Gregory C Henderson & George A Brooks
Department of Integrative Biology, University of California, Berkeley

https://doi.org/10.36866/pn.71.22

Individuals interested in maintaining body weight at a healthy level, as well as individuals seeking to lose body fat, typically look to exercise and dietary modifications to achieve a situation in which daily fat oxidation matches or exceeds daily dietary fat intake. With regard to body fat loss goals in modern society, perhaps it is unfortunate that the evolution of skeletal muscle metabolism over millions of years led to a metabolic preference for carbohydrate oxidation during vigorous muscle contraction. That is to say, when exercise intensity is high, carbohydrate is the predomi-nant fuel in exercising humans. However, during sustained exercise bouts of moderate intensity, the rate of body fat mobilization (rate of lipolysis as measured by the release of glycerol and fatty acids from triacylglycerol) increases to provide fuel energy sources for working muscles and other tissues (Friedlander et al. 1998, 1999). Fat oxidation during exercise is classically considered to be the major way in which regular physical activity can aid in efforts to lose body fat. However, work by Henderson et al.(2007) (Fig. 1) brings insight that body fat mobilization and oxidation during the postexercise recovery period is also of great importance, and in some cases of greater import-ance than the fat oxidation that occurred during the exercise session.

Factors that affect exercise fat oxidation may affect body fat mass. For years it has been known that gender is one such factor with a significant impact upon fat metabolism during exercise (Friedlander et al. 1998, 1999; Tarnopolsky et al. 1990). Although for both women and men carbohydrate is essentially the only fuel at or above the peak aerobic capacity (VO2peak), in comparison to men, women are typically found to derive a significantly larger proportion of energy from lipid during exercise intensities in the moderate range. The ‘crossover’ concept (Brooks & Mercier, 1994) describes the effect of relative exercise intensity (expressed as %VO2peak) upon fuel selection during exercise. As described by the ‘crossover’ concept, fat oxidation can make up a considerable proportion of exercise energy expenditure (EEE) if the intensity is low to moderate, but not if the exercise is quite challenging (e.g. ≥ 75% VO2peak). Importantly though, at moderate intensities, women are able to derive a larger proportion of energy from fat than men, and therefore, while the ‘crossover’ concept applies to both genders in the same qualitative manner, separate curves for men and women represent the relationship between relative exercise intensity and substrate oxidation rates (Fig. 2). Again, to refer back to the classical thought that exercise fat oxidation causes body fat loss, one might naturally assume that exercise must facilitate weight loss more in women than men. However, this assumption would not be correct, and quite the opposite appears to be true. That is, paradoxically, women actually achieve less fat loss than men in exercise programmes (Donnelly & Smith, 2005) even though they oxidize relatively more fat during exercise bouts (Friedlander et al. 1998, 1999; Tarnopolsky et al. 1990).

Exercise is a powerful tool to alter pathways of lipid mobilization and fatty acid partitioning between storage and oxidation, and it is of interest to understand how behavioural choices can alter the rate of lipid oxidation. While it has been known that during moderate intensity exercise the rate of lipid oxidation increases, particularly in women, less is known about how energy substrate mobilization during exercise integrates with that which occurs for the remainder of the day, and the following day, after the exercise session comes to completion and daily activities are resumed. In order to better understand the importance of the postexercise recovery period upon pathways of lipid metabolism and substrate oxidation, we have recently combined stable isotope tracer methodology with indirect calorimetry in both male and female study participants (Henderson et al. 2007). Each volunteer was studied under three different conditions with metabolite tracer infusion and blood and breath sampling: (1) before, during and 3 hours after bicycle exercise at 45% VO2peak; (2) before, during and 3 hours after bicycle exercise at 65% VO2peak (equal energy expenditure as first condition); and (3) in a sedentary condition involving no exercise. In each of the three conditions, the participants were also studied the next morning by breath collection for additional measurement of whole body lipid and carbohydrate oxidation rates. Despite relatively less fat oxidation during exercise in men, lipid mobilization was elevated more in men (Fig. 3A) than in women after exercise (Fig. 3B). Furthermore, the lipid oxidation rate was elevated more in men after exercise, even including approximately 21 hours after exercise on the following day. It has now become apparent that the gender difference in exercise lipid metabolism may actually reverse in the postexercise recovery period such that the total impact of exercise upon lipid metabolism may actually be larger in men than women. These recent results resolve the long-standing paradox associated with the fact that women oxidize fat more than men during exercise, but that women who exercise retain more body fat than do men who exercise similarly.

The ‘crossover’ concept describes the effect of relative exercise intensity in men and women upon fuel utilization. Men and women respond similarly to exercise in terms of the balance of fuel selection, but women utilize more lipid in the moderate intensity range (Fig. 2). Dependence on carbohydrate derived fuels (glycogen, glucose, lactate) leads to depletion of those energy sources, and during the post-exercise recovery period metabolism is altered, especially in men in whom lipid is used to conserve carbohydrate. Extending the study of exercise metabolism to include the postexercise period helped resolve an apparent paradox in the literature. Considering both the exercise and recovery periods, it is now possible to understand why compared to men, women are more capable of conserving fat and total body mass in response to exercise stress.

References

Brooks GA & Mercer J (1994). The balance of carbohydrate and lipid utilization during exercise: the ‘crossover’ concept. J Appl Physiol 76, 2253-2261.

Donnelly JE & Smith BK (2005). Is exercise effective for weight loss with ad libitum diet?Energy balance, compensation, and gender differences. Exerc Sport Sci Rev 33, 169-174.

Friedlander AL, Casazza GA, Horning MA, Budinger TF & Brooks GA (1998). Effects of exercise intensity and training on lipid metabolism in young women. Am J Physiol Endocrinol Metab 275, E853-E863.

Friedlander AL, Casazza GA, Horning MA, Usaj A & Brooks GA (1999). Endurance training increases fatty acid turnover, but not fat oxidation, in young men. J Appl Physiol 86, 2097-2105.

Henderson GC, Fattor JA, Horning MA, Faghihnia N, Johnson ML, Mau TL, Luke-Zeitoun M & Brooks GA (2007). Lipolysis and fatty acid metabolism in men and women during the postexercise recovery period. J Physiol 584, 963-981.

Tarnopolsky LJ, MacDougall JD, Atkinson SA, Tarnopolsky MA & Sutton JR (1990). Gender differences in substrate for endurance exercise. J Appl Physiol 68, 302-308.