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An international comparison of serum 25-hydroxyvitamin D measurements. Lips P, Chapuy MC, Dawson-Hughes B, Pols HA, Holick MF. Osteoporos Int. 1999;9(5):394-7. PMID: 10550457 DOI: 10.1007/s001980050162 The results show that 25(OH)D values from different laboratories can not be assumed to be comparable unless a careful cross-calibration has been performed.

A comparison of vitamin d levels in nondiabetic and diabetic patient populations.\nYoho RM, Frerichs J, Dodson NB, Greenhagan R, Geletta S.\nJ Am Podiatr Med Assoc. 2009 Jan-Feb;99(1):35-41.\nPMID: 19141720

A randomised comparison of increase in serum 25-hydroxyvitamin D concentration after 4 weeks of daily oral intake of 10 microg cholecalciferol from multivitamin tablets or fish oil capsules in healthy young adults. Holvik K, Madar AA, Meyer HE, Lofthus CM, Stene LC. Br J Nutr. 2007 Sep;98(3):620-5. Epub 2007 Apr 24. PMID: 17456248 We conclude that fish oil capsules and multivitamin tablets containing 10 microg cholecalciferol administered over a 4-week period produced a similar mean increase in s-25(OH)D concentration.

Comparison of daily, weekly, and monthly vitamin D3 in ethanol dosing protocols for two months in elderly hip fracture patients. Ish-Shalom S, Segal E, Salganik T, Raz B, Bromberg IL, Vieth R. J Clin Endocrinol Metab. 2008 Sep;93(9):3430-5. Epub 2008 Jun 10. PMID: 18544622 doi:10.1210/jc.2008-0241 CONCLUSIONS: Supplementation with vitamin D can be achieved equally well with daily, weekly, or monthly dosing frequencies. Therefore, the choice of dose frequency can be based on whichever approach will optimize an individual's adherence with long-term vitamin D supplementation.

Randomized comparison of the effects of the vitamin D3 adequate intake versus 100 mcg (4000 IU) per day on biochemical responses and the wellbeing of patients. Vieth R, Kimball S, Hu A, Walfish PG. Nutr J. 2004 Jul 19;3:8. PMID: 15260882 doi:10.1186/1475-2891-3-8 CONCLUSION: The highest AI for vitamin D brought summertime 25(OH)D to >40 nmol/L, lowered PTH, and its use was associated with improved wellbeing. The 100 mcg/day dose produced greater responses. Since it was ethically necessary to provide a meaningful dose of vitamin D to these insufficient patients, we cannot rule out a placebo wellbeing response, particularly for those on the lower dose. This work confirms the safety and efficacy of both 15 and 100 mcg/day vitamin D3 in patients who needed additional vitamin D.

Aging decreases the capacity of human skin to produce vitamin D3. MacLaughlin J, Holick MF. J Clin Invest. 1985 Oct;76(4):1536-8. PMID: 2997282 doi:10.1172/JCI112134 An evaluation of surgically obtained skin (age range, 8-92 yr) revealed that there is an age-dependent decrease in the epidermal concentrations of provitamin D3 (7-dehydrocholesterol). To ascertain that aging indeed decreased the capacity of human skin to produce vitamin D3, some of the skin samples were exposed to ultraviolet radiation and the content of previtamin D3 was determined in the epidermis and dermis. The epidermis in the young and older subjects was the major site for the formation of previtamin D3, accounting for greater than 80% of the total previtamin D3 that was produced in the skin. A comparison of the amount of previtamin D3 produced in the skin from the 8- and 18-yr-old subjects with the amount produced in the skin from the 77- and 82-yr-old subjects revealed that aging can decrease by greater than twofold the capacity of the skin to produce previtamin D3. Recognition of this difference may be extremely important for the elderly, who infrequently expose a small area of skin to sunlight and who depend on this exposure for their vitamin D nutritional needs.

"Had enough about reading/hearing about Vitamin D? Well, it keeps on coming. And for my lack of surprise, the medical community in general is not catching on like wild fire. I really don't understand it. A recent study from the Heart Institute at Intermountain Medical Center in Salt Lake City (click) followed 27,686 patients greater than 50 years of age with no prior history of cardiovascular disease. The Vitamin D levels were checked and classified as such: * normal - greater than 30 ng/ml * low - 15 to 30 * very low - less than 15 The results of the study showed that patients with very low Vitamin D levels in comparison to normal had: * 77% greater risk of death * 45% increased risk of coronary artery disease * 78% increased risk of stroke * twice the risk of developing heart failure"

Vitamin D2 is as effective as vitamin D3 in maintaining circulating concentrations of 25-hydroxyvitamin D. Holick MF, Biancuzzo RM, Chen TC, Klein EK, Young A, Bibuld D, Reitz R, Salameh W, Ameri A, Tannenbaum AD. J Clin Endocrinol Metab. 2008 Mar;93(3):677-81. Epub 2007 Dec 18. PMID: 18089691 Conclusion: A 1000 IU dose of vitamin D2 daily was as effective as 1000 IU vitamin D3 in maintaining serum 25-hydroxyvitamin D levels and did not negatively influence serum 25-hydroxyvitamin D3 levels. Therefore, vitamin D2 is equally as effective as vitamin D3 in maintaining 25-hydroxyvitamin D status.

Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2. Trang HM, Cole DE, Rubin LA, Pierratos A, Siu S, Vieth R. Am J Clin Nutr. 1998 Oct;68(4):854-8. PMID: 9771862 Although the 1.7-times greater efficacy for vitamin D3 shown here may seem small, it is more than what others have shown for 25(OH)D increases when comparing 2-fold differences in vitamin D3 dose. The assumption that vitamins D2 and D3 have equal nutritional value is probably wrong and should be reconsidered.

Differences in vitamin D status between countries in young adults and the elderly. McKenna MJ. Am J Med. 1992 Jul;93(1):69-77. PMID: 1385673 PURPOSE: To compare vitamin D status between countries in young adults and in the elderly. MATERIALS AND METHODS: Reports on vitamin D status (as assessed by serum 25-hydroxyvitamin D) from 1971 to 1990 were reviewed. Studies were grouped according to geographic regions: North America (including Canada and the United States); Scandinavia (including Denmark, Finland, Norway, and Sweden); and Central and Western Europe (including Belgium, France, Germany, Ireland, The Netherlands, Switzerland, and the United Kingdom). RESULTS: Vitamin D status varies with the season in young adults and in the elderly, and is lower during the winter in Europe than in both North America and Scandinavia. Oral vitamin D intake is lower in Europe than in both North America and Scandinavia. Hypovitaminosis D and related abnormalities in bone chemistry are most common in elderly residents in Europe but are reported in all elderly populations. CONCLUSIONS: The vitamin D status in young adults and the elderly varies widely with the country of residence. Adequate exposure to summer sunlight is the essential means to ample supply, but oral intake augmented by both fortification and supplementation is necessary to maintain baseline stores. All countries should adopt a fortification policy. It seems likely that the elderly would benefit additionally from a daily supplement of 10 micrograms of vitamin D.

The same annual dose of 292 000 IU of vitamin D(3) (cholecalciferol) on either daily or four monthly basis for elderly women: 1-year comparative study of the effects on serum 25(OH)D(3) concentrations and renal function. Pekkarinen T, Välimäki VV, Aarum S, Turpeinen U, Hämäläinen E, Löyttyniemi E, Välimäki MJ. Clin Endocrinol (Oxf). 2009 May 25. [Epub ahead of print] PMID: 19486025 DOI: 10.1111/j.1365-2265.2009.03637.x

Vitamin D2 is much less effective than vitamin D3 in humans. Armas LA, Hollis BW, Heaney RP. J Clin Endocrinol Metab. 2004 Nov;89(11):5387-91. PMID: 15531486 Vitamin D2 potency is less than one third that of vitamin D3. Physicians resorting to use of vitamin D2 should be aware of its markedly lower potency and shorter duration of action relative to vitamin D3.

The bioavailability of vitamin D from fortified cheeses and supplements is equivalent in adults. Wagner D, Sidhom G, Whiting SJ, Rousseau D, Vieth R. J Nutr. 2008 Jul;138(7):1365-71. PMID: 18567762 Compared with baseline, serum parathyroid hormone decreased with both fortification (P = 0.003) and supplementation (P = 0.012). These data demonstrate that vitamin D is equally bioavailable from fortified hard cheeses and supplements, making cheese suitable for vitamin D fortification.

Bread fortified with cholecalciferol increases the serum 25-hydroxyvitamin D concentration in women as effectively as a cholecalciferol supplement. Natri AM, Salo P, Vikstedt T, Palssa A, Huttunen M, Kärkkäinen MU, Salovaara H, Piironen V, Jakobsen J, Lamberg-Allardt CJ. J Nutr. 2006 Jan;136(1):123-7. PMID: 16365070 Both fortified breads increased serum 25-hydroxyvitamin D concentration as effectively as the cholecalciferol supplement. Supplementation or fortification did not affect serum intact parathyroid hormone concentration or urinary calcium excretion. In conclusion, fortified bread is a safe and feasible way to improve vitamin D nutrition.