vitamin d3

Vitamin D3 (cholecalciferol) is one of the two forms of vitamin D used in supplements; the other is D2 (ergocalciferol). The NIH Office of Dietary Supplements describes vitamin D as a fat-soluble nutrient the body needs to absorb calcium, a key building block for strong bones, with the muscles, nerves, and immune system also depending on it. The body makes vitamin D when bare skin is exposed to sunlight, and per the NIH both supplement forms raise blood levels, though D3 might raise them higher and keep them up longer than D2.

The strongest evidence is for bone health: vitamin D helps the body absorb calcium, and a long-term shortage of both can lead to osteoporosis. Even here the NIH is measured, noting that vitamin D plus calcium supplements only "slightly" increase bone strength in older adults and that "it's not clear whether they reduce the risk of falling or breaking a bone." For most other conditions the NIH is explicitly negative or uncertain: it reports that vitamin D supplements "do not reduce the risk of developing heart disease," "does not prevent or ease symptoms of depression," "does not improve blood sugar levels" in type 2 diabetes, and that taking the supplement "does not help you lose weight." On cancer, the NIH says vitamin D "does not seem to reduce the risk" of cancer of the breast, colon, rectum, or lung; the effect on prostate cancer risk or survival is unclear; and "more research is needed." The NIH adds two cautions: trials suggest supplements "might slightly reduce your risk of dying from" cancer, while very high blood levels "may even increase the risk of pancreatic cancer."

The NIH lists average recommended daily amounts (from all sources combined) of 10 mcg (400 IU) from birth to 12 months, 15 mcg (600 IU) for ages 1 to 70 including during pregnancy and breastfeeding, and 20 mcg (800 IU) for adults 71 and older. Because vitamin D is fat-soluble, the NIH notes it is absorbed best when taken alongside a meal or snack containing some fat. Talk to your health care provider before starting a supplement; they may check your blood level first, and to treat a deficiency they might recommend doses above the usual upper limits for a limited period.

The NIH sets tolerable upper intake limits (from all sources combined) of 100 mcg (4,000 IU) per day for everyone aged 9 and older, including during pregnancy and breastfeeding, with lower limits for younger children and infants. Per the NIH, sun exposure does not cause toxicity because the skin self-limits how much vitamin D it produces; harmful excess is almost always the result of taking too much from supplements. The NIH notes that in the United States most people have adequate blood levels, but almost one in four has levels that are too low or inadequate for bone and overall health.

At recommended intakes vitamin D is generally well tolerated. The NIH warns that very high blood levels (greater than 375 nmol/L or 150 ng/mL), almost always from over-supplementing, can bring on nausea, vomiting, muscle weakness, confusion, pain, loss of appetite, dehydration, excessive urination and thirst, and kidney stones. Extremely high levels can cause kidney failure, irregular heartbeat, and even death.

The NIH flags several medication interactions. The weight-loss drug orlistat (Xenical, alli) can reduce how much vitamin D the body absorbs; cholesterol-lowering statins such as atorvastatin, lovastatin, and simvastatin might not work as well alongside high-dose vitamin D; steroids such as prednisone can lower vitamin D blood levels; and thiazide diuretics taken with vitamin D could push blood calcium too high. Tell your doctor and pharmacist about every supplement and medicine you take so they can check for interactions.