On Sunday 25 October the clocks go back, and from the Monday after that the walk from the car park to the squat rack will happen in full dark, both ways. Your gym bag, your pre-session coffee and your logbook stay exactly as they were. What changes, quietly and with no symptom you would notice on a top set of five, is that the sun stops doing a job for you. The NHS says that between October and early March we do not make enough vitamin D from sunlight, and government advice is that everyone should consider a daily supplement through autumn and winter. That advice is sound. The pitch that often rides along with it, that the right capsule will add to your deadlift, is a lot shakier, and the research sits somewhere between the two.
Why the window runs from October to March
Your skin makes vitamin D when UVB radiation in sunlight hits it, and across most of the UK that radiation is too weak to do the job for roughly half the year. The NHS puts the productive season at late March or early April to the end of September. The National Diet and Nutrition Survey report for 2019 to 2023 goes further and says that skin production of biologically useful amounts cannot occur in the UK winter, which is why dietary sources and supplements matter then. Across the whole year, 18% of adults aged 19 to 64 had a blood level below 25 nmol/L, the line under which the UK considers the risk to bones and muscles to rise. That average hides a steep seasonal swing: in January to March, the share below it ranged from 21% to 38% across the age and sex groups aged 11 and over, against 0% to 10% in July to September. Men aged 19 to 64 appeared marginally more likely than women to be under the line, by about three percentage points, although the report did not test whether that gap is statistically real. A lunchtime walk in December does not count as a plan.
The same survey has a detail that should calm you down. Average levels in October to December look much like those in April to June, and the lowest readings come in January to March, not in the weeks after the clocks change. If you spent the summer outdoors, you are probably still carrying a reserve today.
What the strength research actually shows
So does a supplement put weight on the bar? Mostly not, unless you start from a low base, and that single qualifier explains most of the contradictory headlines.
The cleanest evidence comes from pooled trials. A 2011 systematic review in Osteoporosis International by Stockton and colleagues combined 17 randomised trials with 5,072 participants and found no significant effect of vitamin D on grip strength or leg strength in adults whose blood level was already above 25 nmol/L. In the two trials on people who started below 25 nmol/L, the effect on hip muscle strength was large. A 2014 meta-analysis by Beaudart and colleagues in the Journal of Clinical Endocrinology & Metabolism, covering 30 trials and 5,615 people, landed in the same place: the strength benefit was significantly bigger in people who began below 30 nmol/L. Neither review was designed around lifters, so treat them as a pattern rather than a verdict on your squat.
Trials in athletes muddy the picture rather than clearing it. A 2019 meta-analysis in PLOS ONE by Zhang and colleagues pooled eight trials with 284 athletes and found no effect on overall muscle strength. Split by limb, lower-limb strength did improve (a standardised effect of 0.55) while upper-limb strength and vertical-jump power did not, and the benefit was larger in athletes who trained indoors. A separate meta-analysis from 2015 by Tomlinson and colleagues, covering seven small trials in healthy 18-to-40-year-olds, did find an effect of 0.32 for both upper and lower limbs. It covered only 310 people, two-thirds of them women, and the doses ran from 4,000 IU a day to 60,000 IU a week, the top of which is well over the NHS ceiling.
The study closest to home is the 2013 paper by Close and colleagues in the Journal of Sports Sciences. They measured winter blood levels in 61 UK-based athletes and 30 healthy non-athletes, and 62% of the athletes and 73% of the non-athletes were below 50 nmol/L. In the second half of the paper, 5,000 IU of vitamin D3 a day for eight weeks improved 10 m sprint time and vertical jump compared with placebo, which showed no change. The group taking the vitamin began with an average of 29 nmol/L, so they were low to begin with, which fits the pattern in the larger reviews. It was also a small trial, and 5,000 IU a day sits above the daily ceiling the NHS sets for adults.
Put together, the honest reading is narrow. If your level is comfortably above 50 nmol/L, a tablet is very unlikely to move your squat, and the claim on the tub is running ahead of the evidence. If you come out of winter at 20 or 30 nmol/L, which in the UK is far from rare, correcting it is worth doing, and the plausible gain is concentrated in the legs. Either way, a few pounds is the whole downside.
The dose, the form and the timing
The NHS dose is 10 micrograms a day, which is 400 IU on labels that still use International Units (1 microgram equals 40 IU). It is what the survey data and the official advice are built around, and it is cheap: Holland & Barrett was listing a 240-tablet pack of 10 µg tablets at £5.59 in early October, roughly 2p a tablet, so one pack covers the whole October-to-March stretch with tablets to spare. Buy that, not a premium blend.
- 10 µg (400 IU) once a day from the clock change in October until around the end of March.
- D3 (cholecalciferol), not D2. A 2012 meta-analysis by Tripkovic and colleagues in the American Journal of Clinical Nutrition found D3 raises blood levels more efficiently.
- With a meal you never skip. Vitamin D is fat-soluble, and a trial by Dawson-Hughes and colleagues found D3 was absorbed better with a meal containing fat than with a fat-free one, although other work disagrees on how much fat helps, so there is no need to engineer the plate.
- Not the 4,000 IU tubs, or any other high-strength blend with muscles and immunity on the label, because you would be sitting on the NHS ceiling without knowing your starting level.
That ceiling is 100 micrograms (4,000 IU) a day for adults, and the NHS is blunt about why: taking too much for a long time can cause calcium to build up in the body, which can weaken the bones and damage the kidneys and the heart. A dose above it, like the 5,000 IU used in the Close trial, is a trial condition rather than a template.
Do not count on food to cover the gap. The NHS lists oily fish such as salmon, sardines, trout, herring and mackerel, red meat, egg yolks and fortified foods like some fat spreads and breakfast cereals, but says it is difficult to get enough from food alone. It also notes that cows' milk in the UK is generally not a good source because it is not fortified. For a lifter who leans on milk for protein, that last line surprises people.
Who should not wait for October
The autumn-and-winter rule has exceptions. The NHS recommends a year-round 10 microgram supplement for adults who are not often outdoors, such as people who are frail or housebound, for people in institutions such as care homes, and for people who usually wear clothes that cover most of their skin when outside. It also says that people with dark skin, for example those with an African, African-Caribbean or south Asian background, may not make enough vitamin D from sunlight and should consider a daily supplement throughout the year. If either description fits you, the October start date does not apply: take it all year.
A blood test is probably not worth chasing, and your GP is unlikely to order one. NICE's clinical knowledge summary for GPs says not to routinely test asymptomatic people for vitamin D deficiency, including those at higher risk, so a symptom-free lifter is left with the plain 10 microgram supplement. Persistent bone pain is a different matter, since the NHS links a lack of vitamin D to osteomalacia, which shows up in adults as bone pain, and that is a conversation for a doctor rather than the supplement aisle. If you do get a result, UK labs report it in nmol/L, and NICE treats below 25 as deficient, with 25 to 50 possibly inadequate in some people. American lifting material usually quotes ng/mL instead; multiply by 2.5 to convert, so the "30 ng/mL" target you see online is roughly 75 nmol/L, well above the UK thresholds.
A simple way to run the winter
Treat the clock changes as bookends. Start when the clocks go back on Sunday 25 October, leave the tub next to whatever you eat at breakfast, and stop once spring is properly underway, after the clocks go forward on 28 March 2027, when the NHS says most people can make what they need from sunlight again. Then go back to worrying about the bench press.