Why Is Vitamin D Deficiency So Common, Especially in Northern Climates?

Humans evolved to get most of their vitamin D from sunlight, but modern living and certain physiological factors have severely curtailed that input for many people. In northerly climates especially, the winter sun isn’t adequate, and people spend a lot of time indoors or covered up. As a result, vitamin D insufficiency has become a global issue, making it crucial to get vitamin D from other sources when sunlight is lacking.

Despite being critical, vitamin D is one of the most commonly deficient nutrients worldwide. An estimated 1 billion people globally have inadequate vitamin D levels in their blood[20]. Even in developed countries, a large portion of the population has suboptimal vitamin D status. There are several key reasons for this widespread deficiency, particularly among people living in northern (high latitude) regions:

Limited Sunlight at High Latitudes: People who live far from the equator (e.g. in Canada, northern US, Northern Europe, etc.) cannot get sufficient sunshine during a good part of the year. Above about ~37–40° latitude, the suns reduced intensity and shorter winter days greatly reduce the opportunity to generate vitamin D naturally through the skin[21]. For example, studies have shown that at around 42°N latitude (Boston, USA), there is essentially zero vitamin D synthesis from November through early March[21]. Further north, the “vitamin D winter” is even longer – at ~52°N (Edmonton, Canada or London, UK), the body cannot make vitamin D from about October through April[21]. This means people in northern climates have a built-in seasonal risk for vitamin D deficiency. They might build up some vitamin D stores in summer, but those stores often dwindle over winter. By late winter/early spring, a large proportion of people at high latitudes are vitamin D deficient[22].

Reduced Sun Exposure (Modern Lifestyles): In addition to latitude, lifestyle greatly impacts vitamin D status. Many individuals simply don’t get outside enough in peak sun. We spend more time indoors (work from offices, indoor entertainment), and when we do go out, we tend to cover up with clothing or sunscreen for skin protection. While sun protection is important (more on that below), it has the side effect of minimizing vitamin D synthesis. Wearing long sleeves or pants, or consistently using sunscreen, blocks UVB from reaching the skin[23]. For instance, properly applied sunscreen with SPF 30 can reduce vitamin D production by more than 95%[24]. Thus, conscientious sunscreen use – necessary to prevent skin cancer – can unintentionally contribute to low vitamin D. Urbanization also plays a role: people living in cities may get less direct sun (tall buildings, pollution haze can reduce UV). In short, many of us don’t receive the “20 minutes of midday sun on ~40% of skin” that some experts cite as needed to maintain vitamin D[25]. The result is that even in sunny climates, indoor lifestyles and sun-safe behaviors are creating vitamin D gaps.

Darker Skin Pigmentation: Individuals with naturally darker skin (higher melanin content) are at higher risk of vitamin D deficiency, especially if they live in northern latitudes[26][27]. Melanin, the pigment that makes skin brown or black, acts as a natural sunscreen – it absorbs and dissipates UV radiation. This is beneficial for protecting the skin’s DNA from UV damage, but it also means that less UVB is available to produce vitamin D in darker-skinned people. Consequently, someone with very dark skin might need significantly more sun exposure to make the same amount of vitamin D as someone with fair skin. One scientific source estimates that, depending on skin type, it can take up to 10 times longer for a person with a dark complexion to synthesize a given amount of vitamin D compared to a light-complexioned person under the same UV conditions[28]. This is why, for example, in the U.S. an estimated ~75% of African-American adults are vitamin D insufficient (serum 25(OH)D <50 nmol/L), compared to ~20% of White adults[20]. The combination of dark skin + high latitude is especially challenging – e.g. a dark-skinned individual living in Canada or the UK is very likely to become deficient without dietary supplementation, because the limited UVB that does reach the earth in those regions is effectively filtered by skin pigment[29].

Other Risk Factors: Age is a factor – older adults have a reduced capacity to produce vitamin D in skin. Aging decreases the concentration of 7-dehydrocholesterol (the vitamin D precursor) in the skin, so a 70-year-old makes substantially less vitamin D than a 20-year-old with the same sun exposure[30]. Compounding this, many elderly people are homebound or in nursing facilities with little sun, and the prevalence of vitamin D deficiency in nursing home residents is quite high (studies in the US and Europe show 50–80% of institutionalized seniors are deficient)[26]. Obesity is another risk factor – vitamin D is fat-soluble and tends to get sequestered in fat tissue. Obese individuals have lower bioavailable vitamin D; in fact, obesity is associated with about a 35% higher prevalence of deficiency, even at the same latitude, compared to non-obese peers[26]. Certain medical conditions can also lead to deficiency. These include fat-malabsorption disorders (like celiac disease, Crohn’s disease, cystic fibrosis) which impair dietary vitamin D uptake[31], and organ failures (kidney or liver disease can disrupt the conversion of vitamin D to its active form)[32]. Medications that accelerate vitamin D breakdown (e.g. some anticonvulsants) are yet another factor[33].

In summary, vitamin D deficiency is common because natural sources are limited – sun exposure may be insufficient due to geography or lifestyle, and very few foods provide enough vitamin D (we will discuss diet shortly). This is why large segments of the population – from high-latitude dwellers, to people with indoor jobs, to those with dark skin, to the elderly or obese – often have suboptimal vitamin D levels[27][26].