Influence of age on effects of endogenous 1,25-dihydroxyvitamin D on calcium absorption in normal women

Genetic, environmental and biomarker considerations delineating the regulatory effects of vitamin D on central nervous system function

Studies show that vitamin D (vit-D) (25(OH)D), the bioactive metabolite (1,25(OH)2D3) and vit-D receptors (vit-D receptor; protein disulphide isomerase, family A member 3) are expressed throughout the brain, particularly in regions pivotal to learning and memory. This has led to the paradigm that avoiding vit-D deficiency is important to preserve cognitive function. However, presently, it is not clear if the common clinical measure of serum 25(OH)D serves as a robust surrogate marker for central nervous system (CNS) homeostasis or function. Indeed, recent studies report CNS biosynthesis of endogenous 25(OH)D, the CNS expression of the CYP group of enzymes which catalyse conversion to 1,25(OH)2D3 and thereafter, deactivation. Moreover, in the periphery, there is significant ethnic/genetic heterogeneity in vit-D conversion to 1,25(OH)2D3 and there is a paucity of studies which have actually investigated vit-D kinetics across the cerebrovasculature. Compared with peripheral organs, the CNS also has differential expression of receptors that trigger cellular response to 1,25(OH)2D3 metabolites. To holistically consider the putative association of peripheral (blood) abundance of 25(OH)D on cognitive function, herein, we have reviewed population and genetic studies, pre-clinical and clinical intervention studies and moreover have considered potential confounders of vit-D analysis.

Mandibular inferior cortex erosion as a sign of elevated total serum calcium in elderly people: a 9-year follow-up study

OBJECTIVES

The mandibular inferior cortex (MIC) classification that was determined by observing the mandible distally from the mental foramen in panoramic radiography can be considered to reflect bone loss in the jaws. The purpose of this 9-year study was to investigate whether there is a relationship between MIC condition and total serum calcium (TSC) in elderly Japanese people.

METHODS

A total of 280 subjects aged 70 years at baseline (137 males and 143 females) took part in this longitudinal study design. TSC and dental panoramic radiographs were carried out at the baseline in 1998 and the final year in 2007.

RESULTS

The results of the MIC measurement were divided into two groups according to changes in MIC after the 9-year study period, namely no change group and change group. A significant relationship was found in females but not in males between the MIC condition and the TSC. The regression showed that female subjects are 3.26 times more likely to have increased erosion of the inferior border of the mandible than male subjects after the 9-year study period.

CONCLUSIONS

These results suggest that an increase in erosion of the inferior border of the mandible was significantly associated with an elevated TSC after 9 years and only in females.

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.

Effect of age on calcium absorption in postmenopausal women

BACKGROUND

It is assumed that calcium absorption decreases with age, but this is not well documented. We report a study that addresses this issue.

OBJECTIVE

The aim was to establish the extent and timing of any age-related change in calcium absorption in postmenopausal women.

DESIGN

We measured radiocalcium absorption (alpha) in 262 healthy postmenopausal women aged 40-87 y. We also measured the serum vitamin D metabolites, parathyroid hormone (PTH), and other biochemical variables.

RESULTS

Radiocalcium absorption decreased with age (P = 0.018); it was 28% lower in the 25 women aged >75 y than in the rest (P < 0.001). It was significantly related to serum 1,25-dihydroxyvitamin D [1,25(OH)(2)D] in the whole set and in both the younger and older subsets, but it was not related to either 25-dihydroxyvitamin D [25(OH)D] or PTH or to any other measured variable. No decrease in 1,25(OH)(2)D was seen with age to account for the decrease in calcium absorption, so radiocalcium absorption corrected for serum 1,25(OH)(2)D decreased significantly after age 75 y. On multivariate analysis, the serum 1,25(OH)(2)D concentration was a positive function of 25(OH)D (P < 0.001), albumin (P = 0.010), and PTH (P = 0.012) and a negative function of serum creatinine (P = 0.003). PTH was a negative function of calculated ionized calcium (P = 0.004) and 25(OH)D (P = 0.009) and a positive function of weight (P = 0.011) and age (P = 0.028).

