New research suggests that active levels of vitamin D may improve muscle function.
Vitamin D is key for the development and maintenance of healthy bones. It also has many positive effects on the immune system, endocrine glands, and cardiovascular system.
Some observational studies have linked vitamin D deficiency with a higher risk of colorectal and breast cancer, while others have found a correlation between vitamin D levels and the risk of autoimmune diseases such as type 1 diabetes and multiple sclerosis.
A new study - carried out by researchers from the University of Birmingham in the United Kingdom - investigates the effect of vitamin D levels on muscle strength.
The findings were published in the journal PLOS One.
Analyzing the effect of vitamin D on muscle mass
Using innovative technology, researchers were able to study both active and inactive vitamin D levels, together with their impact on muscle strength.
Vitamin D - whether it is in D2 or D3 form - is, by itself, biologically inactive, until it is activated by two enzymatic reactions: one in the liver and the other in the kidney.
Dr. Zaki Hassan-Smith, from the University of Birmingham, explains the novelty of the research procedure in the current study:
"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 team examined vitamin D levels in 116 healthy participants aged between 20 and 74. They also measured the participants' body fat and "lean body mass" - a measure of muscle mass, obtained by subtracting the body fat weight from the total body weight.
Supplemental vitamin D may enhance muscle function
The researchers found that women who had a healthy composition and lower levels of body fat were less prone to high levels of inactive vitamin D - a common marker of vitamin D deficiency.
Conversely, women with more body fat tended to have less inactive vitamin D. While this does suggest a relationship between vitamin D and body composition, the active form of vitamin D did not correlate with body fat. Instead, vitamin D levels were linked with lean mass.
People who had a higher lean mass and muscle mass also had higher levels of active vitamin D. This suggests that active vitamin D may help to optimize muscle strength.
Dr. Hassan-Smith explains the findings, which echo previous studies that have suggested a link between a lack of muscle mass and low vitamin D levels:
"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."
The scientists also note that some of the beneficial associations were not found in men, and that future, larger-scale studies are needed to establish whether the differences noticed are purely biological. The researchers plan to work with international colleagues in order to study the mechanisms in laboratory studies and clinical trials.
"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," Dr. Hassan-Smith explains. "When you look at significant challenges facing healthcare providers across the world, such as obesity and an aging population, you can see how optimizing muscle function is of great interest."