It has dampened our spirits, ruined many a gathering and cancelled countless sporting events, but could the cloudy, humid summer we have endured so far now be damaging our health too, by depriving us of vitamin D, a vital nutrient from sunlight?
After last week’s mini heatwave, we’ll be seeing sunshine and rain again for the foreseeable future, which raises an important question, as vitamin D is essential for maintaining the health of not only our bones, but also for our immune system to function effectively, among other things. It’s even being investigated as a way to reduce the risk of dementia.
However, without access to sunlight (or fresh air, it seems), most people will not earn enough to meet their needs.
Our main source of vitamin D is exposure to ultraviolet B (UVB) radiation from sunlight; this causes cholesterol in our skin to begin a series of chemical reactions that create vitamin D.
But the amount of cloud we’ve had so far this summer (the average number of hours of sunshine has been much lower than normal) will raise fears that we’re not getting enough sunlight. The weather has also been so wet (some parts of the UK experienced the wettest May on record, according to the Met Office) that people have been spending more time indoors than usual.
The weather has been very wet – parts of the UK experienced the wettest May on record, according to the Met Office
Even when the weather is more favourable, many of us are deficient in vitamin D. The latest UK National Diet and Nutrition Survey reports that between 2008 and 2012, almost a quarter of adults aged 19 to 64 and one in five adults aged 65 and over had inadequate levels in their blood.
In the dark and cloudy winter months of January to March, this figure rose to four in ten people aged 19 to 64, and three in ten over 65s.
Older people are at greater risk of deficiency because they become less able to absorb the vitamin from food (we get small amounts from oily fish and eggs, for example) and their bodies become less efficient at producing it.
There is good news, though: While our exposure to sunlight is likely to be “a little bit lower” this year, there are multiple factors that affect how much vitamin D we produce, so this may not be a problem, says Adrian Martineau, professor of respiratory infections and immunity at Queen Mary University of London.
“The ability of sunlight to synthesize vitamin D in the skin is related to the intensity of UVB rays, and that intensity is affected by cloud cover and latitude. Therefore, the further away you are from the equator, the more atmosphere the sun’s rays have to pass through before reaching the Earth,” he explains.
So in summer, when our part of the world is tilted towards the sun, the intensity of the rays reaching the UK increases, so we should be able to top up our vitamin D levels.
But there are other factors at play, too, she says. “These include the time of day (UVB radiation is strongest between 10 a.m. and 2 p.m.), how much time we spend outdoors, and whether we use sunscreen” (although some experts say we rarely apply enough sunscreen to block vitamin D production).
Vitamin D is essential for maintaining the health of not only our bones, but also for our immune system to function effectively.
And the behaviour is really significant, because although you might assume that warmer countries have fewer problems with vitamin D deficiency, that’s not the case. “Saudi Arabia has much higher rates of vitamin D deficiency than we do in the UK, because it can be so hot that they don’t go outside in the sun,” says Professor Martineau.
The fact that vitamin D is so crucial to our wellbeing is why in 2016 the Department of Health recommended that between October and March, all adults and children should consider taking a daily 10mcg supplement, as in winter we often don’t get enough sunlight to trigger adequate vitamin D production.
Sunlight exposure in spring is arguably more important, as “it’s in spring that our levels tend to be lowest and when we need the biggest boost,” says Professor Martineau.
While brief exposure to the sun can help boost your levels (since vitamin D is a fat-soluble vitamin, it can be stored in the body and used later), it has a limited shelf life.
“The half-life of calcidiol (the form of vitamin D that circulates in the body) is one to two months, meaning that if you were in a submarine or suddenly deprived of sunlight, your vitamin D stores would be halved every one to two months,” Professor Martineau added.
However, summer weather doesn’t have to be great to replenish vitamin D levels. Susan Lanham-New, a professor of human nutrition at the University of Surrey who was part of the government working group that drew up the 2016 recommendations, says there can be enough UVB even on a cloudy day to help boost levels – as long as you can get outside.
“It doesn’t necessarily matter if it’s a sunny day, you can still get enough UVB exposure,” she told Good Health. “As long as it’s between April and September, the UVB wavelength is strong enough. Even if it’s cloudy in these months, you should still get enough UVB exposure to allow your body to produce enough vitamin D.”
However, he adds that the most important thing is that “our findings suggest that, at a minimum, you should spend between ten and twenty minutes in the sun every day, but under no circumstances should you allow your skin to burn.”
People with dark brown skin who rarely burn may need to spend 25 minutes in sunlight to get adequate exposure because their skin absorbs UVB rays less easily, according to a University of Manchester study published in the journal Nutrients in 2018.
As for taking a vitamin D supplement, Professor Lanham-New says: “Unless you can’t go outside (for example, because you’re frail or older) or you’re not allowed to expose your skin for cultural reasons, you won’t need to take a supplement between April and September.” She adds: “It won’t do you any harm to take a moderate-dose supplement all year round, although you might be wasting your money.”
In fact, research also shows that vitamin D pills are no complete substitute for getting vitamin D in a healthy diet.
In 2022, a five-year Harvard Medical School study that followed 26,000 middle-aged men and women taking daily supplements reported in The New England Journal of Medicine that even in people with low levels of the vitamin, the pills did not prevent fractures or osteoporosis.
The study found that supplements do not provide the same benefits that even small amounts of the vitamin can provide naturally from foods such as oily fish, eggs and red meat.
Other sources include fortified breakfast cereals, breads, and some children’s yogurts.
NHS dietician Catherine Collins adds: “Mushrooms can also be a good source, which is helpful for vegans. They are produced on the undersides of the mushrooms in response to sunlight, so turn them over and leave them on a windowsill for a few days.”
If anyone needs more encouragement to get out in the sun, there are health benefits beyond vitamin D.
A 2014 study by Dr Richard Weller, an honorary consultant dermatologist at the University of Edinburgh, found that sunlight increases our nitric oxide levels.
“This chemical dilates blood vessels and lowers blood pressure,” he says. “My work shows that natural sunlight appears to prevent hypertension and cardiovascular disease, the leading causes of death worldwide.”
All of this makes getting outdoors this summer, even if it’s cloudy and humid, that much more appealing.
Additional information: Lucy Elkins
Recycling starts at home
How your body “repurposes” things. This week: estrogen
In some women, menopause occurs later than the average age of 51. One theory is that they are more efficient at recycling estrogen.
When “old” estrogen reaches the intestine after traveling through the body, it is either expelled in the stool or intestinal bacteria, known as estrobolome, produce an enzyme that “reactivates” it.
“There is some evidence of the impact of female hormones on the gut microbiome, as well as hormones being reabsorbed from the gut,” says Dr Shazia Malik, consultant obstetrician and gynaecologist at the Royal Free London NHS Foundation Trust and Portland Hospital.