CONCLUSIONS

A late age-related decrease in calcium absorption is seen in postmenopausal women in addition to the decline that occurs at menopause. This decrease could be due to a decline in either the active calcium transport or diffusion component of the calcium absorption system.

A longitudinal study of bone-related biochemical changes at the menopause

OBJECTIVE

To evaluate the effects of the menopause on bone-related biochemical variables in a longitudinal study.

DESIGN

Recruitment by advertisement of premenopausal women over the age of 44 for measurement of selected variables and collection of blood and urine samples for deep freezing, followed by annual check of menopausal status and repeat collection of blood and urine samples for deep freezing after the menopausal transition.

PATIENTS

A total of 104 women with confirmed premenopausal status and on no treatment likely to affect calcium or bone metabolism were admitted to the study over a period of 2 years. After 8 years, 43 of the volunteers had passed through the menopause and the study was closed.

MEASUREMENTS

Radiocalcium absorption was measured at the first attendance and again after the menopausal transition. Calcium and other relevant variables were measured consecutively on paired thawed-out samples of blood and urine.

RESULTS

The data were complete in 34 subjects. In these women, there were highly significant correlations between the first and second measurements of most variables - serum calcium and fractions, radiocalcium absorption, vitamin D metabolites, PTH and others - indicating significant 'tracking' of these variables across the menopause. Within that framework there were significant rises in serum total and calculated ionized calcium (both P < 0.001) without change in mean serum parathyroid hormone (PTH). Radiocalcium absorption fell (P < 0.001) without change in serum 1,25D. There was a rise in fasting urinary calcium (P < 0.001) which could not be explained by the rise in filtered load and therefore represented a fall in TmCa (P < 0.001). There were significant rises in urinary bone resorption markers, pyridinoline and deoxypyridinoline (P < 0.001).

CONCLUSIONS

We conclude that the menopausal rise in calculated serum ionized calcium without fall in PTH, indicates a change in PTH set-point, and that the falls in gastrointestinal absorption and renal tubular reabsorption of calcium reflect the loss of an oestrogen action at these two sites. Although these changes are sufficient to explain the rise in calcium requirement at the menopause, the association of high bone resorption with normal serum PTH suggests also an increased sensitivity of bone to the action of parathyroid hormone. There is significant 'tracking' of many variables across the menopause despite very significant changes in their absolute values.

Effects of vitamin D metabolites on intestinal calcium absorption and bone turnover in elderly women

BACKGROUND

The relative importance of vitamin D metabolites in the regulation of gut calcium absorption has not been well studied in elderly women living in an environment with abundant sunlight.

OBJECTIVE

The objective was to examine the determinants of active gut calcium absorption ( +/- SD: 42 +/- 11%) after an overnight fast with the use of a low (10 mg) calcium load.

DESIGN

One hundred twenty elderly women aged 74.7 +/- 2.6 y underwent an active calcium absorption test with a radioactive calcium tracer, dietary analysis, and measurement of markers of bone turnover and calcium metabolism.

RESULTS

The mean serum 25-hydroxyvitamin D [25(OH)D] concentration at the time of the calcium absorption test was 68 +/- 29 nmol/L. Gut calcium absorption was correlated with 25(OH)D but not 1,25-dihydroxyvitamin D (calcitriol), the free calcitriol index, or dietary calcium intake. After adjustment for age, calcitriol concentration, and dietary calcium intake, the significant determinant of fractional calcium absorption was the 25(OH)D concentration (r = 0.34, P = 0.001). When body weight was included in the regression, both 25(OH)D (beta = 1.20 x 10(-3)) and calcitriol (beta = 1.00 x 10(-3)) were significantly correlated with calcium absorption. Despite the strong relation between 25(OH)D and gut calcium absorption, there was no relation with other aspects of bone turnover or calcium metabolism.

CONCLUSION

These data suggest that at low calcium loads, 25(OH)D is a more important determinant of gut calcium absorption than is calcitriol in elderly women exposed to abundant sunlight, but that this relation has little effect on overall calcium metabolism.

