Adipose tissue morphology and energy substrates: Metabolic differences and evolutionary perspectives of white, brown, and beige fats.

Adult mammals contain both white and brown adipose tissue, which differ in both distribution and function. White adipose tissue is abundant and widely distributed under the skin and around internal organs, serving as the body's largest energy storage depot. Brown adipose tissue is mainly present in infancy and gradually decreases with age.

Brown adipose tissue has a rich blood supply and its cells are filled with mitochondria, acting like a "heat generator." When stimulated by cold, it generates heat through uncoupled oxidation. In 2012, scientists discovered "beige adipose tissue," which lies between the two. Beige adipose tissue can produce high levels of thermogenic proteins after exercise or cold stimulation. Studies have shown that exercise-induced hormones can convert white adipose tissue into metabolically active beige adipose tissue.

Fat is the most efficient way for humans to store energy, a capability developed through long-term evolution. In harsh environments, mobilizing fat oxidation was crucial for survival in early humans. In today's world of abundant resources, this instinct has become a burden. Aerobic exercise is the only effective way to burn excess fat. However, in the initial stages, the proportion of energy supplied by fat will not immediately increase.

This is because skeletal muscle follows a "proximity principle" for energy supply, primarily utilizing glycogen. Fat mobilization involves hormonal regulation and complex enzymatic reactions, and its oxidation process requires entry into the mitochondria. During the first 15 minutes of aerobic exercise, muscle glycogen is the primary energy source, while the proportion of fat only begins to increase after 20 minutes. Therefore, aerobic exercise is generally required to last for more than 30 minutes.

Human skeletal muscle fibers are divided into fast-twitch and slow-twitch fibers, and their ratio is determined by genetics. Fast-twitch fibers contract quickly, tire easily, and primarily rely on anaerobic metabolism for energy. Slow-twitch fibers contract slowly, are more sustained, have abundant mitochondria, and have strong aerobic oxidation capabilities. Individuals with a higher proportion of slow-twitch fibers have a stronger ability to utilize fat for energy and are less prone to obesity.

People who engage in speed-based sports like sprinting have a high proportion of fast-twitch muscle fibers, resulting in low fat utilization efficiency. Marathon runners, on the other hand, have a high proportion of slow-twitch muscle fibers and tend to be leaner. Those with a high proportion of fast-twitch muscle fibers are more prone to weight gain after retirement. If individuals with a high proportion of fast-twitch muscle fibers engage in high-intensity exercise, lactic acid buildup can lead to decreased activity of metabolic enzymes.

In the liver, lactic acid can even be reused to synthesize fatty acids, leading to weight gain despite dieting. Therefore, this group of people should engage in low-intensity, long-duration exercise, keeping their heart rate between 100 and 120 beats per minute. Both excessively high and low exercise intensity will affect the results. Among all aerobic intensities, the relatively high intensity that does not produce lactic acid results in the highest calorie expenditure per unit time.

Before developing a plan, an exercise function assessment should be conducted to select appropriate exercise programs. Only by combining scientific exercise intensity with sufficient duration can efficient fat loss be achieved.