Vitamin D (D2 + D3)

Vitamin D (D2 + D3) is a fat-soluble nutrient the body needs to absorb calcium and phosphorus and build strong bones. Unlike most vitamins, the body can also manufacture it in the skin from sunlight exposure, which is why it is sometimes described as a hormone rather than a classic dietary vitamin.

What is vitamin D (D2 + D3)

This value combines vitamin D2 (ergocalciferol, found in some mushrooms and fortified foods) and vitamin D3 (cholecalciferol, found in animal foods and made in human skin) — the two forms the body converts into active vitamin D. Per the Harvard T.H. Chan School of Public Health, The Nutrition Source, vitamin D's best-established role is helping the body absorb and retain calcium and phosphorus for bone mineralization; it also has receptors in many other organs, which researchers are still studying for additional roles in immune and cell function. Very few foods naturally contain meaningful amounts of vitamin D — fatty fish such as Atlantic salmon, swordfish, and eel, along with egg yolks and fish liver oils, are the main natural sources. Most of the vitamin D in the U.S. diet instead comes from fortified milk, plant-based milks, cereals, and (per Harvard) UV-exposed mushrooms that naturally produce D2. Harvard also notes that D3 supplements raise and sustain blood vitamin D levels more effectively than D2.

Health benefits of vitamin D (D2 + D3)

• Bone health. Harvard's Nutrition Source reports that vitamin D supplementation in the range of about 500–800 IU/day has been shown to reduce hip and non-spine fractures by roughly 20% in older adults in some trials, though results across studies are mixed.
• Cancer survival. The same source notes that adequate vitamin D status has been associated with improved survival odds if cancer develops, even though supplementation has not consistently been shown to prevent cancer from occurring in the first place.
• Autoimmune disease risk. Harvard cites the VITAL randomized trial, which found vitamin D supplementation reduced the incidence of autoimmune diseases by about 22% compared with placebo.
• Type 2 diabetes. Harvard reports one study finding a 33% lower risk of type 2 diabetes in women with the highest combined calcium-and-vitamin-D supplement intake versus the lowest.
• Immune function. Harvard notes research suggesting a possible protective association between adequate vitamin D and multiple sclerosis, type 1 diabetes, and respiratory infections, while describing the evidence overall as inconsistent rather than conclusive.

How much vitamin D you need

Per Harvard's Nutrition Source, the RDA for adults ages 19–70 is 15 mcg/day (600 IU), rising to 20 mcg/day (800 IU) for adults over 70. The Tolerable Upper Intake Level (UL) is 100 mcg/day (4,000 IU) for adults and children 9 and older; Harvard's Nutrition Source specifically advises against exceeding 4,000 IU/day without medical supervision. These figures are consistent with the NIH Office of Dietary Supplements' Vitamin D Health Professional Fact Sheet, which corroborating searches show sets RDAs in the 15–20 mcg (600–800 IU) range for adults and 10–15 mcg (400–600 IU) for infants, children, and adolescents depending on exact age.

Signs of deficiency and excess

Deficiency: prolonged inadequate vitamin D causes rickets in children — soft, weakened bones and skeletal deformities such as bowed legs — and osteomalacia in adults, a softening of bone that is generally reversible with adequate vitamin D, per Cleveland Clinic. Adults with deficiency may also experience fatigue, bone pain, muscle weakness or cramps, and mood changes, though Cleveland Clinic notes many adults with low vitamin D have no symptoms at all. People over 65, people with darker skin tones, those with limited sun exposure, exclusively breastfed infants, and people with conditions such as cystic fibrosis, Crohn's disease, celiac disease, obesity, or kidney or liver disease are at higher risk, per Cleveland Clinic.

Excess/toxicity: vitamin D toxicity almost always results from excessive supplement or prescription intake rather than food or sun exposure, per Cleveland Clinic. Because vitamin D increases calcium absorption, toxicity presents mainly through elevated blood calcium: nausea, vomiting, increased thirst and urination, confusion, fatigue, muscle weakness, bone pain, and kidney stones. Harvard's Nutrition Source adds that severe, sustained excess can harden blood vessels and soft tissue from calcium buildup, potentially damaging the heart and kidneys.

Warnings & interactions

Per Cleveland Clinic, several medications can lower vitamin D status or affect its use in the body, including certain laxatives, steroid medications, cholesterol-lowering drugs, seizure medications, rifampin, and orlistat (a weight-loss drug). Separately, the NIH Office of Dietary Supplements' Vitamin D fact sheet (linked above; corroborated via search) describes interactions in both directions: steroids such as prednisone can lower blood vitamin D levels, high-dose vitamin D supplements may reduce the effectiveness of certain statins (including atorvastatin, lovastatin, and simvastatin), and orlistat can reduce vitamin D absorption from food and supplements. Cleveland Clinic also cautions that combining vitamin D supplements with vitamin-A-containing fish oil products should be done carefully to avoid excess vitamin A intake. People with kidney disease, liver disease, or conditions requiring high-dose vitamin D therapy (such as osteomalacia or malabsorption disorders) face higher toxicity risk and should have their vitamin D managed by a clinician rather than self-supplementing, per Cleveland Clinic.

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