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Tuesday, May 09, 2017

The annualized decrease in exercise capacity may be even worse than we thought

The longitudinal decline in aerobic capacity is typically said to be 5 -10% per decade from age 40 to about age 70  then a more rapid decline occurs. Data from the Baltimore Longitudinal study of Aging (BLSA) (1) suggests that the decline is not constant across age ranges  but rather" accelerates markedly   with each successive age decade".

 How so?

In cross sectional studies " each succeeding age decade  represents a more highly selected group than it predecessor, thus healthy 70 to 90 year-olds may have been physiologically superior to current 20-40 years olds when they were on a similar age. In other words there is inherent selection bias in cross sectional data.

I will not attempt to explain the statistical model used but here are their values for the reduction  in men in peak O2 uptake expressed in ml/kilo/min for various decades:

30- 39 minus 7
40-49  minus 10
50-59  minus 15
60-69 minus 20
over 70 minus 26
 A similar pattern was found in women.

They found that the oxygen pulse (oxygen consumed per heart beat) rate of decline mirrored the rate of decline in peak 02 .Since oxygen pulse is a function of stroke volume and peripheral muscle uptake of oxygen it is not possible to determine if it is cardiac output or muscle uptake that is largely responsible for the decline.

Caveat-The published results are from a mixed-effect prediction model and the median followup was only 7.9 years and prediction is the operative word. Though the study is "longitudinal",numbers in their tables do not represent for example following the same people over a lifetime and noting the per decade change in oxygen uptake. Still their data suggest that the often quoted 5-10 % decrease per decade may be too optimistic and rates of changes likely accelerate with aging.The confidence intervals for each age range are quite large and you have to wonder what are the mechanisms responsible for such wide variation in loss of exercise capacity within an age range. If subendocardial  fibrosis is the ( or a) culprit what are the factors that accelerate or retard that process.

1) Fleg, JL Accelerated Longitudinal Decline or Aerobic Capacity in Healthy Older Adults"
Circulation 2005, 112: 674 -682.

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