The Science of Vitamin D & Sun Exposure

Vitamin D synthesis begins in the skin when UVB rays from the sun activate a naturally occurring compound called 7-dehydrocholesterol. When UVB rays reach the skin, they interact with this compound and convert it into pre-vitamin D3, which is then gradually transformed into vitamin D3 through the body’s own heat. The liver and kidneys then convert this into the active form of vitamin D used throughout the body (1).

This active vitamin D supports (2):

• Bone mineralization and growth
• Immune regulation and resilience
• Mood balance and neurological function
• Healthy cell development and repair

It is important to understand that vitamin D synthesis only occurs when UVB rays are present and can reach the skin. This is why certain environmental factors shape how much vitamin D the body is able to make (3) (more on this below).

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UVB rays are strongest when the sun is high in the sky, generally between late morning and early afternoon. Sunlight early in the morning or late in the day may feel warm, but UVB levels are often too low to support vitamin D production. 

This can be especially true during:

• Winter months
• Cloudy seasons
• Times of heavy air pollution
• Northern latitudes where the sun stays lower in the sky

Additionally, glass windows, shade, tall buildings, hats, clothing, and sunscreen all block or reduce UVB exposure. This helps explain why many children and adults develop low vitamin D levels even when they spend plenty of time outdoors (4).

Skin tone affects how quickly vitamin D is produced. Melanin, the pigment that gives skin its color, acts as a natural UV filter. Because of this, individuals with darker skin require more direct sunlight to produce the same amount of vitamin D as individuals with lighter skin (5).

This is one reason vitamin D needs vary widely among families. Two children in the same household may require different amounts of sun exposure to support healthy vitamin D levels.

Healthy vitamin D levels are supported best by short, regular exposure to sunlight rather than long periods in the sun at once. Vitamin D synthesis does not require tanning or burning. A few minutes of midday sunlight on exposed skin several times per week can be helpful for many people (6, 7).

Research suggests that exposing larger areas of skin (such as the arms and legs) allows the body to produce vitamin D far more efficiently than exposing only the face and hands (7). When possible, spending time outside in midday sun, when UVB rays are strongest, supports vitamin D synthesis most effectively (7).

Modern life makes it surprisingly easy to miss the sunlight needed for vitamin D synthesis. Many children spend most of their daytime hours indoors at school, and outdoor play is often brief or weather dependent. Adults frequently experience the same thing: work, errands, and daily routines keep us inside, and in many regions the colder months offer very few opportunities for direct midday sun.

When this pattern continues over weeks or months, vitamin D levels may gradually fall. Low vitamin D can show up as reduced energy, more frequent seasonal illnesses, shifts in mood, slower recovery, and changes in bone or muscle strength over time (8,9). These signs are often subtle, which is why vitamin D levels can decline without us noticing right away.

We can also get vitamin D through food. Vitamin D that appears in food has already been activated by the animal or plant source, so the body does not need UVB exposure in order to use it. These foods can help support vitamin D levels, especially when paired with regular outdoor time.

Some foods naturally contain vitamin D, including:

• Pastured egg yolks
• Salmon, sardines, and cod liver oil
• Mushrooms exposed to sunlight

However, these foods generally offer modest amounts. For growing children, pregnant or breastfeeding mothers, individuals with darker skin tones, or those living in northern climates, food sources alone often are not enough to maintain optimal vitamin D levels throughout the year, especially during winter, or during seasons of indoor living.

To learn more, read this article: Top Sources of Vitamin D + Kid Friendly Recipes

Human breast milk is rich in immune-supportive and developmental nutrients, but it typically contains very low levels of vitamin D. This is because the amount of vitamin D present in breast milk reflects the breastfeeding parent’s own vitamin D status.

For this reason, many breastfed babies benefit from additional vitamin D support. Some families choose to give vitamin D drops directly to their baby. Others choose to support the breastfeeding parent’s vitamin D levels (which can increase the amount of vitamin D passed through milk). Both approaches are supported in the research, and either can work well depending on the family’s preferences, routine, and comfort.

To learn more, read this article: Vitamin D Supplementation For Babies

When regular sunlight is limited or vitamin D needs are higher than can be obtained through sun exposure and diet alone, supplementation can provide gentle, consistent support. Vitamin D3 is the form the body uses most readily. Pairing it with vitamin K2 helps guide calcium into the bones and teeth, where it belongs, rather than allowing it to circulate in the bloodstream. This pairing supports bone strength, cardiovascular health, and overall mineral balance.

It can be helpful to check vitamin D levels periodically, especially if you live in a northern climate or rely on supplementation. A simple blood test called a 25-hydroxy vitamin D (25(OH)D) reflects the body’s vitamin D status. If you’re unsure whether you or your child may need additional support, ask your healthcare provider about this test. 

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Vitamin D is made in the skin when it is exposed to sunlight. Specifically, UVB rays trigger the conversion of a natural compound in the skin into vitamin D, which the body then activates and uses to support bone strength, immune resilience, mood balance, and healthy cell function. 

Because UVB availability depends on the time of day, season, skin tone, and where we live, many children and adults may not consistently produce enough vitamin D from sunlight alone. Foods containing vitamin D can be helpful, but they generally provide only small amounts. For many families, a combination of regular gentle sunlight, nutrient-rich foods, and thoughtful vitamin D3 + K2 supplementation offers the most dependable way to support healthy vitamin D levels throughout the year. 

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