THE ADVICE ON vitamin D keeps changing, but the evidence for upping daily intake appears stronger with every passing year as does evidence for its many health benefits. “In the last two decades, vitamin D has gone from being recognized for its importance in bone health to being investigated…for its potential as a preventive or treatment for more than 100 disorders, from cancer to diabetes and depression,” according to Berkeley Wellness.com.
Higher vitamin D blood levels have a positive impact on genes associated with cancer as well as with autoimmune and infectious diseases. And vitamin D appears to play a role in gaining and losing weight: Higher blood levels at the start of a low-calorie diet are good predictors of weight loss. Also, children given 1,200 IU (International Units) of vitamin D/day for four winter months reduced their risk of catching the flu by more than 40 percent.
Important questions about vitamin D remain: Are “desirable” vitamin D blood levels the cause or result of good health; and, if a healthy person increases blood levels of vitamin D with supplements, as opposed to natural sources like the sun, will they be better protected from disease?
As much as one-half of people worldwide may have deficient or insufficient blood levels of vitamin D , and deficiency rates in the U.S. are rising. Here the most vulnerable live above the 40 degree latitude north of the equator — north of Philadelphia or Denver — where getting adequate levels from the sun, the best vitamin D source, becomes more difficult. In a study of healthy young girls, ages 10 to 13, living in Bangor, Maine, nearly half had insufficient levels in March, when levels tend to be lowest following long winters. (Blood tests for vitamin D should be done in late winter to assess an individual’s lowest levels.)
Thus, women of the DMV can be less vigilant about D levels, although the body’s synthesis of vitamin D from sunshine is lower in those with darker skin and declines with age. From the same sun exposure, a 70-year-old can produce as little as one-quarter as much as a 20-year-old, and vitamin D deficiency is seen in more than three-quarters of hip-fracture patients.
A Danish study of the medical records of almost 10,000 individuals showed that lifelong low vitamin D (traced to genetic variants) increased the risk of cancer death by more than 40 percent and from any other cause by more than 30 percent. In the Danish study, small elevations in vitamin D levels were associated with a decrease in triglycerides and LDL cholesterol, and a corresponding decrease of death by digestive disease, metabolic and nutritional diseases and respiratory disease.
While nutrition experts have put optimal minimum blood levels of vitamin D at 20 ng/ml (nanograms/milliliter), many are now pushing for higher goals of 40 or 50 ng/ml. For cancer prevention, desirable levels appear to be 36 to 48 ng/ml. (On my recent blood test results, “sufficiency levels” are listed as 30 to 100 ng/ml, placing my 36 ng/ml on the low side.)
Because vitamin D from the sun is the best at raising blood levels, the general advice is, two to three times a week, to sit with arms and legs exposed to the sun before applying sunscreen for as little as 5 minutes — although some believe 10 to 20 minutes is more successful. Under the best conditions of weather and age, a fair-skinned person exposing their full body to the sun can synthesize up to 20,000 IU of vitamin D3 in 20 minutes.
The best ultraviolet rays for increasing vitamin D levels occur between 10 a.m. and 3 p.m., but vary depending on cloud cover and smog. And sunscreens with SPF as low as 15 can reduce the body’s production of vitamin D by almost 100 percent. Because vitamin D is fat-soluble, dietary fat is necessary for its absorption. Also, once stored in fat, vitamin D has a half-life of two weeks, so that winter levels can run low.
After sunlight, food is the best source of vitamin D3, especially fish oil and fatty fish, along with maitake mushrooms, eggs, chicken and fortified skim milk. Four ounces of swordfish provides 941 IU, but three ounces of tuna offers only 68 IU and a whole large egg, 44 IU. For the hardy, a tablespoon of cod liver oil tops the list at 1360 IU.
The natural form of vitamin D3, or cholecalciferol, is what the body makes in response to sunlight, and D3 supplements are created from fish oil or lanolin from sheep’s wool. In the past, supplements were composed mostly of vitamin D2, synthesized by irradiating fungus and plant matter, and the two forms were considered equally beneficial. But, perhaps because D3 is what the body produces, recent studies showed a 6% reduced risk of death from D3 supplements and/or higher blood serum levels, compared to a relative increased risk with D2. Most manufacturers have made the switch, but labels should still be checked on fortified milk, which used to contain D2, as well as prescription medicines that include vitamin D.
Daily recommended intake of vitamin D supplements, set most recently by the U.S. Institutes of Medicine (IOM) in 2010, is up to 600 IU for adults up to age 70, and up to 800 IU over age 70. Recent evidence, however, associates better health outcomes with at least 1,000 IU of vitamin D3/day, which can bring blood levels up to 30 ng/ml. Daily supplements as high as 3,000 IU might be recommended if blood levels of D are very low or in cases of weakening bones or bone loss.
According to one study, 800 IU (international units) can reduce the risk of fracture by 20% in populations at risk. Veterans taking more than 645 IU of vitamin D/day along with 4 grams of cereal fiber had a 40% reduced risk of developing precancerous colon polyps. While the upper level limit recommended to avoid toxicity is 4,000 IU/day, more than 2,000 IU could increase risk of kidney stones, kidney damage and other health problems.