TIME 4 FAT LOSS SERIES – PART 10 – Are performing cardio on an empty stomach and training in the ‘fat burning zone’ the best ways to maximise fat loss?

9 May 2019


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This is the tenth in a series of original articles we will be publishing looking at the science behind fat loss. Join our mailing list to be informed when we have added the latest article.


PART 10 – Are performing cardio on an empty stomach and training in the ‘fat burning zone’ the best ways to maximise fat loss?

Let’s begin with exercising on an empty stomach.

Performing cardio in the morning on an empty stomach

The rationale behind performing cardiovascular exercise first thing in the morning on an empty stomach to burn more fat is based on the notion that exercising in a fasted state, when the body’s carbohydrate stores are low, will force it to metabolise fat to fuel the activity and so result in a greater loss of body fat.

There is some evidence to support this concept. For example, the results of a study by Wilcox et al., (1) showed that a kilogram of fat is burned at a greater rate when exercise is performed in a fasted state in the morning in comparison to later in the day. Specifically, the amount of fat burned during cardiovascular exercise amounted to 67% of the total energy expenditure in the morning after a 12 hour fast in comparison to 50% when the same amount of exercise was performed later in the day or after eating. These results are supported by those of other studies, which also found that subjects oxidized more fat when they exercised on an empty stomach in comparison to exercising after eating (2,3).

However, before you set your alarm clock for an early morning cardio session, it is important to consider the findings of a systematic review and meta-analysis by Hackett and Hagstrom (4), which examined a number of studies relating to the effect of overnight-fasted versus fed exercise on weight loss and body composition. The authors concluded that although acute exercise in the fasted state has been shown to result in greater fat oxidation than exercise performed in a fed state, the findings from the present review suggest it does not increase the amount of body fat or weight lost.


If we burn more fat exercising on an empty stomach, why we don’t lose more body fat?

There could be a number of reasons for this: Firstly, it is 24-hour calorie balance is that has the biggest influence on our fat loss (4). For example, the results of one study showed no significant differences among trials when comparing the total caloric expenditures (range: 215-219 kcal). Also, the difference in the actual amount of fat burned was relatively small in real terms: The subjects who exercised on an empty stomach burned 94.3 kcal of fat in comparison to 71.6 kcal burned exercising after a meal (2).

An explanation for the disparity between the acute studies showing increased fat oxidation following fasted exercise and the review findings could be due to a compensatory decrease in fat oxidation in the post-exercise period once a meal is consumed (5).

Another point to consider is that an acute bout of fasted compared to fed exercise has been shown to result in a significantly greater loss in muscle protein (6), which may lead to a reduction in muscle mass if this practice is maintained over a long period. As we saw earlier in this series, the greater the amount of muscle mass we have, the greater the amount of energy we expend at rest.

Studies have shown that consumption of food prior to exercise increases the thermic effect of the bout, leading to greater energy expenditure post-exercise compared to exercise in a fasted state (4). These results suggest that fed compared to fasted exercise may be better for fat loss.

Remember that you will be able to exercise for longer and at a greater intensity, and therefore expend greater amounts of energy, if you fuel your body by eating prior to exercise (7).

Interestingly, Hackett and Hagstrom (4) concluded that findings of the various studies in their review show minimal changes in body mass and composition following cardiovascular exercise in either fasted or fed states. So whether you eat before exercise or not appears to be a matter of personal preference, as performing cardio in a fasted state is no more effective for fat loss than exercising when fed, and may even be counter-productive.


The ‘fat burning zone’

Traditionally, it was believed that exercising at a lower intensity in the ‘fat-burning zone’ (FBZ) was more effective for reducing body fat in comparison to high intensity exercise. This concept came about due to a misunderstanding regarding the optimal exercise intensity at which your body uses a greater percentage of fat as fuel and how exercise reduces body fat levels (4).

Simply explained, at lower exercise intensities your body uses a greater percentage of fat as fuel than at higher intensities. However, the amount of body fat you lose due to exercise is determined by the total energy (kcal) expenditure and not by the fuel source. If that weren’t the case, then athletes who engage in explosive, high intensity exercise, such as sprinting, which burns little, if any, fat during the activity, wouldn’t be so lean and high intensity interval training (HIIT) discussed earlier in this series would not be so effective in reducing body fat.

For example, if an individual exercised at 50% of their maximum oxygen uptake for 30 minutes, they may expend approximately 225 kcal. However, if that individual exercised at 80% for 30 minutes, they will expend approximately 360 kcal.

This does not mean that you have to exercise at a high intensity to burn body fat and that exercise at a lower intensity is ineffective, as similar energy expenditure over 24h has been observed following HIIT and moderate intensity exercise (MICT) (8). Furthermore, recent systematic reviews and meta-analyses have shown that HIIT and MICT can induce similar reductions in body fat, with HIIT offering the advantage of being more time-efficient (4). For many people new to exercise and/or those with certain medical conditions exercising at a lower intensity may be more appropriate and safer. They will just need to perform a greater volume of exercise to achieve the desired level of energy expenditure.

It is important to note that when HIIT or MICT are performed on their own without any dietary changes, neither are likely to result in meaningful fat loss, i.e., more than a 5% reduction (9) unless performed at very high volumes (4).


Although the evidence suggests that there is no advantage in terms of fat loss by exercising in a fasted state or in the ‘fat burning zone’, this does not mean that you can’t engage in either if they better suit your preferences and abilities. The evidence also suggests that regardless of the intensity of the exercise performed or if the exerciser is in a fed or fasted state, meaningful fat loss is only likely to occur if energy intake is also reduced.


CLICK HERE TO READ: Part 11 – Why do we put fat on in particular areas of the body and can we use exercise to specifically reduce them?