Can fasted exercise increase fat oxidation in women?

It’s pretty common to see “fat-burning zone” or “weight loss zone” plastered on cardio equipment. This fabled aerobic exercise intensity is supposed to be the best for melting the fat away. The thinking behind the fat-burning zone has a little bit of science to back it up. The relative contribution of fat versus glucose to fuel activity reaches its peak at moderate intensities^[1][2] of exercise - this is the fat-burning zone. Here, the body is using more fat relative to glucose in order to fuel the exercise.

But fat burning during exercise doesn’t tell the whole story. After all, it’s not really the goal of people training in the “fat-burning zone” to switch up the ratio of fuel their muscles are using in the moment. Presumably, their goal is to lose fat mass. And here’s where the science of the “fat-burning zone” starts to break down.

As shown in Figure 1, in order to lose body fat, the body has to be in a negative fat balance. One way to tip the fat balance scales could be to increase the fat oxidation rate. This concept has been tested, and at this point, the general consensus is that exercise does not^[3] affect 24-hour fat oxidation compared to sedentary controls, at least if energy balance is maintained^[4] (i.e., you eat just enough food to compensate for the greater energy expenditure from exercise). So, while the body uses more fat as fuel during moderate intensity exercise, over the course of a day, fat balance is the same as if the person just laid around all day. In other words, it looks like exercising does not magically make you lose fat. The key to fat loss is burning more energy than you consume.

Figure 1: Fat balance in a nutshell

References: Melanson et al. Exerc Sport Sci Rev. 2009 Apr.

The story doesn’t end there, though. Recently, a series of studies^[5][6][7] have found that if moderate intensity exercise is done while fasted (i.e., before breakfast), 24-hour fat oxidation is indeed increased, even when energy balance is maintained. But there’s a nuance here that limits generalizing these results: the studies were all done in men, which is a common problem^[8] in exercise science. This leaves open the question of whether these findings extend to women.

There are several^[9] physiological differences between the sexes, many of which are related to exercise and fat metabolism. On average, women burn more fat^[10] than men during exercise, but less fat^[11] afterward. Men’s and women’s skeletal muscle also adapts differently^[12] to endurance training. Some of these differences involve how skeletal muscle utilizes fat as a fuel, and can be observed at the genetic level^[13]. So, it’s far from clear that aerobic exercise in a fasting state could burn more fat for women. The study under review aimed to address this gap in the research.

Current consensus is that aerobic exercise in a fed state doesn’t affect fat oxidation over a 24-hour time frame; fat balance is maintained, and so the “fat-burning zone” of cardio exercise doesn’t live up to its name in practical terms. However, recent evidence has suggested that if exercise is done in a fasted state, fat burning increases. But this evidence has only been collected from studies involving men. Physiological differences raise the question of whether this effect would also be seen in women, and the study under review sought to answer it.