Here is the complete guide to vitamin D, a unique hormone-vitamin whose management is crucial, especially at our latitudes and for those following a plant-based diet.

The complete guide to vitamin D: the hormone of sunshine and immunity

Vitamin D is a special case in nutrition. Unlike other vitamins, our diet is not supposed to be our primary source: it is our skin that produces it under the action of the sun’s UVB rays.

For vegetarians and vegans, vitamin D is a major point of attention. It is naturally found in fatty fish and eggs, but very little in the “classic” plant kingdom. Yet it is the conductor of your bone and immune health. How can you ensure optimal levels, especially in winter? This guide provides all the solutions.

What does vitamin D actually do in your body?

Now considered a true hormone, vitamin D is involved in hundreds of genetic processes:

Calcium absorption: This is its number one mission. Without it, the calcium you eat (even in large amounts) is not absorbed by the intestine and cannot be fixed onto your bones.
Immune system support: It activates T cells (the “soldiers” of your immune system) that fight viruses and bacteria. A lack of vitamin D is often associated with increased susceptibility to winter infections.
Muscle health: It prevents muscle weakness and improves balance, reducing the risk of falls and fractures.
Mood regulation: Vitamin D receptors are present in areas of the brain involved in depression. It plays a role in serotonin synthesis.

Nutritional needs: the challenge of lack of sunlight

In France, it is estimated that nearly 80% of the population is deficient in vitamin D at the end of winter.

Recommended nutritional intake (RNI)

Profile	Recommended daily requirement
Adult (18–70 years)	15 µg (600 IU)
Over 70 years old	20 µg (800 IU)
Infant	10 µg (400 IU)

The geographical factor: From October to March, above the latitude of Bordeaux, the sun’s rays are too low in the sky for the skin to synthesize vitamin D, even in clear weather. During these months, intake must come exclusively from food or supplements.

Vitamin D2 or D3: what difference for vegans?

There are two forms of vitamin D, and their origin matters:

Vitamin D2 (Ergocalciferol): It is always plant-derived (mushrooms, yeast). It is effective, but some studies suggest it maintains blood levels for a shorter time than D3.
Vitamin D3 (Cholecalciferol): Long extracted only from lanolin (sheep wool), it now exists in a 100% plant-based form derived from northern lichen. It is the most bioavailable form and the most recommended for vegans.

The rare plant-based sources of vitamin D

The plant kingdom is naturally poor in vitamin D, with the exception of a few specific sources:

UV-exposed mushrooms

Some mushrooms (button mushrooms, oyster mushrooms) contain vitamin D2. If you expose them to sunlight (gill-side up) for 30 minutes before consuming them, their vitamin D content increases dramatically!

Fortified products

This is the most reliable daily source for vegans. Many plant-based milks (soy, rice, almond), plant yogurts, and margarines are fortified with vitamin D2 or plant-based D3. One glass of fortified plant milk provides around 1.5 to 2 µg of vitamin D.

Top 5 plant-based sources of vitamin D

Food	Serving	Vitamin D content (µg)
1. Supplement (Lichen)	1 drop/capsule	10 to 25 µg (depending on dosage)
2. Button mushrooms (UV exposed)	100 g	~10 to 20 µg
3. Fortified plant milk	250 ml	~3.5 µg
4. Fortified soy yogurt	125 g	~1.5 µg
5. Fortified plant margarine	10 g	~0.75 µg

Signs of deficiency: often silent signals

Vitamin D deficiency does not cause immediate pain, but develops gradually:

General fatigue: A persistent feeling of physical and mental exhaustion.
Bone and muscle pain: Constant “aches” or bone fragility (osteomalacia).
Seasonal depression: Mood decline related to lack of light and vitamin D.
Poor dental health: Fragile enamel and sensitive gums.

FAQ: your common questions about vitamin D

Should you supplement all year round?

For most people living in Europe, supplementation from October to April is essential. People with darker skin (who synthesize vitamin D more slowly), older adults, or those who always work indoors may need supplementation all year round.

What is the ideal supplemental dose?

The safe dose for adults generally ranges between 1000 IU (25 µg) and 2000 IU (50 µg) per day. It is preferable to take a small daily dose rather than a mega-dose (ampoule) every three months, as absorption is more stable and physiological.

Is vitamin D better absorbed with fat?

Yes! Like vitamins A, E, and K, vitamin D is fat-soluble. Take your supplement or fortified foods during a meal containing some oil, avocado, or nuts for optimal absorption.

Does tanning bed use help produce vitamin D?

No. Tanning beds mainly use UVA rays, whereas vitamin D synthesis requires UVB. In addition, the risks to the skin (cancer, premature aging) far outweigh any potential benefits.

Scientific references and sources

ANSES: Nutritional references for vitamin D (Updated 2021).
EFSA (European Food Safety Authority): “Scientific Opinion on Dietary Reference Values for vitamin D”.
Academy of Nutrition and Dietetics: “Vitamin D and Vegetarian Diets”.
Journal of Clinical Endocrinology & Metabolism: “Evaluation, Treatment, and Prevention of Vitamin D Deficiency”.
The Lancet Diabetes & Endocrinology: Studies on the benefits of vitamin D for the immune system.