Skin Synthesis of Vitamin D: How It Works, and Is Sunlight a Safe Source?

Cutaneous Production of Vitamin D: When ultraviolet B (UVB) rays from sunlight hit the skin, they trigger a chemical reaction in the epidermis that produces vitamin D. Specifically, UVB photons (in the 290–315 nm wavelength range) convert 7-dehydrocholesterol (a cholesterol precursor in the skin) into previtamin D₃, which then thermally isomerizes into vitamin D₃ (cholecalciferol)[34][35]. This newly made vitamin D₃ enters the bloodstream and is carried to the liver, where it is hydroxylated to 25-hydroxyvitamin D (calcidiol) – the main circulating form. Calcidiol is then converted to the biologically active hormone 1,25-dihydroxyvitamin D (calcitriol) primarily in the kidneys (and to some extent in other tissues)[34]. Calcitriol is the form of vitamin D that actually binds to vitamin D receptors in the body to carry out its functions (regulating calcium absorption, gene expression, etc.)[34]. In essence, sunlight hitting the skin is the first step in a multi-organ process that yields active vitamin D – this is why vitamin D is often called the “sunshine vitamin.”

Factors Affecting Skin Synthesis: The amount of vitamin D your skin makes from sun exposure can vary dramatically based on several factors:

Latitude and Season: As discussed earlier, your geographic location and the time of year determine the UVB intensity available from sunlight. A high sun angle (as occurs at lower latitudes and during summer midday) yields the strongest UVB for vitamin D production[35]. In practical terms, a short midday summer sun exposure (e.g. 10–20 minutes in a t-shirt) can produce a substantial amount of vitamin D (potentially thousands of IU), whereas early morning or winter sun exposure of the same length might produce little to none[21][22].

Skin Pigmentation: Melanin, the pigment in skin, absorbs UV radiation. Individuals with darker skin (higher melanin) have a natural protective buffer against UV, which is why they tend to sunburn less easily. However, that protection also means slower vitamin D production. A deeply pigmented skin can require significantly more UV exposure to make the same vitamin D. One source notes that someone with very dark skin (Fitzpatrick Type V/VI) might need up to 10 times the sun exposure to synthesize an equivalent amount of vitamin D as someone with very fair skin (Type I)[28]. This is because melanin competes with 7-dehydrocholesterol for UVB photons[28]. Thus, ethnic groups with dark skin often show lower vitamin D levels, especially if living in mild-sun climates. It’s important for individuals with dark skin to recognize that they may have a higher dietary requirement for vitamin D or need supplements if they get limited sun.

Age: An older person’s skin is less efficient at making vitamin D. The concentration of the 7-dehydrocholesterol in skin declines with age, so a 70-year-old exposed to the same summer sun as a 20-year-old will produce less vitamin D[30]. Additionally, older adults often spend less time outdoors (due to mobility issues or lifestyle), compounding the risk of deficiency. This is one reason why vitamin D supplementation is especially recommended in the elderly.

Extent of Skin Exposure: The more skin you have exposed, the more vitamin D you can produce. Vitamin D synthesis in skin is roughly proportional to the area exposed (up to a saturation point). Exposing just your face and hands will net you far less D than exposing your arms and legs or your full torso. Clothing therefore plays a big role – a person sunbathing in a swimsuit will make more vitamin D in 10 minutes than a person taking a 10-minute walk in long sleeves and pants.

Use of Sunscreen: Sunscreen is designed to block UV radiation to protect the skin from sunburn and DNA damage. Unfortunately, any sunscreen that is effective at preventing sunburn will also reduce vitamin D synthesis. A high-SPF sunscreen, if applied thoroughly, can block 90–98% of cutaneous vitamin D production[23][24]. For example, SPF 15 lotion filters out ~93% of UVB, and SPF 30 filters ~97%. In practice, people often don’t apply a thick enough layer to achieve the full SPF, so some vitamin D synthesis might still occur when wearing sunscreen, but it is greatly attenuated. Sunscreen use has become widespread (for good reason – it prevents skin cancer and aging), so this is a major factor in why natural vitamin D synthesis is lower in populations today compared to decades past.

Other factors: Weather conditions (heavy cloud cover, air pollution, shade) can reduce UVB availability. Window glass blocks UVB completely – so you will not make vitamin D by sitting indoors by a sunny window[36] (the UV that comes through glass is mostly UVA, which does not help vitamin D but can still age the skin). Altitude (higher elevation has more intense UVB) and surface reflection (UV bouncing off snow or water can increase exposure) also affect the dose of UVB one’s skin gets. These factors are highly variable but worth noting in specific contexts.

It’s clear that producing vitamin D from sunlight is a complex and variable process. Depending on where you live, the time of year, your skin type, age, and habits, you might make a lot of vitamin D on a given day – or virtually none at all.