Vitamin D in Winter: Deficiency, Sunlight, Food Sources, and Prevention

The risk of vitamin D deficiency can increase during the darker months of the year, particularly for people who spend little time outdoors or live in regions where sunlight is not strong enough for adequate vitamin D production. Why does this happen, and what should you consider during winter to maintain a healthy vitamin D level?

Let’s first explain what vitamin D is and why the skin plays an important role in its production.

What Is Vitamin D, and What Is It Suitable For?

Vitamin D is a group of fat-soluble compounds, commonly referred to as calciferols. The two major forms are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol).

Vitamin D helps regulate calcium and phosphorus metabolism and plays an important role in maintaining healthy bones and teeth. It also contributes to normal muscle function and immune system activity.

Vitamin D is involved in many biological processes, including cell growth and gene regulation. However, having adequate vitamin D does not mean that taking additional supplements will prevent or treat every disease associated with vitamin D.

For a broader understanding of the nutrients the body needs, see our guide to nutrients in your diet.

What Does Vitamin D Have to Do With the Skin?

The body can produce vitamin D3 in the skin when it is exposed to ultraviolet B (UVB) radiation from sunlight. The amount produced depends on several factors, including the season, latitude, time of day, skin pigmentation, age, clothing, and the amount of skin exposed.

It is sometimes estimated that a large proportion of vitamin D can come from skin production under suitable conditions. However, the contribution from sunlight varies significantly between individuals and locations.

The remaining vitamin D can come from food and supplements. Natural food sources include fatty fish, egg yolks, and certain mushrooms, while fortified foods can also provide vitamin D.

How Do I Recognize a Vitamin D Deficiency, and What Are the Consequences?

Vitamin D deficiency may not cause obvious symptoms, particularly when it is mild. When deficiency becomes significant, possible symptoms can include:

• Fatigue
• Muscle weakness
• Bone pain
• Muscle aches
• Increased risk of bone fractures
• Impaired bone mineralization

These symptoms are not specific to vitamin D deficiency and can have many other causes.

In children, severe vitamin D deficiency can cause rickets, a condition in which growing bones become soft and weak. In adults, severe deficiency can contribute to osteomalacia, which involves impaired bone mineralization and can cause bone pain and muscle weakness.

How Does a Vitamin D Deficiency Come About?

The body’s production of vitamin D depends partly on exposure to UVB radiation. Depending on the latitude, season, weather, and time of day, the amount of UVB reaching the skin can vary considerably.

Several external and individual factors can affect vitamin D production.

External Factors

• Season and climate: During winter, shorter days and weaker UVB radiation can reduce vitamin D production in some regions.
• Cloud cover: Heavy cloud cover can reduce the amount of UVB reaching the skin.
• Altitude: UV exposure can be greater at higher elevations.
• Air pollution: High levels of air pollution can reduce UVB exposure.

Individual Factors

• Skin pigmentation: Melanin reduces the skin’s production of vitamin D in response to UVB radiation.
• Age: Older adults generally have a reduced ability to produce vitamin D through the skin.
• Clothing: Covering most of the skin reduces exposure to UVB radiation.
• Lifestyle: Spending most of the day indoors can reduce sun exposure.
• Certain medical conditions: Liver, kidney, and digestive disorders can interfere with vitamin D metabolism or absorption.

What Should I Consider in Winter?

The amount of vitamin D stored in the body before winter varies from person to person. Some people may have sufficient stores to maintain adequate levels for part or all of the winter, while others may become deficient.

Spending appropriate time outdoors can support vitamin D production when UVB levels are sufficient. However, sunlight exposure should not replace sensible sun protection, particularly when UV levels are high.

Diet can also contribute to maintaining vitamin D intake. Foods containing vitamin D include:

• Fatty fish such as salmon, sardines, and mackerel
• Egg yolks
• Fortified milk
• Fortified plant-based milk alternatives
• Some fortified cereals
• Certain mushrooms

A balanced diet can help provide vitamin D along with many other essential nutrients. You can find additional dietary guidance in our article on nutritional tips.

For some people, food and sunlight may not be enough to maintain adequate vitamin D levels. This can be more likely in people with limited sun exposure, certain medical conditions, malabsorption problems, or other risk factors.

A healthcare professional can determine whether a blood test or vitamin D supplement is appropriate.

What Role Does Protein Play in a Healthy Diet?

Maintaining overall nutritional health involves more than focusing on a single vitamin. Protein is also essential because it provides amino acids needed for maintaining and repairing tissues, supporting muscle, and carrying out many biological processes.

Including appropriate protein sources in a balanced winter diet can help support overall nutritional adequacy. Learn more about protein requirements and how much protein may be appropriate for different dietary needs.

Conclusion

Vitamin D is important for bone health, calcium metabolism, muscle function, and normal immune system activity. During winter, vitamin D production through the skin may decrease in some regions because of shorter days and reduced UVB exposure.

Spending appropriate time outdoors, eating vitamin D-containing or fortified foods, and maintaining a varied diet can help support adequate nutrient intake. However, some people may still require vitamin D supplementation based on their individual circumstances.

If you are concerned about vitamin D deficiency, particularly if you have risk factors or symptoms, speak with a healthcare professional. They can determine whether testing or supplementation is appropriate and recommend a safe dose when needed.