SAN DIEGO — The Institute of Medicine is reviewing its 1997 guidelines for vitamin D intake, and will likely recommend increased supplementation when new guidelines are published in 2010.
There is a growing consensus that currently recommended intakes—200 IU/day for persons under age 50 and 400 IU for those aged 50–70—are too low, said Connie Weaver, Ph.D., director of the department of food and nutrition at Purdue University, West Lafayette, Ind.
In addition to vitamin D's well known effects on bone metabolism, levels in the range of 1,000 IU/day have been associated with good outcomes in recent controlled and epidemiologic trials that examined risks for falls in the elderly and persons with type 2 diabetes.
Also, 1,000 IU is well below the 2,000-IU upper limit of vitamin D cited in the existing guidelines, Dr. Weaver said at a meeting on natural supplements in evidence-based practice sponsored by the Scripps Center for Integrative Medicine in San Diego.
The IOM is not likely to recommend that people spend a lot more time in the sun as a way of increasing vitamin D, advice which would run counter to skin cancer prevention efforts.
Further, “you cannot eat enough vitamin D-containing foods to get anywhere near 1,000 IU of vitamin D per day,” said Dr. Weaver, who served on the IOM committee for the current guidelines.
Any substantive increase in recommended daily vitamin D intake will probably mean a call for supplementation. But the IOM has been reluctant to make nutrient recommendations that exceed what an individual can reasonably obtain from ordinary food consumption.
“Subsequent research has shown no evidence of harm at levels much higher than 2,000/day. So the upper limits will likely change along with the new intake recommendations,” Dr. Weaver said.” Until the upper limits go up, the ability to fortify foods is hindered. Higher upper limits will engender bolder efforts and greater fortification.”
Osteoporosis prevention is the strongest rationale for increasing vitamin D intake, she added, given the epidemiologic correlations between low vitamin D status and increased risk of fractures in the elderly. Research showed that daily supplementation with 700 IU vitamin D and 500 mg calcium reduced the 3-year rate of first nonvertebral fractures by 5.5%, compared with placebo, in a cohort of 176 men and 213 women aged over 65 (N. Engl. J. Med. 1997;337:670–6).
Dr. Weaver cited a National Health and Nutrition Examination Survey III data analysis showing significant and positive improvements in bone mineral density in a cohort of adults aged 50 years and older as serum vitamin D increased from 20 nmol/L to 40 nmol/L. The improvements continued at values over 80 nmol/L. (Am. J. Med. 2004;116:634–9).
The same researchers published a meta-analysis of six large population studies showing that the relative risk of all bone fractures starts to decline when vitamin D intake increases to 450 IU per day (J. Steroid Biochem. Mol. Biol. 2007;103:614). “You really need to get up around 700 IU per day to achieve the lowest relative risk of hip fractures in this meta-analysis,” said Dr. Weaver.
Vitamin D's fracture-reducing impact may also have something to do with its effects on neuromuscular function and overall health status. Observational data from the NHANES III database showed that those with the highest serum vitamin D levels had the shortest times on the sit-to-stand test in a large cohort of people aged 60–90 years (Am. J. Clin. Nutr. 2004;80:752–8).
Further, a study of 122 Swiss women in a geriatric hospital showed a 49% reduction in incidence of falls among the women taking 800 IU vitamin D plus 1,200 mg calcium for 3 months, compared with those taking calcium alone (J. Bone Miner. Res. 2003;18:343–51).
But bone health is only one aspect of the vitamin D story, said Dr. Weaver. Low vitamin D level is associated with increased risk of cardiovascular disease and, possibly, type 2 diabetes.
A Framingham Offspring study found a hazard ratio of 1.62 for incident cardiovascular events during a 7-year period for subjects with serum vitamin D levels less than 15 nmol/L. The observed effect was largely attributable to the 40% of the total cohort who were hypertensive at baseline. In this subgroup, the hazard ratio was 2.13 for those deficient in vitamin D versus those who were not. The authors noted that vitamin D receptors are plentiful in vascular smooth muscle, endothelium, and cardiomyocytes (Circulation 2008;117:503–11).
According to Dr. Weaver, the emerging story is vitamin D's role in glucose metabolism and its potential for slowing insulin resistance and lowering the risk for type 2 diabetes.
A new Framingham Offspring study involving 808 nondiabetic persons showed a strong inverse correlation between serum vitamin D levels and fasting plasma glucose and fasting insulin, as well as homeostatic model assessment (HOMA) for insulin resistance, after researchers controlled for age, gender, BMI, waist circumference, and smoking. Compared with those in the lowest vitamin D tertile, those in the highest tertile had a 1.6% lower concentration of fasting glucose and a 9.8% lower concentration of fasting insulin, translating into a 12.7% lower HOMA-IR score. Vitamin D correlated positively with insulin sensitivity, plasma adiponectin, and HDL cholesterol (J. Nutr. 2009;139:329–34).
This work builds on prior studies at the division of endocrinology, diabetes, and metabolism at Tufts University, Boston, that showed consistent correlations between low vitamin D status and metabolic syndrome, insulin resistance, and frank diabetes.
In an analysis of data on nearly 84,000 women in the Nurses Health Study, the Tufts researchers found a 13% reduced relative risk of type 2 diabetes in the women taking the highest versus lowest amounts of supplemental vitamin D. Supplementation with 1,200 mg calcium or more and 800 IU vitamin D or more was associated with a 33% lower risk of type 2 diabetes, compared with an intake of less than 600 mg calcium and 400 IU vitamin D (Diabetes Care 2006;29:650–6).
The Tufts researchers published an intervention study involving 314 white, nondiabetic people randomized to supplementation with 500 mg calcium citrate plus 700 IU vitamin D, or placebos, for 3 years. Among the 92 with impaired fasting glucose at baseline, the active supplements attenuated the rise in fasting glucose at the 3-year point, compared with placebo (Diabetes Care 2007;30:980–6).
Overall, the findings suggest that vitamin D and calcium supplementation at moderate doses can slow the progression of hyperglycemia and insulin resistance, Dr. Weaver said.
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