Although the scientists did not evaluate the individual dietary habits of the participants, the observed correlation of low ferritin (iron storage) levels with vitamin D status suggests a major dietary influence on 25(OH)D levels.
This is in accordance with Naama's observation that there was no difference between athletes competing indoors and athletes who exercise outdoors. This strongly suggest that even with adequate sun exposure vitamin D supplementation may be indicated if not in the general public then in athletes of whom it has previously been speculated that they might have increased vitamin D requirements.
These observations are supported by a case study by Rodriguez and Trojan (Rodriguez & Trojan. 2010) who report on a female athletes diagnosed with "recurrent Stress Fracture with low vitamin D and iron stores". With a ferritin level of 14 and Vitamin D 25-OH level of 7, the patient exhibited similar, but more severe deficiencies than the athletes in the Naama-study.
On a side note: In the general context of Vitamin D deficience, another recent study comes handy as well: "Vitamin D deficiency promotes growth of MCF-7 human breast cancer in a rodent model of osteosclerotic bone metastasis" (Ooi. 2010):
"Vitamin D deficiency enhances both the growth of tumors and the tumor-induced osteosclerotic changes in the tibiae of mice following intratibial implantation of MCF-7 cells. Enhancement of tumor growth appears dependent on increased bone resorption rather than increased bone formation induced by these tumors." (Ooi. 2010)The anti-cancer effect of vitamin D (which has been previously addressed by other researchers) is particularly obvious in the grafical representation of the study results (cf. Figure below. Ooi. 2010)
(A) Growth curves for MCF-7 cells when treated with varying concentrations of 1,25(OH)2D3. (B) Cell proliferation was significantly reduced by 10-8M and 10-7M 1,25(OH)2D3. (C) Cell apoptosis was increased by treatment with 10-8M and 10-7M 1,25(OH)2D3.
Whether you are an athlete a dancer or a couch potatoe, on your next blood test, you better ask your doctor to test for vitamin D deficiency, as well.