Aerobic exercise consumes

Aerobic exercise never consumes a single substance, but a mixture of the three major energy-supplying nutrients: glycogen, fat, and protein. The proportions of the three will fluctuate with the intensity, duration, and individual physical condition of the exercise. There is no clear limit for "suddenly switching energy sources at a certain point in time."

It is estimated that many people have heard the classic saying of gym instructors: "If you do not do aerobic for 30 minutes, it will not count. The first half hour only burns sugar and does not burn fat." When I first became a fitness instructor two years ago, I also said the same thing as my predecessors. It was not until I later read the ACSM (American College of Sports Medicine) updated exercise energy guide in 2022 and took members to see the actual effects of hundreds of aerobic programs that I found that this statement is too one-sided.

A very easy-to-understand analogy: the glycogen in your body is like a fast-charging power bank that you carry with you. It has small reserves and fast output. It is the first thing you use when you move a little. When you sprint for 100 meters or climb two floors to catch the bus, almost all glycogen is consumed.； Fat is the mains power socket on the wall at home. It has a large reserve and stable output. It continues to be consumed when you lie down to sleep or sit and check your mobile phone. However, the transfer speed is slow. You have to wait for the power bank to use up part of it before the energy supply ratio will gradually increase. ； As for protein, it is equivalent to the plastic back cover of your mobile phone. It will not be taken out and burned unless it is absolutely necessary (such as complete depletion of glycogen, long-term high-load exercise). When you really burn a large amount of protein, your body is already in a dangerous state of deficit.

The "30-minute fat-burning turning point" in traditional exercise physiology is actually derived based on the premise that ordinary people have sufficient glycogen reserves and perform moderate to low-intensity aerobics: in the first 20 minutes, the energy supply ratio of glycogen is generally more than 60%, and fat only accounts for about 30%. After 30 minutes, glycogen is consumed to a certain extent, and the energy supply ratio of fat will gradually increase to more than 50%. But this is really not an iron rule. Last year, I took care of a girl who was on sugar control and carbon reduction. She didn’t eat refined rice or noodles for a week in a row. She climbed stairs for 15 minutes on an empty stomach on weekend mornings. The body fat scale measured that her fat consumption ratio reached 58% without waiting for the 30-minute turning point. Her “power bank” didn’t have much charge to begin with, so she had to connect it to “mains electricity” from the beginning.

Speaking of which, I have to mention the controversy over "Will fasting aerobics cause muscle loss" that has been quarreling on the Internet for almost ten years? The two factions are arguing fiercely: one side is an ordinary fitness enthusiast, saying that fasted aerobics will consume a lot of protein, lose muscle and lower the baseline, and they will not touch it until death.； The other person is a bodybuilder preparing for competition. He said that climbing stairs for 40 minutes on an empty stomach every morning during the season can remove fat twice as effectively as doing aerobic exercise after meals, and he will not lose muscle at all. In fact, both sides are right. The core difference lies in the individual state: if you have less than 30 kilograms of muscle mass, rarely exercise, and run 10 kilometers on an empty stomach at a pace of 6 minutes, then the protein energy supply ratio can soar to 15%, and muscle loss is inevitable. ； But if you have a year or two of training and sufficient muscle mass, and control your heart rate at 60%-70% of (220-age) when climbing stairs, and protein energy supply accounts for only about 2%, then you can make up for that loss by eating two eggs after training, so there is no need to worry.

A while ago, a fan sent me a private message to complain, saying that he ran for an hour every day, ate less, and had not lost any weight for half a month. He asked if his method was wrong. I asked him to wear a sports bracelet to measure his heart rate, only to find that when he was running, his heart rate soared to 85% of his maximum heart rate, which was almost anaerobic intensity. 90% of his energy supply relied on glycogen. After running, he always felt, "I have been exercising for so long and it's okay to drink a sports drink."

Don’t just focus on the immediate consumption of exercise. In the hours after aerobic exercise, the body will continue to consume fat to replenish the decomposed glycogen, which is often called the afterburn effect (EPOC). Although it is weaker than the afterburn effect of strength training, it is not a small amount. I usually lead members to do aerobics and never get stuck in the 30-minute deadline. Sometimes I get off work late and briskly walking around the neighborhood for 20 minutes is worse than lying down. After all, we ordinary people do not exercise to compete. When we are struggling with the energy supply ratio, it is better to take two more steps.