Role of Ca(2+) and vitamin D in the prevention and treatment of osteoporosis

Osteoporosis is defined as a progressive systemic skeletal disease characterised by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture. The clinical relevance of osteoporosis derives from the fractures that it produces. More than one-third of the adult women will suffer one or more osteoporotic fractures in their lifetime. The lifetime risk in men is approximately one-half that in women. The decrease of the bone mineral density is the most important cause of risk fracture. Among other factors, Ca(2+) and vitamin D deficiencies are important risk factors for a decrease in bone mineral density, consequently inducing osteoporosis. The high prevalence of vitamin D deficiency in healthy elderly people living mainly in southern European countries increase the risk of osteoporotic fractures in these populations above those anticipated for the general elderly population of the European community. In addition, the ageing of the European population will double the number of osteoporotic fractures over the next 50 years, unless adequate preventative measures are undertaken. The efficacy and safety of Ca(2+) and vitamin D supplements at preventing bone loss and reducing the risk of hip and other fractures have been assessed in different clinical trials, which are extensively discussed in this review.

Intestinal calcium absorption in men with spinal osteoporosis

OBJECTIVE

To investigate the role of serum 1,25-dihydroxyvitamin D (1,25D) in the decreased calcium absorption found in men with osteoporosis.

DESIGN

Prospective study of patients referred to a university teaching hospital clinic for investigation of possible osteoporosis.

PATIENTS

Male patients referred for investigation for osteoporosis, from 1981 to 1995, because of specific risk factors or radiological suspicion of osteoporosis. Men with vertebral compression fractures were compared with those without.

MEASUREMENTS

Height and weight, radiocalcium absorption, serum 1,25D and fasting urinary calcium and hydroxyproline excretion.

RESULTS

The men with vertebral fractures had higher fasting urinary hydroxyproline excretion (P = 0.003) and lower calcium absorption (P = 0.002) than the men without. Calcium absorption was positively related to 1,25D in both groups but the estimated calcium absorption at zero 1,25D was lower in the osteoporotic than the normal group. 1,25D was lower in the osteoporotic group than in the normal group. However this difference could only explain about half of the difference in calcium absorption between the groups.

CONCLUSIONS

Calcium absorption is low in men with osteoporosis. About half of the deficit is due to low serum 1,25-dihydroxy vitamin D levels but there appears, in addition, to be some intestinal resistance to its effect on calcium absorption.

Measurement of vitamin D and its metabolites (calcidiol and calcitriol) and their clinical significance

The serum concentration of vitamin D will only give information about the recent exposure to either nutritional vitamin D or to recent vitamin D production in the skin. Within hours vitamin D is removed from the circulation and reappears again a few hours later as 25(OH)D. Measurements of vitamin D therefore are not useful to judge the vitamin D status of man. Plasma concentrations of 25-hydroxyvitamin D (25(OH)D) are the best markers of imminent or existing vitamin D deficiency. Suboptimal vitamin D supply (or 25(OH)D plasma concentrations) are observed in most European countries and North America from November to April. Vitamin D substitution is recommended to persons with a serum concentration of 25(OH)D below 50 nmol/L (20 micrograms/L). Plasma concentrations of 1,25(OH)2D3 (calcitriol) depend mainly on renal function, concentrations of intact PTH and the supply of the organism with calcium and phosphate. High PTH, low calcium and low phosphate supply are the main stimulators of the production of calcitriol. Vitamin D substitution, or if necessary, therapy with active vitamin D metabolites or analogs, e.g. calcitriol or alfacalcidol, are often necessary in hypercalcemic persons with low serum concentrations of calcitriol, e.g. in patients with renal failure. Disorders with low or high vitamin D metabolite concentrations in serum are described in this paper.

