Holick's rule and vitamin D from sunlight

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Abstract

Holick's rule says that sun exposure 1/4 of a minimal erythemal dose (MED) over 1/4 of a body is equivalent to 1000 International Units (IU) oral vitamin D3. Webb and Engelsen recently commented that the ultraviolet (UV) spectrum used to establish Holick's rule is unknown. They consequently used a spring midday Boston solar spectrum to estimate ample sunlight exposures for previtamin D3 (preD3) at various locations. Literature review found the source upon which this rule is based was a fluorescent sunlamp (FS lamp). The FS spectrum is known and its relative weighting against the action spectra for erythema and the preD3 is significantly different from the solar spectrum used to derive the standard vitamin D effective dose (SDD). The preD3 effectiveness of the solar spectrum per unit erythemal hazard is greater than the FS lamp by a factor of 1.32. Consequently, UV exposure estimates based on Boston reference sunlight, instead of the UV lamp employed in the originating experiments, over estimate UV exposure equivalent to ∼1000 IU orally by ∼1/3. This redefinition of SDD impacts risk/benefit assessments of optimal/feasible sun exposure for vitamin D maintenance and the application of Holick's rule to rational public health messages.

Introduction

Holick's rule that exposing 1/4 of our body to 1/4 MED of sunlight will produce vitamin D3 equivalent to 1000 IU of vitamin D taken orally, has been used for several years [1], [2], [3]. In Webb and Engelsen's work on determining periods of sun exposure requisite for healthy vitamin D levels [4], the original UV spectrum upon which this axiom is based was cited as unclear. It is important to establish the spectrum of the UV source upon which Holick's rule is based so that accurate guidelines for needed sun exposure to maintain healthy vitamin D status may be developed. The spectral distribution of the UV source taken together with the action, or response, spectrum for conversion of 7-dehydrocholesterol to previtamin D3 (preD3) in human skin allows calculation of relative preD3 benefit from exposure. The action spectrum for vitamin D production using ex vivo techniques was originally published in 1982 [5], was reviewed in 2006 by a CIE Technical Committee who issued a consensus action spectrum for preD3 in human skin [6]. Webb and Engelsen's 2006 work used an earlier version, similar to the 1982 curve however, we employ the updated consensus version in this work.

Recent studies are suggesting an ever increasing importance of vitamin D for our health and well being. The importance of vitamin D in controlling a number of cancers and other health problems has been extensively reviewed and analyzed [7], [8], [9], [10], [11], [12]. The potential to make adequate vitamin D through sunlight exposure has also been extensively examined [4], [13], [14], [15], [16], [17]. Kimlin and co-workers and Webb and Engelsen emphasized that optimal preD3 is made during solar noon exposures [4]. Sayre and Dowdy have recently concurred that solar noon exposure is optimal and further cautioned against wearing an SPF ≥15 sunscreen during exposure if making vitamin D is desired [18]. Consequently, it has been suggested that several modifications in the current sun safe public health messages are needed if individuals are to make adequate vitamin D during sunlight exposure. Despite this, a number of “safe sun” advocates have persistently, and erroneously, advocated that casual exposure while adhering to current sun protection guidance would allow adequate vitamin D [19], [20], [21]. In this work we identify the UV spectrum that underpins Holick's rule and examine it's implications upon our understanding of sunlight derived vitamin D.

Section snippets

Literature review

Since the axiom referred to as Holick's rule was not coined contemporaneously with the original research we conducted a retrospective literature review, in collaboration with the original author (M. Holick). We traced the origins of the axiom from general use in recent reviews back to the original 1982 research article [22] describing the use of a clinical broadband UVB phototherapy booth containing 16 vertically arranged Westinghouse FS40 fluorescent sunlamps. This is a common broadband UVB

Results

The spectrum of the FS lamp is known and its overlap with the action spectrum for preD3 photosynthesis is shown in Plate 1 (lower). The irradiance of the FS spectrum was rescaled to that of the model solar spectrum to facilitate comparison of relative spectral distributions. The solar spectrum we used to duplicate the spectrum Webb and Engelsen derived their standard vitamin D effective dose (SDD) from is like wise compared in Plate 1 (upper).

The results of our comparative spectral analysis is

Discussion

Holick's rule was developed from empirical studies using almost pure UVB sources rather than intermediate zenith angle sunlight. Guidance provided by Webb and Engelsen [4] overestimates required exposure by roughly a third due to their choice of Boston sunlight to convert Holick's rule into quantitative units of preD3 effective UV. As noted in their work, the MED of most individuals varies with skin type [23] and efficiency of preD3 photosynthesis decreases with increasing pigmentation. We also

Acknowledgment

Supported in part by the UV Foundation.

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