Vitamin D has become one of the most celebrated nutritional supplements in recent years. Many connections have been made between the vitamin and disease prevention that it's hard to overlook. Often we hear about calcium and its role in bone health.

However, we would not be able to absorb and use that calcium without the presence of the D vitamin, which also promotes phosphorus use. Additionally, this vitamin strengthens the immune system, presides over cell growth and differentiation, and is used in the prevention and treatment for osteoporosis.

For the best sources of the D vitamin, try a tablespoon of cod liver oil (for 340% of your daily value), 3.5 ounces of cooked mackerel or salmon (for 90% of your daily value), 3 ounces of tuna canned in oil (50% DV), 2 ounces of sardines canned in oil (70% DV) or a cup of vitamin D fortified milk (25% DV). Other sources include eel, catfish, margarine, pudding, fortified ready-to-eat cereals, egg yolks, liver and Swiss cheese. Of course, taking calcium supplements combined with this vitamin will ensure that you get the best of both worlds for a strong, healthy body.

Numerous studies imply that vitamin D and calcium supplements (used conjunctively) promote bone health. The American Medical Women's Association called for the recommended daily allowance to be raised from 400-600 to 800-1,000 international units per day for everyone over 50.

Their November 2005 study found that nearly 70% of women over 50 and 90% of women over 70 were not getting their recommended allowance of the vitamin. As we age, our bodies slow down in the production of the D vitamin and we may be seeing less sunlight too.

The Journal of the American Geriatrics Society adds that senior citizens also need a higher amount of magnesium. Therefore, for optimal bone health, people over 50 should take a D vitamin/magnesium supplement.

You'll get 20,000 IU of vitamin D after just thirty minutes of sunlight. Yet when it comes to dietary supplements, doctors recommend less than 2,000 units per day. It is possible to develop D vitamin toxicity. Symptoms include poor appetite, weakness, weight loss, constipation, nausea and vomiting, at best.

At worst, the calcium levels can rise in your blood, causing confusion and heart arrhythmias. Sometimes the body has a hard time ridding itself of the excess supplements, which accumulate as kidney stones. You won't suffer this toxicity from the sun, yet you must take precautions when choosing a supplement.