VITAMIN D IN WINTER: ENOUGH IS NOT THE SAME AS OPTIMAL
Summer is nearly over and from October until March, your skin stops making vitamin D. For most people, that comes as a surprise.
Part of the reason is a piece of advice that is everywhere at the moment, which is to get ten minutes of daylight first thing in the morning. It is good advice, and I give it myself. The problem is where most of it comes from. Many of the podcasts that repeat this advice record in California, where the midwinter sun still climbs high enough to do things our sun simply can’t, and they never seem to discuss that detail.
Here in the UK, in December, the sun is not up until around 8am and it is setting before 4pm. Plenty of us have had breakfast, been to the gym, dropped the kids off and at our desks before the first light of the day, and that is assuming the weather gods haven’t sent clouds! So every time I hear that advice, I wonder why nobody in the UK has put their hand up and shouted, “Yes, but what do we do instead?”.
And it works through a completely different route. Vitamin D is made in your skin. The morning light signal is read by your eyes, and it never becomes a nutrient at all. Light lands on a set of cells in the retina that are not the ones you see with, and they send a signal along a dedicated nerve pathway straight to a cluster of around 20,000 cells in the hypothalamus called the suprachiasmatic nucleus. That is your master clock, and it sets the timing for everything downstream, including the hormones.
Same sunshine. Two organs. Two entirely separate jobs.
And it does that job whatever the angle of the sun, which is why it still works in January. We know this because of what happens when you take the cue away. Put people somewhere with no natural light and no clocks, and their sleep and wake times do not hold to the day outside. They drift later, little by little, until they are completely out of step. The internal clock runs at roughly 24.2 hours rather than 24, so without a daily reset it slides. Morning light is that reset.
This is where more understanding needs to come in. That advice was never about vitamin D. It is about exactly that reset, about letting your body know the day has started so that hormones arrive when they should and digestion falls into place. It does that job whatever the angle of the sun, which is why it still works in January. People may believe they are protected by equating morning light with vitamin D production, but these are two different things.
But back to the task at hand...
“Your skin stops making vitamin D”, let me reiterate - not slows down, it stops (more or less). At our latitude, the sun sits too low in the sky through the winter months for the UVB wavelengths that drive the reaction to reach us in any useful quantity. You can stand outside all day in October, feel perfectly warm, even catch a bit of colour in your cheeks, but still make almost zero Vitamin D. Which means whatever you have stored as of the end of summer is roughly what you are carrying into winter, minus whatever you use along the way.
Do you know what your levels are? No, kinda? You are not alone. Most people have no idea what their number is, and of those who do know, must have been told it is normal without ever being told what normal means.
That concerns me because the knock-on effects run further than people realise. Without a baseline, you are guessing about supplements, taking something you may not need or skipping something you do. You might change your diet on the strength of that guess, cutting foods out or adding them in, none of is warranted. And underneath all of it, a process that depends on a nutrient you are short of carries on running slowly. That tends to show up as the ordinary things people put down to life. Tiredness a good night's sleep does not fix, recovery taking longer than it used to, and catching everything going round the office. None of it dramatic but all of it adding up to performing below what you capable of.
This is the part of my work I care most about changing.
Vitamin or hormone?
It’s worth getting this out of the way because it explains why vitamin D turns up in conversations about so many things. Vitamin D behaves like a hormone rather than a nutrient. Your skin produces it, your liver converts it to 25(OH)D, which we measure in blood tests, and then the kidney converts it again into its active form. There are receptors throughout the body, including in skeletal muscle, in immune cells, and in the lining of the gut. That is why it is involved in calcium absorption, bone mineralisation, immune function, and in muscle. It is not a bone supplement that does other things, it is a signalling molecule that fundamental in bone health. If vitamin D were a player on a football team it would be the midfielder, the one everything goes through, involved in almost every play without being the one who gets on the front page.
The Manchester study
They took 120 adults aged between 20 and 60 and gave them simulated summer sunlight at the recommended level, calibrated to the sun available at 53.5 degrees north, the latitude of Manchester. Thirteen minutes around midday to about a third of the skin, roughly what you would have uncovered in a t-shirt and shorts. Three times a week for six weeks, built to copy the type of sun exposure you might experience over the school summer holiday. They measured blood 25(OH)D throughout the study instead of only once at the end.
No clouds, no missed sessions, nobody forgetting. Everyone got the full recommended dose perfectly for six weeks. This is the best case, and it is not real life.
So you would think the numbers would be good, right? No, it got most people to sufficiency, which they defined as 20 ng/ml, or 50 nmol/L in the units UK labs use. Sufficient, not good, just sufficient. It did not get them to the proposed optimal of 32 ng/ml, or 80 nmol/L. To the researchers’ credit, the title of the paper spells this out.
But we just had the hottest summer on record?
Apologies if you are reading this from somewhere else in the world, but we had quite a summer in the UK. Record temperatures, air conditioning selling out, and my parents, who escaped the Egyptian summer, reckoned it was hotter here.
Here is the first thing that surprises most people. Heat on its own tells you very little about vitamin D. Synthesis depends on how high the sun sits in the sky, not on how warm the air is, and a hot day is not automatically a high UVB day. This summer was not only hot, it was provisionally the hottest on record, it was also England’s second sunniest summer since records began in 1910.
So, the conditions for making vitamin D were about as good as this country has ever offered. But when it hits 38 degrees, people go indoors, blinds go down, air conditioning turns up and if you venture out you tend to avoid the middle of the day, which unfortunately is the peak time for when your skin does its Vitamin D work.
