Vitamin D in winter: do you need a supplement?

Why your skin produces hardly any vitamin D in winter

Vitamin D is produced in the skin under the influence of UVB radiation from sunlight. For this to happen, sufficient UVB radiation must reach the Earth's surface.

In the Netherlands and Belgium, the sun is so low in the sky during the winter months that the UVB radiation travels a longer distance through the atmosphere and is largely absorbed. As a result, from approximately October through March, there is insufficient UVB radiation to produce a meaningful amount of vitamin D through the skin.

This is not primarily related to the temperature. Even on a clear and sunny winter day, the sun is too low for sufficient vitamin D production. A one-hour walk outside in January is of course healthy for all sorts of other reasons, but in the Netherlands or Belgium, it yields virtually no vitamin D through the skin.

How much of a reserve do you still have at the beginning of winter?

Vitamin D is fat-soluble, and your body stores a portion in fat tissue and muscles. That summer supply slowly depletes, usually over a period of two to three months. This means that most people run out of their summer supply sometime in November or December, after which blood levels drop further until spring restarts production.

Exactly how much supply you have built up depends on how much exposed skin you have had in the sun during the summer, for how long and how often, and on your body composition. With a higher percentage of body fat, relatively more vitamin D is retained in the fat tissue, meaning there is less freely available in the blood. People who spend the summer largely indoors, for example due to work, start the winter with a lower supply than people who have spent a lot of time outdoors.

For whom is a vitamin D supplement necessary according to the Health Council?

The Health Council does not advise everyone to supplement with vitamin D. For a number of groups, however, the advice is to take a vitamin D supplement daily throughout the year:

• All children up to 4 years of age: 10 micrograms per day.
• Pregnant women: 10 micrograms per day.
• People with dark or tanned skin: 10 micrograms per day. Due to the higher amount of pigment, the skin produces less vitamin D with the same exposure to sunlight.
• People who are insufficiently exposed to sunlight: 10 micrograms per day. For example, people who spend little time outdoors during the day or largely cover their skin.
• All women aged 50 to 69: 10 micrograms per day.
• Everyone aged 70 and older: 20 micrograms per day.

If you fall into one of these groups, the advice therefore applies not only in winter, but throughout the entire year.

Do you not have any of these risk factors? Then the situation is more nuanced.

For a healthy adult without the above risk factors, the Health Council does not recommend a mandatory daily supplement. The premise is that the summer reserves, built up by regularly being outdoors between April and September, bridge a large part of the winter for many people.

In practice, however, many general practitioners and dietitians in the Netherlands and Belgium still choose to recommend a low-dose supplement in winter, even to people without specific risk factors. The underlying rationale: a low dose of vitamin D3 has hardly any disadvantages, while quite a few people turn out to have a lower winter supply than expected, for example due to an office job, spending a lot of time indoors, or a summer with little outdoor activity. This is therefore more of a practical consideration than a strict medical necessity.

Blood test or just preventative supplementation?

For most healthy people without symptoms, a low, preventative dose of vitamin D3 throughout the winter is more practical and cheaper than having a blood test done first. A supplement of 10 micrograms (400 IU) per day is a safe, standard maintenance dose for most adults.

A blood test is particularly useful if you recognize one of the following situations:

• You have symptoms that may indicate a deficiency, such as persistent fatigue, muscle pain without a clear cause, or being sick more often than normal.
• You fall into a risk group and want to know if the standard dose is sufficient for you.
• You are considering a higher dose than the standard maintenance dose, for example, because a previous measurement already showed a deficiency.
• You want to check after a few months of supplementation whether it is actually working.

A blood test measures the 25-hydroxyvitamin D concentration in your blood and provides clarity at once, instead of leaving you guessing based on symptoms alone.

Conclusion

Not everyone automatically needs a supplement in the winter, but most people in the Netherlands and Belgium do have less vitamin D available during the winter months. than they think. If you fall into one of the groups mentioned by the Health Council, a supplement is recommended year-round. If you do not, a low preventive winter dose is a sensible, accessible choice for most people.

Are you unsure whether your vitamin D level is actually low, or are you considering a higher dose? A blood test will give you clarity instead of guessing.