Conditions Expert Dr. Otis Brawley Chief Medical Officer,
American Cancer Society

Expert answer

You ask a fascinating question that is currently of great interest to medical researchers. Vitamin D is commonly found in fish, eggs, fortified milk, cod liver oil and in supplements. It helps regulate blood calcium levels and maintain bone health. Very recently the Institute of Medicine of the prestigious U.S. National Academy of Science published a review of the scientific literature and noted that vitamin D may be linked to heart disease, immune function, cancer prevention and diabetes. It is definitely linked to bone health.

Type 2 diabetes, which is also called adult onset diabetes, is a growing problem in the Western world and is most common in the overweight and the obese. It occurs when cellular tissues develop resistance to insulin stimulation. This is referred to as "insulin resistance." The result is insulin is not able to move glucose from blood into the cells and the blood glucose level rises.

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There is some data to suggest that vitamin D may decrease insulin resistance. For example, in one study of 124 people with type 2 diabetes, researchers found 113 (91.1%) had vitamin D deficiency. hemoglobin A1C is used to determine blood glucose control over the past three months or so. The average hemoglobin A1C level was higher (meaning less than optimal control) in patients with the most severe vitamin D deficiencies. This is not adequate evidence to show that vitamin D deficiency causes diabetes nor that vitamin D supplementation is a legitimate treatment for diabetes. It simply shows that there is a correlation between vitamin D level and glucose control. This being said, it is not inappropriate for a diabetic to ask their physician to check their vitamin D level every year or so and for a physician to prescribe supplements if it is low. Calcium levels in the blood are routinely checked in most standard laboratory panels.

There are also several studies published in the past 10 years that correlate a low magnesium intake or low level of magnesium in the blood with greater risk of development of diabetes. The design of these studies is very similar to the vitamin D studies. They show a correlation and not a causation.

It is a good idea to read nutrition content on food labels to see if you are getting enough vitamin D, calcium or magnesium. The recent Institute of Medicine study recommends everyone age 1 to 70 take 600 international units (IU) per day of vitamin D and those over 71 take 800 IU. The institute recommends adolescents age 9 to 18 receive 1,300 mg per day of elemental calcium. Women age 19 to 50 should take 1,000 mg of elemental calcium. Women should increase to 1,200 mg starting at age 51. Men should take 1,000 mg of elemental calcium from age 19 to age 70 and increase to 1,200 mg at age 71. Too much vitamin D and/or calcium can lead to kidney stones. No more than 4,000 IU of vitamin D should be consumed daily and no more than 2,000 mg of calcium.

Magnesium is found in nuts, beans, whole grains and green leafy vegetables magnesium levels are generally good in people who have a balanced diet. Like calcium, most physicians check it routinely and will obtain dietary counseling for moderately low levels or prescribe replacement if the level is very low. The recommended daily requirements for adolescent and adult males are 270 to 400 mg per day. Adolescent and adult females should get 280 to 300 mg per day. Pregnant and breastfeeding women should get more: 320 to 355 mg.

Ultimately, what we need is a study that randomizes people at high risk of developing diabetes into four groups. One to receive supplementation with vitamin D, one with magnesium, one with both, and one with placebo (an inactive drug). After several years of follow-up and comparison of the four groups we can determine if vitamin D or magnesium is useful in preventing diabetes. Smaller and shorter term prospective studies could be done to see if vitamin D and magnesium are good treatments for people who already have diabetes.