Vitamin D is an essential nutrient that many people are slight short on, especially in winter time. A small vitamin D shortage does not have immediate bad effects, but in the long run it means you are slightly less healthy and have a slightly larger chance to get various diseases. Since vitamin D supplements are likely to have a small benefit, are extremely unlikely to cause harm and are cheap, everyone should just take them.
It doesn’t matter whether you take vitamin D2 or D31. The officially recommended daily dose is 15 μg/day, but
In 2011, The Endocrine Society [stated that] at least [37.5-50 μg/day] of supplemental vitamin D might be required in adults
It is known that up to 100 μg/day is safe to take1, so taking a supplement of 25 or 50 μg/day is a good idea.
Our summary of the references2,3,4 is that increasing blood levels of the hormone7 produced from vitamin D slightly reduces total mortality, including deaths due to heart disease and some cancers. Taking 25 μg a day (1000 IU) in the summer and twice as much in winter is very likely to be beneficial and very unlikely to cause harm, so why not add a small pill to your daily diet?