Essential vitamin · D2 and D3 · Food literacy
Vitamin D
A vitamin with a second life.
Vitamin D supports calcium absorption and normal bone mineralization. [1]
An egg, fortified milk and UV-treated mushrooms lead into a story about different chemical forms and different kinds of evidence. Measuring vitamin in a food, a blood marker or a clinical outcome answers three different questions.
Meet the family
Nutrient first, hormone precursor too.
D2 is ergocalciferol; D3 is cholecalciferol. These are distinct forms of a fat-soluble vitamin. The body converts vitamin D first in the liver to 25-hydroxyvitamin D, then mainly in the kidneys to the active form, 1,25-dihydroxyvitamin D. The latter, also called calcitriol, acts as a hormone. It is not another name for dietary D3. [1]
Vitamin D supports calcium absorption and normal bone mineralization.
On the plate
Naturally present or added?
Fatty fish supply vitamin D naturally; egg yolks contain smaller amounts. Mushrooms provide variable D2, increased by UV treatment. Many US milks and plant drinks are fortified, but products differ. Check the actual label, including for yogurt or kefir. Fermentation alone does not establish fortification. [1]
Original food research
The cooking method belongs in the story.
In a 2018 experiment by Ložnjak and Jakobsen, researchers measured vitamin D after household-style cooking. They tested D2 and D3 added to sunflower oil, D3 in rainbow trout, and D2 in button mushrooms. The methods included boiling, steaming, microwaving, pan-frying and oven baking. [2]
Mushrooms retained 62–88% of their D2 under the tested conditions. Trout generally showed no statistically significant loss, except with pan-frying, where retention was 85%. D2 and D3 did not differ significantly in retention in the oil model.
What retention measurements can establish
This was a food-composition experiment, not a human absorption study. Its percentages cannot be applied to every recipe or used to calculate a meal's vitamin D without knowing its starting content. “Cooked” is not one uniform treatment.
Original human research
A fortified food can change a blood marker.
Tripkovic and colleagues' 2017 trial enrolled 335 healthy South Asian and white European women aged 20–64 in the UK. During winter, participants were randomly assigned to placebo or 15 micrograms daily of D2 or D3, delivered in specially prepared juice or biscuits for 12 weeks. Neither participants nor researchers knew the assignments. [3]
The D3 groups had a greater increase in total blood 25-hydroxyvitamin D than the D2 groups. That is evidence about improving a vitamin-status marker in this population and these preparations.
How far the fortified-food result reaches
It is not evidence that every D3-containing food improves skin, prevents disease or outperforms every D2 product. Nor does the experiment establish the vitamin content of an ordinary biscuit or juice. The fortification and the measured endpoint are essential parts of the finding.
A different question
More intake did not mean fewer fractures.
The 2022 VITAL fracture study followed 25,871 US adults, men aged 50 or older and women aged 55 or older, for a median 5.3 years. Daily D3 at 2,000 IU was compared with placebo. Participants were not selected for vitamin D deficiency, low bone mass or osteoporosis. [4]
There was no significant reduction in total, nonvertebral or hip fractures. For total fractures, the hazard ratio was 0.98, with a 95% confidence interval of 0.89–1.08: the results were compatible with no difference.
Why a necessary nutrient is not a guaranteed extra benefit
This answers a prevention question in generally healthy adults. It does not test treatment of established deficiency. A nutrient's necessary biological role and the benefit of adding more are separate questions; the study dose is not a recommendation.
Read the label
One amount, two units.
1 microgram (mcg) of vitamin D = 40 international units (IU). [1]
For a unit conversion, 5 micrograms equals 200 IU. These are two descriptions of the same amount, not quantities to add. The conversion changes the unit; it does not measure how much is absorbed or predict an outcome.
- Food question
- What amount is present in this actual serving?
- Study question
- Was the endpoint retention, blood status or a health outcome?
- Product question
- Does the evidence test this finished preparation?
Keep the boundaries clear
Food literacy, not a sun prescription.
Skin can make vitamin D, but UV exposure also raises skin-cancer risk. CDC points to foods and supplements as sources that avoid that UV risk. Keep sun protection in place rather than using intentional exposure as a vitamin strategy. [6]
Excessive supplemental vitamin D can cause dangerously high blood calcium. [1]
Sources to follow
Revised October 25, 2026. Sources below distinguish chemical identity, food composition and the particular preparations or populations studied. Research methods and interpretation remain alongside the findings.
