SAN DIEGO —Over the last decade there’s been a bumper crop of studies documenting the benefits of vitamin D in preventing and ameliorating chronic disease. And epidemiologic data show that a majority of Americans are vitamin D-deficient, sometimes severely so.
These two signals are likely to prompt the Institute of Medicine to increase its recommended daily intake of the Sunshine Vitamin.
Previous guidelines, published in 1997, consider a daily intake of 200 IU as “adequate” for people under 50, and 400 IU the recommended level for those aged 50-70 years. Both figures are far too low, say many nutrition scientists and clinicians. The big question is, how much of an increase will IOM advocate?
Physicians using vitamin D for heart disease and osteoporosis prevention often advocate for daily intake of at least 1,000 IU. This falls well below the 2,000 IU upper limit in IOM’s 1997 guidelines, but it is still five times higher than the current recommendations, said Connie Weaver, PhD, at the 6th annual Evidence-Based Update on Natural Supplements, sponsored by the Scripps Center for Integrative Medicine.
Dr. Weaver, Director of the Department of Food & Nutrition, Purdue University, West Lafayette, IN, has served on IOM vitamin D advisory committees. She says she expects a substantial increase not only in the recommended daily intake but also in the safe upper limit.
“When we evaluated the research for the upper limit determination in 1997, we set it at 2,000 IU/d. Subsequent research has shown that levels much higher showed no evidence of harm.”
Impact at the Grocery
New recommendations could have a big impact at the grocery store. The food industry is, to a large degree regulated in accord with IOM recommendations. “Until the upper limits go up, your ability to fortify foods is hindered. Higher upper limits will engender bolder efforts and greater fortification,” Dr. Weaver said.
IOM has never been particularly supplement-friendly, and seldom recommends that people take nutrients at levels beyond what one could reasonably get from ordinary foods. But in the case of vitamin D, the Institute has little choice. “You cannot eat enough vitamin D-containing foods to get anywhere near 1,000 IU per day,” she stressed.
Sun exposure is always an option, but not one the Institute is likely to recommend: telling people to spend more time in the sun runs counter to decades of skin cancer prevention effort.
Make No Bones About It
Osteoporosis prevention is the strongest rationale for increasing vitamin D. In the years since the last IOM recommendations, nutrition scientists have revealed much about how this vitamin influences bone metabolism.
One of its most important effects is to reduce parathyroid hormone (PTH), a big promoter of bone resorption. Vitamin D deficiency almost always leads to PTH elevation. In one study, people with serum D levels less than 15 ng/mol, had two-fold higher PTH as similarly aged people with vitamin D levels of 75 ng/mol or better (Thomas, et al. N Engl J Med. 1998.338: 777).
How much vitamin D is really needed to reduce fractures? At least 700 IU per day, say the epidemiologic studies. A metanalysis by Bischoff-Ferrari and colleagues of 6 large population studies showed that relative fracture risk starts to drop once daily intake goes above 450 IU and continues to decline as intake increases (Bischoff-Ferrari, et al. 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,” said Dr. Weaver, commenting on the data.
There may be more to vitamin D’s effect than preservation and promotion of bone mineral density; the vitamin may also improve neuromuscular function and overall health status. Data from the NHANES III database show that among people aged 60-90, those with the highest vitamin D serum levels had the shortest Sit-to-Stand times, indicating the best neuromuscular coordination, and that may translate into reduced risk of falls ((Bischoff-Ferrari, et al. Am J Clin Nutr. 2004; 80:752-758).
A study of 122 Swiss women in a geriatric hospital showed a 49% reduction in falls among those taking 800 IU vitamin D plus 1,200 mg calcium for 3 months, compared with those taking the calcium alone (Bischoff HA, et al. J Bone Miner Res. 2003; 18: 343-351.)
Think “D” for Diabetes Prevention
Bone health is only part of the vitamin D story, said Dr. Weaver. The latest vitamin D buzz is all about its potential to reverse insulin resistance and prevent type 2 diabetes.
The Framingham Offspring study, involving 808 non-diabetic individuals, showed a strong inverse correlation between serum vitamin D and both plasma glucose and fasting insulin. Homeostatic model assessment (HOMA) scores, an indicator of insulin resistance, also showed an inverse correlation with vitamin D, after controlling for age, gender, BMI, waist circumference and smoking.
Compared with subjects in the lowest vitamin D tertile, those in the highest tertile had a 1.6% lower fasting glucose and a 9.8% lower fasting insulin, translating into a 12.7% lower HOMA-IR score. There was a strong, positive correlation between serum vitamin D and insulin sensitivity, plasma adiponectin, and HDL cholesterol (Liu E, et al, J Nutr. 2009; 139 (2): 329-34).
Three years ago, researchers at the Division of Endocrinology, Diabetes and Metabolism at Tufts University showed a strong correlation between low vitamin D status and metabolic syndrome, insulin resistance and frank diabetes, based on analysis of data from nearly 84,000 women in the Nurses Health Study.
They found a 13% reduced relative risk of type 2 diabetes among the women taking the highest versus the lowest amounts of vitamin D. The women taking 800 IU or more, plus 1,200 mg calcium or more had a 33% lower risk of diabetes, compared with those who got less than 400 IU vitamin D and 600 mg calcium (Pittas AG, et al. Diabetes Care. 2006; 29(3): 650-6). Dr. Weaver pointed out that this lower level is the current recommended adult intake.
Following on those findings, the Tufts group undertook an intervention study involving 314 White, non-diabetic people randomized to supplementation with 500 mg calcium citrate plus 700 IU vitamin D or placebos, for three years.
There were 92 people with impaired fasting glucose (IFG) at baseline, and those taking the active supplements showed a much slower rise in glucose levels after 3 years in the trial, compared with those in the placebo group. They also had a smaller increase in HOMA-IR scores as well. Among those with normal fasting glucose at baseline, there was no measurable vitamin D benefit (Pittas AG, et al. Diabetes Care. 2007; 30(4): 980-6).
These studies all seem to point to a role for both vitamin D and calcium supplementation in preventing hyperglycemia, insulin resistance and diabetes. The doses are moderate, but definitely higher than the IOM’s 1997 recommended levels, a point not lost on the Institute’s review committee. “This is a very interesting story, to be watched closely,” Dr. Weaver said.
The Institute of Medicine’s new guidelines are expected early in 2010.
