Vitamin D3 is one of five hormone-like substances that constitute the vitamin D group. D3 is found in some foods naturally, such as fatty fish, used to fortify foods and beverages, available as a supplement and produced within the skin in response to certain frequencies of sunlight. D3 is needed in the body for strong immune function, mood regulation and healthy bones. All forms of vitamin D, including D3, promote the absorption of calcium, which is needed for bone growth and maintenance.
Vitamin D Conversion
All forms of vitamin D, whether obtained from sunlight, food or supplements, are biologically inert and must undergo two conversions in the body for activation and to impact calcium absorption, according to the National Institutes of Health. The first occurs in the liver and converts vitamin D into calcidiol. The second occurs in the liver and kidneys and forms the more physiologically active calcitriol, a hormone that works synergistically with parathyroid hormone, or PTH.
Vitamin D and Calcium
Vitamin D3 promotes calcium absorption in the intestines and maintains adequate serum calcium and phosphate concentrations to promote normal mineralization of bone and to ensure appropriate muscle tone, as cited in "Biochemical, Physiological and Molecular Aspects of Human Nutrition." D3 is also needed for bone growth and bone remodeling by specialized cells called osteoblasts and osteoclasts. Vitamin D deficiency can lead to thin, brittle and deformed bones, a condition termed rickets in children and osteomalacia in adults. A lack of vitamin D leads to poor absorption of calcium, which increases the risks of osteoporosis and cardiovascular diseases.
Biochemistry of Calcitriol
Vitamin D3 not only regulates calcium absorption, it also controls its excretion when calcium intake is low. Calcium is important for strong bones, but it is crucial for heart contraction, which is why the body preserves blood calcium levels at the expense of bone calcium, as cited in "Biochemistry of Human Nutrition." Thus, when calcium levels in the blood drop, the hormone PTH is released. PTH causes calcium to be released from bones, which raises the calcium levels in the blood. Vitamin D3, as calcitriol, which works synergistically with PTH. Specifically, calcitriol increases absorption of calcium and phosphorus, another component of bone, from the intestines and decreases their excretion in the urine. As a result, calcium levels in the blood rise and PTH levels drop.
Sources of D3
In addition to being produced within the skin in response to certain frequencies of sunlight, D3 is found naturally in some foods. The flesh of fish -- particularly salmon, mackerel, sardines, cod and tuna -- is an excellent source of D3, according to the "American Dietetic Association Complete Food and Nutrition Guide." Wild varieties of fish are richer in D3 than organically farmed varieties. Fish oils such as cod liver oil, beef liver and egg yolks are also good sources of D3. Most dairy milk in the United States is fortified with D3, and has been for decades. Breakfast cereals often contain added D3, as do some brands of orange juice, yogurt and margarine.
References
- National Institutes of Health: Office of Dietary Supplements: Vitamin D Fact Sheet
- "Biochemical, Physiological and Molecular Aspects of Human Nutrition"; Martha Stipanuk; 2006
- "Biochemistry of Human Nutrition"; George Gropper; 2000
- "American Dietetic Association Complete Food and Nutrition Guide"; American Dietetic Association; 2006
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