Vitamin D For Dummies
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The medical community has known of the benefits of vitamin D on bone health for decades. In recent years, scientists have discovered that vitamin D may play a role in many other aspects of our health. This overview provides a look at some areas in which vitamin D may improve health and prevent diseases.

Building bone with vitamin D

The work of calcitriol (active vitamin D) is intimately linked to the way your body uses calcium. Active vitamin D levels increase when you regularly eat a diet low in calcium. When elevated, the role of calcitriol begins in the intestine, where it promotes increased absorption of calcium in an effort to overcome your low dietary calcium intake.

Calcitriol also influences the kidneys, where it keeps calcium from leaving in the urine. In the skeleton, calcitriol causes both the production of the framework of the bone and the mineralization of that framework with calcium and phosphate. On the other hand, abnormally high levels of calcitriol cause bone to break down and too much calcium to be absorbed by the intestines; this can cause toxic levels of blood calcium.

Normal levels of calcitriol promote the breakdown of old bone and the creation of new bone. Another way that calcitriol protects bone is by influencing the production of the parathyroid hormone. If you have a deficiency of vitamin D, you can’t make enough calcitriol. As a result, the parathyroid gland makes more parathyroid hormone which goes to bone and breaks it down to release calcium into the bloodstream. If this goes on too long, the increase in parathyroid hormone is detrimental, leading to weakened bones.

Restoring vitamin D and calcitriol levels to normal allows the skeleton to regain lost calcium and strength. Maintaining the calcium level in the blood is important for the body’s muscle function: heart muscles, skeletal muscles, and all other muscles.

As they grow, children add more bone than they break down, so bone mass increases. When you’re a kid, calcium absorption from the diet has to be very efficient to meet the needs of growing bone, so active vitamin D is important at this stage of life. When you stop growing, there is still a lot of activity going on in the bone. About 10 to 30 percent of the bone in your body is renewed yearly.

After you reach your 30s, you begin to lose slightly more bone than the amount you make, so you have a net loss of bone. At menopause, women lose bone mass even more rapidly. Because of all this bone loss during adulthood you need to build up plenty of bone at a younger age so that by the time you start to lose more bone mass than you gain, you can avoid osteoporosis, a condition in which the bones are fragile and can fracture.

Reducing your risk of cancer with vitamin D

One promising role for vitamin D is in the prevention of cancer. In some studies, the rates of certain but not all cancers appear to be lower the closer you live to the equator. Some scientists think that this is because you make more vitamin D in your skin the closer you live to the equator. Other studies show that high blood levels of vitamin D (25OH D) are associated with lower rates of a number of cancers.

Scientists estimate that higher blood vitamin D levels could cause:

  • A 50 percent reduction in the risk of colon cancer

  • A 30 percent reduction in the risk of breast cancer

  • A 30 percent reduction in the risk of ovarian cancer

  • A 43 percent reduction in the risk of pancreatic cancer

Calcitriol has been shown to slow the growth of cancer cells isolated from the breast, the prostate, and the colon, and it can kill cancer cells in culture. Unfortunately, it isn't yet known whether this ability to slow or even kill cancer cells occurs in humans. The high doses of calcitriol needed in cell culture studies would cause toxic, high levels of blood calcium if they were used in humans.

Preventing heart disease and diabetes with vitamin D

Still other studies are pointing to a possible role for high blood vitamin D levels in the prevention of other chronic diseases like diabetes and heart disease.

If you looked at a graph comparing the average blood pressure of the population with the distance from the equator, you’d see that blood pressure rises the farther you get from the equator and its strong sun rays (and, therefore, greater skin production of vitamin D). The change in blood pressure might have nothing to do with vitamin D, but it seems reasonable to assume that it does.

Studies in animals show that calcitriol can lower blood pressure and decrease the risk of an enlarged heart. Calcitriol also relaxes blood vessels, which further lowers blood pressure.

There is also evidence that higher blood vitamin D levels might also protect against the development of diabetes. This might be related to observations that calcitriol can alter the cells of the immune system to suppress autoimmunity, the reaction of the body against itself.

Type 1 diabetes mellitus is an autoimmune disease, so active vitamin D might help limit the development of this disease. At the same time, studies in animals and cell cultures suggest that calcitriol improves insulin secretion from the pancreas and increases the sensitivity of cells to the action of insulin. These actions might help prevent and treat type 2 diabetes.

About This Article

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About the book author:

Alan L. Rubin, MD has been a physician in private practice for more than 30 years. He is the author of several bestselling health titles, including Diabetes For Dummies, High Blood Pressure For Dummies, and Thyroid For Dummies.

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