Vitamin D: A Science-Based Guide
What is vitamin D?
Vitamin D is a fat-soluble vitamin that the body produces in the skin when exposed to sunlight (UVB rays). It is sometimes called the "sunshine vitamin". There are two main forms: D2 (ergocalciferol), found in some plant foods and supplements, and D3 (cholecalciferol), produced in the skin and the most common supplement form.
In northern climates such as Estonia, where sunlight is limited from October to March, vitamin D supplementation may be particularly relevant.
As a supplement, vitamin D is available in capsules, tablets, drops, and chewable tablets. In the European Union, only EFSA-approved health claims may be used in the marketing of vitamin D supplements.
How does it work?
Vitamin D goes through a multi-step process before it becomes active in the body. Vitamin D formed in the skin or obtained from food and supplements is first transported to the liver, where it is converted to calcidiol (25-hydroxy-vitamin D). It is then converted in the kidneys to the active form — calcitriol (1,25-dihydroxy-vitamin D).
Active vitamin D functions like a hormone, binding to vitamin D receptors found in many tissues throughout the body. It participates in calcium and phosphorus metabolism, bone mineralisation, normal muscle function, and immune system processes.
Supported benefits (EFSA-approved claims only)
The following benefits are based on health claims authorised under Commission Regulation (EU) No 432/2012. All claims are valid when the supplement provides at least 0.75 micrograms (15% of the Nutrient Reference Value) of vitamin D per 100 g, 100 ml, or per serving:
Immune system
- Vitamin D contributes to the normal function of the immune system — this is an EFSA-approved claim. Note that it is not permitted to say "boosts immunity" or "prevents colds".
Bone health
- Vitamin D contributes to the maintenance of normal bones — vitamin D is important in the bone mineralisation process.
Muscle function
- Vitamin D contributes to the maintenance of normal muscle function — this is particularly relevant for physically active individuals and older adults.
Calcium and phosphorus absorption
- Vitamin D contributes to normal absorption and utilisation of calcium and phosphorus — vitamin D is necessary for the efficient absorption of these minerals from the intestine.
Teeth
- Vitamin D contributes to the maintenance of normal teeth — linked to calcium metabolism.
Cell division
- Vitamin D has a role in the process of cell division — a general physiological function.
Blood calcium levels
- Vitamin D contributes to the maintenance of normal blood calcium levels — vitamin D regulates calcium balance in the blood.
Important note: Claims such as "boosts immunity", "prevents colds", "prevents osteoporosis", or "treats vitamin D deficiency" are not permitted. These are either overly broad or medical claims that may not be used for food supplements.
How to take it
Vitamin D is typically taken with fat-containing food, as it is a fat-soluble vitamin and dietary fats improve its absorption.
Guidelines:
- Always follow the recommended daily serving stated on the label — do not exceed it
- Take vitamin D with a meal containing fats (e.g., butter, olive oil, nuts, avocado)
- The European Food Safety Authority has set the tolerable upper intake level for adults at 100 micrograms (4000 IU) per day — this is not a recommended dose but an upper safety limit
- Vitamin D tablets or capsules can be taken at any time of day — choose a convenient time that helps you remember consistently
- Some people prefer the drop form, which allows more flexible dosing
Northern climate note: From October to March, UVB radiation in Estonia and other northern countries is too weak for the body to synthesise vitamin D in the skin. During this period, vitamin D supplementation may be particularly practical.
Who should use it?
A vitamin D supplement may be relevant for adults who want to ensure adequate vitamin D intake, especially in conditions of limited sunlight exposure.
Vitamin D supplementation may be of interest to:
- Residents of northern countries (including Estonia) who experience long, dark winters
- People who spend most of their day indoors (office work)
- Older adults whose skin synthesis of vitamin D is reduced
- People with darker skin tones, where melanin reduces UVB absorption
- Athletes who want to support normal muscle function and bone health
Who should exercise caution:
- People with hypercalcaemia (elevated blood calcium levels)
- People taking medications that affect calcium metabolism
- People with kidney disease
When in doubt, consult your doctor. Vitamin D levels can be measured with a blood test.
Frequently Asked Questions
What is the difference between vitamin D2 and D3?
D2 (ergocalciferol) is of plant origin and D3 (cholecalciferol) is of animal origin or synthesised from lanolin. D3 is the most common supplement form, as it has better bioavailability. Vegan D3 derived from lichen is also available.
Do I need vitamin D in summer?
In summer, if you spend time outdoors in sunlight regularly, the body produces vitamin D in the skin. However, if you spend most of your time indoors or use high-SPF sunscreen, synthesis may be limited. Each individual should assess this for themselves.
Can I take too much vitamin D?
Vitamin D is fat-soluble and the body stores it. The European Food Safety Authority has set the tolerable upper intake level for adults at 100 micrograms (4000 IU) per day. Always follow the recommended dosage on the label.
How do I know if I am vitamin D deficient?
Vitamin D levels can be measured with a 25-hydroxy-vitamin D blood test. Consult your general practitioner, who can order the test. Only a test result can determine whether your levels are adequate.
Can I take vitamin D with other supplements?
Yes, vitamin D can generally be combined with other supplements. Vitamin D contributes to the normal absorption of calcium, which is why they are often taken together. If you take medications, consult your doctor regarding potential interactions.
Which form of vitamin D should I choose: tablets, capsules, or drops?
The form is a matter of personal preference. Drops allow more flexible dosing, capsules are convenient to carry. What matters is the amount of active ingredient per serving, not the form. Always check the vitamin D content per serving on the label.
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