In many practical situations such as the treatment of hypertension, it is important to determine whether an improvement of condition following exercise prescription is due to an increase in aerobic fitness, or whether it simply reflects a reduction in body fat content (1) A previous review of 61 studies of training-induced changes in resting blood pressure (2) concluded that any reduction in resting pressures could not be attributed to concomitant weight loss, since the changes in systolic and diastolic readings showed very small and statistically non-significant correlations with changes in body mass.
In their recent paper, Barrone et al. (1) wished to test whether the same was true of exercise hypertension, and in support of such a conclusion they claim to have demonstrated independent correlations of delta fitness and delta fat with changes of pressure through the use of generalized estimating equations. A variety of valid measures of body fatness were obtained on their subjects, but unfortunately an inappropriate measure of aerobic fitness was chosen for the analysis. The outcome is reported as a change in peak oxygen transport, expressed in ml/kg/min. This is dimensionally incorrect, but let us assume that the authors intended to indicate a relative change of oxygen transport, expressed in ml/[kg.min]. Aerobic fitness is in fact the overall ability of the cardio-respiratory system to transport litres of oxygen to the working tissues. Any accumulation of body fat reduces the utility of this transport in terms of daily activities (including treadmill running). To take a practical example, a man with a body mass of 70 kg and an aerobic fitness of 3.5 l/min has a relative VO2max of 50ml/[kg.min]. If that same person accumulates an extra 14 kg of body fat, the aerobic fitness may remain at 3.5 l/min. but the relative VO2max decreases to 41.7 ml/[kg.min]. Plainly, the relative units of oxygen transport confound the influence of fitness and fatness, and cannot be used to distinguish the importance of changes in fitness relative to changes in fatness.
The authors must have the data to make a more convincing independent analysis of the two variables, and I would encourage them to do so.
1. Barone BB, Wong N-Y, Bacher AC et al. Decreased exercise blood presure in older adults after exercise training: contributions of
increased fitness and decreased fatness. Br J Sports Med 2009; 43: 52-56.
2. Hagberg JM, Park J-J, Brown MD. The role of exercie training in the treatment of hypertension. An Update. Sports Med 2000; 30: 193-206.