Q. Glen, Does vitamin D help with prostate cancer?
A.( Yes )Vitamin D’s best-known role is to keep bones healthy by increasing the intestinal absorption of calcium. Without enough vitamin D, the body can only absorb 10% to 15% of dietary calcium, but 30% to 40% absorption is the rule when vitamin reserves are normal.
Low levels of vitamin D lead to low bone calcium stores, increasing the risk of fractures. If vitamin D did nothing more than protect bones, it would still be essential. But researchers have begun to accumulate evidence that it may do much more. In fact, many of the body’s tissues contain vitamin D receptors, proteins that bind to vitamin D. In the intestines, the receptors capture vitamin D, enabling efficient calcium absorption. But similar receptors are also present in many other organs, from the prostate to the heart, blood vessels, muscles, and endocrine glands. And work in progress suggests that good things happen when vitamin D binds to these receptors. The main requirement is to have enough — but many Americans don’t.
Vitamin D has an important role in regulating cell growth. Laboratory experiments suggest that it helps prevent the unrestrained cell multiplication that characterizes cancer by reducing cell division, restricting tumor blood supply (angiogenesis), increasing the death of cancer cells (apoptosis), and limiting the spread of cancer cells (metastasis). Like many human tissues, the prostate has an abundant supply of vitamin D receptors. And, like some other tissues, it also contains enzymes that convert biologically inactive 25(OH)D into the active form of the vitamin, 1,25(OH)2D. These enzymes are much more active in normal prostate cells than in prostate cancer cells. Do the results from these experiments translate into clinically important effects? Possibly. Several lines of evidence are relevant.
First, the calcium connection. In 1998, Harvard’s Health Professionals Follow-up Study of 47,781 men reported that a high consumption of calcium, either from food or supplements, was associated with an increased risk of advanced prostate cancer; the risk was greatest in men taking more than 2,000 mg of calcium a day. Since then, other studies have confirmed a link between very high levels of dietary calcium and increased risk, but they have exonerated modest calcium consumption. The Harvard scientists speculate that the problem is not calcium itself but a relative lack of active vitamin D. That’s because high levels of calcium can reduce the body’s production of the active form of the vitamin.
Second, the sunlight connection. Prostate cancer is more common in North America and northern Europe than in the sunnier climates of Asia, Central America, or Africa. In the United States, the disease is more prevalent in African Americans than Caucasian Americans.
Of course, sunlight is just one of the many differences between these groups of men, and various genetic, dietary, lifestyle, and health care disparities could account for the prostate cancer gap. Indeed, early studies failed to demonstrate a clear association between blood levels of 25(OH)D and prostate cancer. But in 2005, a 13-year Harvard study of 2,399 men reported that those with high blood levels of both forms of vitamin D — 25(OH)D and 1,25(OH)2D — enjoyed a 45% lower risk of developing aggressive prostate cancer than those with below-average levels. And certain genetic variations in the vitamin D receptor appeared to enhance the protective effect of the vitamin.
Can vitamin D reduce your risk of prostate cancer? It’s too early to say — but it’s a possibility that requires additional study.
“D” right amount
Until 1997, the recommended dietary allowance (RDA) for vitamin D was 200 IU for all adults. Faced with growing evidence of vitamin D deficiencies in Americans, the RDA for 51- to 70-year-olds was increased to 400 IU, and to 600 IU for people older than 70.
Is more better? New research suggests that it is, and many authorities are recommending 800 or even 1,000 IU a day. Remember, though, that you can get too much of a good thing. Like the other fat-soluble vitamins, vitamin D is stored in the body’s adipose (fat) tissue. That means your body can mobilize its own reserves if your daily intake falters temporarily — but it also means that excessive doses of vitamin D can build up to toxic levels. At those extremes, vitamin D can raise blood calcium to levels that can cause grogginess, constipation, and even death. But it takes massive overdosing to produce toxicity, and doses up to 2,000 IU a day are considered safe.
Diet can help, but it’s very hard to approach the new goals with food alone. Fish and shellfish provide natural vitamin D (oily fish are best), but you’ll have to eat about 5 ounces of salmon, 7 ounces of halibut, 30 ounces of cod, or nearly two 8-ounce cans of tuna to get just 400 IU. An egg yolk will provide about 20 IU, but since it also contains nearly a day’s quota of cholesterol, you can’t very well use eggs to fill your tank with D. Other foods have even less D, which is why manufacturers fortify milk, some yogurt, some orange juice, and many cereals with vitamin D. In general, a serving will provide about 100 IU; that means drinking a quart of fortified milk to get 400 IU.
Most people require supplements to get the vitamin D they need. It’s the main benefit of a daily multivitamin; most provide 400 IU. Remember to read the labels carefully so you won’t get too little or too much. And although cod liver oil is rich in vitamin D, it has too much vitamin A for regular use.
Resource: Harvard Medical
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