Calcium and osteoporosis

Calcium is an essential nutrient that is involved in most metabolic processes and the phosphate salts of which provide mechanical rigidity to the bones and teeth, where 99% of the body's calcium resides. The calcium in the skeleton has the additional role of acting as a reserve supply of calcium to meet the body's metabolic needs in states of calcium deficiency. Calcium deficiency is easily induced because of the obligatory losses of calcium via the bowel, kidneys, and skin. In growing animals, it may impair growth, delay consolidation of the skeleton, and in certain circumstances give rise to rickets but the latter is more often due to deficiency of vitamin D. In adult animals, calcium deficiency causes mobilization of bone and leads sooner or later to osteoporosis, i.e., a reduction in the "amount of bone in the bone" or apparent bone density. The effects of calcium deficiency and oophorectomy (ovariectomy) are additive. In humans, osteoporosis is a common feature of aging. Loss of bone starts in women at the time of the menopause and in men at about age 55 and leads to an increase in fracture rates in both sexes. Individual fracture risk is inversely related to bone density, which in turn is determined by the density achieved at maturity (peak bone density) and the subsequent rate of bone loss. At issue is whether either or both of these variables is related to calcium intake. The calcium requirement of adults may be defined as the mean calcium intake needed to preserve calcium balance, i.e., to meet the significant obligatory losses of calcium through the gastrointestinal tract, kidneys, and skin. The calcium allowance is the higher intake recommended for a population to allow for individual variation in the requirement. The mean requirement defined in this way, calculated from balance studies, is about 20 mmol (800 mg) a day on Western diets, implying an allowance of 25 mmol (1000 mg) or more. Corresponding requirements and allowances have been calculated for pregnancy and lactation and for children and adolescents, taking into account the additional needs of the fetus, of milk production, and of growth. There is a rise in obligatory calcium excretion at menopause, which increases the theoretical calcium requirement in postmenopausal women to about 25 mmol (1000 mg) and implies an allowance of perhaps 30 mmol (1200 mg) or even more if calcium absorption declines at the same time. At issue here, however, is whether menopausal changes in calcium metabolism are the cause or the result of postmenopausal bone loss. The first interpretation relies on evidence of a positive action of estrogen on the gastrointestinal absorption and renal tubular reabsorption of calcium; the latter interpretation relies on evidence of a direct inhibitory effect of estrogen on bone resorption. The calcium model for postmenopausal bone loss tends to be supported by the effect of calcium therapy. An analysis of the 20 major calcium trials in postmenopausal women reported in the last 20 years yielded a mean rate of bone loss of 1.00% per annum (p.a.) in the controls and 0.014% p.a. (NS) in the treated subjects (P < 0.001). However, trials in which calcium and estrogen have been directly compared have shown that the latter is generally more effective than calcium in that it produces a small, but often significant bone gain. This superiority of estrogen over calcium could be due to the former's dual action on calcium absorption and excretion or to a direct action of estrogen on bone itself. In older women, the importance of calcium intake is overshadowed by the strong association between vitamin D insufficiency and hip fracture. Whether this insufficiency arises primarily from lack of exposure to sunlight or to a progressive failure to activate the vitamin D precursor in the skin or both is uncertain but it is compounded by a general decline in dietary vitamin D intake with age. The biological effect is probably an impairment of calcium absorption and c

Ageing and calcium metabolism

Ageing alters the metabolism of calcium and vitamin D in a number of ways. Intake of calcium and vitamin D, exposure to sunlight, cutaneous production of vitamin D3, renal production of 1,25-dihydroxyvitamin D (1,25(OH)2D3), intestinal absorption of calcium and the ability to adapt to a low calcium diet may all be reduced in elderly subjects. As a consequence, secondary hyperparathyroidism often occurs with ageing and can contribute to accelerated bone loss. In fact, alterations in calcium and vitamin D metabolism may be widespread in the ageing population and play a central role in the pathogenesis of senile (age-related) osteoporosis. From a preventive point of view, recent intervention studies have indicated the need to optimize calcium intake and to maintain serum 25(OH)D3 levels within the normal range in elderly people.

Aging, neuroendocrine function, and osteoporosis

Osteoporosis may occur in both women and men. Osteoporosis is defined as age-related involutional bone loss that exceeds that observed in the normal population of the same age and sex. The hallmark of the disease is a reduction in mineralized bone matrix. Osteoporosis is a multifactorial disease and can be considered the result of impaired coordination of bone remodeling ("resorption/formation coupling"). This review describes the role of systemic hormones and local acting mediators, including neuropeptides and neurotransmitters, in the control of bone turnover. Furthermore the significance of age-related (neuro)endocrine dysfunction for bone remodeling and consequently for development of osteoporosis is discussed.