Vitamin D is stored in the body's fat tissue. In obesity, large fat stores can sequester vitamin D, leading to lower serum levels of the major circulating form, 25(OH)D, despite higher total stores—resulting in nearly double the risk of deficiency compared to normal-weight individuals. The risk of vitamin D deficiency also rises in winter when sunlight (UVB exposure) is limited.

However, exercise can help elevate vitamin D levels—particularly during winter months when UVB exposure is low and deficiency risk is high, in part due to its ability to mobilize vitamin D stores from fat tissue. Just one hour of indoor aerobic exercise can boost circulating levels of active vitamin D by 35%. Regular exercise also appears to be beneficial.

A new study found that performing around 160 minutes per week of mixed-intensity exercise completely prevented the seasonal decline in the active form of vitamin D (1,25(OH)₂D₃). Exercisers also experienced a significantly smaller drop (15%) in overall vitamin D status compared to non-exercisers (25%).

Interestingly, exercise didn't simply "release" stored vitamin D from fat tissue. Instead, it improved the body's vitamin D metabolism, increasing the hormone's availability in ways that supplementation alone often can't—especially in obesity.

Regular aerobic exercise–along with adequate sunlight exposure and vitamin D supplementation if needed—might be your best tool for maintaining optimal vitamin D levels year-round.

Why is Vitamin D Deficiency So Common?

Vitamin D is unique among vitamins because our bodies primarily produce it through sunlight exposure. Specifically, UVB radiation from the sun kick-starts the synthesis of vitamin D in our skin. However, several factors severely limit our exposure to UVB rays:

• Sunscreen: Sunscreen blocks UVB rays, which are necessary for vitamin D synthesis in the skin. SPF-15 and SPF-50 sunscreens filter out 93% and 90% of UVB rays, respectively, allowing 2–7% of UVB to reach the skin.
• Northern latitudes and winter months: If you live above a certain latitude (like much of North America or Europe), during winter the angle of the sun significantly reduces UVB availability. This alone can cause vitamin D levels to drop by 37–50%.
• Skin pigmentation: Melanin serves as a natural sunscreen, but it also blunts the body's ability to make vitamin D from UVB radiation. People with darker skin may need up to 10 times longer sun exposure to produce the same amount of vitamin D as those with lighter skin.
• Indoor lifestyles: Most of us spend the majority of our days indoors, away from sunlight, further limiting vitamin D synthesis.
• Obesity: Studies show obesity can double your risk of vitamin D insufficiency. Obese adults produce 57% less circulating vitamin D₃ after UV exposure compared to their leaner counterparts; however, weight loss can increase vitamin D levels.

Supplementation can indeed boost vitamin D levels; however, vitamin D supplements don’t significantly impact the levels of circulating 1,25(OH)₂D₃—the active form of vitamin D responsible for its beneficial effects on your bones, immunity, metabolism, and muscles.

But here's some good news: Recent research points to an effective strategy for optimizing vitamin D levels—exercise.

Exercise Releases Stored Vitamin D

We store vitamin D in our body's adipose (fat) tissue. Losing weight can improve vitamin D status by releasing these stored reserves, with some studies showing that losing 15% of one's body weight can increase vitamin D levels nearly three times more than losing just 5–10%. But you don't necessarily need to shed pounds to reap vitamin D benefits—exercise alone can trigger a similar liberation of stored vitamin D, thanks to its ability to mobilize fat.

Just a single 60-minute session of moderate-intensity exercise can boost circulating 25(OH)D levels by about 10% and the biologically active form, 1,25(OH)₂D₃, by a striking 35%. While 25(OH)D quickly returns to baseline, elevated levels of the active form can last up to an hour post-exercise.

Short-term boosts are promising, but regular exercise offers even greater benefits–even when it's not accompanied by weight loss. One recent study provided compelling evidence.

Long-term Benefits of Regular Exercise on Vitamin D

Researchers conducted a 10-week exercise trial during winter—precisely when UVB exposure is minimal. Participants were overweight or obese adults split into two groups. One group was an exercise group who performed four sessions weekly (all indoors), including two treadmill walking sessions (targeting maximal fat oxidation rate), one steady-state aerobic cycling session (60–75% VO₂ max for 30–40 minutes, e.g., "zone 2 training"), and one session of high-intensity interval training (HIIT; 8–10 intervals of one minute at 90–95% maximal heart rate alternating with one minute of recovery).

Participants who exercised regularly throughout the winter fully maintained their levels of the active form of vitamin D. In contrast, those who didn't exercise saw their active vitamin D levels drop by about 15%.

Interestingly, both groups experienced a decline in overall vitamin D status (25(OH)D)—but exercisers had a significantly smaller decrease (15% drop) compared to the non-exercisers (25% drop).

Contrary to the researchers' hypothesis, these benefits occurred without significant changes in the vitamin D stored in adipose tissue. This means exercise didn’t merely pull stored vitamin D from fat—it actively optimized its metabolism throughout the body. Exercise seemed to make the body’s vitamin D metabolism more efficient, allowing better utilization of available vitamin D substrates.