Vitamin D in Winter: Not a Muscle Shortcut

Written by INNER G

A 1988 study set out to find when winter sunlight stops making vitamin D in skin. On cloudless days in Boston (42.2 degrees north), it made none from November through February. In Edmonton (52 degrees north), none from October through March (Webb et al., 1988).

Vaughan sits at about 43.8 degrees north (Environment and Climate Change Canada, n.d.). That's a bit past Boston, and nowhere near Edmonton. Nobody tested Vaughan, so I can't give you an exact date. But it's fair to read it this way: from about now until March, the sun isn't doing much for your vitamin D.

That's when vitamin D talk picks up, and a lot of it is about muscle and strength. Here's what the research backs up, and what it doesn't.

Please read first. This post is general information, not medical advice. How much vitamin D you need, and whether a supplement makes sense for you, is a question for your doctor or pharmacist. If you have new muscle weakness or pain that won't go away, get medical care.

When winter sun makes no vitamin D

Months when sunlight made no vitamin D in skin, from a 1988 study that tested it on cloudless days at several latitudes. Three are shown. Gold months worked. Grey months did not. Pick a place.

Source: Webb AR, Kline L, Holick MF. J Clin Endocrinol Metab 1988;67(2):373-378. Read the study. Vaughan is at about 43.8 degrees north (Environment and Climate Change Canada station list, Woodbridge 43°47′ N). It was not tested in this study.

Why winter sun doesn't count

Health Canada says your body can make vitamin D from sunlight, and it lists what cuts that down: smog, season, time of day, cloud cover, sunscreen, limited time outdoors and how much skin is showing (Health Canada, n.d.). In a Canadian winter, several of those hit at once. The NIH adds that the UVB part of sunlight doesn't pass through glass, so a sunny window doesn't count either (NIH Office of Dietary Supplements, n.d.).

This isn't a reason to chase sun or tanning beds. UV is linked to skin cancer, and the NIH says limiting skin exposure to sunlight and tanning beds is prudent.

What that does to blood levels in Canada

Less sun doesn't mean most people end up low. A national survey of 21,770 people found an average blood level of 57.9 nmol/L. About 19.0% were below 40 nmol/L, which that study called inadequate, and 8.4% were below 30 nmol/L, which it called a risk of deficiency. The authors concluded that most people in Canada have adequate vitamin D status (Weiler et al., 2023).

Some groups were more likely to be inadequate. People who didn't take a supplement had about 5 times the odds of being inadequate (adjusted odds ratio 5.21). Not eating fish weekly raised the odds 1.6 times. A BMI of 30 or more and some racialized groups also had higher odds, so "most people" doesn't mean everyone (Weiler et al., 2023).

Near Ottawa, researchers measured blood levels in spring to stand in for winter and in fall to stand in for summer, in 669 people. The seasonal gap was small in both groups they studied, and only clear enough to count statistically in one of them (2.9 nmol/L) (Nagasaka et al., 2018). Winter makes it harder to get vitamin D. It doesn't crash everyone.

Where vitamin D comes from besides sun

Health Canada's advice is to include vitamin D from food or a supplement every day. Very few foods have it naturally, and most dietary vitamin D comes from fortified foods. Its list includes egg yolk, soft margarine, fatty fish like salmon, arctic char and rainbow trout, unsweetened lower fat milk and unsweetened fortified plant drinks (Health Canada, n.d.).

The calculator below uses NIH food values. For scale, the NIH daily recommended amount for most people is 600 IU, and the upper limit for food and supplements together is 4,000 IU. Too much vitamin D can be harmful, and muscle weakness is one of the signs (NIH Office of Dietary Supplements, n.d.). The NIH tables break these amounts down by group, so the numbers here are shortened.

What a day of vitamin D foods adds up to

Pick how many servings you eat in a day. Values are IU per serving from the NIH Office of Dietary Supplements. It's a rough guide, not a test.

400600

Source: NIH Office of Dietary Supplements, Vitamin D fact sheet for health professionals, Table 3. See the data. Milk is the Canadian minimum of 35 IU per 100 mL, worked out for 250 mL. Plant drinks vary, so check the label. 400 IU is the daily supplement amount in Health Canada guidance. 600 IU is the NIH daily recommended amount for most people (their table has other amounts for some groups). The upper limit for food and supplements together is 4,000 IU. Talk to your doctor or pharmacist about what is right for you.

We won't give you a dose. That's for your doctor or pharmacist. If you want the basics on vitamin D next to other supplements, read our post on which supplements are worth it.

