Dietary vitamin D intake is not associated with 25-hydroxyvitamin D3 or parathyroid hormone in elderly subjects, whereas the calcium-to-phosphate ratio affects parathyroid hormone☆
Introduction
In addition to the role of vitamin D in osteoporosis, there is evidence that vitamin D is also involved in the pathogenesis of other widespread diseases such as hypertension and certain types of cancer [1], [2]. Elderly people have an increased risk of developing vitamin D deficiency because of age-related declines in cutaneously produced vitamin D, sun exposure, and dietary intake [3]. In Germany, the dietary vitamin D intake of elderly subjects is usually well below the reference value [4], [5]. Thus, sun exposure or the use of supplements may be required to achieve adequate vitamin D status. In addition to vitamin D intake, the amounts of calcium and phosphate in the diet may interfere with vitamin D metabolism. Elevated intact parathyroid hormone (iPTH) levels, which can result from inadequate calcium intake or high phosphate intake, induce the formation of 1,25-dihydroxyvitamin D3, which may promote the turnover of circulating 25-hydroxyvitamin D3 [25(OH)D3]. The existing evidence is not sufficient to determine whether dietary calcium and phosphate and widespread low vitamin D intake affect 25(OH)D3 and/or iPTH in free-living elderly individuals and whether the use of supplements impacts the vitamin D and/or iPTH status of these individuals. Previous studies have provided inconsistent results [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]; phosphate intake and the dietary calcium-to-phosphate ratio (Ca/P ratio) were not frequently taken into consideration as influencing factors.
To date, several studies focused on nutrient intake without considering biochemical parameters [4], [17] such as 25(OH)D3 or iPTH levels, whereas others reported nonfasting measurements [8] and/or did not simultaneously examine intakes of vitamin D, calcium, and phosphate [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. In addition, earlier reports often exclusively focused on 25(OH)D3 or iPTH without considering the interaction between both [6], [16]. Overall, studies with elderly people who are in general good health and do not have vitamin D deficiency and severely elevated iPTH values are scarce. Currently, there is a great debate on higher recommended dietary intakes of vitamin D for elderly people. At present, it is unclear to what extent all elderly subjects, independent of their health, vitamin D status, or body mass index (BMI), would benefit from such a strategy. Other dietary factors, for example, calcium and phosphate, may have a stronger impact on 25(OH)D3 or iPTH. We hypothesized that (1) in free-living elderly subjects, habitual dietary intakes of vitamin D, calcium, and phosphate show no independent associations with 25(OH)D3 or iPTH and (2) BMI confounds these associations. The objectives of the present study were to (1) evaluate vitamin D, calcium, and phosphate intakes; (2) investigate whether intakes of vitamin D, calcium, and phosphate or the Ca/P ratio contribute to 25(OH)D3 and/or iPTH, independent of potential confounders such as age, sex, body composition, sun exposure, physical activity, and smoking behavior, in an aging population; (3) examine the impact of a mutual adjustment for 25(OH)D3 and iPTH in this context, respectively; and (4) assess the influence of BMI on the associations between 25(OH)D3 and iPTH and vitamin D, calcium, and phosphate intakes by comparing normal-weight with overweight/obese individuals. The present study provides a comprehensive examination of the relationships between 25(OH)D3 and iPTH concentrations and vitamin D, calcium, and phosphate intakes in free-living elderly subjects, a population that was rarely the focus of previous investigations.
Section snippets
Study population
The present investigation reports cross-sectional data from the longitudinal study on nutrition and health status in senior citizens of Giessen, Germany (50°35´North), (GISELA study) obtained in 2008. The GISELA study is an ongoing prospective cohort study initiated in 1994. To be enrolled in the study, participants had to be at least 60 years of age and physically mobile. Investigations took place in the Institute of Nutritional Science in Giessen from July to October. Written informed consent
Results
The characteristics of the subjects are presented in Table 1. Both sexes failed to meet the recommended daily intake of 20 μg vitamin D [5]; only 1 woman had an intake greater than 20 μg/d. A vitamin D intake less than 10 μg/d and less than 5 μg/d was observed in 93.9 % and 62.6 % of the women and 90.2 % and 56.1 % of the men, respectively. The median calcium intake for both sexes was close to the recommendation of 1000 mg/d [25], and only 19.2 % of the women and 14.6 % of the men had a calcium
Discussion
The results of our study confirm that in noninstitutionalized elderly subjects, habitual dietary intakes of vitamin D, calcium, and phosphate exhibit no independent associations with 25(OH)D3 or iPTH, although the Ca/P ratio was negatively related to iPTH, even after multiple adjustments. Therefore, our first hypothesis must be partially rejected as well as the second hypothesis that BMI functions as a confounder in this context. The second hypothesis was only confirmed for the association
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The authors do not have any competing interests to declare. This investigation received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.