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First identified in the early twentieth century, vitamin D is an essential component of a healthy body. Two forms of Vitamin D include Vitamin D2 and Vitamin D3. Vitamin D3 can be made in human skin upon exposure to sunlight but many factors affect how much you can make. Vitamin D can also be consumed via foods and supplements.

For vitamin D3 to be utilized by the body, it must first be converted into its active form. Vitamin D3 is considered inactive until it undergoes two main reactions following its acquisition from sunlight or when consumed from a food or supplement source. First it travels through the liver, where it encounters certain enzymes that alter its structure, and then it passes through the kidneys where a similar process takes place with a different set of enzymes. The result is the biologically active hormone calcitriol. Calcitriol plays a vital role in calcium and phosphorous absorption in the digestive system and calcium reabsorption in the kidneys, which increases the amount of calcium ions in the bloodstream. Healthy calcium levels are critical for maintaining bone and muscle health and supporting communication between certain cells. Calcitriol also plays an important role in the maintenance of many organ systems. Without vitamin D, our bodies cannot effectively make use of calcium, which results in a number of health problems. To learn more about how vitamin D3 and calcium interact, read the article “How Calcium and Vitamin D3 Work Together.”

Vitamin D3 supplementation can be used by individuals who don’t get a lot of sun. During the spring and summer months it can be relatively easy to get adequate sunlight, the shortened days and decreased UV levels can make it more difficult during winter months. According the Institute of Medicine, the Recommended Dietary Allowance (RDA) for adults 19-50 is at least 600 IUs per day, with some studies suggesting the need for more. The upper intake level is 4,000 IUs per day and excessive vitamin D levels can have harmful effects on the health, so make sure to read the labels of the foods and supplements you consume.


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For maximum absorption