To Your Health

Q. My mother has been taking 400 IU of vitamin D for several years. She has severe osteoporosis that was still getting worse until she increased her vitamin D supplement to 800 IU per day. What is the safe limit for a vitamin D supplement?

A. Vitamin D is not a true vitamin because individuals with adequate exposure to sunlight do not require any extra vitamin D in their diet. There are dietary sources of vitamin D, such as egg yolk and fish oil, but most diets do not contain sufficient quantities of vitamin D to prevent deficiencies.

The term "vitamin D" actually refers to several substances related to steroid hormones. The vitamin D we make in our skin on exposure to light is called cholecalciferol or vitamin D3. Vitamin D3 is the safest type of vitamin D to use as a supplement, though it is still by far the most toxic of all the vitamins. The first symptoms of toxicity can occur on as little as 2,000 IU per day or five times the recommended amount of 400 IU per day, although some individuals have used daily doses of 25,000 IU per day over long periods of time without apparent harm.

As with most vitamins, we activate vitamin D, converting it into 1,25-dihydroxycholecalciferol or calcitriol, the hormone form of vitamin D that is involved in mineral metabolism and bone growth. Its most significant effect is on intestinal absorption of calcium, although it also stimulates absorption of phosphate and magnesium. It also induces the expression of a hormone known as osteocalcin, important for activity of the bone-building cells.

Although it is a fat-soluble vitamin, vitamin D is not stored in the body as are most other fat-soluble vitamins. Instead, cholecalciferol is produced in generous quantities (5,000 to 15,000 IU per day), but its conversion to calcitriol is tightly regulated so that we do not develop toxicity. Even excessive exposure to sunlight does not lead to vitamin D poisoning because, in addition to this regulation, prolonged exposure to sunlight results in formation of biologically inactive relatives of vitamin D.

The crucial effect of vitamin D is on bone health, but it turns out that vitamin D receptors are present in most body cells. Experiments using tissue-culture cells have demonstrated that vitamin D has potent effects on the growth and differentiation of many types of cells, suggesting that it has effects beyond that of bone formation.

There are several reasons that vitamin D might be deficient. Elderly people who perpetually stay inside may not get sufficient sunshine. Habitual use of sunscreen with SPF greater than 8, although protective against skin cancer, effectively blocks synthesis of vitamin D in the skin. Some cultures dictate that women be heavily veiled when outside the home. Even when sunshine is adequate, severe liver or kidney disease can interfere with generation of calcitriol, the biologically active form of vitamin D.

A physician should supervise doses of vitamin D over 2,000 IU per day. High doses are linked to an increased risk for premature heart attack, atherosclerosis, and kidney stones, but there is wide variation among individuals in their tolerance. The physician can monitor your mother's serum calcium, maintaining a level less than 11 mg per decaliter, permitting the benefits of improved bone health with minimal risk of toxicity.