, randomized trial of calcium supplementation, in which change in weight during the course of study was evaluated as a function of treatment status. Significant negative associations between calcium intake and weight were found for all three age groups, and the odds ratio for being overweight (body mass index, >26) was 2.25 for young women in the lower half of the calcium intakes of their respective study groups (P: < 0.02). Relative to placebo, the calcium-treated subjects in the controlled trial exhibited a significant weight loss across nearly 4 yr of observation. Estimates of the relationship indicate that a 1000-mg calcium intake difference is associated with an 8-kg difference in mean body weight and that calcium intake explains approximately 3% of the variance in body weight.

Dietary protein and phosphorus do not affect calcium absorption

BACKGROUND

Variation in absorption efficiency explains more of the variability in calcium balance than does actual calcium intake. Several investigators have suggested that the relatively high phosphorus and protein intakes of the diets of industrialized nations reduce calcium absorption and thereby aggravate the problem of calcium deficiency.

OBJECTIVE

My objective was to test whether variation in phosphorus and protein intakes is associated with variation in calcium absorption.

DESIGN

One hundred ninety-one Roman Catholic nuns aged 48.7 +/- 7.0 y were studied approximately 3 times each over a >20-y period with a full metabolic balance regimen; controlled, chemically analyzed diets; and double-tracer measurement of calcium absorption.

RESULTS

Although the expected associations with absorption were found for age, calcium intake, and estrogen status, no association was observed for intakes of either phosphorus or protein.

CONCLUSION

Phosphorus and protein intakes do not contribute to the wide variability in calcium absorption efficiency.

Factors associated with calcium absorption efficiency in pre- and perimenopausal women

BACKGROUND

The amount of calcium ingested by an individual may affect several chronic conditions, including osteoporosis, hypertension, and colon cancer. However, individuals vary in their ability to absorb the calcium they consume.

OBJECTIVE

The purpose of this study was to examine sources of interindividual variation in the efficiency of calcium absorption in women.

DESIGN

Fractional calcium absorption was estimated in 142 healthy pre- and perimenopausal women. Dietary habits, lifestyle factors, calciotropic hormones, and vitamin D receptor gene polymorphisms were also assessed.

RESULTS

Calcium absorption values averaged 35% and ranged from 17% to 58%. Fractional calcium absorption was positively associated with body mass index (r = 0.22, P = 0.007), dietary fat intake (r = 0.29, P = 0.001), serum 1,25 dihydroxyvitamin D [1,25(OH)(2)D] concentrations (r = 0.23, P = 0. 006), and parathyroid hormone concentrations (r = 0.21, P = 0.015). Fractional calcium absorption was inversely associated with total calcium intake (r = -0.18, P = 0.030), dietary fiber intake (r = -0. 19, P = 0.028), alcohol consumption (r = -0.14, P = 0.094), physical activity (r = -0.22, P = 0.007), and symptoms of constipation (r = -0.16, P = 0.059). In stepwise regression analysis, dietary fat, dietary fiber, serum 1,25(OH)(2)D, and alcohol consumption emerged as independent predictors of calcium absorption, explaining 21.02% of the observed variation. Women in the lowest tertile of the ratio of dietary fat to fiber had 19% lower fractional calcium absorption values than did women in the highest tertile of ratio of dietary fat to fiber (test of trend, P < 0.001).

CONCLUSIONS

There is a wide range of calcium absorption values in healthy women. The amount of dietary fat consumed relative to dietary fiber appears to have an important role in determining differences in calcium absorption performance among individuals.

Effects of increased consumption of fluid milk on energy and nutrient intake, body weight, and cardiovascular risk factors in healthy older adults

OBJECTIVE

To assess the impact of increased consumption of milk, without other dietary advice, on older adults' energy and nutrient intakes, weight, cardiovascular risk factors (blood pressure, plasma lipid levels), and quality of life.

SUBJECTS/SETTING

Two hundred four healthy men and women, aged 55 to 85 years, who consumed fewer than 1.5 dairy servings per day were chosen from six US academic health centers.

DESIGN

Randomized, controlled open trial.

INTERVENTION

Advice to increase skim or 1% milk intake by 3 cups per day (n = 101) or to maintain usual diet (n = 103) for 12 weeks after a 4-week baseline period.

MAIN OUTCOME MEASURES

Changes in energy and nutrient intake assessed from 3-day food records, body weight, blood pressure, and plasma lipid levels.

