osteomalasia, auto-immune diseases, and certain cancers (Holick, 2001, p. 28; Garland et
al, 2006, pp. 252-256; Grant et al, 2003, p. 372).
To account for the relationship between vitamin D and stature, a measure is
constructed that accounts for solar radiation. Insolation is the incoming direct sunlight
that reaches the earth, its atmosphere, and surface objects.2 Insolation and ultraviolate B
are also the primary source of vitamin D production (Holick, 1981, p. 590; Holick, 2007,
p. 270). Because of its distance from the equator, European insolation is comparatively
low, and before their migration to North America, Europeans at low insolation latitudes
had to be more efficient in vitamin D production. As early hominids migrated out of
Africa to Northern latitudes, they received less solar radiation, and through the process of
natural selection, darker pigmented hominids were less successful hunter-gatherers in
Northern latitudes and were selected-out (Loomis, 1967, pp. 503-504).
Because US historical insolation is unavailable, a modern insolation index (1993-
2003) is constructed, and monthly insolation values are measured from January through
June. The insolation index measures statewide average insolation levels across each of
the states based on the hours of direct sunlight per day at county centroids in each state.3
2 Insolation is an acronym for incident solar radiation, and is a measure for sunlight energy received for a
given surface area at a given time. If w equals watts, m equals meters, and i equals insolation,
w kwh
i = —— = —2------. Data for US insolation is available from the National Aeronautics and Space
m2 m2 ∙ day
Administration at http://eosweb.larc.nasa.gov/cgi-bin/sse/sse.cgi.
3 Insolation is not the insolation in the county that surround’s the state’s centroid, but insolation in each
county’s geographic center. The range of state insolation values extends from Maine’s minimum of 3.43
hours of direct sunlight to Arizona’s maximum of 5.22 hours of direct sunlight per day.
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