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Fat Burning Zone

The Fat Burning Zone

By Dr. L. Lee Coyne, the Healthy Professor

Dr. L. Lee CoyneExercise burns calories and the more calories you burn the more fat you lose. At least that is the common argument for increased physical activity in any weight management program.

More recent trends have attempted to fine-tune the issue of fat burning exercise by establishing the exercise intensity most conducive to fat burning vs. mainly carbohydrate burning. Still several “experts” with media exposure, have tried to dismiss the “fat burning zone” concept by using some simple arithmetic to calculate energy expenditure during exercise and during the recovery phase. These critics will tell you that a calorie is a calorie is a calorie and it won’t matter if you run three miles in 20 minutes, 30 minutes or 40 minutes, it will still add up to approximately 350 to 400 calories (depending on how big you are) and therefore assuming a certain fat loss.

These assertions are made without accounting for or measuring something called the Respiratory Exchange Ratio (RER). The RER is a ratio of carbon dioxide produced over oxygen used. This ratio is a biochemical reflection of what substrate (fat, carbohydrate or protein) is being oxidized (metabolized or burned) during rest or exercise.

Some people tend to be better fat burners than others and some are primarily carbohydrate burners. But the only way to tell is to measure the RER. The good news is that you can apparently train your system, over time, to become more of a fat burner. It involves more long, low intensity exercise and a reduction in carbohydrate intake. That is an over simplified version of course.

The fallacy of the critic’s argument and the foundation of a fat burning zone lies in individual metabolic rates that in turn dictate the source of energy.

Calories come from the metabolism (burning) of carbohydrates, fats and proteins. Science has established that when you exercise at or near 100% of your aerobic capacity (a metabolic indicator of your relative rate of energy expenditure), most of your immediate calories will be derived from carbohydrates.

As you recover from this bout of intense exercise, the relative amount energy from carbohydrate will decrease and you will slowly return to a mixed source (carbohydrate, fat and protein) of energy.

However, scientists at the University of Wisconsin demonstrated over 10 years ago that during intense prolonged exercise the percent of calories derived from protein progressively increased. This further demonstrates that not all energy is derived from the same source.

When you are a “couch potato” for several hours (existing at near your Basal Metabolic Rate – the minimum energy used to stay alive) you find that more than 85% of your energy is derived from fat, less than 15% from carbohydrate and none from protein.

As you progress from total rest through increasing intensities of exercise there is a reduction in energy from fat and an increase in energy from carbohydrate with protein contributing to the picture at higher intensities.

There are two ways to stop or slow your fat burning system. One, and the most common, is consumption of high carbohydrate (sugar and starch) snacks and meals. This will raise insulin production and insulin is the “fat storage” hormone (also know as a fat burning inhibitor). When insulin remains elevated even though blood sugar has dropped, you become hungry and that leads to the need for more calories. Unfortunately, the most common source of snack calories tends to be simple carbohydrates and that leads to more insulin production and more fat storage.

The second fat burning inhibitor is the production of lactic acid. Lactic acid is produced during high intensity exercise known as “anaerobic” (meaning with inadequate oxygen) exercise. The lactic acid seems to interfere with the fat mobilizing hormones and the fat burning enzymes of the body.

Therefore the concept of a “fat burning zone” is referring to any exercise intensity that is aerobic (metabolizing in the presence of adequate oxygen) that does not produce significant lactic acid. The appearance of lactic acid in the blood is known as the “Lactate Threshold”. Naturally the lactate threshold is higher among the more fit and the more fit you are the more fat you burn over a wide range of exercise intensity.

So, in your personal fitness-training program you will find it wise to engage in two types of exercise training. During a low intensity warm-up and long slow running (just get the miles covered style of running) you will be burning primarily fat. Even during the typical recreational marathon where most runners will perform at 50 to 65% of their aerobic power, 50 to 55% of their energy is still coming from fat.

The fat burning zone is usually estimated (although to be sure you could have it tested at a well equipped exercise laboratory) by using the formula of 180 minus your age as a training heart rate. That should keep you below your lactate threshold.

To increase your lactate threshold you would want to engage in interval training or hill running following a god warm-up (fat burning) 2 to 3 times per week to increase your cardiovascular health and your lactate threshold so you can burn fat over a wider range of intensities.

If fat loss is your goal then aerobic (sub-lactate threshold) exercise should be your primary form of training. If you wish to become a better fat burner then aerobic training and reduced carbohydrate eating should be your plan. If your want more cardiovascular fitness and a high lactate threshold you would need to perform some higher intensity – more anaerobic training.


Lee Coyne, Ph.D. is a nutritional consultant, lecturer and author of Fat Won't Make You Fat and the Lean Seekers coaching program. He may be reached at 1-800-668-4042 or by e-mail

Copyright Dr. L. Lee Coyne, reprinted with permission.