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

Abstract

This cross-sectional study investigates whether serum 25-hydroxyvitamin D₃ [25(OH)D₃] and intact parathyroid hormone (iPTH) are affected by vitamin D, calcium, or phosphate intake in 140 independently living elderly subjects from Germany (99 women and 41 men; age, 66-96 years). We hypothesized that habitual dietary intakes of vitamin D, calcium, and phosphate are not associated with 25(OH)D₃ or iPTH and that body mass index confounds these associations. Serum 25(OH)D₃ and iPTH were measured by an electrochemiluminescence immunoassay. Dietary intake was determined using a 3-day estimated dietary record. The median dietary intake levels of vitamin D, calcium, and phosphate were 3 μg/d, 999 mg/d, and 1250 mg/d, respectively. Multiple regression analyses confirmed that dietary vitamin D and calcium did not affect 25(OH)D₃ or iPTH; however, supplemental intakes of vitamin D and calcium were associated with 25(OH)D₃ after adjustment for age, sex, body composition, sun exposure, physical activity, and smoking. In addition, phosphate intake and the calcium-to-phosphate ratio were associated with iPTH after multiple adjustments. In a subgroup analysis, calcium and vitamin D supplements, as well as phosphate intake, were associated with 25(OH)D₃ and/or iPTH in normal-weight subjects only. Our results indicate that habitual dietary vitamin D and calcium intakes have no independent effects on 25(OH)D₃ or iPTH in elderly subjects without vitamin D deficiency, whereas phosphate intake and the calcium-to-phosphate ratio affect iPTH. However, vitamin D and calcium supplements may increase 25(OH)D₃ and decrease iPTH, even during the summer, but the impact of supplements may depend on body mass index.

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.