Sources of vitamin D - what is worth eating and how to absorb it?

Differences between D2 and D3

Vitamin D occurs in two main forms: D2 (ergocalciferol) and D3 (cholecalciferol). Although both are prohormones capable of transforming into the active form of calcitriol, they differ in origin and effectiveness of action. D2 is synthesized mainly by plants and yeast under the influence of UV radiation, while D3 is formed in the skin of people and animals from 7-dehydrocholesterol, and also appears in animal products.

Studies indicate that D3 has a higher biopotency than D2-it is faster and more effectively transformed in the liver into caldene (25-hydroxyvitamine D), and then in kidney to kalcitriol (1,25-dihydroxyvitamine D). Ergocalciferol is characterized by a shorter semi -period of elimination and less ability to bind to plasma transport proteins, which results in lower circulation concentrations at comparable supplementation doses.

In clinical practice, this means that supplements and products rich in D3 better increase level 25 (OH) D in the blood than those containing D2. However, in enriched products, we often find the form of D2 - mainly for technological and cost reasons. People monitoring vitamin D levels should pay attention to the appropriate form on the label, especially if they care about faster deficiency correction or maintaining a stable level throughout the year.

To sum up, although both forms show the ability to act, D3 is a preferred form - more effective, staying longer in the body and better raising serous concentration. That is why it is worth choosing products and supplements containing cholecalciferol, especially during a period of limited skin synthesis.

Natural sources in the diet

The richest natural carriers of vitamin D3 are fatty sea fish. Wild salmon provides an average of 15-20 µg (600–800 IU) D3 per portion 150 g, mackerel 10-15 µg, and sardines and follow around 5-10 µg. Regular consumption of two portions of fatty fish per week can significantly contribute to covering daily demand, especially in the sunny season.

Tran - cod liver oil is another noteworthy source. Already one tablespoon of fish oil (15 ml) can provide 25-35 µg (1000–1400 IU) vitamin D3, which makes it a convenient complement to the diet during a period of weakened skin synthesis. However, pay attention to the cleanliness and quality of fish oil, choosing products with low heavy metal content.

Free-range chicken eggs contain approximately 1–2 µg of D3 in one egg - a lot depends on the hens' diet. Egg yolk is also a source of lecithin and choline, which supports the absorption of other nutrients. Including eggs in your diet several times a week is a simple way to enrich your diet with cholecalciferol.

Smaller amounts of D3 can be found in poultry liver (1–2 µg/100 g) and in some species of mushrooms, especially those subjected to UV irradiation. Mushrooms or shitake often appear on the shelves as "enriched with vitamin D2" - but it is worth remembering that it is ergocalciferol, less active than D3.

The effect of fat on the absorption of D

Vitamin D is fat-soluble, which means that its absorption occurs in the presence and participation of lipids. In the small intestine, vitamin D is part of micelles made of bile acids and dietary fats, and then transported in ionized form to enterocytes. Without the presence of an adequate amount of fat - especially polyunsaturated omega-3 fatty acids and monounsaturated omega-9 fatty acids - the bioavailability of vitamin D may decrease by as much as 30-40%.

To optimize absorption, it is worth consuming D3 sources with a meal containing vegetable or animal fats. For example, salmon salad with olive oil or avocado will improve the absorption of cholecalciferol. Similarly, a free-range omelette with clarified butter or eggs cooked in the context of full-fat natural yogurt will provide better vitamin D availability than low-fat meals.

Studies have shown that D3 supplements taken without fatty foods have a lower effectiveness in raising 25(OH)D levels by approximately 20–25%, which emphasizes the importance of consumption with a full-fat meal. Even a bit of fat, e.g. 5-10 g of olive oil, nuts or avocado oil, is enough to ensure optimal conditions for absorption.

Fortified products

For people who do not consume fish or cod liver oil regularly, fortified products are an excellent supplement to the diet. In Poland, cow and plant milks (soy, almond, oat) are increasingly enriched with vitamin D3 at a level of 2.5–3 µg (100–120 IU) per 100 ml. Thanks to this, a glass of plant milk provides approximately 5–6 µg (200–240 IU) D3, which makes it easier to meet your daily needs.

Margarines and vegetable butters often contain added cholecalciferol, and breakfast cereals and porridges for children - ergocalciferol or cholecalciferol - at a level of 2-5 µg per serving. However, it is worth reading labels because the level of enrichment may vary even several times between brands.

An interesting alternative are mushrooms irradiated with UV lamps - some varieties of champignons and shitakes reach up to 10-15 µg D2 per 100 g. Although this is a form of ergocalciferol, it can be a complement to a plant diet. People looking for a comprehensive dietary solution may also consider a complete powdered meal SMART MEAL, which contains both vitamin D3 and other essential vitamins and minerals in optimal doses.

