Header for Osteoporosis Digest

Vitamin D Absorption

Vitamin D absorption varies significantly both with the source and with each person processing the vitamin.

To view comments about vitamin D dosage and supplements visit our Vitamin D Forum.

The Vitamin D Council recommends that healthy people receive 5,000-10,000 IU of vitamin D daily to maintain optimal health. A variety of strategies are needed to maintain this intake in regions north of the 42 latitude which includes Boston, Rome and Beijing. The UV energy at this latitude is insufficient for vitamin D synthesis from November through February and further north this “vitamin D winter” may last six months or longer.

There are three ways of ensuring adequate intake in our northern regions:

  • expose as much of the skin as possible (without burning) to the midday sun during the late spring, summer, and early fall
  • use a sun bed during the winter months.
  • take 5,000 IU per day for 2–3 months, then obtain a vitamin D blood test and adjust your dosage to maintain blood levels between 50–80 ng/mL (125–200 nmol/L) year-round.

During the winter months in northern regions, 5,000 IUs daily of vitamin D are often required to maintain optimal blood levels.


Our body will automatically regulate how much vitamin D it produces from sunshine. Studies show that sunbathing in the summer for about 30 minutes can result in the production of over 10,000 IU of vitamin D and that once our skin makes about 20,000 IUs, the ultraviolet light begins to degrade the vitamin so that we do not “overdose” on the sun.

Vitamin D absorption from the sun can be affected by a number of factors.

  • Complete cloud cover can reduce ultraviolet energy (UV) B by 50%
  • Shade (including that produced by severe pollution) can reduce it by 60%
  • Ultraviolet B radiation does not penetrate glass
  • Sunscreens with a sun protection factor of 8 or more appear to block vitamin D
  • People over 50 synthesize vitamin D less efficiently and their kidneys are less able to convert vitamin D to its active hormone form
  • People with darker skin produce less vitamin D from sunlight than people with lighter skin
  • Obesity causes some vitamin D to sequester in the fat and prevents it from circulating in the body

People suffering from gastrointestinal problems may have difficulty absorbing vitamin D supplements and prefer to use a sunlamp.


The designation of "USP" or "CL" on a label helps to guide us to a quality supplement. Supplements that bear The United States Pharmacopeia (USP) or Consumer Lab (CL) abbreviations meet voluntary industry standards for quality, purity, and tablet disintegration or dissolution. But how does vitamin D absorption compare in the different types of supplements?


Capsules tend to be “purer” than tablets because they do not have excipients, lubricants, or other ingredients used to aid the manufacturing process. Most capsules also disintegrate rapidly in the stomach and release their contents quickly while tablets may dissolve incompletely or not dissolve at all. Generally speaking, capsules are a better choice than tablets.


Flaxseed and olive oil are often added to vitamin D softgel and drops in order to improve absorption. Caution must be taken when putting vitamin D drops in a drink or on food as they may adhere to the side of the container and escape ingestion.


Many brands of cod liver oil go through a process that removes all of the natural vitamins including vitamin D... although some manufacturers restore vitamins to the final product. While manufacturers argue that their oils provide more absorbable nutrients because of the high vitamin A content, this is widely disputed as vitamin A deficiency is rare in developed countries. During the winter months in northern regions several tablespoons of cod liver oil would be required each day to maintain adequate blood levels.


As a fat-soluble vitamin, vitamin D absorption is improved when taken with meals or with a snack that includes some dietary fat. The Vitamin D Council advises that a number of co-factors also help the body to use vitamin D properly. These include:

Magnesium is the most important of these co-factors as raising vitamin D intake may worsen an underlying magnesium deficiency. In fact, problems with vitamin D supplements may be a sign of magnesium deficiency, which is common in North America.

For information on supplements that can support vitamin D absorption and also help to prevent osteoporosis... visit Best Calcium.