Vitamin D for endurance athletes: why UK winters cost you strength
From October to March the UK sits too far north for the sun to help. UV-B reaches us at such a low angle that the atmosphere filters most of it out, so your skin makes almost no vitamin D for about half the year. Do most of your training indoors through those months, like most Hyrox athletes, and you're running on summer stores and whatever's on your plate.
Vitamin D matters for endurance athletes because it acts on skeletal muscle. It behaves like a hormone, with receptors in muscle tissue and a hand in how well those fibres contract.
What low vitamin D does to a trained athlete
The muscle effect is the one to pay attention to. Deficiency is linked to weakness in the big muscles around the hips and shoulders and to smaller type II fibres. Those are the fibres pushing the sled, throwing the wall balls and holding your 1km runs together when you're tired.
A study on collegiate athletes found that poor vitamin D status tracked with lower strength and lower power output.
Bone is the other half. A 2021 review of vitamin D in athletes links poor status to stress fractures and other musculoskeletal injuries, and to slower recovery from the muscle damage that follows hard sessions. If you're stacking running volume on top of Hyrox strength work, take that seriously.
Supplementing only helps if you're low
This bit saves you money. A meta-analysis of randomised controlled trials on vitamin D3 and limb strength found the benefit sat mostly in athletes who started out deficient. Fix a real shortfall and strength improves. Pile more on top of a healthy level and very little happens.
So should you take vitamin D? Probably yes if you live in the UK and it's winter, and definitely yes if a blood test says you're low. Past that point you're buying expensive urine.
How common is low vitamin D in athletes
It's more common than you'd guess. Studies of professional footballers in Liverpool and of American football players have found deficiency in a sizeable share of squads. The pattern across the research is consistent: status drops in winter, and athletes who train indoors sit lower than those who train outside.
Two things put Hyrox athletes in the risk group. Most of the training happens under a gym roof. And race season runs right through the months when UK sunshine does nothing for you.
The dose, and the ceiling
The NHS position is 10 micrograms (400 IU) a day for adults, and everyone in the UK is advised to consider a daily supplement through autumn and winter. The upper limit is 100 micrograms (4,000 IU) a day. Going above that without a doctor involved is a bad trade.
A few practical notes:
Take it with fat. Vitamin D is fat-soluble, so take it with a breakfast that has eggs, yoghurt or oil in it.
Take it daily. Big weekly or monthly doses are popular and poorly supported. Small and consistent is the better habit anyway.
Magnesium is a cofactor in vitamin D metabolism, so a diet that's always short on it makes correction harder. Check your intake if you're supplementing and your numbers aren't moving.
Food gets you part of the way
Very few foods carry much vitamin D, which is why UK advice goes straight to supplements.
Oily fish is the best source by a distance. A 100g portion of salmon, mackerel or sardines gives you roughly 8-15 micrograms depending on whether it's farmed or wild. Eat it twice a week and you've covered a decent chunk of what you need.
Egg yolks add a little. Fortified cereals and some plant milks add a bit more. UV-exposed mushrooms are the only real plant source, and you need the ones labelled as UV-treated, because standard button mushrooms grown in the dark contain almost none.
Add it all up on a normal British diet in January and you'll likely land well short of 10 micrograms a day. The supplement is there to close that gap.
Who's most at risk
Four groups tend to sit lower than average, and endurance athletes often tick more than one box.
People with darker skin need a lot more UV exposure to make the same amount of vitamin D, and in a UK winter there's no amount of time outside that covers it. Anyone training almost entirely indoors gets less sun than they think, even in summer. Athletes in a long-term energy deficit tend to have poorer status across most micronutrients. And anyone who covers up for religious or practical reasons gets less skin exposure all year.
If two or more of those describe you, get tested this year.
Get tested
A 25(OH)D blood test costs about £30 privately in the UK and needs a finger prick. Book it for late February or early March, when your level bottoms out. That number tells you whether you actually have a problem, so your plan is built on data.
If you test one thing this year, make it this or ferritin. They're the two that most often come back surprising in endurance athletes, and both are cheap to check and fixable.
Where it sits in a Hyrox stack
Vitamin D sits in the boring daily tier: things you take for months and never feel. That tier also includes creatine at £12.99, enough sodium and enough total food. You won't feel any of it on the day, and all of it changes what you can do over 12 weeks.
The other tier is what you time around a session, which is where Pre-Shot sits at £10.99 for 6. It does a different job on a different timeline.
Get the daily stuff right first. It's cheaper, and it's the half most people skip because they never feel it working.