Vitamins for children and adults

The need for vitamin D varies depending on age, health condition and lifestyle. For breastfed infants, WHO recommends 400 IU (10 µg) D3 daily from the first days of life until the age of 12 months to support normal bone development. In the case of bottle feeding, it is worth choosing mixtures enriched with cholecalciferol.

Children aged 1–10 years need 600–1000 IU D3 daily to prevent rickets and support bone mineralization. It is worth introducing fatty fish, eggs and fortified products into their diet - yogurts, cheeses and plant drinks. Regular testing of 25(OH)D levels every 6–12 months will allow you to monitor your nutritional status and quickly respond to deficiencies.

Adults up to age 70 should aim to consume 800–2,000 IU (20–50 µg) per day. Seniors over 70 years of age - due to reduced skin synthesis and often lower dietary intake - may require higher doses, up to 2000-4000 IU per day. In the autumn and winter period, it is recommended to use supplements or fortified products to maintain the level of 25(OH)D in the serum in the optimal range of 30-50 ng/ml.

Supplementation and sun exposure

The best way to supplement vitamin D is to combine short-term sun exposure with appropriate supplementation. In the summer season - between May and September - it is recommended to expose exposed skin (arms, legs) to UVB radiation for 10-20 minutes 3 times a week between 10:00 a.m. and 3:00 p.m. For people with darker skin or seniors, this time may increase to 30-40 minutes.

In months with insufficient sunlight, it is necessary to take supplements - cholecalciferol in the form of drops, soft capsules or spray. Recommended maintenance doses are 1,000–2,000 IU per day, and in the case of confirmed deficiency - up to 10,000 IU for several weeks, under medical supervision and with 25(OH)D level testing.

When supplementing vitamin D, it is worth remembering the synergy with calcium, magnesium and vitamin K2 - these ingredients together support bone mineralization and prevent unfavorable calcium deposition in the arteries. Regular monitoring of vitamin D levels and consultations with a dietitian or doctor will allow you to select the optimal dose, tailored to individual needs and living conditions.

Sources

• National Institutes of Health (NIH) – Vitamin D Fact Sheet for Health Professionals.
• European Food Safety Authority – Dietary Reference Values ​​for vitamin D.
• World Health Organization (WHO) – Vitamin D and Calcium: Supplementation Guidelines.
• Polish Society of Dietetics – Recommended intake of vitamin D.
• Holick, M.F., "Vitamin D: Physiology, Molecular Biology, and Clinical Applications", 2020.

FAQ

What are the differences in bioavailability of vitamins D2 and D3?

Vitamin D3 (cholecalciferol) has a much higher biopotency than D2 (ergocalciferol). D3 is converted into calcidiol faster in the liver and stays in circulation longer, which translates into a more effective increase in the level of 25(OH)D in the blood. D2 has a shorter half-life and weaker binding to transport proteins, therefore, at comparable supplementation doses, the effectiveness of D3 is higher.

How often should you eat oily fish to meet your vitamin D needs?

To naturally support vitamin D levels, it is recommended to eat two portions of oily sea fish per week - e.g. salmon, mackerel, herring or sardines. Two servings of 150 g cover approximately 50-70% of the daily requirement for D3, depending on the type of fish and its cholecalciferol content.

Do calcium and magnesium affect the absorption of vitamin D?

Vitamin D supports the absorption of calcium and phosphorus in the intestines, and in combination with magnesium and vitamin K2, supports proper bone mineralization. Magnesium is a cofactor of enzymes that activate vitamin D, therefore an optimal level of magnesium promotes the effectiveness of cholecalciferol and prevents unfavorable calcium deposition in soft tissues.

How to safely combine sun exposure with skin cancer prevention?

Short, controlled skin exposure (10-20 minutes without sunscreen) to UVB rays 3 times a week is sufficient for D3 synthesis and at the same time minimizes the risk of burns. After this time, use sun protection or stay in the shade to prevent skin damage and increase safety.

What fortified products are worth introducing into your child's diet?

For children from 1 to 10 years of age, fortified cow and plant milks, yogurts and breakfast cereals with added vitamin D are recommended. Look for products containing at least 2-3 µg (80-120 IU) D3 per serving, which will support proper bone development and prevent rickets.

When should the level of 25(OH)D in the blood be checked?

It is recommended to test the vitamin D level before starting supplementation and after 3-6 months of its use in order to assess the effectiveness of the therapy. People from risk groups - seniors, people with dark skin, patients with malabsorption diseases - should check their 25(OH)D concentration once a year or more often, according to the doctor's recommendations.

Can plant products meet the demand for vitamin D?

Natural plant sources of vitamin D are limited to D2 in UV-irradiated mushrooms, which does not provide sufficient cholecalciferol. Therefore, vegans and people on plant-based diets should choose products enriched with D3 or D2 and consider supplementation to avoid deficiencies and maintain optimal 25(OH)D levels.