Vitamin D3

What it is

Vitamin D is made in your skin from sunlight and found in a few foods. It acts more like a hormone than a vitamin, helping you absorb calcium and supporting immune and muscle function. Low levels are common, especially in winter, at higher latitudes, and with limited sun exposure or darker skin.

How it works

Vitamin D acts more like a hormone than a vitamin. Made in your skin from sunlight or absorbed from food, it's converted by the liver and kidneys into an active form that tells your gut to absorb calcium and phosphorus, helps regulate bone remodeling, and influences immune-cell behavior. Receptors for it sit on tissues throughout the body — which is why low levels have been linked to many systems, even though correcting a genuine deficiency is where the clear benefits lie.

What it's been studied for

Used to support immune defenses, bone strength (with calcium), muscle function, and overall health, particularly when levels are low.

What the evidence shows

Pooled trial data show supplementation modestly reduces the risk of respiratory infections, with the biggest benefit in people who were deficient. With calcium, it supports bone health and fracture reduction in at-risk groups. Benefits are clearest for correcting a genuine deficiency rather than adding more on top of adequate levels.

Who should consider it

Low vitamin D is common, so many people benefit — especially those with limited sun exposure, who live at higher latitudes or spend most of the day indoors, have darker skin (melanin reduces skin production), are older (skin makes less), carry more body fat (which sequesters vitamin D), or have conditions that impair fat absorption. Testing your blood level (25-hydroxyvitamin D) is the most rational way to know if and how much you need.

Amounts commonly studied

Common maintenance doses are 1,000-2,000 IU (25-50 mcg) per day; higher short-term doses are sometimes used to correct a deficiency under guidance. Testing your level takes the guesswork out.

How to take it

A common maintenance dose is 1,000-2,000 IU (25-50 mcg) of D3 daily; a deficiency is sometimes corrected with higher short-term doses under guidance. Because it's fat-soluble, take it with your largest meal. It's often paired with vitamin K2 and works alongside adequate calcium and magnesium. If you started out deficient, retest after a few months to confirm you're in range.

Forms & absorption

D3 (cholecalciferol) raises blood levels more effectively than D2. It's fat-soluble, so take it with a meal containing some fat. Often paired with vitamin K2.

Food sources

Few foods contain much: fatty fish (salmon, sardines, mackerel), cod liver oil, egg yolks, and UV-exposed mushrooms. Milk, some yogurts, orange juice, and cereals are often fortified. For many people, sun and food together still fall short in winter.

Safety & interactions

Safe at typical doses. Because it's fat-soluble, very high doses over time can build up and raise calcium levels, so don't megadose without testing and guidance.

Common questions

D3 or D2?

D3 (cholecalciferol) raises and maintains blood levels more effectively than D2 (ergocalciferol), so it's the preferred supplemental form.

Do I need vitamin K2 with it?

They're commonly paired on the idea that K2 helps steer the calcium D helps absorb into bone rather than arteries. It's reasonable, though the long-term outcome evidence is still developing.

What blood level should I aim for?

Most labs consider 20-50 ng/mL sufficient. It's worth reviewing your target with your provider rather than chasing very high numbers.