STATISTICAL ANALYSES PERFORMED

Group-by-time analysis of variance with repeated-measures, chi 2 test.

RESULTS

Compliance with the intervention was good. Compared with controls, participants in the milk-supplemented group significantly increased energy, protein, cholesterol, vitamins A, D, and B-12, riboflavin, pantothenate, calcium, phosphorus, magnesium, zinc, and potassium intakes. Prevalence of nutrient inadequacy, assessed for nutrients with Estimated Average Requirements, decreased among women in the milk group for magnesium (40% at baseline vs 13% at 12 weeks, P < .001) and vitamin B-12 (6% vs 0%, P < .05) and tended to decrease (P < .10) for protein and thiamin (women) and magnesium and vitamin B-6 (men). The milk group gained 0.6 kg more than control group (P < .01); however, weight gain was less than predicted, which suggests some compensation for the added energy from milk. Blood pressure decreased similarly over time in both groups. Total and low-density lipoprotein cholesterol levels, and the ratio of total cholesterol to high-density lipoprotein cholesterol, were unchanged. Triglyceride levels increased within the normal range in the milk group (P = .002). Quality of life scores were high at baseline and remained high throughout.

APPLICATIONS/CONCLUSIONS

Older adults can successfully increase milk intake, thereby meaningfully improving their nutrient intakes. Dietitians can play a key role in disseminating this advice.

Bioavailability of the calcium in fortified soy imitation milk, with some observations on method

BACKGROUND

Calcium-fortified soy milk is growing in popularity, particularly among vegetarians, but the bioavailability of its calcium was not previously known. Additionally, the validity of isotopic labeling methods for fortified liquid products had not been established.

OBJECTIVES

The objectives of this study were to compare the bioavailability of the calcium in fortified soy milk with that of calcium in cow milk and to evaluate the method of labeling soy milk for bioavailability testing.

DESIGN

A within-subject comparison of extrinsically labeled cow milk with intrinsically and extrinsically labeled soy milks was undertaken in 16 healthy men. In all tests, 300-mg Ca loads were given as a part of a light breakfast after an overnight fast. The milks were physically partitioned into liquid and solid phases to enable evaluation of tracer distribution.

RESULTS

Calcium from intrinsically labeled soy milk was absorbed at only 75% the efficiency of calcium from cow milk. Extrinsic labeling of soy milk did not produce uniform tracer distribution throughout the liquid and solid phases and resulted in a 50% overestimate of true absorbability.

CONCLUSION

Calcium-fortified soy milk does not constitute a calcium source comparable to cow milk, and extrinsic labeling of such calcium particulate suspensions does not produce the uniform tracer distribution needed for bioavailability testing. Hence, intrinsic labeling of the fortificant is required for such liquid suspensions.

Calcium, dairy products and osteoporosis

Osteoporosis is a multifactorial disorder in which nutrition plays a role but does not account for the totality of the problem. 139 papers published since 1975 and describing studies of the relationship of calcium intake and bone health are briefly analyzed. Of 52 investigator-controlled calcium intervention studies, all but two showed better bone balance at high intakes, or greater bone gain during growth, or reduced bone loss in the elderly, or reduced fracture risk. This evidence firmly establishes that high calcium intakes promote bone health. Additionally, three-fourths of 86 observational studies were also positive, indicating that the causal link established in investigator-controlled trials can be found in free-living subjects as well. The principal reason for failure to find an association in observational studies is the weakness of the methods available for estimating long-term calcium intake. While most of the investigator-controlled studies used calcium supplements, six used dairy sources of calcium; all were positive. Most of the observational studies were based on dairy calcium also, since at the time the studies were done, higher calcium intakes meant higher dairy intakes. All studies evaluating the issue reported substantial augmentation of the osteoprotective effect of estrogen by high calcium intakes. Discussion is provided in regard to the multifactorial complexity of osteoporotic response to interventions and to the perturbing effect in controlled trials of the bone remodeling transient, as well as about how inferences can validly be drawn from the various study types represented in this compilation.

Low fractional calcium absorption increases the risk for hip fracture in women with low calcium intake. Study of Osteoporotic Fractures Research Group

BACKGROUND

Decreased ability to absorb calcium with age limits adaptation to low calcium intake and is thought to lead to secondary hyperparathyroidism and increased risk for hip and other fractures. However, the associations between fractional calcium absorption, dietary calcium intake, and risk for fracture have never been studied.

