Vitamin D deficiency is common, but a review [1] of hundreds of well-done studies suggests that vitamin D supplements are generally not helpful for problems other than bone health and respiratory illness.  Current data suggest that:
In most circumstances we should not take more than 1,000–2,000 units of vitamin D3 dailyBlood level of vitamin D should be about 20-40 ng/ml; higher levels are associated with harm The authors of the review point to evidence indicating that low vitamin D levels are the consequence, not the cause, of ill health.  This is particularly concerning in light of the evidence that high blood levels of vitamin D seem to increase risk of death, falls, and cancer.  
 My recommendation:
Get vitamin D through natural sources rather than supplements.  A blood level of 20–40 ng/ml is a reasonable target, but I only recommend supplementing with 1,000–2,000 IU of vitamin D3 daily if you are not able to get enough vitamin D from natural sources like oil-rich fish (salmon, tuna, sardines, etc.) and non-burning sunlight on as many square inches of bare skin as is socially acceptable.  Mushrooms which have been exposed to ultraviolet light for a few hours may emerge as another useful vitamin D supplement.  Despite decades of research it is still not known if taking vitamin D supplements will provide the same benefits, or if people who naturally have higher blood levels of vitamin D are healthy for some other reason.  For more detail and some useful general resources on nutritional supplements, see page 89 of Good Food, Great Medicine, 3rdedition – and then check out Salmon Cakes(page 227), Tuna and White Bean Salad(page 232), and Sardine Pâté (page 152) for a few of our favorite ways to improve vitamin D levels! 

Miles Hassell MD 
[1] Autier, P. et al. Lancet Diabetes Endocrinol 2017;5:986-1004