Microscopic pathways of lipid metabolism: triglyceride mobilization, β-oxidation, and the "local energy supply" principle

Fat is an important energy storage substance in the human body, mainly stored in the subcutaneous fat layer, but also distributed around internal organs and in intermuscular spaces. Fat is oxidized and broken down in the body through a series of complex processes to produce energy; fat breakdown occurs directly within cells. Although fat has high chemical energy, its molecular composition contains relatively little oxygen, requiring more oxygen to oxidize and break down. Compared to carbohydrate metabolism, fat metabolism involves more steps and is less efficient than carbohydrate metabolism in the short term. Therefore, the body tends to choose carbohydrates as the primary energy source for high-intensity exercise, while using fat as a reserve energy source and for endurance.

The oxidative breakdown of fats in the human body follows a highly complex pathway. Under the action of lipases, fats (triglycerides) are hydrolyzed into glycerol and fatty acids. Glycerol undergoes phosphorylation and dehydrogenation to convert into dihydroxyacetone phosphate, which is then incorporated into the carbohydrate metabolism pathway. The conversion of fatty acids is even more complex: fatty acids react with adenosine triphosphate (ATP) and coenzyme A (CoA) under the action of acyl-CoA synthase to generate acyl-CoA. Acyl-CoA then enters the mitochondrial matrix via the carnitine transport system on the inner mitochondrial membrane, where it undergoes β-oxidative degradation.

The β-oxidation process involves four consecutive steps: dehydrogenation, hydration, re-dehydrogenation, and thiolysis. Each β-oxidation cycle generates one molecule of FADH2, one molecule of NADH, one molecule of acetyl-CoA, and produces an acyl-CoA molecule with two fewer carbon atoms. The generated acetyl-CoA then enters the tricarboxylic acid cycle, ultimately undergoing complete oxidation to produce water and carbon dioxide, releasing a large amount of ATP. This efficient energy storage method is a result of human evolution, ensuring survival by mobilizing fat in environments with food scarcity.

However, skeletal muscle energy supply follows a "proximity principle." During exercise involving the aerobic oxidative energy supply system, skeletal muscle will first utilize muscle glycogen because the oxidative breakdown of muscle glycogen is relatively simple and the enzyme system mobilizes quickly. Fat is mobilized from fat stores into the bloodstream, then into skeletal muscle cells, and finally into the mitochondria; this series of processes is strictly controlled by hormones and enzyme activity. The oxidative breakdown and mobilization of fatty acids is very slow; the activity of relevant metabolic enzymes only reaches its peak some time after the start of exercise.

Recent physiological studies show that the aerobic oxidation of fat for energy typically requires 20-30 minutes of mobilization time. Moreover, this mobilization rate exhibits a significant gender difference: at the same age, women generally require a longer time to mobilize fat for energy than men. This means that if an exercise session lasts less than 20 minutes, the proportion of fat used for energy is extremely low, with muscle glycogen and blood glucose being the primary energy sources. This is the biochemical reason why fragmented, short-duration, intense exercise is unlikely to produce significant fat-loss effects.

Changes in the ratio of energy supplied by carbohydrates and fats during exercise can be monitored using the "respiratory quotient" (RQ). Fatty acid molecules have a high hydrogen-to-oxygen ratio, requiring a large amount of oxygen for oxidation, thus resulting in a lower RQ. If the RQ gradually decreases during moderate-intensity exercise, it indicates that the proportion of fatty acid utilization is increasing. If the RQ increases in the later stages of exercise, it may suggest that the exercise intensity is too high, inducing anaerobic glycolysis of carbohydrates, and the resulting lactic acid inhibits the activity of lipoxygenases.

Therefore, to improve the efficiency of aerobic fat oxidation, exercise for weight loss must be sustained for a sufficient duration. Generally, a single exercise session should last at least 40 minutes, and if physical ability permits, 60-120 minutes is more suitable. Within this time window, the proportion of fat oxidation reaches its optimal level. At the same time, warm-up activities before exercise are essential; by raising muscle temperature, the inertia of fat mobilization can be reduced, allowing the body to enter a fat-burning state more quickly.

In summary, fat metabolism is a complex but slow-starting pathway. Understanding the microscopic logic of β-oxidation helps dieters overcome the initial fatigue of exercise, persevere through the glycogen-depleting phase, and ultimately reach a truly stable state of efficient fat burning.