ReviewGeographic location and vitamin D synthesis
Introduction
Ultraviolet (UV) radiation is a known carcinogen. Excessive exposure causes at least 20% of melanoma and 99% of non-melanoma skin cancer (Longstreth et al., 1995). The numerous deleterious effects of UV exposure also include cataracts, photokeratitis, aging of the skin and sunburn (Diffey and Davis, 1978). 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 (Lucas et al., 2008).
Paradoxically, adequate sun exposure is essential for human health. Practically our entire requirement of vitamin D is satisfied by exposing our skin UV radiation, causing its synthesis in the skin (Holick and Chen, 2003). Vitamin D regulates calcium absorption and, in conjunction with the parathyroid hormone, bone mineralization. Vitamin D insufficiency causes reduced bone mass, leading to the debilitating diseases of osteoporosis and osteomalacia in adults and rickets in children (Holick, 2006). More recently, the role of vitamin D in internal cancer prevention has been suggested, leading to an increase in the research activity in this area.
The interactions between solar UV exposure and subsequent vitamin D synthesis are confined to a few studies (Kimlin et al., 2007a, Kimlin et al., 2007b). However, the associations between location of residence and risk of disease development, particularly for some internal cancers such as prostate and colorectal cancer, indicate that there may be a role for UV exposure in preventing such diseases (Giovannucci et al., 2006, Gorham et al., 2005). This paper will overview the main factors researchers should consider when undertaking research into location of residence (geographic location) and subsequent vitamin D synthesis potential.
Section snippets
Ultraviolet radiation
Ultraviolet radiation (UV) is primary sourced from the sun, along with radiation from all the other regions of the electromagnetic spectrum. Specifically, ultraviolet radiation is most responsible for almost all the detrimental human health effects of the sun, such as skin cancer and photoaging (Armstrong and Kricker, 2001). Ironically, it is also responsible for the beneficial health effects also, namely, vitamin D synthesis.
Ultraviolet radiation is divided into three categories:
UVA (320–400 nm)
Ultraviolet radiation and vitamin D synthesis
The synthesis of vitamin D (pre-vitamin D) from the cholesterol pre- cursor, 7-dehydrocholesterol is governed by the vitamin D action spectrum (reference CIE here). This presents the impact of pre-vitamin D synthesis as a function of wavelength. This action spectra’s unique characteristic is that it lies entirely within the UVB (280–320 nm) part of the UVB spectrum. Most importantly, the vitamin D action spectra, as will all biological action spectra, are independent of the source of UVR. The
Impact of latitude on vitamin D effective UV radiation
Latitude influences the range of solar zenith angles, hence, the UVR levels. Fig. 2 shows the impact of latitude only on vitamin D effective UV radiation as a function of the month for north latitudes only. All other parameters such as ozone, altitude and aerosols (pollution) are kept constant. For 0° latitude (equator) seasonal variations are noted, however two (2) peaks in UV occur in February–March and October–November with a minima in July. This is due to the SZA variation at the equatorial
Impact of season vitamin D effective UV radiation
Due to the orbit of the earth around the sun resulting in changes in the earth sun distance, combined with a changing solar zenith angle, results in seasonal variations in vitamin D effective UV radiation. For the data shown in Fig. 3, the northern hemisphere locations are only considered. Again, as was done in Fig. 2, for the model input, a standard ozone and altitude were considered and the only variable was location and time of year. Data presented in Fig. 3 shows the seasonal vitamin D
Impact of month of year vitamin D effective UV radiation
The aforementioned data presented in Fig. 3 indicated that the season of interest is important when considering vitamin D effective UV. However, when the data is presented as a function of month, with respect to the July (which can be assumed to be the highest values throughout the northern hemisphere year) vitamin D effective UV values, patterns emerge with respect to month of year and are shown in Fig. 4. The data presented in Fig. 4 uses a fixed ozone amount for all calculations (320 DU), 0 m
Impact of ozone
To investigate the impact of ozone variability on the monthly total vitamin D effective UV radiation, the data presented is for, in this example, a fixed southern hemisphere site (27.5° south, 151° E longitude) in clear sky conditions, at 0 m above sea level, using varying ozone values of 280 DU, 300 DU and 320 DU (DU = Dobson Units) as the variable. This data is shown in Fig. 5. Lower ozone values result in higher vitamin D effective UV, particularly in the summer months when the pathlength of light
Impact of cloud
Cloud, both on a diurnal and longer timescale can have significant impact on the surface UV levels. Cloud acts, in most cases as a moderating (reducing) influence on surface UV levels. In rare and short-lived cases, clouds can enhance surface UV levels, however, these typically are in the order of minutes. In this example, presented in Fig. 6, data is presented for monthly total vitamin D Effective UV at a fixed southern hemisphere site (27.5° south, 151° E longitude) at 0 m above sea level,
Conclusions and discussion
Many factors must be considered when associating geographic location with potential for vitamin D synthesis. Latitude, season, month of year, cloud and ozone all impact on the surface UV levels at a particular site. A general rule is that latitude by itself is not a good indicator for potential for vitamin D production, or indeed a population’s vitamin D status. Through the examples presented in this paper, it the impact of ozone and cloud in reducing (and with ozone losses, increases) in
Acknowledgement
Michael Kimlin is supported through a Senior Fellowship from the Cancer Council Queensland (TCCQ).
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