Vitamin D Functions
Vitamin D is a fat-soluble vitamin that helps control the level of calcium and phosphorus in the blood to help the body build and maintain strong bones and teeth. It also helps increase the body’s absorption of calcium.
Recent research suggest that getting adequate amounts of vitamin D (even more than currently recommended) can help protect against osteoporosis, hypertension, cancer, and autoimmune diseases.
The two main sources of vitamin D include the diet and sunlight.
Dietary
Vitamin D is found naturally in only a few foods, most notably in fish. Most vitamin D we consume, however, comes from fortified foods including milk, margarine, and ready-to-eat cereal.
Sunlight
Vitamin D (in the form of vitamin D3, the kind found in foods) can be made in the body when the skin is exposed to ultraviolet-B (UVB) rays from sunlight for as little as 10 or 15 minutes.
Deficiencies and Excesses
Some people are at high risk for not getting or making enough vitamin D to meet their daily needs. These include:
• Older people
• People with dark skin
• Obese people
• Infants who are exclusively breastfed
• People who live in environments or climates in which sunlight is limited
• People with certain conditions that limit absorption of fat, such as cystic fibrosis and Crohn’s disease
• People who follow vegetarian or vegan diets and avoid fish, fish oils, and D-fortified dairy and other foods like milk and orange juice.
Without enough vitamin D, blood levels of calcium and phosphate can get too low, which can cause the body to produce hormones that release calcium and phosphate from the bones, causing bones to soften and weaken. In children, a vitamin D deficiency causes rickets. In adults, it causes osteomalacia.
Too much vitamin D from supplements can cause symptoms such as nausea, vomiting, decreased appetite and weight loss, weakness, confusion, altered heart rhythm, and calcium and phosphate deposits in soft tissues. However, D toxicity is now believed to be much less likely than previously believed unless intake routinely exceeds the 1,000 to 2,000 IU a day that experts now recommend.
Cystic fibrosis is an inherited disease that causes endocrine glands (organs that make and secrete hormones into the bloodstream) to malfunction. Mucus becomes thick and sticky and damages other body organs and can lead to serious problems with the lungs, pancreas, and digestive system.
Crohn’s disease is a lifelong inflammatory bowel disease that causes swelling and ulcers (deep sores) in the intestinal tract.
Rickets is a bone disease that occurs in infants and children who have a vitamin D deficiency; symptoms include soft, bendable bones.
Osteomalacia is a bone disease caused by a vitamin D deficiency in adulthood; bones foten and cause bowed legs and curvature of the spine.
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While typical medical guidelines have advised that serum levels over 30ng/mL are adequate, scientific evidence is building that an optimal level may be significantly higher. Thus, a healthy target range for most people would be at least 40-60ng/mL. Because relatively small amounts of vitamin D are obtained through the diet and so many lifestyle factors reduce endogenous vitamin D synthesis, supplementation is critical to achieve and maintain an optimal vitamin D status, particularly during the winter. However, the daily dose of vitamin D required to achieve specific serum levels of vitamin D, in the background of other lifestyle characteristics, remain in question.
Deficiencies of vitamin D are common. It is currently estimated that more than 1 billion people worldwide and 30 to 40% of the population between 15 and 49 years of age in the United States suffer from vitamin D deficiency. Additionally, the “Vitamin D Winter” occurs from approximately November until March at latitudes greater than 35-37° north. During this time, the wavelength of light required to synthesize vitamin D is filtered out due to the change in incident angle of sunlight resulting from seasonal tilting of the earth. Thus, for people living in the affected region, sun exposure is not sufficient to produce adequate levels of vitamin D during the winter months. Moreover, for most of the U.S. population, our modern lifestyles keep us out of the sun during the prime vitamin D-producing hours, even in the most southern latitudes.
Vitamin D is unique in that not only is very little obtained through a normal diet, but the principal source of serum vitamin D is endogenous production in the skin upon exposure to the UVB portion of the solar spectrum. Thus, factors that limit sun exposure and/or endogenous vitamin D synthesis greatly reduce serum 25-hydroxyvitamin D (25[OH]D) concentrations. Some of these limiting factors include geographic latitude, season of the year, melanin content of the skin, use of sunscreen or sun obscuring clothing, and lack of outdoor activity. Obesity, diet, and age can further limit serum vitamin D levels. Any of these factors, among others, can lead to chronic vitamin D deficiency.