Vitamin D exists in its natural form 25-hydroxyvitamin, however must be converted in the kidneys, liver and skin to its active form so that the body can use it. Vitamin D is used by the body in many ways but the most known is for bone mineralization and growth. Vitamin D is required not only for calcium utilization into bone, but also for its absorption from the diet. The role of calcium in bone growth is so strongly linked that it helped lead to milk being fortified with vitamin D in the 1930’s. Vitamin D is also used in protein modulation and cell growth, an important aspect in neuromuscular development and your immune system.
The requirement of vitamin D remains about 400 IU throughout life and the recommended intake for all age groups is 600 IU’s (except those over 70 in which 800 IU’s are recommended).
-It is estimated that the average male gets 204-288 IU’s daily in his diet.
-It is estimated that the average female gets 144-276 IU’s in her diet.
Therefore it is important that vitamin D is a fat soluble vitamin that can be stored in our liver and adipose tissue and can be utilized when the diet is deficient for short periods of time or when sun exposure is limited. It is also why it is important to consume fortified foods and allow for cautious sun exposure (15 minutes twice daily). The other source of vitamin D is from our skin, where 7-dehydrogenase (a cholesterol) is converted by sunlight to 25 hydroxyvitamin D. Things that can interfere with UVB penetration are cloud cover, shade/smog, windows and sunscreen of SPF 8 or greater.
Vitamin D deficiency may be caused by:
- Decreased dietary intake
- Impaired absorption
- Increased requirements
- Increased secretion
- Amount of skin melanin (pigmentation),
- Medications (Prednisone, weight loss drugs, cholestyramine, Phenobarbital and Dilantin are the most common)
- Renal impairment or liver disease.
Vitamin D deficiency in infants and children can lead to Rickets (poor bone mineralization and bone deformities). Vitamin D deficiency in adults leads to osteomalacia (decreased bone mineralization which can lead to weakened bone), muscle weakness, bone pain, neuromuscular pain and hypocalcemia. There is also ongoing research that demonstrates an increased risk of Type 1 & 2 diabetes and some cancers (colon, breast and prostate).
It is suggested that 15 minutes of sun exposure twice daily, a proper diet under optimal circumstances (without factors which decrease vitamin D conversion or absorption) and a proper diet is all that is needed for a healthy individual. The only groups routinely recommended to supplement are infants who are breast feeding (200 IU’s daily) and the elderly . Unfortunately very few people live in ideal circumstances.
It should be noted that prolonged sun exposure may not have any benefit to increase vitamin D production because it increases the skin temperature which degrades vitamin D.
The best sources of vitamin D are fish oils, fish, fortified dairy products, fortified cereals, fortified juices, beef liver, egg yolks and raw mushrooms (go figure).
Vitamin D toxicity is rare and is estimated that an intake of 40,000 IU’s daily is needed to cause toxicity. The recommended safe intake of vitamin D supplementation is 1,000 IU’s daily (for both men and women), 1,500 IU’s (infants), 3,000 IU’s (elderly -age over 70), 4,000 IU’s (in pregnancy) and 4,000 IU’s (lactating women). If you know your diet is deficient and you do not get sunlight exposure, supplementation at the doses above are considered safe. However, you will only benefit from the supplementation if you are deficient in vitamin D. It is recommended that if you are taking a calcium supplement due to being at risk for osteoporosis or having osteopenia that the calcium should be taken with a vitamin D supplement to maximize absorption and utilization.
Routine vitamin D testing is only recommended in the elderly, people who are house/bed bound, signs of osteoporosis, renal insufficiency, individuals with malabsorption issues (Celiac disease, ulcerative colitis, Crohn’s disease) and liver disease. Talk to your primary care physician if your think you are at risk. Vitamin D testing can be done through simple blood test, however,not all insurance companies will pay for this test without symptoms, signs or risk factors.