How do I know when I'm exercising hard enough to burn fat?

Actually, you're almost always burning fat at one rate or another, but you burn most when your body is in its aerobic range. A good rule of thumb is that after 20 minutes in your aerobic zone, you will be burning more fat than carbos. Covert Bailey, in Smart Exercise, states that you will be burning fat after only twelve minutes of aerobic exercise. If you can increase your aerobic activity to 30 minutes or longer, you will be burning a larger percentage of calories from fat. There is still some disagreement as to which is better - longer duration at lower intensity, or shorter duration at higher intensity. If you are limited in time, then the higher intensity will maximize your aerobic benefits in a shorter amount of time. If you can work for a longer duration at a lower intensity, you will decrease your chance of injury. The object is to burn more calories than you take in. 3,500 calories equals l pound of fat. Your muscles will continue to burn fat after both aerobic and anaerobic (muscle training) exercise.

(from Michael G. Kurilla

This is perhaps the most common question raised by individuals exercising for the purpose of either weight loss or simply weight control. This stems from the recognition that aerobic exercise is a significant adjunct to any weight loss program, that is diet plus aerobic exercise produces more weight loss than diet alone. In addition, the weight lost with exercise tends to be a higher percentage of fat.

Exercise can be grouped into three broad levels of intensity, mild, moderate, and high. Mild intensity is a comfortable walking pace and can be sustained almost indefinitely, moderate intensity is equal to an average cardiovascular conditioning workout (able to talk, but not sing) and can be sustained (in a trained individual) for upwards of 3 - 4 hours, and high intensity is not able to talk and can only be sustained for 30 - 45 minutes.

Based on recent and very detailed research studies, in terms of absolute fat burning, a moderate intensity workout burns the most fat. At a heart rate equal to about 75% of max, fat burning will approach 0.5 grams - 1.0 grams of fat per minute. There is a weight dependence with the lower end referring to a 100 pound individual and the upper end to a 200 pound person. As the duration continues (greater than 1 hour), fat burning can increase slightly (another 10%).

At a mild intensity, the majority of calories expended (85 - 90%) are fat calories, but the absolute level is only about 60% of the moderate intensity. At high intensity levels, fat burning declines to a level of about 65% of the moderate pace, as sugar burning supplies the rest. The high rate of sugar burning exhausts the limited sugar supply in muscles and causes muscular failure.

The only caveats for the above burn rates are that these numbers are derived from individuals who were already aerobically trained and were conducted in the AM before breakfast. Less fit individuals are known to burn less fat and more sugar (part of aerobic conditioning is greater reliance on fat burning for energy). Exercising after a meal will tend to promote more sugar burning. Consumption of sugar during an exercise session will also tend to retard fat burning in favor of the sugar. These numbers were derived from cycling and so the absolute numbers can be increased if exercises that involve more muscle groups are utilized (running, rowing, etc.). From peak energy production rates for various exercises, rowers might reach about 40% higher.

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