Journal of Photochemistry and Photobiology B: Biology
Location and Vitamin D synthesis: Is the hypothesis validated by geophysical data?
Introduction
Ultraviolet (UV) radiation is a carcinogen. Excessive exposure causes at least 20% of melanoma and 99% of non-melanoma skin cancer [1]. The numerous deleterious effects of UV exposure also include cataracts, photokeratitis, aging of the skin and sunburn [2]. Together, the global burden of diseases (BOD) due to excessive UV exposure accounts for the loss of 1.7 million disability-adjusted life-years (DALYs) annually [3].
Paradoxically, adequate sun exposure is essential for human health. Practically, our entire requirement of vitamin D is satisfied by exposing ourselves to UV radiation, causing its synthesis in our skin [4]. Vitamin D regulates calcium absorption and, in conjunction with the parathyroid hormone, bone mineralization. Vitamin D insufficiency leads to reduced bone mass, which can be manifested as the debilitating diseases of osteoporosis and osteomalacia in adults and rickets in children [5].
Recently, the global burden of these UV deficient diseases was estimated for the WHO at 3.3 billion life-years annually, almost 2000 times greater than the BOD of excessive UV exposure [3], [6]. Vitamin D insufficiency is widespread, highlighted by the recent claim by Prof Roger Bouillon of University of Leuven that one billion people worldwide are vitamin D insufficient. Vitamin D insufficiency occurs in up to half of free-living adults in New Zealand [7], one-quarter of Australians [8], 14% of French [9], 36% of US young adults and 57% of US general medicine inpatients [10], and particularly in the elderly, including up to 90% in UK [11] and 86% in Switzerland [12]. Dietary intake and artificial fortification of foods is a trivial and ineffectual proportion of vitamin D intake for most populations. Adequate UV exposure would alleviate the sizeable burden of vitamin D deficiency [4], [13], [14], [15].
However, it is critical to measure levels and trends of UV radiation before healthy sun exposure is to be advocated. So far, the most important determinant for vitamin D levels is said to be where you live, due to the dependence on geographical location of the availability of UV radiation for vitamin D synthesis determined from the previtamin D action spectrum (“vitamin D UV”). Hence, it has been assumed that vitamin D levels in populations follow latitude gradients (increasing with closer proximity to the equator). Latitude gradients of cancer (breast, colo-rectal, prostate), autoimmune diseases (multiple sclerosis, type II diabetes), coronary heart disease and mental disorders correlate with these hypothetical vitamin D latitude gradients. Such correlations have been used as evidence to assert the protective nature of UV exposure for these diseases [6], [16], [17], [18], [19]. Ad hoc increases in sun exposure are now being hastily promoted by media and in the literature [20], [21], [22], [23], [24].
This report uses actual data of ground-level UV measurements across the USA from year 2000, in conjunction with computer modeling, to challenge current perceptions on vitamin D latitude gradients. We will investigate latitude gradients of vitamin D relative to erythemal UV, and discuss implications for healthy sun exposure. We then highlight the need to monitor and report on levels of UV for vitamin D photoproduction (not just erythemal UV), especially for high latitude locations.
Section snippets
Action spectrum for previtamin D synthesis
An action spectrum A(λ) describes the wavelength-dependence of a biological effect arising from exposure to UV radiation. Convoluting an action spectrum with the measured solar spectrum S(λ) on each day gives the weighted “effective irradiance” for inducing that effect. The paradoxical effects of sun exposure are erythema (reddening of the skin after sun exposure) and the positive impact of vitamin D synthesis. The erythemal action spectrum was established by CIE [25] (Fig. 1)
Results
UV data will be reported as ratios of vitamin D UV to erythemal UV. Normalizing vitamin D UV in this way is physiologically appropriate because exposing different skin types to the same amount of UV, relative to the skin type’s particular MED (the minimal erythemal dose of UV required to produce erythema), will produce the same quantity of vitamin D [36].
The distribution of the data is shown in Fig. 2, in which all the ratios of vitamin D UV to erythemal UV are plotted from the seven locations.
Discussion and conclusions
Using data from the US EPA Brewer network and computer modeling, we have investigated the seasonal dependence of vitamin D UV levels relative to erythemal UV levels. The results can be confirmed by computer modeling and generalized to latitudes from 0°N to 70°N. During the 8 warmer months of the year (March–October), relative vitamin D UV levels are practically independent of latitude. We conclude that there is practically no latitude gradient of relative vitamin D UV for the entire USA during
Acknowledgement
Michael Kimlin is funded through a Queensland Government “Smart State” Fellowship.
References (37)
High prevalence of vitamin D inadequacy and implications for health
Mayo Clin. Proc.
(2006)- et al.
Considering the potential benefits as well as adverse effects of sun exposure: Can all the potential benefits be provided by oral vitamin D supplementation?
Prog. Biophys. Mol. Biol.
(2006) Epidemiology of disease risks in relation to vitamin D insufficiency
Prog. Biophys. Mol. Biol.
(2006)The challenge resulting from positive and negative effects of sunlight: How much solar UV exposure is appropriate to balance between risks of vitamin D deficiency and skin cancer?
Prog. Biophys. Mol. Biol.
(2006)Who, what, where and when – influences on cutaneous vitamin D synthesis
Prog. Biophys. Mol. Biol.
(2006)- et al.
Indian and Pakistani immigrants have the same capacity as Caucasians to produce vitamin D in response to ultraviolet radiation
Am. J. Clin. Nutr.
(1986) - (2004)
Solar ultraviolet radiation effects on biological systems
Phys. Med. Biol.
(1991)- et al.
The Global Burden of Disease due to UVR Exposure, UV Radiation and its effects – an update 2006
(2006) Vitamin D: a millenium perspective
J. Cell. Biochem.
(2003)
Comparisons of estimated economic burdens due to insufficient solar ultraviolet irradiance and vitamin D and excess solar UV irradiance for the United States
Photochem. Photobiol.
Serum 25OHD Concentrations of New Zealanders Aged 15 Years and Older, UV Radiation and its effects – an update 2006
Vitamin D insufficiency in south-east Queensland (letter)
Med. J. Aust.
Prevalence of vitamin D insufficiency in an adult normal population
Osteoporos. Int.
Vitamin-D status in long-stay geriatric patients
Lancet
Calcidiol, calcitriol and parathyroid hormone serum concentrations in institutionalized and ambulatory elderly in Switzerland
Int. J. Vitam. Nutr. Res.
Ultraviolet irradiation corrects vitamin D deficiency and suppresses secondary hyperparathyroidism in the elderly
J. Bone Miner. Res.
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