What the research says about vitamin D and strength training

Vitamin supplements

For most of us living in the UK, it’s probably sensible to be supplementing with Vitamin D - at least in the winter months. Adequate levels of vitamin D play a role in bone health, immunity, and more with research ongoing for its effects on muscle repair. 

What is vitamin D

Vitamin D is not really a vitamin, because it doesn’t really come from our diet much like other vitamins. There are two main forms: vitamin D2 and vitamin D3. D3 is 9.5 times more potent than vitamin D2. D2 comes from food (oily fish, fortified milks, some mushrooms) and we produce D3 when we’re exposed to sunlight (UVB rays in particular). 

Our bodies are very efficient at producing vitamin D3 from sunlight, however the amount we can produce is impacted by a variety of factors. The angle of the sun, where we are in the world (latitude), skin pigmentation, cloud cover, and clothing/suncream. Vitamin D synthesis (the process where we convert sunlight into vitamin D3) is weaker in those with darker skin; melanin acts as a natural sunscreen so will need to spend longer in the sun to produce equivalent levels of vitamin D.

Biological role of vitamin D

Vitamin D acts more like a hormone. It can control gene expression which is the process of telling our cells how to function. Vitamin D plays a role in four key areas:

  1. Bone health
  2. Immune function
  3. Oxidative function (how we create energy in the body) 
  4. Skeletal muscle

Effect on muscles

Our muscles are responsive to vitamin D. Recent studies have found that vitamin D supplementation (for those athletes with a deficiency) had a positive impact on muscle regeneration. Vitamin D influences the stem cells that are responsible for repairing muscles. This is particularly important after training and when recovering from an injury. 

There is still ongoing research to understand if and how vitamin D impacts performance. The general stance is that you need to have very low levels of vitamin D for muscle function to be affected.

How do I know if I'm vitamin D deficient?

Most of our lifestyles will mean that we don't get adequate sun exposure, especially during winter months. Low levels of vitamin D are also associated with a weakened immune system, so if we’re constantly getting ill it might be a sign that your vitamin D levels are low.  

The best way to tell is to take a blood test - they will measure your status of 25[OH]D, the major circulating form of vitamin D in your body. If you decide to supplement without testing your levels it won’t cause any real health issues, however you may not get the optimum results from taking the standard recommended allowance. 


In winter months, at least in the UK, oral supplementation of vitamin D is advised. If you already take fish oils, multivitamin supplements, or tend to use sunbeds, you may not need additional supplementation.

The recommendation is a daily dose of vitamin D3, anywhere from 800 - 2,000 IU (International Units). This should correct for most deficiencies and will maintain your vitamin D levels throughout winter, supporting regular function of the body. It will take 2-3 months to achieve a steady state concentration of vitamin D.

Menstrual cycles 

There has been some research done (still early days) looking at the role vitamin D plays in menstrual health. Some findings suggest those deficient in vitamin D may suffer from amenorrhea (absent periods) or infrequent periods. High dose vitamin D supplementation has been found to reduce the prevalence of PMS. More research is needed. 


Vitamin D and the athlete with Daniel Owens 
Vitamin D with Graeme Close
Vitamin D and menstrual cycles