Do you burn more fat on an empty stomach?

14 11 2012

There has been much debate as to whether cardio on an empty stomach can burn fat more effectively. Some argue it limits fat burning potential with a decrease in performance.  I really wanted to understand this topic more and so the below, research will highlight some key findings that may help us better understand this.

Carbohydrate is the main nutrient that fuels our exercise, while fat can fuel low intensity exercise for long periods of time. Therefore these two components are used to generate energy for muscle contraction during exercise. For endurance based exercise performed at a moderate intensity, you obtain more than half of the energy needed from glycogen (stored energy from carbohydrates). The rest will come from the fats. Taking this into account, once your glycogen stores are depleted by fasting overnight, or going several hours without eating, fatty acids break down in the body and are used as a secondary energy source. As workout intensity increases your body will then rely more on carbohydrates to fuel it.

One study reviews the efficacy of performing cardio on an empty stomach early in the morning. The theory, as mentioned above, is that there is a greater shift in energy utilization from carbohydrates allowing more stored fat to be used as fuel.

What did the study find?

Essentially a study was conducted into the fat burning response of six moderately trained individuals in a fed versus fasted state to different training intensities. The participants cycled for 2 hours at varying intensities on four separate occasions. During two of the trials, they consumed a high-glycemic carbohydrate meal at 30, 60, and 90 minutes of training, once at a low intensity and once at a moderate intensity. During the other two trials, participants remained fasted for 12–14 hours before exercise and for the duration of training.

The results in the low-intensity trials showed that although lipolysis (the breakdown of fats) was suppressed in the fed state compared with the fasted state, fat oxidation (utilization of the fats) remained similar between groups until 80–90 minutes of cycling. Only after this point was a greater fat oxidation rate observed in fasted participants. On the other hand, during moderate-intensity cycling, fat oxidation was not different between trials at any time.

More recent studies show the effect of pre-exercise and during exercise carbohydrate consumption on fat oxidation.

In another study seven endurance- trained participants cycled for 120 minutes at a moderate intensity, followed by a high intensity cycle where participants pedalled as fast as possible. There were four separate tests and participants were given a placebo and a carbohydrate drink at different intervals before and during exercise. The study was carried out in a double- blind fashion with trials performed in random order. As shown in the first study discussed, results showed no evidence of impaired fat oxidation associated with consumption of carbohydrate either before or during exercise.

What can we conclude from the studies?

Currently the studies do not support the theory that training early on an empty stomach burns any more fats than if carbohydrates were ingested. Although Lipolysis (breakdown of fats) was shown to increase in low-intensity exercise on an empty stomach and suppress in the fed state compared with the fasted state, fat oxidation (utilization of the fats) remained similar between groups. Taking this into account, training in the morning with depleted glycogen stores not only impairs your physical performance it has also been shown to have negative effects on those concerned with muscle strength and size. Concentration levels may be lower and you will not be able to work to your full capacity with the net result being less calories burned and lowering fat loss potential.

I hope this has given some insight into doing cardio fasted vs. after food and although the limitations with research are apparent there are some key points highlighted that I hope will help you with your training.





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