What it does
The NIH consumer sheet says vitamin D helps the body absorb calcium, one of the main building blocks for strong bones. It also says muscles need vitamin D to move, nerves need it to carry messages between the brain and the body, and the immune system needs it to fight off invading bacteria and viruses.
What research can and cannot say
The sheet separates getting enough vitamin D from what a finished bottle does. It says vitamin D and calcium supplements slightly increase bone strength in older adults, and that it is not clear whether they reduce the risk of falling or breaking a bone. Both supplement forms raise vitamin D in the blood. The sheet says D3 might raise it higher and for longer than D2.
A vitamin D label does not show that a product changes mood, bone health, or any other outcome. This page does not turn a bottle into a recommendation. A blood level is a lab result, not a label claim, and this page does not read your level.
Forms and label reading
The two supplement forms are D2 (ergocalciferol) and D3 (cholecalciferol). A label that only says vitamin D still uses one of the two. Read which form the Supplement Facts panel names. Labels may print mcg or IU. On the NIH sheet, 1 mcg of vitamin D is 40 IU, so 15 mcg is 600 IU. The sheet says vitamin D is fat soluble and is taken with a meal or snack that includes some fat.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Amount and upper limit
Recommended amount. The NIH sheet lists 15 mcg (600 IU) a day for people 1 to 70 years old, and 20 mcg (800 IU) for people older than 70. Cited sheet
Upper limit. The sheet lists 100 mcg (4,000 IU) a day as the upper limit for adults, counting vitamin D from food, drinks, and supplements. Cited sheet
Safety
The sheet says too much vitamin D can be harmful. Very high blood levels can cause nausea, vomiting, muscle weakness, confusion, and kidney stones, and high levels are almost always from getting too much from supplements. You cannot get too much vitamin D from sunshine. The upper limit is on this page so it is not hidden behind the source link. The sheet also names medicines that can interact, including steroids and some weight-loss and cholesterol medicines.
Food sources
The sheet names fatty fish such as trout, salmon, tuna, and mackerel, fortified milk and breakfast cereals, and mushrooms. It also says skin makes vitamin D when bare skin is in sunlight. Sun exposure is not a dose.
