Physiological threshold for fat mobilization: the biochemical logic of continuous exercise for more than 30 minutes and gender differences

For exercise to burn fat, certain conditions must be met, one important condition being that the exercise needs to be sustained for a certain period of time. Whether at rest or in motion, the human body requires an energy supply. For human movement, the main energy sources are carbohydrates (sugars), fats, and proteins. These three nutrients each have their specific functions: carbohydrates are direct energy sources, fats are energy storage substances, and proteins are primarily the building blocks of the human body.

Based on the energy supply characteristics of carbohydrates and fats, the number of enzyme systems required for the oxidation and decomposition of carbohydrates is generally relatively small, and the steps involved in oxidation and decomposition are relatively simple. The process from muscle glycogen to the production of acetyl-CoA is relatively rapid. After acetyl-CoA is produced in the cytoplasm, it enters the tricarboxylic acid cycle, producing water and carbon dioxide. In contrast, fats are mobilized from storage depots into the bloodstream and then into skeletal muscle cells. This mobilization process requires the participation of certain hormones, and the enzyme systems involved in its oxidation and decomposition are highly complex.

The oxidation of fatty acids involves many steps, and many processes require entry into the mitochondria to ultimately produce acetyl-CoA, which then enters the tricarboxylic acid cycle and is finally oxidized to produce water and carbon dioxide. Skeletal muscle energy supply follows a "proximity principle," using whichever energy supply system is most convenient. Clearly, during exercise involving aerobic oxidation, skeletal muscle will first utilize the oxidation and breakdown of muscle glycogen for energy because it is relatively convenient.

The mobilization rate of fatty acids through oxidation is very slow; the proportion of fat participating in oxidation for energy only begins to gradually increase 20 to 30 minutes after exercise. Recent research shows that aerobic oxidation of fat for energy generally requires 20 to 30 minutes of mobilization time. Furthermore, there are gender differences in fat mobilization rates; under the same age conditions, women require significantly longer times to mobilize fat for energy than men. Therefore, in most cases, fat only begins to "burn" and provide energy after more than 30 minutes of continuous exercise.

This is why exercise lasting less than 20 minutes is unlikely to have a significant weight loss effect. While high-intensity exercise burns energy quickly, its high intensity primarily relies on the anaerobic glycolysis system for energy, and the resulting lactic acid inhibits fat oxidation and breakdown. During prolonged high-intensity exercise, although a significant proportion of aerobic oxidation is involved, glucose remains the primary energy source because fat mobilization takes longer.

Prolonged, high-intensity exercise inevitably affects the relative stability of blood sugar levels due to the significant consumption of glucose, leading to a noticeable drop in blood sugar and a pronounced feeling of hunger. Therefore, using high-intensity exercise for weight loss often fails to produce good results and may even result in weight gain. Studies have shown that during moderate-intensity sustained exercise, the proportion of energy supplied by glucose begins to decrease after 20 to 30 minutes, while the proportion supplied by fat oxidation begins to increase.

When exercise lasts for about 3 hours, almost all the energy required for muscle contraction comes from the oxidation and breakdown of fat. Therefore, for optimal weight loss, moderate-intensity, long-duration aerobic exercise is recommended. However, this doesn't mean high-intensity exercise is completely off-limits. In the later stages of a weight loss program, to increase skeletal muscle volume and weight, it's necessary to supplement with high-intensity exercise to increase the body's metabolic rate and boost energy expenditure during non-exercise periods, thus consolidating the weight loss results.