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Should Popeye have taken a vitamin D supplement?
Regular readers of my blog will be aware of the importance of an adequate vitamin D status in the older adult, as well as throughout a person's lifetime. So those of us at the British Nutrition Foundation welcomed the recent publication of a special vitamin D edition of our nutrition journal, Nutrition Bulletin, which explores many facets of this fascinating and essential nutrient.
An interesting article included in the edition with relevance to the older population is a review of vitamin D and muscle strength. Muscle strength is important in ageing, as it is key to physical performance; poor muscle strength or muscle weakness is associated with risk of falls. Typically muscle strength in the older person is measured by a number of functional tests which include walking speed and gait tests, chair stands and tandem tests (walking or standing with foot heel to toe).
Does the evidence show an association between vitamin D and muscle strength?
Studies in older people with higher vitamin D blood concentrations have shown better lower extremity functioning (improved muscle strength in legs), compared to those with lower vitamin D status. However findings from vitamin D supplement randomised controlled trials are more inconsistent. It has been suggested that marked differences in the study designs used has made it difficult to draw firm conclusions. These include differences in vitamin D status of participants, how long the subjects were receiving the supplements, the dose and types of vitamin D supplement used and the use of additional supplements in the studies, particularly calcium.
In areas of uncertainty, to obtain a more robust conclusion, it is often useful to undertake systematic reviews and combine studies in a meta-analysis. Three such analyses reported a beneficial effect of vitamin D supplementation on muscle strength and function in older adults. For example a meta-analysis of 13 intervention studies, in a total of 2,268 subjects with a mean age 78 years, assessed the efficacy of vitamin D supplementation on muscle strength, gait and balance. A significant improvement was reported in postural sway, timed up and go test (stand up from sitting, walk on a 3 metre line, turn, return to char and sit)and lower extremity muscle strength but not on gait. A 2011 systematic review that pooled the results of 17 trials involving 5,072 participants suggested the benefits on muscle strength were mainly limited to those with vitamin D deficiency. However, further to these reviews, a number of subsequent randomised controlled studies have not supported an effect of vitamin D supplementation on muscle strength.
SACN review supports a beneficial association between vitamin D and muscle strength
Usefully the Scientific Advisory Committee on Nutrition (SACN), the body that advises government agencies on nutrition and related health issues, has recently looked at the evidence. It concluded, like the article in our journal, that although evidence from the supplement studies is mixed, overall it is suggestive of a beneficial effect of vitamin D on muscle strength in older adults. Evidence from observational studies is also supportive of an association between vitamin D blood concentration and muscle strength and function.
The current government recommendation is that people aged 65 years and over should take a daily supplement containing 10 micrograms (0.01mg) of vitamin D. But it’s important in discussions on vitamin D to always remember there are some, although not many, natural food sources. One of the richest sources of vitamin D in our diets is oily fish – so include it in your dishes.