As summer ends in the northern hemisphere, we spend less time in the sun and the sun’s intensity decreases. Both of these factors decrease the amount of vitamin D our skin is able to produce. Vitamin D is both a nutrient that we eat and a hormone that our bodies make when our skin is exposed to ultraviolet B (UVB) rays. Because foods naturally rich in vitamin D are limited (fatty fish like salmon, tuna & sardines), we are dependent on fortified foods (milk and breakfast cereals in the U.S.) and supplements to meet our needs. But we still may not be getting enough. Worldwide, vitamin D deficiencies are found on all continents, in all ethnic groups, and across all ages. Some surveys suggest that half of the world’s population is vitamin “D-ficient” which research is showing may increase the risk of a host of chronic diseases like osteoporosis, heart disease, some cancers and multiple sclerosis, as well as infectious diseases like tuberculosis and seasonal flu.
Vitamin D is best known for its role in bone health. Without the “sunshine vitamin” the body is not able to absorb the calcium it ingests, so it steals calcium from bones; increasing the risk of osteoporosis and bone fractures. Vitamin D also helps maintain normal blood levels of phosphorus, another bone-building mineral. For this contribution alone, vitamin D is essential, but research over the past 10 years has led to the discovery of vitamin-D-receptors on essentially all tissues and cells in the body, expanding its benefits beyond bone health. Laboratory studies reveal that vitamin D can reduce cancer cell growth, increase muscle strength and play a critical role in controlling infections and immune response.
Vitamin D deficiency is defined as a blood level less than 20 nanograms per milliliter or 20ng/mL. However, levels that low have been linked to poor bone density falls, fractures, cancer, immune dysfunction, cardiovascular disease, and hypertension. Many experts recommend >30ng/mL and suggest that 800 to 1000 IU of vitamin D per day is required to maintain that level. Many people may need 2,000 IU per day (or more) for adequate blood levels particularly if they have darker pigmented skin, spend winters in higher latitudes, spend little time in the sun or regularly use sunscreen, are elderly or obese. If you fall into one of these groups, taking 2000 IU per day is reasonable and well within the safe daily range for adults as determined by the Institute of Medicine’s (IOM) most recent recommendations for dietary reference intakes (Nov. 2010). Note, the IOM also recommends vitamin D supplementation for breastfed infants and children in smaller doses.