Vitamin D is a fat-soluble vitamin which is present in some foods, either naturally, or put in as an additive, and also available as a dietary supplement. It is also synthesized in the skin as a result of sunlight exposure.
In order for it to become metabolically active, vitamin D needs to undergo a transformation. These transformations are referred to as “hydroxylations”.The first hydroxylation is in the liver and converts vitamin D to 25-hydroxyvitamin D. The second hydroxylation occurs in the kidney and forms 1,25-dihydroxyvitamin D.
Vitamin D helps with calcium absorption from the gut. It is also needed for normal bone growth and bone remodeling. Vitamin D deficiency causes bones to become fragile. In children, this condition is referred to as rickets and in adults it is called “osteomalacia.”
When taken with calcium supplementation, vitamin D protects adults from getting osteoporosis.
Other activities Vitamin D has are regulation of cell growth, maintenance of immune function, neuromuscular conditioning, and reduction of inflammation.
The blood concentration of 25- hydroxyvitamin D is the best indicator of a patient’s vitamin D status.
There is debate over the exact level of vitamin D necessary for both bone as well as overall health. A steering committee of the Institute of Medicine stated that people with levels of vitamin D (25-hydroxyvitamin D) less than 30 nmol/L are vitamin D deficient. Levels above 50 nmol/L are sufficient for most people.
Food sources for vitamin D include fatty fish (salmon, tuna, and mackerel) and fish liver oils are among the best sources. Lesser amounts of vitamin D are found in beef liver, cheese, and egg yolks, and mushrooms. Most vitamin D, in food form, comes by way of fortified foods such as milk, orange juice, yogurt, and margarine.
Fortunately, most people meet vitamin D requirements through sunlight exposure. There are multiple factors that modulate the effect of sunlight though. These include the season, duration of sunlight exposure, smog or cloudiness, melanin content of the skin, and the use of sunscreen.
People who have limited sun exposure need to have good dietary intake of vitamin D.
Obviously, the need for sunlight exposure in order to achieve good vitamin D levels conflicts with the fact that ultraviolet light exposure is a carcinogen responsible for skin cancers and the 8,000 deaths due to metastatic melanoma that occur annually in the United States (data from the National Institutes of Health).
Older adults are at increased risk of developing vitamin D insufficiency because they do not synthesize vitamin D efficiently, they tend to spend more time indoors, and they may have an inadequate dietary intake.
Other high risk groups include people with darkly pigmented skin, people who wear long robes and head coverings for religious reasons, patients with fat malabsorption syndromes such as inflammatory bowel disease, liver disease, or cystic fibrosis, obese individuals, and people who have undergone gastric bypass.
Approximately 40 million adults in the United States have osteoporosis, a disease characterized that increases bone fragility and fractures. Insufficient vitamin D contributes to osteoporosis by reducing calcium absorption.
Osteoporosis can develop as a result of inadequate calcium and vitamin D. Adequate levels of vitamin D might help prevent osteoporosis in older adults, postmenopausal women, and people who take chronic steroids. Some studies have shown that inadequate vitamin D levels can cause an increased risk of falls and subsequent fracture.
Some data also indicates vitamin D could affect cancer risk. However, studies remain inconclusive.
Bottom line:
Levels of vitamin D (25-hydroxyvitamin D) less than 30 nmol/L are vitamin D deficient
Levels above 50 nmol/L are sufficient
Vitamin D intake should be at least 600 IU to 1,000 IU for most individuals 50 years of age or older. This may need to be increased in individuals who are D deficient. Some individuals who are very deficient may need to be placed on pharmacologic doses of vitamin, meaning 50, 000 units once or twice weekly. Blood levels of vitamin D can be used to gauge response.