Vitamin D is one of the fat soluble vitamins, and along with other fat soluble vitamins, it is possible to take too little, but it is also possible to take too much. And taking too much can have negative health effects, just as taking too little can.
For many of us we get enough vitamin D naturally from exposure to the sun (roughly 15 minutes of exposure to the face, arms and hands twice a week without sunscreen is enough) and from food (good sources are many types of fish, soybeans, soymilk, milk, mushrooms, and others).
Vitamin D intake is important for bone health (making stronger bones), preventing stroke and heart attacks, maintaining good balance in old age, and possibly preventing depression. If you are concerned about where you stand, get your doctor to check a “serum 25-hydroxy vitamin D level.” For most people levels between (roughly) 15 ng/mL and 30 ng/mL are fine. For people who are particularly worried about optimal bone health a narrower range of 24 ng/mL to 30 ng/mL is ideal.
Once you have your blood level you can figure out the right amount to take. For most people take 100 IU of vitamin D per day for every 1 ng/mL increase in blood level you want. (People who are obese may need to take as much as double this dose, since it is stored in body fat). So, for example, if your blood level is 16 ng/mL and you want to get the level to 24 ng/mL you should take 8 x 100 IU per day.
(A word of warning. vitamin D assays are not all equally good. A recent study suggested that two newer assays, made by Abbott and Siemens, may not be as good as other tests).
To be sure that you are taking the right dose, get another blood level in a month. And then just keep taking that dose. By the way, since vitamin D is stored in your body fat, you can take it less often than once a day. This is good news because it can be hard to find supplements that have the smaller amounts that most people are likely to need when taken daily. So, in the example above, where the person needed 800 IU per day, it would be fine to take 1000 IU on weekdays only (none on weekend days).
For most people it doesn’t matter which of the two forms of vitamin D you take (vitamin D2 – ergocalciferol or vitamin D3 – cholecalciferol) but if you need large amounts of the supplement, vitamin D3 is probably somewhat better.
Consumers Lab does ongoing tests of vitamin D and other supplements, and about a quarter of all supplements tested fail one of their quality standards.