Vitamin D

Vitamin D is one of thirteen vitamins that are necessary in the human diet to help ensure proper health, particularly as it relates to calcium absorption and regulation, as well as bone development and maintenance. It also plays a role in the immune system, by helping to regulate genes that fight bacteria and viruses. Vitamin D actually occurs in two distinct forms: vitamin D2 (also called ergocalciferol) and vitamin D3 (cholecalciferol). These two versions of vitamin D are essentially the same in function, with the exception of their molecular structure.

Although the body naturally produces most of its vitamin D through exposure to the ultraviolet rays from the sun, it is also important for humans to obtain a sufficient amount of vitamin D through foods and/or beverages. Inadequate levels of vitamin D can lead to serious health defects. Since the 1930s, for example, lack of vitamin D has been known to cause rickets, a childhood condition that results in an abnormal weakening of bones. In recent years, however, a variety of new health disorders have been attributed to insufficient levels of vitamin D in the body. At the other extreme, prolonged excessive intake of vitamin D can have negative impacts on a person’s health as well.

Brief History

Vitamin D is necessary for the body to absorb calcium, a process that occurs in the intestines. Adequate levels of calcium are important to promote strong, healthy bones, as well as the health of other bodily tissues. Humans naturally produce vitamin D through exposure to sunlight, as the sun’s ultraviolet rays penetrate the surface of the skin and stimulate the production of vitamin D.

Physical anthropologists have speculated that the variations human skin color throughout the world have been closely related to the role of sunlight in promoting the production of vitamin D. Humans whose recent ancestry (within the past 500–1,000 years or so) comes from tropical regions generally have the darkest skin pigmentations of all human groups, while those whose recent ancestry comes from cooler, northern latitudes—such as Scandinavia—tend to have the lightest pigmentations. Populations whose recent ancestry lies between these two geographic extremes tend to have intermediate complexions. Scientists have theorized that this pattern is correlated with the intensity of the sun’s ultraviolet rays in these different regions; as sunlight becomes less intense, lighter skin evolved in order to allow enough ultraviolet rays to penetrate the skin to produce vitamin D (darker skin blocks out higher volumes of ultraviolet rays). This pattern is generally true throughout the world, although certain notable exceptions do exist—such as the Inuit of Alaska and Canada and Aboriginal Australians, both of whom have relatively darker complexions for their geographic habitats.

Vitamin D deficiency became a major health concern in many societies beginning with the Industrial Revolution, as ever-growing numbers of people began spending most of their daily routines indoors and removed from sunlight. This problem continues to the present, as the Centers for Disease Control (CDC) estimates that approximately one-third of all Americans are vitamin D deficient. Other non-governmental estimates place the rate of vitamin D deficiency in the United States much higher, at one-half to two-thirds of the population. Senior citizens and persons who live in northern states, of all ages, are most likely to be deficient. A person’s level of vitamin D is determined through a blood test; approximately 20 ng/ml (nanograms per milliliter) is considered the normal range, while 50 ng/ml or higher is considered excessive vitamin D.

Overview

Growing awareness of vitamin D deficiency led the United States to fortify milk beginning in the 1930s. In addition to milk, some juices and many cereals are also fortified with vitamin D. Foods that are naturally rich in vitamin D include salmon, tuna, swordfish, eggs, and sardines. Just one tablespoon of cod liver oil contains more than three times the recommended daily value of vitamin D. However, very few foods are good sources of vitamin D, and even today, the best source of vitamin D for humans remains exposure to sunlight. Experts disagree on just how much sunlight—and length of exposure—is best, as individual tolerance to ultraviolet rays varies considerably. Many health professionals recommend all vitamin D needs be provided by food or supplements, as the risk of skin cancer from prolonged sun exposure is significant. Others suggest between 5 and 30 minutes of sunlight on exposed skin for two or three days per week often results in adequate vitamin D for most individuals.

The federal government recommends that both male and female adults obtain 600 IU (International Units) of vitamin D daily, with the figure rising to 800 IU for persons over seventy years of age. Other research organizations recommend higher intake levels, though most suggest 10,000 IU per day as an upper limit. However, the effects of supplements continue to be debated.

Persons who are at increased risk for vitamin D deficiency include persons with darker skin pigmentation (since darker skin blocks out more of the sun’s ultraviolet rays), persons who spend much of their day indoors, senior citizens, and individuals who are overweight or obese. Since vitamin D is soluble in fat, persons who carry excessive body fat need additional vitamin D to maintain an adequate daily level. Vitamin D deficiency is also a concern for individuals with kidney failure or other forms of kidney damage. In addition, newborns who are exclusively breastfed may develop low vitamin D if their mother is deficient in vitamin D herself. on the other hand, excessive vitamin D can lead to a number of unpleasant side effects, including increased urination, nausea and vomiting, dizziness, headaches, tiredness and fatigue, and severe cramps in the abdomen.

A study published in the journal Gut and reported on in the New York Times in early 2015 claimed that higher levels of vitamin D may reduce the risk of colon cancer. The researchers contend that higher vitamin D may work in conjunction with the body’s immune system to stave off the growth of tumors in the colon. A different study that appeared in BMJ (formerly the British Medical Journal) reported that a lifetime level of low vitamin D increased the risk of death from cancer by over 40 percent.

Bibliography

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