Life-style factors associated with winter serum 25-hydroxyvitamin D levels in elderly adults

Age and Ageing, July, 1995 by Robert Scragg, Kay-Thee Khaw, Sean Murphy

Introduction

Mean blood levels of 25-hydroxyvitamin D, a measure of body vitamin D [1], are low among elderly people during winter [2, 3]. There are two reasons for this. First, solar ultraviolet radiation, the major source of vitamin D [1, 4], is low in winter [5] and at UK latitudes the sun is not strong enough for vitamin D synthesis during winter [5]. Second, the capacity of skin to synthesize vitamin D in response to ultraviolet radiation decreases with age [6]. Reduced blood levels of 25-hydroxyvitamin D have been found in patients with coronary heart disease [7], colon cancer [8], and fractured neck of femur [9] compared with controls. Moreover, the occurrences of fractured neck of femur [10] and coronary heart disease [11] increase in winter. Thus, the maintenance of winter vitamin D levels in elderly people may be important to prevent the winter increase in some or all of these conditions.

There is evidence that alterations in vitamin D metabolism are associated with several coronary heart disease risk factors. Oral supplementation with a synthetic analogue of 1,25-dihydroxyvitamin D (alphacalcidol) lowers blood pressure in people with impaired glucose tolerance [12]. Administration of 25-hydroxyvitamin [D.sub.3] improves left ventricular function in patients with end-stage renal disease [13]. Decreased serum levels of 25-hydroxyvitamin [D.sub.3] have been described in newly detected cases of diabetes and impaired glucose tolerance compared with controls [14]. People who do vigorous leisure-time physical activities have higher levels of 25-hydroxyvitamin [D.sub.3] compared with those who are inactive [15].

Because of the limited research in this area, we carried out a cross-sectional survey of free-living elderly volunteers, with interviewing restricted to a 5-week period during early winter to prevent the confounding effect of season on serum vitamin D levels. Our aim was to determine whether cardiovascular and other potentially modifiable life-style factors were associated with serum levels of 25-hydroxyvitamin D during this period.

Methods

The cross-sectional data presented in this paper come from baseline information collected from 191 volunteers (104 women, 87 men) aged 63-76 years who were enrolled in a clinical trial of vitamin D supplementation [16]. They were selected from participants in a population-based survey of bone density who were originally recruited from general-practitioner age-sex registers of people living in the Cambridge Health District. The only exclusion criteria (for purposes of the trial) were the use of vitamin D medication or supplements (including cod liver oil) or antihypertensive medication.

Participants were interviewed at Addenbrook's Hospital during December 1991. Prior to interview they answered a short self-administered questionnaire which included information on demographic variables, medication and past medical history. Current smoking status was assessed with regard both to cigarettes and pipes. The frequency of ingestion, during the previous 4 weeks, of margarine, which is the most important source (40%) of dietary vitamin D in Britain, was assessed [17]. Participants were asked the number of hours spent outdoors each week during the previous month, and whether they had a holiday at the seaside within the last 6 months and, if so, in which country. Regular leisure time physical activity (at least weekly) in the last 4 weeks was recorded and classified as vigorous if it caused shortness of breath, and otherwise as moderate.

After a wait of 30 minutes inside the hospital, to allow standardization of body temperature, radial pulse was counted for 30s by hand. Blood pressure was measured twice with a mercury sphygmomanometer (diastolic pressure recorded at cessation of sounds) and averaged. Forearm muscle strength (nearest kg) was measured twice for each hand using the Harpenden Handgrip Dynamometer and all four readings were averaged. Height (nearest 0.5cm) and weight (nearest 0. 2 kg) were measured after removal of shoes and heavy clothing, Finally, a blood sample was collected, centrifuged within 2h, divided into aliquots and stored at -70[degrees]C. Serum 25-hydroxyvitamin D ([D.sub.2] plus [D.sub.3]) was measured by the method of Preece et al. [18] (intra-assay coefficient of variation 2.7%); serum intact parthyroid hormone (1-84) was estimated using a two-site immunoradiometric assay [19]; serum total cholesterol and high-density-lipoprotein(hdl) cholesterol were measured by standard laboratory techniques on the RA1000 (Technicon) analyzer [20, 21]. Low-density-lipoprotein(LDL) cholesterol was determined by the method of Friedwald [22] after measurement of triglycerides [23]. Body mass index (BMI) was calculated from weight(kg)/square of height(m).

All analyses were carried out with SAS (SAS Institute, Cary, NC, 1987, Version 6). Analysis of variance was used to calculate adjusted means. Statistical significance was set at p < 0.05 (2-tail).

Results

Table I shows the characteristics of the participants whose ages ranged from 63 to 76 years. Women had significantly higher mean levels of serum total cholesterol, HDL-cholesterol and LDL-cholesterol. In contrast, mean serum 25-hydroxyvitamin D was higher in men than in women, with the result that women had a higher (albeit nonsignificant) mean serum PTH level. Men also had higher handgrip strength than women. Otherwise, mean levels of blood pressure, pulse and BMI were similar (p > 0.05) for men and women.