A loss of body fat, and hence burning body fat stores for fuel, is often the goal when exercising to lose weight, and thus it would seem advantageous to perform more low-intensity exercise to stay in the fat burning zone. Because fitter runners have an easier time producing energy aerobically at increasing exercise intensities, fitter runners burn a larger percentage of fat and use less carbohydrate than newer runners. However, the higher the intensity of an exercise the more total fat calories will be burned due to an overall increase in caloric expenditure.
According to David Swain, Ph.D., Director of the Wellness Institute and Research Center for Old Dominion University, "You burn five calories for every liter of oxygen you consume." Therefore, the harder you work, the more oxygen you consume, and the more overall calories you will burn regardless of percentage of calories metabolized as fat.
The Jury Is Out
The bottom line is that it's hard to say whether a new runner or a trained runner burns more calories. The fitter you become, the more efficient you become at using calories so it sounds like this would actually hinder weight loss efforts. Likewise, the more weight you lose the less calories you burn doing the same exercise, which also seems counterproductive.
However, it is helpful to think about it this way. The easier exercise becomes due to gains in fitness, the easier it is to exercise for longer and at higher intensities, ultimately resulting in more calories burned.
It's clear that the long-term benefits of weight loss and cardiovascular fitness accompany a long-term training program. The key continuing or maintaining weight loss through running is understanding how calories burned is affected by the factors discussed above and to continue challenging yourself in your running routine.
You can't expect to lose weight if you don't change the distance or intensity you are running at due to the adaptations your body makes as you get fitter. But, if you embrace your new fitness and continue to find different ways to challenge yourself and improve, there's no limit to what goals you can accomplish.
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