OBJECTIVE

To determine whether low fractional calcium absorption in women with low calcium intake increases the risk for subsequent hip and other nonspine fractures.

DESIGN

Prospective cohort study.

SETTING

Four clinical centers in Baltimore County, Maryland; Portland, Oregon; Minneapolis, Minnesota; and the Monongahela Valley, Pennsylvania.

PARTICIPANTS

5452 nonblack women 69 years of age or older participating in the fourth examination of the Study of Osteoporotic Fractures.

MEASUREMENTS

Fractional calcium absorption was measured by using a 3-hour single isotope (45Ca) technique. Incident fractures were identified prospectively and were confirmed by radiographic report.

RESULTS

During an average of 4.8 years, 729 women (13%) experienced at least one nonspine fracture; 153 of these women had hip fractures. After adjustment for age, women with lower fractional calcium absorption were at increased risk for hip fracture (relative risk per 1-SD [7.7%] decrease in fractional calcium absorption, 1.24 [95% CI, 1.05 to 1.48]). Women with low fractional calcium absorption and low calcium intake were at greatest risk for subsequent hip fracture; among women whose dietary calcium intake was less than 400 mg/d, those who had fractional calcium absorption at or below the median value of 32.3% had a 2.5-fold (CI, 1.29-fold to 4.69-fold) increase in risk for hip fracture compared with those who had greater absorption efficiency. Fractional calcium absorption was not related to risk for other nonspine fractures (relative risk per 1-SD [7.7%] decrease in fractional calcium absorption, 1.05 [CI, 0.96 to 1.14]).

CONCLUSIONS

In elderly women, low fractional calcium absorption in the setting of low calcium intake increases the risk for hip fracture. Our findings support the hypothesis of type II osteoporosis, which postulates that decreased calcium absorption is an important risk factor for hip fracture in older persons.

Study subjects and ordinary patients

Clinical trials of treatment agents impose strict and often necessary inclusion and exclusion criteria, while patients presenting to physicians for treatment frequently exhibit complicating features that would have excluded them from entry into study. To quantify the degree of discordance between ordinary patients and study subjects, a retrospective chart review was carried out of all new patients with osteoporosis seen in an academic medical center within a consecutive 40-month period, meeting clinical treatment criteria. Each patient chart was reviewed for the inclusion and exclusion criteria of four large, multicenter study protocols. There were 120 consecutive female patients seeking health care, with bone density T-scores below -2.0 and/or with one or more low-trauma fractures. The four trials would have accepted 4, 5, 25 and 8 of our 120 patients. The trial with the most liberal inclusion criteria would have taken only 21% of the total. Principal reasons for ineligibility were comorbidity, prior treatment with bone-active agents, and current therapy with glucocorticoids, anticoagulants and anticonvulsants. Some of these exclusions inevitably reflect the patient mix of a referral center; nevertheless, comorbidity and its therapy are common in the age range in which osteoporosis is prevalent and would, therefore, be expected to be present in patients in general medical practice as well. Thus a large fraction, perhaps the majority, of patients with diagnoses of osteoporosis who are candidates for treatment by their physicians, are not eligible for entry into typical treatment trials. The results of such trials may, therefore, have uncertain applicability to types of patients excluded, both for safety and for efficacy.

The role of calcium in health and disease

Skeletal fragility at the end of the life span (osteoporosis) is a major source of morbidity and mortality. Adequate calcium intake from childhood to the end of the life span is critical for the formation and retention of a healthy skeleton. High intakes of calcium and vitamin D potentiate the bone loss prevention effects of hormone replacement therapy in postmenopausal women. Pregnancy and lactation are not risk factors for skeletal fragility, although lactation is associated with a transient loss of bone that cannot be prevented by calcium supplementation. Low calcium intake has been implicated in the development of hypertension, colon cancer, and premenstrual syndrome, and it is associated with low intakes of many other nutrients. Encouragement of increased consumption of calcium-rich foods has the potential to be a cost-effective strategy for reducing fracture incidence later in life and for increasing patients' dietary quality and overall health.

Dietary changes favorably affect bone remodeling in older adults

OBJECTIVE

To determine whether dietary counseling to increase milk intake could produce useful changes in the calcium economy and what, if any, other nutrition-related changes might be produced.

