howtosupplement
Deficiency/Longevity/Immunity

Vitamin D

Fat-soluble vitamin that corrects low vitamin D status and supports bone and immune health in adults with low sun exposure.

Vitamin D

Vitamin D

95
score
A
evidence
Caution
risk
Use with caution

This supplement may interact with medications, medical conditions or sensitive populations. Review safety before use.

Quick Take

Worth it if you get little sun or test low — strong for deficiency correction, modest beyond that.

Vitamin D is a fat-soluble vitamin made in skin after sun exposure and found in fatty fish, egg yolks, fortified dairy, and UV-exposed mushrooms. In the body it is converted to calcitriol, a hormone-like compound that regulates calcium and phosphorus balance and gene signaling in bone, muscle, and immune cells. Best-supported uses are correcting low vitamin D status, supporting bone and muscle function, and modestly lowering respiratory infection risk. It helps most people with low sun exposure or low blood levels.

Proven Benefits

01
Corrects vitamin D deficiency
02
Supports bone health
03
Reduces respiratory infection risk
04
Improves muscle function
05
May reduce depressive symptoms
06
May lower CRP and IL-6
07
May improve insulin sensitivity

Protocol

Amount
1000-2000 IU
Frequency
Once daily
When
With a meal containing fat to improve absorption.

Onset Time

2-4 weeks if deficient; 8-12 weeks for blood level change

Who Should Consider

Adults with little midday sun exposure
People with darker skin at higher latitudes
Adults 50+ who spend most time indoors
People with overweight or obesity
People with confirmed 25(OH)D < 30 ng/mL

Food Sources

  • Salmon or sardines (~300-600 IU per 100 g)
  • Cod liver oil (~1300 IU per teaspoon)
  • Egg yolks (~40 IU each)
  • Fortified milk (~100-120 IU per cup)
  • Fortified plant milk (~100-150 IU per cup)
  • UV-exposed mushrooms (~400 IU per 100 g, mostly D2)

How It Works

Vitamin D is converted in the liver to 25(OH)D and then in the kidney and other tissues to calcitriol. Calcitriol binds the vitamin D receptor to increase intestinal calcium and phosphorus absorption and alter gene expression in bone, muscle, and immune cells.

Updated 5/19/2026