Vitamin D has several important functions. It helps to regulate the amount of calcium and phosphate in the body, which in turn keep bones and teeth healthy. Recent research suggests it may have other benefits, too, such as protecting against colds and fighting depression.
According to the Institute of Medicine (IOM) most people get enough vitamin D from a balanced diet and summer sunshine. Those at risk of not getting sufficient vitamin D are children under 5yrs, people over 65yrs, pregnant & breastfeeding women, people who are not exposed to much sun – such as people who cover up their skin when outdoors, or those who are housebound or confined indoors for long periods, and finally people who have darker skin, such as those of African, African-Caribbean and South Asian origin.
We can obtain it from foods including oily fish (e.g. sardines and salmon), meat, eggs and fortified cereals & some powdered milks. In the UK, cows’ milk is generally not a good source of vitamin D because it isn't fortified, as it is in other countries.
During the summer months we make vitamin D under our skin when we are exposed to sunlight. The amount of vitamin D synthesised varies from person to person. Those with darker skin my require longer in the sun than those with lighter skin. In the UK there is not sufficient UV from November to March for vitamin D synthesis.
A lack of vitamin D can lead to bone deformities such as rickets in children, and bone pain and tenderness as a result of a condition called osteomalacia in adults.
In excess vitamin D can be harmful (vitamin D toxicity). This is because it is a fat soluable vitamin, so we find it hard to excreet exesses in our body. This causes high levels of calcium in the blood (hypercalcemia) which can be deposited in and damage the kidneys. Excessive intake of vitamin D can also encourage calcium to be removed from bones, which can soften and weaken them.
Vitamin D toxicity usually happens if you take 40,000 IU per day for a couple of months or longer, or take a very large one-time dose. A blood test to measure your 25(OH)D levels can tell you whether your vitamin D levels are too high.
As a guide, if you take vitamin D supplements, do not take more than 25 micrograms (0.025mg) a day.