DESIGN

Randomized, open trial.

SUBJECTS/SETTING

Two hundred four healthy men and women, aged 55 to 85 years, who habitually consumed fewer than 1.5 servings of dairy foods per day. Six academic health centers in the United States.

INTERVENTION

Subjects were instructed to consume 3 servings per day of nonfat milk or 1% milk as a part of their daily diets, or to maintain their usual diets, for a 12-week intervention period, which followed 4 weeks of baseline observations.

MAIN OUTCOME MEASURES

Energy and nutrient intake assessed from milk intake logs and 3-day food records; serum calciotrophic hormone levels at baseline and at 8 and 12 weeks; urinary excretion of calcium and N-telopeptide at 12 weeks.

STATISTICAL ANALYSES

Repeated-measures analysis of variance.

RESULTS

In the milk-supplemented group, calcium intake increased by 729 +/- 45 mg/day (mean +/- standard error), serum parathyroid hormone level decreased by approximately 9%, and urinary excretion of N-telopeptide, a bone resorption marker, decreased by 13%. Urine calcium excretion increased in milk-supplemented subjects by 21 +/- 7.6 mg/day (mean +/- standard error), less than half the amount predicted to be absorbed from the increment in calcium intake. All of these changes were significantly different from baseline values in the milk group and from the corresponding changes in the control group. Bone-specific alkaline phosphatase level (a bone formation marker) fell by approximately 9% in both groups. Serum level of insulin-like growth factor-1 (IGF-1) rose by 10% in the milk group (P < .001), and the level of insulin-like growth factor binding protein-4 (IGFBP-4) fell slightly (1.9%) in the milk group and rose significantly (7.9%) in the control group (P < .05).

APPLICATIONS/CONCLUSIONS

The changes observed in the calcium economy through consumption of food sources of calcium are similar in kind and extent to those reported previously for calcium supplement tablets. The increase in IGF-1 level and the decrease in IBFBP-4 level are new observations that are beneficial for bone health. Important improvements in skeletal metabolism can feasibly occur in older adults by consumption of food sources of calcium. Dietitians can be confident that food works, and that desired calcium intakes can be achieved using food sources.

The effect of low-dose continuous estrogen and progesterone therapy with calcium and vitamin D on bone in elderly women. A randomized, controlled trial

BACKGROUND

Hormone replacement therapy (HRT), the mainstay of osteoporosis prevention, is limited because of dose-related risks, side effects, and patient acceptance. The bone-sparing efficacy and tolerability of the lowest available doses of HRT have not been adequately studied in elderly women.

OBJECTIVE

To determine the bone-sparing effect of continuous low-dose HRT in elderly women.

DESIGN

Randomized, double-blind, placebo-controlled trial.

SETTING

University osteoporosis research and clinical center.

PATIENTS

128 healthy white women (age > 65 years) with low bone mass recruited by word of mouth and by local advertisement. The principal eligibility criterion was spinal bone mineral density of 0.90 g/cm2 or less.

INTERVENTION

Continuous therapy with conjugated equine estrogen, 0.3 mg/d, and medroxyprogesterone, 2.5 mg/d, or matching placebo. Sufficient calcium supplementation was given to bring all calcium intakes above 1000 mg/d in both groups; supplemental oral 25-hydroxyvitamin D was given to maintain serum 25-hydroxyvitamin D levels of at least 75 nmol/L in both groups.

MEASUREMENTS

Bone mineral density of the spine, hip, total body, and forearm; serum total alkaline phosphatase and serum osteocalcin levels at 6-month intervals; and 24-hour urine creatinine and hydroxyproline excretion at baseline, 12 months, and 42 months.

RESULTS

During 3.5 years of observation, spinal bone mineral density increased by 3.5% (P < 0.001) in an intention-to-treat analysis and by 5.2% among patients with greater than 90% adherence to therapy. Significant increases were seen in total-body and forearm bone density (P < 0.01). Symptoms related to HRT (breast tenderness, spotting, pelvic discomfort, and mood changes) were mild and short-lived.

CONCLUSIONS

Continuous low-dose HRT with conjugated equine estrogen and oral medroxyprogesterone combined with adequate calcium and vitamin D provides a bone-sparing effect that is similar or superior to that provided by other, higher-dose HRT regimens in elderly women. This combination is well tolerated by most patients.