Men's Osteoporosis Support GroupAn excellent health newsletter; sunshine and vitamin D; the very elderly and serum vitamin D Health newsletter. I discovered this website and its newsletter the other day and really like the quality of information, particularly regarding dual-energy X-ray absorptiometry (DXA) interpretation. Check it out to learn more about that topic and others there that might interest you. Sunshine and vitamin D. One of the main methods recommended for maintaining adequate levels of vitamin D is sun exposure. The required amount of time in the sun varies according to skin type, darker-skinned individuals requiring more time to produce adequate vitamin D than light-skinned ones. As such it is hard to give a single figure for the time that will produce adequate vitamin D for everyone. Most articles I've read suggest being in the sun about three times per week from 10AM to 3PM for a time period that doesn't produce skin reddening, perhaps 10-20 minutes. And exactly how high should your serum vitamin D levels be from sun exposure? Holick MF suggests that vitamin D deficiency, insufficiency and sufficiency are defined as <20, 21 to 29, and > 30 ng/mL, respectively. If measuring in nmol/L these figures would be <50, 52.5 to 72.5, and >75. See PMID: 18550652. With these figures in mind, I want to cover two studies that monitored young, healthy individuals who got abundant sun exposure where solar UVB rays were optimal to produce vitamin D. Some of the results are quite interesting, and don't fall exactly in line with expected norms. J Clin Endocrinol Metab. 2007 Jun;92(6):2130-5. Epub 2007 Apr 10. Low vitamin D status despite abundant sun exposure. Binkley N and others. PMID: 17426097. Free full text available here. The study population included a total of 93 men and women, mean age of 24 years, living in Hawaii, who volunteered and were reimbursed for their time. All had self-reported sun exposure of 3 or more hours per day for 5 or more days/week. Serum vitamin D was analyzed by precise HPLC assay in all individuals and by RIA in a subset of 19 individuals. Additionally serum parathyroid hormone levels were tested. The results showed the subjects were outside for a mean time with no sunscreen 22.4 hours/week, and 28.9 hours/week with and without sunscreen. 40% reported never using sunscreen. The authors reported, "Mean serum 25(OH)D concentration was 31.6 ng/ml. Using a cutpoint of 30 ng/ml, 51% of this population had low vitamin D status. The highest 25(OH)D concentration was 62 ng/ml." They concluded, "These data suggest that variable responsiveness to UVB radiation is evident among individuals, causing some to have low vitamin D status despite abundant sun exposure. In addition, because the maximal 25(OH)D concentration produced by natural UV exposure appears to be approximately 60 ng/ml, it seems prudent to use this value as an upper limit when prescribing vitamin D supplementation." Editor's comments. Some of the other interesting findings include, ". . . regardless of the amount of sun exposure, the serum 25(OH)D concentration does not increase to more than approximately 60 ng/ml." When I had my vitamin D tested, the lab results showed the normal range to be "32 to 100 ng/mL." How could the normal range go as high as 100 ng/mL if young healthy adults spend 29 hours in the sun per week and can only get serum vitamin D levels to a maximum of 62 ng/mL? Another comment from the authors is, "This implies that the common clinical recommendation to allow sun exposure to the hands and face for 15 min may not ensure vitamin D sufficiency." They also point out that their results suggest it would be unwise to automatically assume that low serum vitamin D levels are due to inadequate sun exposure. There appears to be much more involved with regulating serum vitamin D levels than is readily apparent. Thus, even if you feel you are getting plenty of sunshine, it is probably prudent to have your serum vitamin D tested occasionally. I've also read that there is quite a bit of variability in the response to oral vitamin D supplements--another reason for testing serum levels. The 62 ng/mL maximum level may also relate to the test method used. The authors point out that in at least one study (to be discussed below) there were some subjects who got extensive summer sun and had serum vitamin D levels of 81 and 84 ng/mL. Apparently they had a different assay method used, ". . . one that measures other vitamin D metabolites in addition to 25(OH)D." In summary, there are a couple of conclusions to draw from this study: 1) Don't assume that high amounts of sunshine guarantee adequate serum vitamin D levels; 2) If vitamin D levels are low, the maximum goal for replacement should be about 60 ng/mL. J Clin Endocrinol Metab. 2002 Nov;87(11):4952-6. Effects of above average summer sun exposure on serum 25-hydroxyvitamin D and calcium absorption. Barger-Lux MJ, Heaney RP. PMID: 12414856. Full text article here. This was another study involving young, healthy individuals, all males, who had extensive time in the sun during the summer in the American midwest. Their serum vitamin D was tested at the end of summer and 175 days later. As well their calcium absorption was tested. The results showed "Median serum 25(OH)D decreased from 122 nmol/liter in late summer to 74 nmol/liter in late winter." Despite the summer sun exposure, the authors found, ", , , at the late winter visit, 25(OH)D was less than 50 nmol/liter in 3 subjects and less than 75 nmol/liter in 15 subjects." They also note, "However, we found only a trivial, nonsignificant seasonal difference in calcium absorption fraction and no change in fasting urinary calcium to creatinine ratio." Editor's comments. In this study they also estimated the percent of body surface area (BSA) exposure to the sunlight and the sun index (sun index = hours of sun exposure per week x fraction of BSA exposed to sunlight). Both those factors were found to be statistically significant for raising serum vitamin D levels. These results would be expected: the more surface area you expose to the sun, the greater should be the production of vitamin D per unit of time in the sun. This could be a significant difference between this study and the one above by Binkley and others which didn't mention the body surface area exposed to sunlight. So 18 of the subjects were either deficient or insufficient in serum vitamin D levels 175 days after a summer of extensive exposure to sunlight. An important aspect of serum vitamin D is that it helps absorption of calcium from the intestine. There was no difference in calcium absorption no matter which time of year the tests were done, making one wonder about the importance of a very high serum vitamin D level--at least to promote calcium absorption. In spite of that the authors suggest that a supplement or food source of vitamin D might be needed for those who spend most of the time indoors. For those wishing to raise serum vitamin D levels with summer sun exposure, this study suggests that doing so for short periods with maximal body surface area exposed might be best. For those wishing to get an equivalent vitamin D dose from supplements, the authors developed a formula that found 2780 IU/day would be the same as the summer sun their study's participants got. Once again, having your serum vitamin D level tested, ideally a couple of times a year, is likely needed, too, since there is great individual variation. To display that variation note the highest serum vitamin D at the end of summer was over 200 nmol/L and the lowest was 50 nmol/L. Exp Gerontol. 2008 Feb;43(2):79-87. Epub 2007 Jul 4. Calcium metabolism and vitamin D in the extreme longevity. Passeri G and others. PMID: 17698310. Free full text here. To go to the the complete other end of the extreme regarding serum vitamin D levels, see this study. Here are the findings that got my attention, "An even more evident finding was the severe hypovitaminosis D. The 25-OH vitamin D, measured by RIA (Nichols Institute Diagnostics,CA,USA), was measurable only in 5 subjects, and one of these has had therapeutic supplementation of this vitamin. The values of the other 99 centenarians were below the limit of sensibility of the method used (5 nmol/L)." These were 99 humans older than 100 years of age who were still alive, with several of them able to walk on their own, who had no measurable serum vitamin D.
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