If you’re health conscious, chances are you know a thing or two about vitamin D.
You may know, for example, that it’s impossible to enjoy optimal health if you’re low in this nutrient.
But getting enough is harder to do than you might think, which is why leading vitamin D expert Dr Michael F. Holick estimates that vitamin D deficiency is “the most common medical condition in the world.”
Among other things, vitamin D is needed for mental and emotional health, and for a peak-functioning immune system.
There is also a growing body of research demonstrating a link between sub-optimal levels of vitamin D and cancer.
You’ll already know that the best way to get vitamin D is via sunlight.
But what you probably know less about is how easy or otherwise that is to do where you live.
This article is a mini guide to working that out.
I’ve met many health enthusiasts in the UK and similar climates who believe that if they go out jogging in shorts and a T-shirt on a bright winter’s day, that sun exposure will be topping up their vitamin D levels.
In the UK we don’t get enough of the kind of sunlight that causes our bodies to manufacture vitamin D under the skin.
Only one kind of solar radiation does this: UVB sunlight.
Vitamin D is synthesised only when we’re exposed to UVB rays – and unless UVB rays are present it doesn’t matter how warm it is nor how brightly the sun is shining: your skin cannot synthesise vitamin D.
“Vitamin D winter” is when no vitamin D production is possible due to the atmosphere blocking all UVB.
And what few realise is that in the UK, that winter lasts for at least six months of the year, from October to March inclusive.
In much of the US it lasts for four months, from November to February inclusive (though, as I’ll explain shortly, some experts consider these to be underestimates).
Location, location, location
Latitude is a measure of distance from the equator, either north or south.
Humans evolved in the low latitudes of the tropics, an area of year-round UVB sunlight – that is our ideal environment.
Here are a few quick examples to help you understand the connection between latitude and UVB sunlight (the figure after each location is the latitude, either north or south of the equator):
Latitudes with year round UVB sunlight
Bali – 8
Honolulu – 21
Miami – 25
Latitudes with UVB sunlight every day, but only for a couple of hours during the middle of the day in winter
Cape Town – 33
Sydney – 33
Los Angeles – 34
Latitudes with insufficient UVB sunlight for around four months of the year (from November to February in the northern hemisphere)
Madrid – 40
Rome – 41
New York City – 41
Latitudes with insufficient UVB sunlight for around 6 months of the year (from October to March in the northern hemisphere)
Anywhere north (or south) of 50 degrees.
The latitude of the UK? It ranges from 50 to 58, with London at 51 and Aberdeen at 57.
So getting enough vitamin D from sunlight is easily done if you live in Miami; impossible if you live in Manchester – unless you make a point of spending time at lower latitudes during the October to March period.
If your location isn’t listed above and you want to know its latitude, that couldn’t be simpler: just type into Google the name of the town or city where you live (or the nearest to you) followed by the word latitude.
And depending on whether it’s below 30, 30-40, 40-50, or above 50, you’ll know where you are on the scale above.
It is the opinion of Harvard Medical School that, “Except during the summer months, the skin makes little if any vitamin D from the sun at latitudes above 37 degrees north or below 37 degrees south of the equator. People who live in these areas are at relatively greater risk for vitamin D deficiency.”
37! Remember, in the UK we’re at 50 to 58! Sultry Ibiza is at 38, sun-soaked Naples in southern Italy is at 40, and Cannes on France’s balmy Riviera is at 43.
Yet Harvard Medical School is not the only credible authority saying that even these Mediterranean locations – much nearer the equator than we are – don’t have the right kind of sunlight year round.
In fact, some would tell you that the figures I gave you above understate the lengths of the “vitamin D winters” in the various locations.
For example, according to Krispin Sullivan, a certified nutritionist and vitamin D researcher: “In much of the US […] six months or more during each year have insufficient UVB sunlight to produce optimal D levels. In far northern or southern locations, latitudes 45 degrees and higher, even summer sun is too weak to provide optimum levels of vitamin D.”
Sullivan is among the vitamin D experts who say that 30 is the magic number when it comes to latitude and vitamin D production: if we are further from the equator than this, we won’t be meeting our D needs year round from sunlight alone.
How long can we store vitamin D?
Vitamin D can be stored in fatty tissue during times of plenty, but when stores are not being regularly replenished, they quickly decline. In one study, submariners with zero UV exposure had vitamin D levels that declined by half in a period of two months.
Also, the body is quite limited in how much D it can produce each day and once this threshold is reached, more exposure won’t lead to more D production.
So those of us in the UK can worship the sun at every opportunity, and soak it up on a summer holiday in Benidorm, Barbados or Bali, but our stores will have fallen below optimal long before our six-month vitamin D winter is out.
Therefore, we need to have a plan in place for replenishing our vitamin D stores during those months. And if we’re not able to jet off somewhere sunny, the two choices are dietary sources of D (including oily fish such as salmon and sardines) and supplementation.
When UVB rays are present, how much sun exposure is “enough”?
There is no simple answer to this. It depends on how close to the equator you are, and on your skin colour, among other things.
If it’s summer where you are (or you’re in the tropics) and you have very fair skin, as little as 10 minutes a day could do it, assuming you are exposing most of your skin to the midday sun (i.e. wearing a bathing suit but no sunblock). You’ll need substantially longer if you’re only exposing hands, arms and face.
And the darker your skin, the more sun exposure you’ll need before you’ll synthesise enough vitamin D.
Here’s the take-home advice about vitamin D and sun exposure:
- Wherever you live, if you don’t yet know exactly when in the year UVB rays are and are not present in your location, find out.
- When there are UVB rays present where you live, try to expose your skin to the sun every day, using the guidelines above, and your own research, to determine how much sun exposure you need each day to keep your vitamin D stores replenished.
- Unless you can afford to take holidays year round, don’t go on holiday when there are UVB rays where you live – go when there are not. If you’re in the UK, this means holidaying between October and March (in locations where there is UVB sunlight at the time).
- Get your vitamin D level tested using the 25(OH) D test.
- If it is lower than the 50 ng/ml leading authorities now say is necessary for optimum health, it’s important to turn this around by including vitamin-D-rich foods in your daily diet and also supplementing for a while. This is best done under the supervision of a qualified nutritionist.