Seasonal variation of serum vitamin D and the effect of vitamin D supplementation in Irish community-dwelling older people

Abstract

Background: Ireland is at 53°N, and its population risk of vitamin D deficiency is high. Previous Irish studies suggested a significant seasonality of serum 25-hydroxyvitamin D [25(OH)D] and a beneficial effect of supplementation in raising 25 (OH)D levels. However, in Irish older people, little is known about the magnitude of the supplementation effect and whether supplementation affects 25(OH)D seasonality. Design: cross-sectional observational. Setting: outpatient clinic. Subjects: five hundred and forty-six community-dwelling subjects (mean age 73.0, SD 7.4; 68.5% females) were assessed between

Make Vitamin D While the Sun Shines, Take Supplements When It Doesn′t: A Longitudinal, Observational Study of Older Adults in Tasmania, Australia

“…The current study confirms observations of two previous studies, [21], [24] regarding continuance of summer vitamin D sufficiency into winter, in participants taking 800 IU of vitamin D per day. Even so, both of these studies were cross-sectional, assessing different individuals at different time points.…”

“…about two weeks longer than that observed by previous studies [15], [21], [32], [33]. This difference may be due to a number of potential mitigating factors including: variation in sun avoidance behaviours, the amount of solar UVB available for vitamin D synthesis (dependant on local weather, pollution, ozone concentration,[33] latitude, and season), and the age range of study populations.…”

“…Furthermore, cutaneous synthesis of vitamin D diminishes with age, [9], [14], [15] yet natural dietary sources of vitamin D are few [4], [5], [16] and their contribution to vitamin D status is minimal. [17], [18], [19], [20] Nevertheless, recent research [15], [21], [22], [23], [24], [25] suggests ageing populations, especially those with limited sunlight exposure, have a greater dependency on dietary sources to maintain optimal vitamin D status, be it through increased consumption of vitamin D-rich foods, food fortification or vitamin D supplementation.…”

“…Previous research was either cross-sectional in design, [15], [21], [22], [23], [24] or did not assess participants over consecutive seasons. [25] The strength of the longitudinal design of the current study is that it is less sensitive to variation within the study population (and thus within a wider population) because it focuses on changes within each study participant.…”

“…[25] The strength of the longitudinal design of the current study is that it is less sensitive to variation within the study population (and thus within a wider population) because it focuses on changes within each study participant. [28] Much of the previous research has been undertaken in the Northern hemisphere at higher latitudes (>45π N), [21], [22], [23], [24], [25] whereas in the Southern Hemisphere, the majority of the population live at latitudes between 45π south and the equator. The participants of the current study lived in and around Launceston, Tasmania (41π S).…”