Vitamins are essential for normal function of our bodies. The word comes from the roots vit- meaning life and –amine which refers to a chemical component of many biological molecules. They are “vital” because most vitamins cannot be made by the body. These nutrients must be obtained by outside sources, that is, food.
Vitamins function in many ways within the human body. Most act as co-enzymes or catalysts, helping to regulate metabolic reactions that are necessary for life. In the case of vitamin D, it is a hormone involved in controlling the body’s level of calcium and phosphorus. These two elements are very important in maintaining bone structure.
The body makes the initial precursor to vitamin D from cholesterol. In the skin, exposure to UV light (sunlight) changes this pro-vitamin to vitamin D3. So, vitamin D3 can be manufactured in the body or it can be obtained from eating vitamin D containing foods. In the liver, vitamin D3 is converted to the active compound, calcitriol. It is this hormone that tells the intestinal cells to make specific proteins to absorb calcium.
In 1645, Daniel Webster described a condition in children that was common in England where there was little sunlight for many months of the year. The disease was called “rickets”, from the Old English word wrickken, to twist. He wrote: “The whole bony structure is as flexible as softened wax, so that the flaccid legs can hardly support the superposed weight of the body.”
Due to a deficiency of vitamin D from inadequate dietary intake and lack of sunlight, the children had impaired calcification of cartilage and bone. Webster referred to it as the “Children’s disease of the English”. In adults, softening of the bones due to vitamin D deficiency is called osteomalacia.
Today, the most reliable source of vitamin D is fortified foods (dairy and cereals). For example, fortified milk has 400 IU (international units) of vitamin D per quart. The current U.S. recommended dietary allowance for vitamin D is 400 IU per day.
Most natural foods are deficient in adequate amounts of vitamin D, except for fish liver oils. For this reason, cod liver oil was commonly used in the past to supplement children’s diets. One tablespoon of cod liver oil provides 1,360 IU of vitamin D. Other “fatty fish” that have a large amount vitamin D include herring, catfish, salmon, mackerel, sardines, and tuna.
Since calcium absorption depends on adequate levels of vitamin D, there are some specific populations of individuals who may be at risk for low vitamin D and calcium levels. This includes people who are not able to get regular sunlight, those who have liver or kidney impairment, and those who have intestinal disorders who cannot adequately absorb calcium in the intestine. Vitamin D levels may also diminish with age.
Blood tests can check both calcium and vitamin D levels in at-risk individuals. For those who are deficient, vitamin D and calcium supplements may be recommended.
Finally, too much vitamin D can also be a problem. Symptoms are that of high calcium levels including nausea, vomiting, weakness, frequent urination, and excessive thirst. For the general population, a balanced diet and sunshine is all that is needed to maintain healthy levels of vitamin D.
The content in this column is for informational purposes only. Consult your physician for appropriate individual treatment. Dr. Reynolds practices Family Medicine in Chesterfield.