Exercise Intensity for Optimal Fat Oxidation

Overweight boy tries to shed some pounds by running.

For those exercising to shed pounds (like this boy to the right) finding the optimal exercise intensity for burning fat may aid in weight loss. It appears that the although the exercise intensity that peak fat oxidation rate occurs is similar in males and females, but is influenced by training and obesity.

During exercise the body has three primary sources of energy to choose from: carbohydrates, fats and proteins. Carbohydrates are broken down faster and require less oxygen consumption per calorie than fats or proteins. For this reason, carbohydrates stored in the muscle or liver are the body's primary source of energy during intense exercise lasting 1 to 20 minutes in duration. During exercise shorter than a minute, the body utilizes phosphocreatine in the muscle. Phosphocreatine transfers its high energy phosphate group to ADP forming ATP, the muscles direct source of energy. Because glycogen in the muscle and liver is rapidly depleted during intense exercise, the body must begin using fats for ATP production during exercise lasting longer than 20 minutes in duration. Because the rate of energy substrate utilization and oxygen consumption are not the principal limiting factors during less intense exercise, the body uses fats for energy in an effort to conserve its stores of precious glycogen. Fats are used for energy production as free fatty acids in a process called beta oxidation. Proteins generally play a more minor role during exercise because they are not readily broken down for energy.

Understanding the optimal exercise intensity for maximum fat utilization is useful for patients trying to lose excess fat. The exercise intensity must balance between achieving a high enough exercise intensity so that significant calories are used, but not such a high exercise intensity that carbohydrates become the preferred source of energy. Several research groups have looked into the optimal exercise intensity to maximize fat oxidation. Apparently it differs for the exercise performed and the demographics of the subjects. This suggests that the optimal exercise intensity for fat oxidation should be tailored to the individual.

A study in 2008 found that the absolute peak fat oxidation was similar in males and females when scaled to fat-free mass, but lower in overweight individuals (Peak fat oxidation rate during walking in sedentary overweight men and women, 2008. Bogdanis, et al.). In this study the exercise activity was walking and the subjects were sedentary overweight men and women. Exercise intensity was measured as a percent of maximal heart rate and oxygen consumption (%VO2max). VO2max is the velocity at which maximum oxygen is consumed and therefore represents the maximum aerobic energy output.  The energy substrates can be determined based on the ratio of oxygen consumed and carbon dioxide expired. This is called the respiratory quotient and is represented as O2 consumed divided by CO2 expired. The figure below shows how the contribution of fats and carbohydrates varies with exercise intensity (measured with %VO2max).

Percent Fat and carbohydrate (CHO) contribution to the energy expenditure at each stage of the treadmill test for men and women, and peak fat oxidation rates (as %fat), plotted against the relative exercise intensity (%VO2max). Values are mean± SE.

The peak fat oxidation rate in the overweight men and women was similar, about 40% of VO2max. This corresponded to about 59% of maximal heart rate.  A large amount of variation in the results suggests that exercise intensity ideally would be individually tailored. The absolute peak fat oxidation, when scaled to body mass, was lower than similar studies looking at subjects who are not overweight. The figure below demonstrates the distribution of exercise intensities that peak fat oxidation occurred displayed in box plots.

Optimal exercise intensity for fat oxidation.  Box plots of relative exercise intensity that elicited peak fat oxidation (PFO) rate in men and women, expressed as a percentage of maximal heart rate (%HRmax) and maximal oxygen uptake (%VO2max).

Why overweight persons have a lower fat oxidation capacity is not known. It is possible that it is a factor that leads to obesity; after all, a reduced capacity to utilize fat would theoretically make it harder to burn fat off. However, it could also stem from being overweight from a currently unknown pathway. 

A more recent study found that exercise training increases the exercise intensity slightly for peak fat oxidation rate (Effects of supervised exercise training at the intensity of maximal fat oxidation in overweight young women, 2012.  Tan, et al.). This study used running, rather than walking, to measure exercise intensity. The subjects were young, overweight, Chinese women. The experimental group completed a 8-week training program. The training program was 5 days a week and consisted of a 10 minute warm-up (stretching and jogging), 40 minutes running at the peak fat oxidation velocity as determined for each individual, followed by a 10 minute cool down (walking and stretching). The maximal fat oxidation occurred at 34.1% of VO2max prior to training and 36.9% of VO2max after the 8-week training period. Both exercise intensities are lower than the aforementioned study with walking as the exercise type. Fat mass was significantly reduced; subjects lost an average of 13% of their fat mass. No significant change was observed in the controls who did not participate in the training program. 

Both these studies suggest that understanding the exercise intensity at which maximal fat oxidation occurs may be able to make exercise more effective at reducing body fat. Although the exercise intensity at which maximal fat oxidation occurs varies considerably in the general population, having a general idea of where it occurs could aid individuals in burning fat during exercise. As these two studies show, and other studies lend support, the exercise intensity, as measured by percent of VO2max and percent of maximum heart rate, vary across exercise character.

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