I am not a big fan of supplements. Unless you have an acknowledge deficiency or a restricted diet lacking of specific nutrients, I tend to believe that a varied diet should be able to provide you with all the nutrients your body needs... except for vitamin D. And this is because our main source of vitamin D is not dietary - it comes from the sun. And in the UK, comes October and suddenly the sun is not strong enough for our body to synthesise vitamin D from it anymore, and thus until April.
This is why this summer the SACN (Scientific Advisory Community on Nutrition, UK) made new recommendations, advising everyone in the UK , children and adults alike to take vitamin D supplement from October to April to a dosage of 10 micrograms a day (400 IU).
These new recommendations have been welcomed by the scientific community even if some pointed out that the dosage was on the conservative side, and that bigger dose would be safe and might be desirable*.
Vitamin D is an essential compound that our body requires to fix calcium and build strong bones and muscle. It is also suspected to play other beneficial roles in various functions of the body although strong evidence is currently missing (e.g., immune system, incidence of cancer, asthma, depression).
If you are interested in finding more about vitamin D and why we need to take a supplement, have a look at this excellent fact sheet from the BDA (Association of UK Dietitians).
*Although hypervitaminosis D is rare, it can happen with supplement intake. A safe upper limit for vitamin D supplement intake is 4,000 IU or 100mcg a day (vitamin D intoxication were observed in patient with above 250mcg/day intake)