A dose of sunshine could boost the strength of muscles in the elderly and the obese, a new study found.
Vitamin D made from sunshine or found in oily fish, red meats and eggs, regulate the amount of calcium and phosphate in the body and these nutrients keep bones, teeth and muscles healthy.
As we age and become less active muscles become weaker and lose mass.
There is increasing concern about the 'sarcopenia', age-associated declines in muscle strength and function, which in turn affects balance, gait and the ability to perform everyday tasks.
But a new study by researchers at Birmingham University found increasing the levels of active vitamin D improves muscle strength.
The findings could help guide doctors in prescribing vitamin supplements and begin to answer questions as to the optimal levels of vitamin D required for healthy muscles.
The study builds on previous knowledge showing levels of inactive vitamin D to be associated with a lack of muscle mass.
Researchers used state of the art technology that allowed both active and inactive forms of vitamin D to be assessed alongside their impact on various muscle functions.
Clinical Lecturer in Endocrinology Dr Zaki Hassan-Smith at the Institute of Metabolism and Systems Research said: "We have a good understanding of how vitamin D helps bone strength, but we still need to learn more about how it works for muscles.
"When you look at significant challenges facing healthcare providers across the world, such as obesity and an ageing population, you can see how optimising muscle function is of great interest.
"Previous studies have tested for the inactive forms of vitamin D in the bloodstream, to measure vitamin D deficiency.
"Here, we were able to develop a new method of assessing multiple forms of vitamin D, alongside extensive testing of body composition, muscle function and muscle gene expression."
The study published in PLOS ONE involved 116 healthy volunteers aged between 20 and 74
All had both active and inactive levels of vitamin D measured alongside physical characteristics including body fat and 'lean mass', a measure of muscle bulk.
It found women with a healthy body composition, and lower body fat, were less likely to have high levels of inactive vitamin D, a marker of vitamin D deficiency.
This was echoed by the finding that levels of inactive vitamin D were lower in women with increased body fat.
Dr Hassan Smith said this suggested a relationship between vitamin D and body composition.
However, the active form of vitamin D was not associated with body fat, but was associated with lean mass.
Individuals with an increased lean mass, and muscle bulk, had a higher level of active vitamin D in the bloodstream.
Dr Hassan Smith explained: "By looking at multiple forms in the same study, we can say that it is a more complex relationship that previously thought.
"It may be that body fat is linked to increased levels of inactive vitamin D, but lean mass is the key for elevated levels of active vitamin D.
"It is vital to understand the complete picture, and the causal mechanisms at work, so we can learn how to supplement vitamin D intake to enhance muscle strength."
In this study some of the positive associations between active vitamin D and muscle bulk were not seen in men and further research was need to identify if this is due to biological differences.
The study was published in PLOS ONE.