Vitamin D is essential to — or very helpful to — a broad range of body systems and functions.

These include bone, immune, metabolic, heart, developmental, muscle/strength, eye, and brain health.

Vitamin D’s uniquely broad effects and benefits stem from hormone-like characteristics that distinguish the so-called “sunshine-and-seafood” nutrient from all other vitamins.

But a new study confirms that many people who live in northerly latitudes lack this long-overlooked micronutrient: the only vitamin humans can make internally — but only in response to sun exposure.

Even with extended sunlight exposure, people in central-to-northern regions of the U.S. make very little vitamin D during winter months, due to the sun’s weaker intensity.

But many studies find unhealthfully low vitamin D levels among people dwelling in northerly states, during the fall and spring — with some suffering deficiencies even in summer months.

A new Irish study, involving people living in England, confirms prior indications that people who live in northerly states run a substantially higher risk of having insufficient or deficient vitamin D levels.

Irish study of English people confirms the risk for vitamin D deficiency
The new study, from Ireland’s Trinity College Dublin, involved 6,004 middle-aged and older adults participating in the English Longitudinal Study of Ageing (Aspell N et al. 2019).

More than half (57%) of its participants had “insufficient” vitamin D blood levels while more than one in four (26%) had even lower, “deficient” blood levels — levels linked to significant risk of chronic health conditions.

Insufficiency is primarily a concern for bone health — although it’s tied to other health conditions — while deficiency is a risk factor for a broader range of health conditions, including breast and other cancers.

Vitamin D insufficiency is defined as having a vitamin D blood level below 30 ng/mL (nanograms per milliliter), while vitamin D deficiency is defined as having a vitamin D blood level below 20 ng/mL. Using a different measure called nanomoles per liter (nmol/L), insufficiency is defined as a blood level below 75 nmol/L, while deficiency is defined as a blood level below 50 nmol/L.

Those most likely to be deficient in vitamin D were women, people aged over 80, smokers, people of non-white ethnicity, obese people, and people who reported that they suffered from poor health, including digestive problems.

The reasons for some of these risk factors are well known:

  • Non-white people tend to have darker skin that blocks more UVB sunrays.
  • Obese people accumulate vitamin D in their body fat, making it unavailable.
  • Older people often have weaker kidneys, where vitamin D gets converted into its “active” form.
  • Smokers, older people, and people in poor health tend to spend less time outdoors in D-generating sunlight.
  • Digestive problems impair absorption of vitamin D from the very few good food sources (e.g., fortified milk and fatty fish).

Conversely, the attributes linked to higher, “sufficient” vitamin D levels were being of healthy weight, retired, physically active, taking vitamin D supplements, traveling to sunny areas during the past 12 months, and traveling (presumably to sunnier places like Spain) during England’s notoriously cloudy, rainy summers.

As study co-author Dr. Eamon Laird, said, “The high rates of deficiency [in England] are similar to rates seen in other high latitude countries such as Ireland. However, other more northern countries such as Finland have implemented a successful vitamin D fortification policy which has all but eliminated deficiency in the population.”

The English participants in the study lived between 50 and 55° latitude north. In North America, this equates to a North-South expanse centered about 300 miles north of the Western US-Canada border.

Put another way, these latitudes roughly equate to the North-South expanse between Vancouver, British Columbia and Alaska’s southern border or to the North-South expanse between Québec City and Labrador City.

Exposure to UV having B-type sunrays is a key determinant of vitamin D status, so the English participants in this study likely got less and weaker sun compared with people in northern American states — so, the rate of deficiency is likely lower among such Americans.

Given that difference in the latitudes, it’s reasonable to estimate that one out of five people living in northern American states would be deficient in vitamin D, while the proportion of southern-dwelling Americans deficient in vitamin D would be lower.

Nonetheless, even if only one out of five people living northern American states suffer from vitamin D deficiency or insufficiency, that’s a staggeringly high proportion.

That estimate is backed by the most recent study on vitamin D status in America, which found that 5% of children and adults were at risk of vitamin D deficiency, while 18.4% (nearly one in five) were at risk of vitamin D insufficiency.

Perhaps unsurprisingly, Americans whose self-reported diets would result in higher vitamin D intakes than average, or who reported taking supplemental vitamin D were at lower risk for vitamin D deficiency or insufficiency (Herrick KA et al 2019).

Supplements, sun, and seafood raise blood levels
Fortunately, vitamin D deficiency can be pretty easy to remedy.

You can either get more sun, eat more fatty fish, and/or take supplemental vitamin D3 — the form in fatty fish and many supplements. Vitamin D3 is a more potent, beneficial form than the vitamin D2 found in mushrooms and some supplements.

The fish highest in vitamin D provide per-serving “doses” comparable to those found in supplements. These fish include wild salmon, sardines, sablefish, halibut, albacore tuna.

Compared with wild salmon, farmed salmon have much less vitamin D: see Wild Salmon Beats Farmed for Vitamin D (Again).

Does sunscreen block creation of vitamin D in the skin?
Despite its anti-cancer power, scientists debate the wisdom of routinely applying sunscreen.