What vitamin D does for muscle

When your level is truly low, it can matter. The NIH says low vitamin D can affect muscle strength and cause weakness and pain. It also says trials of supplements for muscle have had inconsistent results (NIH Office of Dietary Supplements, n.d.).

The reviews show that mixed picture. A review of 30 trials with 5,615 people found a small gain in strength (standardized mean difference 0.17). It found no clear effect on muscle mass or muscle power. The strength gain was larger in people who started with a blood level under 30 nmol/L (Beaudart et al., 2014). A review of 7 trials with 310 healthy people found a larger strength gain for upper and lower body (0.32 each). For the lower body, the range of possible values barely cleared zero (0.01 to 0.63) (Tomlinson et al., 2015).

Then there's the group most likely to take it for performance. A review of 11 trials with 436 athletes found no statistically significant effect on maximal strength or power, and the authors said more research is needed (Sist et al., 2023).

So vitamin D may give a small strength gain, mostly if you're low to begin with. Those numbers are small next to what training does.

What vitamin D supplements did in trials

Pooled results from reviews of randomized trials. Pick an outcome.

Sources: Beaudart C, et al. J Clin Endocrinol Metab 2014;99(11):4336-4345 (study). Bolland MJ, et al. Lancet Diabetes Endocrinol 2018;6(11):847-858 (study). Massé O, et al. BMJ 2026 (study). A difference of 0 or a ratio of 1.00 means no effect.

What it does for falls

You'll hear that vitamin D keeps people steady on their feet. The two big reviews don't back that up. One pooled 37 trials with 34,144 people and found no effect on falls (risk ratio 0.97, range 0.93 to 1.02) (Bolland et al., 2018). A 2026 review in the BMJ pooled 69 trials with 153,902 people. It found little to no effect on fractures and falls, mostly at moderate to high certainty (Massé et al., 2026).

One 2024 analysis of 35 trials disagrees in part. It found fewer falls at 800 to 1,000 IU a day (risk ratio 0.85), and the benefit showed up in people whose blood level was 50 nmol/L or lower (0.69), not in people above that. It also found that doses over 1,000 IU a day came with more falls than 800 to 1,000 IU (Tan et al., 2024). Put the reviews side by side and one point holds up: if vitamin D helps with falls, it looks limited to people who start low. Your doctor can tell you where you stand.

Balance is something you can train. Our post on balance exercises and fall prevention covers that, and we also offer one on one training focused on balance and strength.

What builds the muscle

In a 16 week trial at 56 degrees north, 40 healthy men who hadn't trained before were randomly given vitamin D plus calcium or calcium alone, from December to April when sunlight is low. For the last 12 weeks, they did progressive leg training. Both groups gained muscle size and strength, with no difference between them. The authors did note some changes in muscle fibre type and muscle quality in some of the men, so they didn't call the effect zero for everything (Agergaard et al., 2015).

A 12 week trial found the same thing from another angle. Men with low vitamin D did a supervised strength program, and half took 8,000 IU a day. Their blood levels rose 2.6 times. Strength and lean mass gains were still similar in both groups (Savolainen et al., 2024). It was small, with 28 men, and the groups weren't truly randomized, so treat it as a pointer. And 8,000 IU a day is double the NIH upper limit, so it's not a dose to copy.

In both trials, the training did the work. Canada's movement guidelines call for muscle strengthening activities at least twice a week (Ross et al., 2020). If you want a supplement that's been studied alongside lifting, read our post on creatine and strength training.

What to do this winter

  1. Eat a vitamin D food most days. Use the calculator above to see where you land.
  2. Ask about it if you're unsure. If you rarely eat these foods, or you're worried about your level, talk to your doctor or pharmacist about a blood test and whether a supplement makes sense.
  3. Lift at least twice a week. That's what Canada's guidelines call for.
  4. Watch for weakness. Muscle weakness or pain that won't go away is worth a visit to your doctor.

Get stronger this winter with a coach in Vaughan

Dark afternoons make it easy to skip training. A set time in the studio with a coach fixes that, and it's where the muscle comes from.

Start with our small group classes, where a coach adjusts every exercise to you and each class is capped at 16 people. The studio is open from 5:45 AM to 8 PM on weekdays and from 8:30 AM to 4 PM on weekends, and there's parking on site.

If you'd rather work one on one, see personal training in Vaughan. Our nutrition coaching can help you work vitamin D foods into your week.

Try the $30 seven day trial: 7 days of unlimited classes for $30 (+ HST). Join by day 5 and your $30 is credited toward your membership.

Call (289) 536-8066 or email g-squad@innerg.ca.

References

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