The surprisingly satiating effects of fasted cardio

Despite exercise increasing daily energy expenditure, and thus, in theory, creating negative energy balance, interventions that have focused on using exercise as a tool for weight loss have revealed^[1] lower weight loss than predicted based on the amount of extra energy expended. This is mostly due to the fact that some^[2] people compensate for the energy expended during exercise by consuming more calories, thus negating any effect of exercise on energy balance. Another compensatory behaviour that can occur^[3] in response to exercise is a reduction in non-exercise physical activity, meaning that people move less in their daily lives. These responses limit the ability of exercise to cause negative energy balance and weight loss. On the other side of the equation, skipping breakfast has been shown^[4] to promote a lower energy intake compared to eating breakfast (see NERD #53, Volume 1, “Will eating breakfast keep you lean?”), but it appears to reduce morning physical activity compared to eating breakfast in lean^[5] and obese^[6] participants.

It has been proposed^[7] that the body’s glucose stores, in particular hepatic glycogen (the primary source of blood glucose between meals), plays an important role in regulating energy intake (serving as a kind of “sensor” of energy availability). As depicted in Figure 1, if glucose oxidation is too high, and hepatic glycogen levels are reduced, this could trigger a compensatory increase in energy intake to maintain levels of glycogen above certain levels. Accordingly, there is some evidence^[8] in humans showing that higher levels of glucose oxidation during exercise are associated with higher energy intake after exercise. Therefore, increasing the proportion of energy derived from fat oxidation during exercise (and reducing glucose oxidation), like what happens during fasted exercise^[9], might prevent the compensatory increase in energy intake.

The main goal of the study under review was to assess the role of carbohydrate availability during morning exercise sessions on 24-hour energy balance, in normal weight men.

Weight loss interventions that have used exercise as a weight loss tool have shown mixed results as a consequence of either energy intake compensation or a subsequent reduction in non-exercise physical activity. There is some evidence that suggests how much glucose one uses during exercise is associated with how many calories one consumes after exercise. As performing exercise in the fasted state reduces the relative contribution of glucose to the overall energy demands of exercise, exercising before breakfast, or “fasted cardio,” might prevent or at least ameliorate compensatory increases in energy intake.