The Winter Vitamin D Problem (And How Exercise Helps Fix It)

TOO LONG, DIDN’T READ (TL;DR):
Winter makes it harder for your body to maintain healthy vitamin D levels. A winter-based study found that people who exercised regularly lost less vitamin D than those who didn’t—even without weight loss, extra sun exposure, or supplements. Movement may help your body better store and use vitamin D, making exercise an underrated tool for supporting bone health, immune function, muscle repair, and overall well-being during the darker months.

Let’s talk about another benefit of exercise.

Not weight loss.
Not aesthetics.
Not even stress relief (though yes, that one still counts).

This one’s a little less obvious—and a bit more science-y—but it matters, especially in the winter when energy is low, motivation is shaky, and a lot of people feel “off” without knowing exactly why.

We’re talking about vitamin D.

Most of our vitamin D comes from sunlight, specifically UVB rays. But here’s the catch: those same rays are also linked to skin cancer. So many of us do what we’re supposed to do—wear sunscreen, cover up, spend more time indoors—and unintentionally limit how much vitamin D our bodies can make.

Then winter hits.

Shorter days.
Weaker sunlight.
More time inside.

And for anyone living in northern latitudes, vitamin D levels tend to drop across the board.

Yes, you can get vitamin D from food and supplements. But research is starting to suggest something interesting: movement itself may help your body hold onto vitamin D during the winter months.

Before we get into the study, a quick story.

A quick detour through history

If you’ve ever read A Christmas Carol or watched one of its many adaptations, you probably remember Tiny Tim. Frail, soft-spoken, but full of heart.

Since Charles Dickens published the story in 1843, many historians and medical experts have suggested that Tiny Tim may have suffered from rickets—a bone-weakening disease we now know is caused by severe vitamin D deficiency.

Victorian-era London was almost perfectly designed for that outcome: dark skies from coal smoke, cramped housing, very little sunlight, and limited access to nutrient-dense food. If rickets was what Dickens was describing, Tiny Tim became an early and powerful example of why vitamin D matters so much.

Not just for bones—but for muscle repair, immune health, calcium absorption, and metabolic regulation.

Fast-forward to today, and while we’ve solved some problems, winter vitamin D deficiency is still incredibly common.

So researchers started asking a different question.

Instead of focusing only on how much vitamin D we consume, what if we looked at how well our bodies use and maintain it?

That question led to a winter-based study in the UK.

What the researchers wanted to know

A research team at the University of Bath recruited 51 healthy but sedentary adults. The average age was 49, and all participants were classified as overweight or obese.

That detail matters because people with higher body fat are almost twice as likely to have low circulating vitamin D levels. Vitamin D gets stored in fat tissue, which makes it harder for the body to access and use.

Because the study took place during the Covid-19 pandemic, only 41 participants completed it. They were split into two groups: one group followed a structured exercise program, and the other continued life as usual.

The study ran for 10 weeks during winter. The exercise group completed four workouts per week, each lasting about 40 minutes.

A detail that makes this study different

Here’s where things get interesting.

The researchers did not want weight loss to influence the results. Losing fat releases stored vitamin D, which could artificially boost levels.

So participants were instructed to eat more as their activity increased to keep their weight stable.

It worked—almost too well.

Both groups gained about a pound on average. Body fat stayed the same in the exercise group and increased slightly in the non-exercising group.

In other words, any changes in vitamin D weren’t coming from fat loss.

What actually happened to vitamin D levels

As expected, vitamin D levels dropped in both groups over the winter.

But the drop wasn’t equal.

The exercise group saw about a 15 percent decrease.
The control group saw closer to a 25 percent decrease.

Even more notable? The exercise group maintained levels of calcitriol—the most metabolically active form of vitamin D.

Calcitriol acts more like a hormone than a vitamin, and every single cell in the human body has receptors for it. This is the form responsible for many of vitamin D’s benefits throughout the body.

Ideas to move forward

So what does this mean for real life?

First, exercise may help your body protect vitamin D levels during winter—even without weight loss, extra sun exposure, or supplements.

The researchers suggested a few reasons why. Exercise increases fat oxidation, which may release some stored vitamin D. It may also improve how efficiently the body metabolizes and uses the vitamin D already available.

And as a bonus, participants improved their cardiovascular fitness in a meaningful way—without chasing weight loss.

Second, muscle may play a bigger role in vitamin D storage than we once thought.

Blood tests measure calcifediol, an intermediate form of vitamin D that sticks around in circulation for weeks or even months. One hypothesis suggests it moves in and out of muscle tissue during that time.

That makes sense. Vitamin D supports muscle repair, mitochondrial function, and recovery. Resistance training, in particular, activates vitamin D receptors in muscle cells.

We don’t yet know whether strength training has the same effect on vitamin D levels as the cardio used in this study—but if it does, it’s yet another reason why varied, consistent movement matters.

Not because exercise fixes everything.

But because the body adapts—often in ways we’re only beginning to understand.

And sometimes, simply moving your body helps it hold onto what it needs most, even when the sun is hard to find.