Is ageing 'highly individualistic'?

Abstract

The study of human ageing is important for its intrinsic interest as well as for practical and personal reasons. Out of necessity, most studies are cross sectional, few have very large numbers and it is difficult to control for variables such as habitual exercise. Consequently, we were pleased and interested to read the paper by Pollock et al. (2015) which addresses two of these issues; they have a reasonable sample size and the participants all had high levels of habitual activity. In addition, the authors measured an impressively wide range of physiological variables. However, we remain a little puzzled as to why the authors were searching for a way of ascertaining the age of their subjects: we find that asking them is a fairly reliable method. But to be serious, in the final ‘Key point’ of the present paper it is suggested that the ageing process is modified by inactivity. Removing the uncertainty about levels of physical activity, if it were an important factor, should reduce the variance of the data but, unfortunately, there is no control group in the present study against which to test this idea. It is notable that the variance of the present results for is very similar to that of a recent large survey of fit, but not specifically trained, subjects by Loe et al. (2013). In addition, Loe and colleagues report a very weak, non-significant, relationship between physical activity and (R2 = 0.057), suggesting that habitual physical activity might not be such an important confounding factor in studies of healthy ageing. We fully recognise, however, that physical activity is most important in protecting against cardiovascular disease and that there can be significant improvements in aerobic capacity with short-term training, but the present paper is concerned with the, admittedly difficult, concept of ‘healthy’ ageing and habitual activity. We would also like to comment on the conclusions concerning the consequences of ageing for muscle mass. The authors used dual-energy X-ray absorptiometry (DXA) to measure muscle mass and expressed the data as a skeletal muscle mass index (SMI). We have shown that DXA underestimates the age-related loss of thigh muscle mass compared with magnetic resonance imaging (MRI) (Maden-Wilkinson et al. 2013) and that expressing the data relative to height squared, as with SMI, is not the best way of normalising the data. We suggest that relating thigh muscle mass to the volume of the femur is a better way of taking into account differences in body size. In young subjects there is a strong relationship between muscle and bone volume, and the outer dimensions of the femur are relatively unaffected by age. Consequently the bone size of an older subject can be used to estimate their muscle size when young and comparing this with the actual muscle volume gives a measure of the age-related muscle wasting of a given person (Maden-Wilkinson et al. 2014). We have shown that the standard deviation of the muscle to bone ratio was similar in young and old, suggesting that wasting affected all our healthy older subjects (mean age 72 years) to a similar extent, at least as far as the thigh muscles were concerned. This does not tally with the other suggestion of the final ‘Key point’ that the ageing process is ‘highly individualistic’. There is other evidence that contradicts the notion of a wide variation in the ageing process or even that habitual activity makes much impression on the rate of healthy ageing. In the Loe paper the standard deviations of the data are remarkably constant from 20 to 70+ years, again suggesting little variation in the rate of loss with age. Moreover, the rate of loss of of 3–4 ml kg−1 per decade they report is somewhat less than the 4.7–6.4 ml kg−1 per decade reported in the present paper (Fig. 1B) for subjects with high levels of habitual activity. While it is undoubtedly true that some aspects of ageing, such as greying of hair, vary considerably between people we would like to suggest that this is not true for all body systems, certainly not for muscle and, most likely, not for . The scatter of data seen with older subjects does not necessarily indicate highly individualistic rates of ageing and for some body systems it probably mirrors the scatter seen with young subjects and is consistent with a uniform rate of ageing.