In the present study, the vitamin D status and its determinants were investigated in a group of physically active elderly in the summertime. The main findings were that physically active elderly who do not take supplements have high average 25(OH)D blood concentrations in the summer, with only ~ 2% of the population demonstrating a 25(OH)D concentration < 50 nmol/L. Dietary intake of vitamin D did not significantly contribute to vitamin D status, whereas lower age, lower BMI, higher alcohol intake and more outdoor physical activity were significantly associated with a higher vitamin D status in the multivariate model.
The average vitamin D status of 88.8 nmol/L in elderly aged 65–93 years, determined in July in the Netherlands, is substantially higher than reported in comparable studies. Brouwer-Brolsma et al. investigated vitamin D status in community-dwelling elderly aged ≥ 65 year, and reported a mean 25(OH)D concentration of 70 nmol/L in blood samples that were collected in July [5]. Moreover, in our population, only 2% had a blood 25(OH)D value of < 50 nmol/L, whereas Brouwer-Brolsma reported that 37% of the population had a blood 25(OH)D value < 50 nmol/L. Furthermore, van Dam et al. reported a mean 25(OH)D concentration of 61.3 nmol/L in the summer months with 33.7% < 50 nmol/L in an elderly population with a mean age of 69 year [12]. The dietary intake of vitamin D in the current study (4.0 ± 1.9 µg/day) is comparable to what is found previously by Brouwer-Brolsma (~ 4.0–4.5 µg/day) [5], and therefore it is unlikely that dietary intake explains the differences in vitamin D status between the study populations. A more plausible explanation for the higher average 25(OH)D concentration in the present study is that our population spent more time on outdoor physical activity. Previous studies have shown that (outdoor) physical activity is associated with a higher vitamin D status [13, 15]. In the current study, elderly spent on average 12.4 h/week on outdoor activities compared to an average < 7 h/week as reported by Van Dam [12]. Therefore, in all age categories (65–74 year, 75–84 year and 85–93 year), the substantially better vitamin D status in physically active elderly may be explained by higher levels of outdoor physical activity. This suggests that despite the age-related lower rate of cutaneous vitamin D synthesis [4], a high level of outdoor physical activity can compensate for this. Another explanation for the high vitamin D status in this population is the relative low BMI. A high BMI and/or adiposity is associated with a lower vitamin D status or response to supplementation, which is explained by volumetric dilution and/or sequestration in the adipose tissue [6, 12, 25,26,27]. Our group of physically active elderly had a mean BMI 25.0 ± 2.9 kg/m2, compared to a mean BMI of 27.5 ± 4.3 kg/m2and 26.8 ± 3.6 kg/m2 that was reported for Dutch elderly [5]. Possibly, the high level of (outdoor) physical activity may lead to a high vitamin D status through exposure to UV light as well as lowering the BMI.
Generally, elderly are considered a group at risk for vitamin D deficiencies, which has led to generalized vitamin D supplementation guidelines for elderly [2, 6]. Although we observed that significantly more elderly between 65 and 84 years had a 25(OH)D value ≥ 75 nmol/L compared to the 85–93 year group, the vitamin D status in the entire population is good considering that only 2% of the population had a blood 25(OH)D value < 50 nmol/L. These observations put general vitamin D supplementation guidelines to question, as it shows that physically active elderly seem to reach a sufficient vitamin D status without supplementation, at least in the summertime. It is important to note that we did not measure vitamin D status in winter months. Brouwer-Brolsma investigated the year time fluctuation of vitamin D status in elderly and reported a mean value of ~ 42 nmol/L in January as the lowest value, and ~ 70 nmol/L as the highest mean in July [5]. If this finding is extrapolated to our population and 30 nmol/L is subtracted from the summer values, the mean 25(OH)D value would be > 55 nmol/L in the winter, with 34% < 50 nmol/L and 8% < 30 nmol/L. A follow-up evaluation in the winter would be useful to determine to what extent 25(OH)D values will drop in the winter months in physically active elderly who in general remain physically active in winter months as well [5, 12]. The vitamin D status in physically active elderly is high in summertime, which suggests that vitamin D supplementation strategy should take lifestyle factors into account, such as outdoor physical activity, leading to a more personalized and targeted supplementation.
In both the univariate and the multivariate models, age, BMI and outdoor physical activity were associated with 25(OH)D concentrations. These results are in agreement with what has been reported in literature for adults and (community-dwelling)elderly [5, 12, 13, 15, 28], where negative associations were found between age, BMI and vitamin D status, and positive associations were found between physical activity and vitamin D status.
To our surprise, alcohol intake appeared as a significant contributor to vitamin D status in the multivariate regression model. A positive association between moderate alcohol consumption and vitamin D status has been reported in the literature before [29]. The average alcohol consumption in our population was 14.4 gr/day, and ranged between zero consumption up to 79.3 gr/day, meaning that the population contained non-drinkers, low, moderate and some high drinkers. Van Grootheest et al. observed a positive correlation between both moderate and high alcohol consumption and 25(OH)D blood levels in a healthy adult population in the Netherlands [25]. Similar associations were observed in a German and Finnish population of (elderly) adults [30, 31]. These findings have not been discussed extensively and their relevance for humans is as yet not known. It is possible that the association is explained by drinking outdoor rather than the alcohol itself. Considering that alcohol may also be consumed during, e.g., dinner or later in the evening (when UV-based vitamin D synthesis is no longer active) we believe that outdoor drinking certainly does not fully explain the association. In addition, literature suggests that alcohol itself may alter vitamin D metabolism. Experiments with female rats have demonstrated that chronic ethanol consumption leads to reduced renal CYP27B1 expression, with subsequent lower concentrations of 1,25-dihydroxy vitamin D (1,25(OH)2D, the active vitamin D metabolite), and higher 25(OH)D blood concentrations [32]. It is relevant to know whether the same occurs in humans, as this may lead to overestimation of vitamin D status, while the levels of the active vitamin D metabolite may in fact be decreased. Thus, more research is needed to determine whether the observed positive association between vitamin D status and alcohol intake in humans can be explained by altered vitamin D metabolism.
A limitation of the current study is that our questionnaire did not specifically determine the level of outdoor physical activity and exposure to UV radiation. However, we included participants who were training for a multi-day long-distance walking event and therefore most physical activity was performed outside. Furthermore, all vitamin D data were collected within 48 h, which enabled us to assess determinants of vitamin D status without seasonal effects in vitamin D concentrations. A potential problem of this approach is that we assessed vitamin D status in summer only, and we do not know to what extent these values decrease in winter months.
In conclusion, this study demonstrates that physically active elderly without any supplements have a good vitamin D status in the summer with a low prevalence of deficiencies. From the explored potential determinants of vitamin D status, age, BMI, alcohol intake, and outdoor physical activity contributed significantly to vitamin D status. This report shows that current generalized supplementation recommendations for elderly might lead to unnecessary supplementation in physically active subpopulations in the summer. More research is needed to understand the observed association between alcohol intake and vitamin D status.