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Exercise Intensity, Fat Burning, and Caloric Deficit

Intensity and Fuel Source

Exercise intensity directly affects which fuel source the body uses. At low intensities (below 65% of maximum heart rate), the body relies primarily on fat oxidation. At higher intensities, it shifts progressively toward carbohydrates (glycogen) as the primary fuel. At maximum effort, carbohydrates provide nearly all the energy because they can be metabolized much faster than fat.

This is the basis of the "fat-burning zone" concept: performing prolonged low-to-moderate intensity exercise maximises the proportion of calories derived from fat. However, the total calories burned per minute at low intensity are also lower. A 30-minute moderate walk might burn 120 calories with 60% from fat (72 fat calories), while a 30-minute run burns 340 calories with 35% from fat (119 fat calories). For total fat loss, higher-intensity exercise burns more fat in absolute terms even though its proportional fat contribution is lower.

Caloric Deficit and Weight Loss

One pound (0.45 kg) of body fat stores approximately 3,500 calories. To lose one pound per week requires a deficit of 500 calories per day from some combination of reduced intake and increased activity. In practice, diet accounts for the majority of a sustainable caloric deficit because exercise increases appetite and the total energy expenditure from exercise is modest compared to dietary modifications.

Factors That Affect Individual Caloric Expenditure

The MET formula was calibrated for a 70 kg, healthy 40-year-old male. Several factors cause individual variation around this estimate:

• Body composition: Muscle tissue burns more energy than fat tissue at rest and during exercise. Two people of the same total weight with different muscle-to-fat ratios burn different amounts of energy at the same MET.
• Fitness level: A trained individual performs the same exercise more efficiently, with lower heart rate and lower metabolic cost per unit of work, than an untrained person. This means a fit person burns fewer calories doing the same activity at the same absolute pace.
• Age: Resting metabolic rate declines with age, primarily due to loss of lean muscle mass. Older individuals burn slightly fewer calories for the same MET activity.
• Temperature: Exercising in heat slightly increases caloric expenditure due to thermoregulatory demands. Cold environments can also increase expenditure if shivering occurs, but less so during sustained exercise when the body generates sufficient heat.