Don't Miss Out on Vitamin D

Summarized by Robert W. Griffith, MD
May 19, 2004

Introduction

Lack of sufficient vitamin D can increase the risk of many common and serious conditions - some cancers, type 1 diabetes, cardiovascular disease, and osteoporosis. Dr Michael Holick, Professor of Medicine, Dermatology, Physiology and Biophysics at the Boston University School of Medicine, is an expert on vitamin D, and has recently published a review on its importance in the American Journal of Clinical Nutrition.

Sources of vitamin D

This vitamin is not widely available in foods. Oily fish, such as salmon, mackerel, and sardines are good sources, as well as cod liver oil. Some foods in the USA may be fortified with vitamin D; these include milk, orange juice, cereals, and breads. Easting oily fish three or four times a week will provide enough vitamin D for most people. Otherwise, the body must rely on exposure to sunlight.

The skin can produce large amounts of vitamin D3, although this ability decreases with age. The vitamin D3 produced by casual exposure to sunlight during the spring, summer, and fall, is stored in the body fat, and becomes available in the winter, when required.

Who's at risk of vitamin D deficiency?

Lack of vitamin D is seen commonly in the very young and the very old. Encouraging breast feeding of infants is partially responsible for a resurgence of this deficiency, as breast milk contains very little vitamin D. Infants usually require a vitamin D supplement.

The elderly are at risk, because of poor dietary intake, decreased exposure to sunlight, and reduced formation in the skin; by age 70, the amount of vitamin D3 formed in the skin decreases by as much as 75%.

Race also plays a role, especially in the elderly. Deficiency was reported in white, Hispanic, and black elderly Bostonians at the end of August in 30%, 42% and 84%, respectively; presumably the differences are largely related to skin pigmentation.

Obesity is another risk factor. It's thought that the vitamin D deposited in large body fat stores is not readily accessible to the rest of the body.

Consequences of vitamin D deficiency

Without vitamin D, the small intestine only absorbs about 10% to 15% of dietary calcium, instead of the normal 30% in someone without a deficiency. (In fact, absorption of calcium usually increases to about 80% during growth, lactation, and pregnancy; these periods are obviously time of increased risk for deficiency, too.)

Lack of vitamin D in childhood causes rickets. In adults bone growth stops and deformities can occur, and there is an attempt by the parathyroid glands to counteract this, by producing more parathormone. This hormone tries to maintain the serum calcium, but it causes loss of phosphorus in the urine instead; this results in softening of the bone (osteomalacia), and the risk of fractures is increased.

Apart from effects on bone, lack of vitamin D has been associated with an increased risk of colon, prostate, and breast cancer. This was discovered because of the increase in these tumors seen in people living at high latitudes, i.e. with fewer hours of sunshine. And it's been shown that a breakdown product of vitamin D can slow the multiplication rate of very active cell growth. This action of vitamin D has led to its successful use in treating the skin disease psoriasis, in which the skin cells multiply too quickly.

Although the way it works is not always understood, there is some evidence that vitamin D is able to reduce the chances of development of autoimmune diseases such as type 1 diabetes, rheumatoid arthritis, and multiple sclerosis.

An interesting finding is the reduction in blood pressure in hypertensive patients when they are exposed to ultraviolet B radiation that raises their circulating vitamin D. Exposure to ultraviolet A radiation had no effect on these patients' vitamin D levels, or their raised blood pressure.

Finally, some people are mistakenly diagnosed with fibromyalgia, when they really have a deficiency of vitamin D deficiency. Muscle weakness and bone pain, often reported in fibromyalgia patients, may be presenting symptoms of this vitamin deficiency.

What should be done to avoid vitamin D deficiency?

Without exposure to sunlight, Professor Holick recommends a minimum of 1000 IU vitamin D daily. And he believes that the blood concentration of 25(OH)D - the actual active form of vitamin D - should be measured once a year, just as we measure cholesterol levels. Early correction of a deficiency may prevent some of the conditions outlined in the previous section.

The best way to get enough vitamin D is by moderate exposure to sunlight. Exposure of the hands, face and arms, or arms and legs to sunlight for a period of time equal to a quarter of that required to make the skin pink is quite sufficient. There's no need to increase the risk of skin damage and skin cancer by 'overdosing'. And it can't hurt to consume foods fortified with vitamin D, as well as oily fish a few times a week.

Professor Holick warns against taking more than one multivitamin a day, as too much vitamin A would be consumed, increasing the risk of birth defects and osteoporosis. But one daily multivitamin containing 400 IU vitamin D is quite appropriate.

Source

• Vitamin D: Importance in the prevention of cancers, type I diabetes, heart disease, and osteoporosis. MF Holick, Am J Clin Nutr, 2004, vol. 79, pp. 362--371

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