Age-associated decline in muscle function represents a significant public health burden. Vitamin D-deficiency is also prevalent in aging subjects, and has been linked to loss of muscle mass and strength. New research published in the journal PLOS shows specific forms of active vitamin D can help to optimise muscle strength.
In the absence of Vitamin D from sunlight, disease increases more than 1000 percent. Experts suggest that the chances of getting vitamin D from your diet are very low. And if you are a vegetarian, it is all the more difficult to gain vitamin D. What they repeatedly suggest is exposure to sunlight is the best source of vitamin D. Humans spend less time in the sun today than at any point in human history — which is why more than 1 billion people worldwide are vitamin D deficient.
Humans make 90 percent of our vitamin D naturally from sunlight exposure to our skin — specifically, from ultraviolet B exposure to the skin, which naturally initiates the conversion of cholesterol in the skin to vitamin D3.
Several studies have reported that the D3 form of the vitamin is more potent that D2, with a study led by Robert Heaney, MD, from Creighton University in Nebraska reporting that D3 was 87% more potent than D2.
While vitamin D supplements are an alternative means of producing vitamin D when regular, non-burning sun exposure is not possible, oral supplementation of vitamin D is not nature’s intended means of producing this vitamin.
A systematic review of almost 200 population-based studies shows that more than a third of populations worldwide suffer from low levels of vitamin D.
Scientists from the University of Birmingham found that the active form of blood vitamin D in this study 1,25-dihydroxyvitamin D3 (1beta,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 Vitamin D Levels
Collectively these observations indicate that the link between vitamin D, muscle function and sarcopenia is more complex than originally thought, and cannot be defined simply through measurement of 25OHD3.
“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 1beta,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 1beta,25(OH)2D3 on measures of as efficiency and skeletal muscle gene expression.
“One potential explanation for this is that systemic and locally-generated 1beta,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 1beta,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.