Scientists from the University of Birmingham found that the active form of blood vitamin D in this study 1,25-dihydroxyvitamin D3 (1α,25(OH)2D3), was strongly linked to aspects of lower limb strength. This included power, velocity and jump height.
While vitamin D-deficiency in the elderly has been linked to loss of muscle mass and strength (sarcopenia), the precise role of specific vitamin D metabolites in determining muscle phenotype and function is still unclear.
“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,” said Dr Zaki Hassan-Smith, clinical lecturer in Endocrinology at the Institute of Metabolism and Systems Research at the University of Birmingham and lead study author.
The researchers enrolled 116 volunteers (79 women, 37 men; aged 20–74 years) and had their vitamin D levels identified and evaluated.
Medical history was recorded as were physical characteristics including body fat and lean mass, a measure of muscle bulk.
Procedures were carried out over a 1-day visit. Patients arrived fasted in the morning, having collected a complete urine sample over the preceding 24 hours.
Grip strength testing was carried out using a hand-held dynamometry while the height of jump was assessed using jump-plate mechanography.
Along with the primary findings, Dr Hassan-Smith and his team also found that females with a lower body fat composition were unlikely to have high levels of inactive vitamin D - a marker of deficiency for the nutrient.
The active form of vitamin D was found to be associated with lean mass and not with body fat as expected.
Subjects with an increased lean mass and muscle bulk had increased levels of active vitamin D circulating in the bloodstream.
‘Lean mass key to active vit 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,” commented Dr Hassan-Smith.
“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."
Much of the work that echoes Dr Hassan’s Smith’s research has focused on circulating concentrations of 25OHD3,
Many describe positive correlations with select muscle function measures while others describe no significant associations.
Interestingly, the study pointed out that circulating 1α,25(OH)2D3, appeared to be a better indicator of muscle strength than its precursor 25OHD3.
In contrast, serum 25OHD3 had a greater impact than 1α,25(OH)2D3 on measures of as efficiency and skeletal muscle gene expression.
“One potential explanation for this is that systemic and locally-generated 1α,25(OH)2D3 have different effects on muscle phenotype and function,” the study commented.
“It is interesting that both 25OHD3 and 24,25(OH)2D3 are associated with body fat in addition to efficiency,” it continued.
“It may be that the added burden of fat in generating power and interaction with these metabolites plays a role in these relationships.
The study also pointed out that a few of the positive links between active vitamin D and muscle bulk were not observed in males.
“As 1α,25(OH)2D3 was positively correlated with Pmax (a measure of power) but does not reach significance with Esslinger (a measure of performance controlled for body weight), it is possible that age or sex impact these effects,” the study concluded.
“It is also possible that observed sex differences are due to lack of statistical power in view of smaller numbers of men recruited to the study.”
Source: PLOS One
Published online ahead of print: doi.org/10.1371/journal.pone.0170665
“25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 exert distinct effects on human skeletal muscle function and gene expression.”
Authors: Zaki Hassan-Smith, Carl Jenkinson, David Smith, Ivan Hernandez, Stuart Morgan, Nicola Crabtree, Neil Gittoes, Brian Keevil, Paul Stewart, Martin Hewison