1. Sunlight exposure draws down the body's pool of cholesterol.
Vitamin D production begins in the oil glands of our skin, where the steroid 7-dehydrocholesterol is converted to previtamin D3 after absorbing specific wavelengths of light in the ultraviolet range. Exposure to heat converts previtamin D3 to vitamin D3 or cholecalciferol. All of this occurs on/in skin.
2. Sunlight exposure creates reservoirs of vitamin D3 throughout the body that may be tapped during winter or when needed (that is, when calcidiol or 25(OH)D3 is low). Dietary vitamin D does not get distributed in this way.
Much of the vitamin D3 manufactured in skin is deposited throughout the body in muscle and fat cells before it arrives in the liver. Much of the vitamin D3 absorbed from the digestive tract (food or pill) is delivered directly to the liver.
3. Absorption of vitamin D from diet is poor. Only about 50% of what we eat gets absorbed. In addition:
- Diseases that compromise health of intestinal cells (e.g. irritable bowel syndrome, lactose intolerance, inflammatory bowel disease (Crohn's), infectious agents, cancer) lower absorption.
- Bile is required for absorption, so liver conditions (hepatitis, cirrhosis, hepatic steatosis or fatty liver) lower absorption.
- Fat is required for absorption, so if there is steatorrhea or fat malabsorption (many causes, e.g. anemia, cystic fibrosis, celiac, Whipple), absorption will be lower. (There is usually adequate fat in food to ensure absorption.)
- Some medications, such as bile acid sequestrants which are used to lower cholesterol, lower absorption.
"To offer some perspective here, an adult with white skin [pigmented skin takes longer to manufacture vitamin D], exposed to summer sunshine while wearing a bathing suit, generates about 250 µg (10,000 IUs) of vitamin D3 in 15 to 20 minutes."
- Vitamin D Insufficiency: No Recommended Dietary Allowance Exists For This Nutrient, Reinhold Vieth and Donald Fraser, CMAJ, 2002
"Sunshine alone can bring 25(OH)D concentrations to 210 nmol/L in normal people and vitamin D intakes of 30 mg (1200 IU)/d contribute only a negligible fraction of this."5. Relying upon dietary sources to supply vitamin D necessitates consuming higher amounts of environmental pollutants.
- Vitamin D Supplementation, 25-Hydroxyvitamin D Concentrations, And Safety, Reinhold Vieth, AJCN, 1999
Both vitamin D and many organic pollutants (e.g. pesticides) are fat soluble. As such, they reside in fat tissue. Organic pollutants have been shown to bioaccumulate in animal tissue and are found in greater amounts higher up the food chain.
6. Some oil-based formulations of vitamin D may be harmful to the liver.
There is thinking that fish oil, and some forms of oil-based vitamin D, can be harmful to the liver.1 This is related to number 2 above, in that oral D is sent directly to the liver. Anecdotally - some people experienced elevated liver enzymes while taking fish oil (a common source for vitamin D) only to have them fall back into the normal range when they stopped. More study is needed there.