And some prominent vitamin D experts dispute dermatologists’ common advice to always wear hats and clothes or apply sunscreen when outdoors, especially in midday during the months of strongest sun.

Both sides of the debate agree that excessive sun exposure can cause skin cancer, so the concern is whether sunscreen significantly reduces creation of vitamin D in the skin, and by how much.

Finding a single answer to that question is difficult because sunscreens vary in their “sun protection factor” or SPF, and by the fact that they can be made to protect against UVB rays, UVA rays, or both.

That distinction matters a great deal, because the body produces vitamin D in the skin only in response to UVB sunrays, not UVA sunrays.

As we reported several years ago, a Danish study found that people who applied sunscreen in accordance with WHO guidelines produce little or no vitamin D following sun exposure (see Sunscreen Advice Called Overkill.)

Surprisingly, while some recent clinical studies confirmed that sunscreen reduced vitamin D production very substantially, other recent studies found that sunscreen did not significantly affect vitamin D production.

And the studies that found little effect from sunscreen on vitamin D production were of short duration, which leaves open the question of long-term effects of routinely wearing sunscreen when outdoors.

For example, a Belgian clinical study found that although an SPF 50 sunscreen didn’t affect vitamin D blood levels in the short term, sunscreen slashed production of vitamin D in the skin: covering 96% of total skin area cut vitamin D production by 92.5%, and even covering just 9% of total skin area (head and hands) cut vitamin D production by 83%.

Although the Belgians found that sunscreen didn’t significantly affect vitamin D blood levels in the short term, they expressed caution about the long-term effects: “Short-term sunscreen use probably does not affect circulating vitamin D levels and hence does not increase the risk for osteoporosis. The effect of long-term sunscreen use remains however to be determined.” (Libon F et al. 2017)

Prominent vitamin D researchers like Boston University’s Dr. Michael Holick caution people to protect their skin against strong sunrays with clothing or sunscreen — most of the time.

But — unlike most dermatologists — Dr. Holick and other vitamin D researchers urge people to give their skin five to 10 minutes of substantial sun exposure daily, to help ensure adequate blood levels of vitamin D.

Harvard Medical School’s Edward Giovannucci, M.D., said this in an address to fellow researchers at a 2005 meeting of the American Association for Cancer Research: “I would challenge anyone to find an area or nutrient or any factor that has such consistent anti-cancer benefits as vitamin D. The data are really quite remarkable.”

At the time, Dr. Giovannucci told NPR's Patricia Neighmond that for every person who dies from skin cancers caused by excessive sun exposure, an estimated 30 people die of cancers related to vitamin D deficiency.

Even some dermatologists are now persuaded. As Memorial Sloan-Kettering Cancer Center's chief dermatologist, Allan Halpern, M.D., said in 2005 “I find the evidence [that the benefits of sun outweigh the risks] to be mounting and increasingly compelling.”

To a limited extent, the length of safe, unprotected sun exposure depends on skin color, because darker skin blocks more UVB sunrays, which are responsible for sunburns and less- dangerous forms of skin cancer.

But darker skin doesn’t block UVA rays, which is why — compared with fair-skinned people — folks with darker skin are just about as likely to develop sun-related melanoma tumors.

Sunscreen basics: UVA, UVB, and SPFs
It’s clear that sunburns — to which fair skinned people are most vulnerable — promote skin cancer, especially when they occur earlier in life.

Likewise, excessive exposure to strong sunrays — and routine use of tanning beds — is linked to higher rates of skin cancer.

Sunburns and less-dangerous forms of skin cancer are typically caused by UVB sunrays, while the deadliest type of skin cancer — melanoma — is linked to deeper-penetrating UVA sunrays. (For reasons that remain unclear, melanoma can occur in the absence of excessive sun exposure and on skin areas rarely exposed to any sun.)

A recent University of Sydney study found that Australians aged 18-40 years who routinely used sunscreen in childhood were 40% less likely to develop melanoma later in life, compared with those who rarely used sunscreen.

Importantly, SPF numbers on a sunscreen refer only to UVB sunray protection, not to UVA protection. The term “broad-spectrum” on a sunscreen means that it blocks both UVB and UVA sunrays — but only a broad-spectrum sunscreen that employs zinc oxide or another physical barrier to sunrays provides total protection against UVA rays.

At least in theory, applying a sunscreen with an SPF of 30 means that — compared with unprotected skin — your skin can be exposed to sun 30 times longer before you’ll develop a sunburn. However, the SPF number only remains valid if you reapply the sunscreen as often as directed on the label, and whenever it gets washed off by sweat or water.

And most dermatologists agree that most people don’t need an SPF higher than 30, as these UVB- filtering figures reveal:

  • An SPF 15 sunscreen filters about 93% of UVB rays.
  • An SPF 30 sunscreen filters about 97% of UVB rays.
  • An SPF 50 sunscreen filters about 98% of UVB rays.

As you can see, an SPF 50 sunscreen only filters 1% more of the sun’s rays than an SPF 30 sunscreen — a distinction that makes very little practical difference.


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