To produce vitamin D, the body needs direct sunlight, especially ultraviolet B rays, but during the winter in northern climates, sunlight is often not strong enough to trigger vitamin D production. If you live in one of these areas, you could go months without making any vitamin D naturally. And the more you weigh, the higher your risk of deficiency.
Vitamin D isn’t just for your bones. It helps regulate immune function, inflammation, insulin sensitivity, and muscle health. The most active form causes these effects by binding to receptors in almost every organ system in the body. Not getting enough increases your risk of fatigue, poor blood sugar control, depression, chronic pain, and reduced resilience to infections.
Vitamin D deficiency affects approximately 1 billion people worldwide.1 People with a lot of body fat are especially at risk because vitamin D gets trapped in fatty tissue and becomes more difficult to access. Here’s what makes this issue even more urgent: Most people who are overweight or obese show some degree of vitamin D dysfunction year-round, which gets worse in the winter.
Randomized controlled trials provide solutions that do not require supplements or weight loss.2 Let’s take a look at how exercise can change the way your body processes vitamin D and help keep your activity levels stable even in the darkest times, even without weight loss.
Exercise helps convert stored vitamin D into its most active form
In a study published in Advanced Science, researchers set out to answer important questions: Does regular exercise during the winter preserve vitamin D metabolism in overweight or obese people, even without supplements or sunlight exposure?3 The research team conducted a 10-week experiment to measure the effects of physical activity on different forms of vitamin D in the body.
• Participants were inactive adults who were overweight or obese. The study included adults who lead a sedentary lifestyle and whose body mass index was in the overweight or obese category. None of them took vitamin D supplements, and the study took place in the winter. This means that there is virtually no sunlight that can trigger vitamin D production in the skin. This allowed the researchers to isolate the effects of exercise alone.
• Only the exercise group maintained active vitamin D levels. While both the exercise and sedentary groups experienced a slight decrease in vitamin D, only the exercise group was able to maintain levels of the active form that actually has health implications. This is the version that interacts with receptors throughout the body to control calcium balance, immune response, and cell repair.
• Sedentary participants lost 15% of their active vitamin D. Those who did not move had a noticeable decrease in their active vitamin D levels over just 10 weeks. Meanwhile, people who followed a moderate-intensity indoor cycling and resistance training program were able to maintain their body fat levels steadily during the study period, despite not losing any.
• Exercise Changed Vitamin D Status Even Without Weight Loss — One of the most surprising findings was that participants did not need to lose fat to reap these benefits. This is important because obesity is known to trap vitamin D in fat cells, reducing its availability. However, physical activity has been shown to override these effects by improving the body’s internal regulation of vitamin D.
Exercise Affects How Fat Tissue and Muscle Manage Vitamin D
Researchers found that physical activity increased the expression of vitamin D-related enzymes in fat tissue and muscle. This enzyme helps convert stored vitamin D into the active form that your body actually uses, and it also helps prevent the active form from breaking down too quickly. In other words, exercise doesn’t just slow vitamin D loss. It reprogrammed how the body processes and preserves it.
• The greatest benefit was seen in people with higher levels of fat tissue. Researchers found that individuals with the highest body fat mass benefited the most. Their adipose tissue was inherently less efficient at converting stored vitamin D to its active form, but after the intervention, that dynamic changed. The more you exercise, the better the conversion rate, even though the amount of fat tissue does not decrease.
• Two key enzymes control how vitamin D is used. Researchers investigated two important enzymes. One is to convert vitamin D into an active, usable form, and the other is to break down vitamin D. In people who did not exercise, enzymes that destroy vitamin D decreased while enzymes that destroy vitamin D increased. But for people who exercised, their bodies did the opposite. The enzyme that activates vitamin D increased, and the enzyme that breaks it down decreased.
• Muscle tissue plays a role in helping maintain vitamin D balance. Although the study focused on fat tissue, exercise is known to affect muscle metabolism as well. Although the researchers did not directly measure changes in muscle gene expression, the overall improvement in vitamin D activation suggests that systemic metabolic health, including muscle function, is improved, which helps preserve vitamin D during the winter.
• Exercise changes the way your body processes vitamin D — By changing the activity of certain enzymes in fat tissue, regular exercise helped participants maintain more active vitamin D without losing weight or taking supplements. This challenges the common belief that supplements or sunlight exposure are the only solutions, and provides a new avenue for people with high body fat to naturally protect their vitamin D levels during the winter.
