Scientists reviewed 17 studies to understand how vitamin D and strength training work together to build muscle. They found that when people do resistance exercises (like lifting weights), their bodies produce more of a growth hormone called IGF-1, which helps muscles get stronger. When people also take vitamin D supplements, especially if they don’t have enough vitamin D in their blood, the benefits get even better. The research shows that combining these two approaches—strength training plus vitamin D—may help people of all ages build stronger muscles and improve their physical abilities, particularly those who are deficient in vitamin D.

The Quick Take

  • What they studied: How vitamin D supplements and resistance training (like weight lifting) work together to help muscles grow stronger and produce more of a natural growth hormone called IGF-1.
  • Who participated: The review looked at 17 different studies involving children, adults, older adults, and people with health conditions affecting hormones or metabolism. The studies included both men and women of various ages.
  • Key finding: Resistance training alone increased muscle-building hormones by 20-30% in older women. When vitamin D supplements were added to the training, muscle growth and strength improved by about 22%. The combination worked especially well for people who didn’t have enough vitamin D in their bodies.
  • What it means for you: If you do strength training and take vitamin D supplements (especially if you’re deficient in vitamin D), you may see better muscle growth and strength gains than with either approach alone. However, talk to your doctor before starting supplements, as this research is still developing.

The Research Details

Scientists conducted a systematic review, which means they searched for all high-quality studies on this topic and analyzed them together. They followed strict guidelines called PRISMA to make sure they did this fairly and thoroughly. Out of the 17 studies they found, 11 were randomized controlled trials (the gold standard where people are randomly assigned to different groups) and 6 were observational studies (where researchers just watched what happened without controlling the conditions). The studies looked at different age groups and included people with various health conditions, which helps show whether the findings apply to many different people.

By combining multiple studies, researchers can see patterns that might not show up in just one study. This approach is important because it helps us understand whether the benefits of vitamin D and strength training together are real and consistent across different groups of people. The fact that they included studies from children to elderly people means we can see if the benefits work for everyone or just certain ages.

This is a systematic review, which is a strong type of research because it combines evidence from many studies rather than relying on just one. The researchers followed strict guidelines (PRISMA) to reduce bias. However, the quality depends on the studies they reviewed—some were high-quality randomized trials while others were observational. The fact that they included 11 randomized controlled trials (the most reliable type) strengthens the findings. The review is recent (2025) and published in a respected medical journal, which adds credibility.

What the Results Show

The research shows that resistance training (strength exercises like weight lifting) increases the body’s production of IGF-1, a natural hormone that helps muscles grow. In older women, this increase was between 20-30%, which is substantial. When vitamin D supplements were added to the resistance training program, the benefits got even better—muscle growth and physical function improved by about 22%. The studies also found a positive connection between vitamin D levels in the blood and IGF-1 levels, meaning people with more vitamin D tend to have higher IGF-1. This suggests that vitamin D plays an important role in helping the body use IGF-1 effectively for muscle building.

The research also looked at IGF-binding proteins (IGFBPs), which are molecules that carry IGF-1 around the body. The studies found limited evidence suggesting that vitamin D supplementation may slightly reduce these binding proteins, which could make IGF-1 more available for muscles to use. This is important because it suggests one way vitamin D and resistance training might work together—vitamin D helps make the growth hormone more accessible to muscle tissue.

Previous research has shown that both vitamin D and resistance training individually improve muscle health and strength. This systematic review builds on that knowledge by showing that combining them appears to create a stronger effect than either one alone. The finding that vitamin D helps improve IGF-1 availability is relatively new and adds to our understanding of how these two factors interact at the biological level.

The review has some important limitations to consider. First, the exact number of total participants across all studies wasn’t clearly stated, making it hard to judge the overall strength of the evidence. Second, the studies included people of different ages and health conditions, which is good for showing broad applicability but makes it harder to say exactly who benefits most. Third, the research on IGF-binding proteins was limited, so we can’t be completely confident about that finding. Finally, most studies were relatively short-term, so we don’t know if the benefits last over many months or years.

The Bottom Line

If you’re doing resistance training and have low vitamin D levels (which your doctor can test), vitamin D supplementation appears to enhance muscle-building benefits. The evidence is moderate to strong for this combination. However, vitamin D supplementation alone without exercise probably won’t give you these muscle-building benefits. Talk to your doctor about your vitamin D levels and whether supplementation is right for you before starting.

This research is most relevant for people who want to build muscle strength and are doing or planning to do resistance training. It’s especially important for older adults (who often have low vitamin D), people with hormonal or metabolic conditions, and anyone who has tested low for vitamin D. People who already have adequate vitamin D levels may see less dramatic benefits from supplementation. Pregnant women, people with certain kidney conditions, and those on specific medications should consult their doctor before taking vitamin D supplements.

Based on the studies reviewed, improvements in muscle strength and function typically appear within 8-12 weeks of combining resistance training with vitamin D supplementation. However, some benefits may take longer to develop, and the timeline can vary depending on your starting fitness level, age, and how deficient you are in vitamin D. Consistency with both training and supplementation is key.

Want to Apply This Research?

  • Track your resistance training sessions (type of exercise, weight used, number of repetitions) alongside your vitamin D supplementation (dose and frequency). Also monitor subjective measures like how strong you feel during workouts and your overall energy levels. If possible, track objective measures like how many push-ups or squats you can do, or how much weight you can lift.
  • Set a daily reminder to take your vitamin D supplement at the same time each day (consistency matters). Schedule your resistance training sessions 3-4 times per week and log each session in the app. Create a simple goal like ‘Complete 3 strength training sessions this week + take vitamin D daily’ and check it off as you go.
  • Every 4 weeks, assess your progress by testing your strength (can you lift more weight or do more repetitions?). Every 8-12 weeks, consider asking your doctor to retest your vitamin D levels to confirm they’re improving. Track your overall energy, muscle soreness recovery time, and how your clothes fit as additional indicators of progress. Use the app to create a simple chart showing your strength improvements over time.

This research summary is for educational purposes only and should not replace professional medical advice. Before starting vitamin D supplements or a new exercise program, consult with your healthcare provider, especially if you have existing health conditions, take medications, are pregnant or breastfeeding, or have a history of kidney disease or hypercalcemia. Vitamin D supplementation is not appropriate for everyone, and your doctor can determine the right dose based on your individual needs and blood test results. The findings in this systematic review represent current scientific understanding but are not definitive medical recommendations.