Normal is not Optimal
This is the part I want you to pay close attention to as it applies well beyond vitamin D.
Sufficient means you do not have a deficiency disease. Below roughly 25 nmol/L, you reach the point at which bone damage starts to occur in measurable ways. 50 nmol/L is the level considered adequate for most of the population, which is a distinct thing: not ill, just covered. Optimal is a third claim, again, the level associated with your body’s capacity to perform rather than with the absence of a problem.
So, when your results come back and somebody says they are normal, they mean you are above the floor. But it is a different question from whether you are where you would want to be; most people are never told that two questions exist.
Honesty time. People strongly debate what is truly optimal. The Institute of Medicine sets at 50 nmol/L adequacy. The Endocrine Society suggests 75. The Vitamin D Society recommends 100 to 150. Three bodies, three answers, a threefold spread between the lowest and the highest. When somebody quotes you an optimal number, including me, ask who’s optimal they mean.
There is also a point here that cuts against most of what you will read in sports. Correcting a real deficiency matters a great deal. The research shows that pushing levels higher in someone who is already sufficient has not reliably improved performance, which means much of the optimisation conversation targets the wrong part of the problem.
But the distinction between a floor and a target is real, whatever number you put on the target. And knowing which side of it you are on is not contested at all.
The same advice does not work for everybody
The same group published a companion study asking whether the recommended summer sunlight exposure produced sufficient vitamin D in UK adults of South Asian origin. It did not. Not optimal, and not sufficient either. The numbers are worth showing you as they are fascinating! The identical exposure amounts raised blood vitamin D by an average of 10.5 ng/ml in the white group and 4.3 ng/ml in the South Asian group. Less than half! 90% of the white group reached sufficiency.
Melanin absorbs UVB, and its role is protective, which is the point of it. The trade-off is that more melanin in the skin means more exposure is needed to make the same amount of vitamin D. Public health advice is written as though skin is a constant, and it is not. That’s one recommendation, given to everybody, producing adequate status in some people and inadequate status in others, depending on something nobody is ever asked about.
That is personalised nutrition in a single example, and it is why I find general advice so unsatisfying as a starting point.
Why this matters if you train
Most vitamin D articles stop its importance at bone level. For anyone training seriously, there are three further reasons to care:
Muscle. Vitamin D receptors are present in skeletal muscle, and deficiency is associated with weakness, characteristically in the large muscles closest to the trunk. If you are putting in the work and the output is not matching it, this is one thing worth ruling out.
Bone. Stress fractures are an actual cost in high-volume training, and bone mineralisation depends on vitamin D more than it depends on calcium intake alone. You can eat and drink all the calcium you like and absorb only a fraction of it.
Immune function. Illness and low vitamin D peak in the same months, and a heavy training load is already an immune demand. Correlation rather than proof, but the mechanism is plausible, and the timing is not a coincidence.
One honest caveat to all of this. The evidence that correcting a deficiency improves these things is reasonable. However, the evidence that taking more when you are already replete improves anything is much weaker. More is not better; enough is better, and then the benefit stops. Which is exactly why knowing your number matters more than knowing the recommendation.
What these studies do and do not establish
They used controlled exposure rather than observing people living their normal lives, so what happens in real life may differ in either direction.
The optimal threshold is proposed rather than agreed.
And the studies tell us about status achieved from sunlight. They do not tell us what happens to clinical outcomes at different levels, things like fractures, infections, or muscle strength, which is a separate and much harder question. What they establish is narrower and still useful. Following the guidance gets most people to sufficiency and no further, and for some skin types it does not reach sufficiency at all.
What to actually do with this - 3 key points
If you are going into winter and you have never measured it, this is the month to find out, because this is the month your stores stop being replenished.
If you supplement, pay attention to magnesium, because the enzymes that activate vitamin D require it. Taking one without adequate status in the other is a common inefficiency.
And if somebody tells you your level is normal, it is entirely reasonable to ask them which number they are comparing it against.
If you would rather know than guess
Everything above comes down to one thing. You cannot make a sensible decision about vitamin D, or about most things, without knowing where you are starting from. Not whether you are deficient. Where you actually are.
That is the work I do. Reading a test properly against optimal ranges rather than just the reference range, alongside everything else going on for you, and then working out what is worth doing and what is not.
I offer a free discovery call. It works both ways, so you are deciding whether I am the right person to help as much as I am deciding whether I can.
References:
Rhodes LE, Webb AR, Fraser HI, Kift R, Durkin MT, Allan D, O'Brien SJ, Vail A, Berry JL. Recommended summer sunlight exposure levels can produce sufficient (≥20 ng/ml) but not the proposed optimal (≥32 ng/ml) 25(OH)D levels at UK latitudes. Journal of Investigative Dermatology. 2010;130(5):1411-1418.
Farrar MD, Kift R, Felton SJ, Berry JL, Durkin MT, Allan D, Vail A, Webb AR, Rhodes LE. Recommended summer sunlight exposure amounts fail to produce sufficient vitamin D status in UK adults of South Asian origin. American Journal of Clinical Nutrition. 2011;94(5):1219-1224.
Czeisler CA, Duffy JF, Shanahan TL, Brown EN, Mitchell JF, Rimmer DW, et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science. 1999;284(5423):2177-2181.
Met Office. Summer 2026 provisionally hottest on record for the UK. 1 September 2026, updated 17 September 2026.
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