Along with vitamins A, E, and K, vitamin D is one of the fat-soluble vitamins. Vitamin D is actually more of a hormone, acting at distant receptors (the vitamin D receptor or VDR) and when combined with retinoid acid X receptors (RXR) this complex can interact with DNA at vitamin D responsive elements (VDRE) thus changing transcription of genes. It is in these ways that vitamin D is important for cell differentiation in the immune system and being important for the development of T-cells.
Vitamin D3 enters the body from the diet or the skin, where 7-dehydroxycholesterol in converted by the UVB of sunlight to vitamin D3. Vitamin D3 is converted to 25-hydroxy-D3 (calcidiol), in the liver. This is the major circulating form of Vitamin D3. 25-OH-D3 is then converted in the kidneys to 1,25 hydroxy D3 (calcitriol) the active form of vitamin D that increases calcium absorption from the intestines and released calcium from bones. The parathyroid glands also sense when calcium is low and release PTH (parathyroid hormone) which stimulates this activation of vitamin D into the active 1,25-OH-D3 in the kidneys. Thus the common known function of vitamin D being that for normal bone formation.
SYMPTOMS OF DEFICIENCY
Rickets is the classical disorder of low vitamin D. There is a failure of bone mineralization, and thus growth. Those bones growing most rapidly are affected the most, resulting in bowed legs in infants and soft bones (osteomalacia) in adults and children which can result in bone pain and the obvious lack of mineralization.
There is an increasing rate of cancers (prostate, colorectal, and breast) and autoimmune disorders (insulin dependent diabetes or IDDM, multiple sclerosis, and rheumatoid arthritis) in populations that have less sunlight (northern latitudes and darker skinned individuals). Those with minimal sun exposure are at the highest risk as there is virtually no vitamin D in food unless fortified or taken as a supplement. Infants tend to not be in the sun, the elderly often avoid sun, and some individuals wear clothing or sun screen to prevent sun exposure. Cystic fibrosis and fat malabsorption conditions and inflammatory bowel disease all reduce the absorption of vitamin D resulting in deficiencies.
IU Vit D Micrograms
Cod liver oil (tsp) 450 11
Herring (3 oz) 760 19
Salmon (3 oz) 425 10.6
Sardines 93 oz) 255 6.4
Milk- fortified (8 oz) 100 2.5
Shrimp (3 oz) 90 2.3
Egg yolk 25 0.6
I have found that infants need at least 1000 IU and adults 3-5,000 IU daily. This would mean drinking over a gallon of milk for the infant and many gallons for an adult and that is just not possible!
This is the one vitamin/nutrient that everyone (except perhaps a lifeguard or laborer with their shirts off in the tropics) should take as a supplement.
SAFETY AND TOXICITY
As a fat-soluble vitamin, it is possible to take toxic amounts and develop hypervitaminosis D. The symptoms of this seem to be related to the elevated calcium that results; nausea and vomiting, loss of appetite, thirst, excessive urination, itchiness, joint pain and muscle weakness, and ultimately, disorientation and coma. Supplementation at doses up to 10,000 IU daily for years has not resulted in any toxicity. As I reviewed panels of vitamin D experts, all seem to feel that up to 8,000 IU daily is safe for adults. I put that limit at 5000 unless you have been taking 5000 IU daily consistently and still do not have your vitamin D at optimal levels (50-80 ng/dl).
TOLERABLE UPPER LIMITS
Set at 1000 IU daily for infants and 2000 IU daily for children and adults. I am completely comfortable with the following upper limits:
Infants 40 lbs1000 IU / day
40-80 lbs 2000 IU / day
80-120 lbs 3000 IU / day
Adults 4-5,000 IU / day
If you have been taking Vitamin D supplements for over a year at the higher levels, get your level tested.
Here are some of my previous blogs on Vitamin D: