Location and Vitamin D synthesis: Is the hypothesis validated by geophysical data?

https://doi.org/10.1016/j.jphotobiol.2006.10.004Get rights and content

Abstract

The literature reports strong correlations between UV exposure and latitude gradients of diseases. Evidence is emerging about the protective effects of UV exposure for cancer (breast, colo-rectal, prostate), autoimmune diseases (multiple sclerosis, type II diabetes) and even mental disorders, such as schizophrenia. For the first time, the available levels of vitamin D producing UV or “vitamin D UV” (determined from the previtamin D action spectrum) and erythemal (sunburning) UV from throughout the USA are measured and compared, using measurements from seven locations in the USA are measured and compared, using measurements from seven locations in the US EPA’s high accuracy Brewer Spectrophotometer network. The data contest longstanding beliefs on the location-dependence and latitude gradients of vitamin D UV. During eight months of the year centered around summer (March–October), for all sites (from 18°N to 44°N latitude) the level of vitamin D UV relative to erythemal UV was equal (within the 95% confidence interval of the mean level). Therefore, there was no measured latitude gradient of vitamin D UV during the majority of the year across the USA. During the four cooler months (November–February), latitude strongly determines vitamin D UV. As latitude increases, the amount of vitamin D UV decreases dramatically, which may inhibit vitamin D synthesis in humans. Therefore, a larger dose of UV relative to erythemal UV is required to produce the same amount of vitamin D in a high latitude location. However, the data shows that at lower latitude locations (<25°N), wintertime vitamin D UV levels are equal to summertime levels, and the message of increasing UV exposure during winter is irrelevant and may lead to excessive exposure. All results were confirmed by computer modeling, which was also used to generalize the conclusions for latitudes from 0° to 70°N. The results of this paper will impact on research into latitudinal gradients of diseases. In particular, it may no longer be correct to assume vitamin D levels in populations follow significant latitude gradients for a large proportion of the year.

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” Eeff=λS(λ)A(λ)dλ 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.

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