How much of vitamin D?

Vitamin D is a steroid, fat-soluble vitamin that encourages metabolism and absorption of phosphorous and calcium. Vitamin D is naturally present in some dietary food source and is found as a dietary supplement. Vitamin D is known to produce in sufficient amount when sun’s ultraviolet rays strike the skin and synthesise Vitamin D. Vitamin D that are derived form food,, sun exposure and supplements is biologically inert and undergo two hydroxylations within body for activation. Hydroxylations first occurs in liver, they convert vitamin D into calcidiol or 25-hydroxyvitamin D [25(OH)D]. Kidney is the second body part to show hydroxylations forming 1,25-dihydroxyvitamin D [1,25(OH)2D] or calcitriol. Five forms of vitamin D has been discovered till date from D1 to D5. The most two forms of vitamin utilised by humans are vitamin D2 and vitamin D3. As per researchers at the University of Minnesota, presence of vitamin D level in the body starts with low-calorie diet.

Recommended daily allowance (RDA) of vitamin D is 400 IU and doctors recommend use of nutritional supplements that contain vitamin D, especially in winter when vitamin D deficiency in the body is considerable. If proven deficiencies, children will be faced with rickets and adults with osteoporosis or osteomalacia. A deficiency can lead to symptoms: loss of appetite, diarrhea, insomnia, vision problems, weight loss.

Most people in the U.S. and Canada — from age 1 to age 70 — need to consume no more than 600 international units of vitamin D a day to maintain health, the report found. People in their 70s and older need as much as 800 IUs. The report set those levels as the "recommended dietary allowance" for vitamin D.

The easiest way is to get your 25(OH)D vitamin D level checked with a postal blood spot test. UK readers can get them done for 25 or 30 international from CityAssays Birmingham (UK) NHS PATH LAB. Most people will find it takes 1000iu for each 25lbs they weigh. So somewhere between 5000~10,000iu daily. You need to check 25(OH)D after 3~6months to be sure you are taking sufficient.
Ideally around 50ng/ml or 125bnol/l is the target to aim for.