How to Keep Your Vitamin D Levels Strong All Winter
If you’re worried about your vitamin D levels dropping during the winter, especially if you’re overweight, there are proven ways to combat it. There is no need to take medication or chase sunlight. Studies show that it’s not just low sunlight exposure or diet that’s the root cause of the problem. This is how your body processes and activates vitamin D already stored in fat and muscle tissue. This is where exercise makes a real difference.
You don’t need to lose weight or spend hours at the gym. The important thing is to consistently perform the right kinds of movements. Studies have shown that even without fat loss, people who exercised regularly were able to maintain active vitamin D levels throughout the winter. Here’s how to do the same:
1. Start moving every day — Aim for regular moderate-intensity exercise and daily movement. This study used indoor cycling and resistance training, but you can also use brisk walking, body weight cycling, or cycling. The key is consistency. The body needs repeated activation to continually convert stored vitamin D into an active form that cells can use.
2. Combine moderate aerobic exercise with strength training — Don’t choose one or the other. Aerobic exercise supports circulation and metabolic health, and strength training improves how your muscles process vitamin D. Together, they reprogram fat and muscle tissue to activate more vitamin D instead of leaving it unused. Don’t overdo it. Excessive high-intensity exercise can often have adverse effects, but moderate exercise should not be overdone.
Use the dark months as a window to build your health from within. Most people lose vitamin D between October and March, especially in northern regions. By staying active with these stretches, you can help combat the seasonal decline and get a head start on spring energy, boosted immunity, and metabolic function.
3. Sunlight is your best option for optimizing vitamin D — Remember, your skin is designed to produce vitamin D from the sun. So, spend time in direct sunlight every day with your bare skin exposed during the summer months and winter months if you live in a warm climate.
If you’re still using canola oil, soybean oil, sunflower oil, or regular “vegetable oils,” you’ll be flooding your cells with linoleic acid (LA), a polyunsaturated fat that oxidizes easily and accumulates in the skin, increasing your risk of skin damage if you’re exposed to the sun during peak hours (10 a.m. to 4 p.m.). Exclude these oils from your diet for at least six months before your heaviest exposure to sunlight.
4. If you take supplements, combine vitamin D3 with magnesium and K2. If you are often indoors or live far from the equator, vitamin D3 supplementation is a reasonable strategy for optimizing your vitamin D levels in the winter. However, vitamin D3 works best when combined with magnesium and vitamin K2.
In fact, people who did not use these cofactors needed 244% more vitamin D to maintain healthy blood levels.4 These three ingredients work as a team to improve absorption, reduce arterial calcification, and help the liver process fat more efficiently.
5. Test your vitamin D levels and personalize your approach — Test your vitamin D levels at least twice a year and aim for a level of 60 to 80 ng/mL (150 to 200 nmol/L). It’s not just about bone health, it’s about reducing insulin resistance, calming liver inflammation and restoring immune balance. The test gives you a starting point and helps you track your vitamin D levels throughout the year.
FAQs about Exercise and Vitamin D
cue: Why does vitamin D drop in winter?
no way: The body relies on sunlight, especially UVB rays, to produce vitamin D through the skin. In winter, especially in northern regions, sunlight is not strong enough to trigger this process, leading to a sharp decline in vitamin D production.
cue: How does excess body fat affect vitamin D levels?
no way: Vitamin D is stored in fat cells, making it more difficult for the body to access and activate vitamin D. This lowers the level of the form your body actually uses, especially in the winter.
cue: Will exercising without supplements improve my vitamin D levels?
no way: yes. Studies have shown that sustained, moderate-intensity exercise helps the body convert stored vitamin D into its active form without the need for weight loss or supplements.
cue: What exercise is best to maintain vitamin D?
no way: A mix of moderate cardio and strength training four times a week was most effective in the study. Intense exercise is not necessary. All you need is regular exercise to support vitamin D metabolism.
cue: Should I continue taking vitamin D supplements?
no way: If you live in an area that doesn’t get much sunlight or spend most of your time indoors, a vitamin D3 supplement may be helpful, but it works best when combined with magnesium and vitamin K2 to support proper absorption and function. However, safe sun exposure is the best way to optimize your vitamin D levels.
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