The order in which aerobic exercise consumes body energy

There is no absolute order of “consuming glycogen first, then fat, and finally protein.” From the first second you start doing aerobic exercise, the three major energy-supplying substances, carbohydrates, fats, and proteins, are already participating in the energy supply at the same time. The only difference is that the proportion of energy supply is different under different exercise intensities and durations.

I know that many people have heard the saying when they first came into contact with fitness, "the first 30 minutes of aerobic exercise are in vain, and only after 30 minutes will you burn fat." This statement is not completely nonsense, but it is definitely taken out of context. It has indeed been observed in early exercise physiology tracking experiments that when healthy ordinary people do medium- and low-intensity aerobics with a heart rate of 60%-70% of their maximum heart rate, the energy supply ratio of carbohydrates can reach 60%-70% in the first 20-30 minutes. After that, as glycogen reserves are gradually consumed, the energy supply ratio of fat will slowly overtake it, accounting for up to more than 70% of the total consumption. But this does not mean that you are just fishing for fat in the first half hour. I once taught a student who worked 9 to 5. After getting off work every day, he squeezed the subway to the gym and only had 25 minutes to exercise, so he stepped on the elliptical machine. He persisted for two months and still lost 3 points of body fat. To put it bluntly, the proportion of fat energy supply in the first half hour was slightly lower, and the total consumption was real.

To use an analogy, sugar is like the change you carry with you. You can spend it when you take it out, and the amount you can use quickly is limited.； Fat is a fixed deposit that you have saved for several years. You have to go through some procedures when withdrawing it, but the reserve amount is enough. A person with a normal weight has enough fat reserve to run dozens of marathons in a row. ； Protein is like the furniture and appliances in your home. Under normal circumstances, no one will sell it for money. They will only touch it when they are desperate.

When it comes to variables that affect the proportion of energy supply, the first thing that comes to mind is definitely the duration of exercise. In addition, the impact of exercise intensity is actually much greater than you think. For example, if you usually jog at a pace of 6 minutes, and by the time you run for 40 minutes, the energy supply from fat has accounted for more than half, and then you suddenly speed up to 4 and a half minutes to run a kilometer, gasping for breath, you can't even speak. At this time, no matter how long you have been running, the energy supply from carbohydrates will immediately return to more than 70% - after all, high-intensity exercise requires rapid energy supply, and "change" is easy to use. The last time I accompanied a student who was preparing for a half-marathon to do intervals, he had been running at a constant speed for 45 minutes. When he was running three sets of 1-kilometer intervals, the energy supply ratio of fat measured by the sports watch he carried dropped directly from 58% to 22%. This is the reason.

Speaking of which, I have to mention fasting aerobics, which has been particularly controversial recently. The two schools of thought have been arguing for almost ten years, but there is still no absolute conclusion. One group of supporters believes that after a night of fasting, most of the liver glycogen reserves have been consumed. At this time, if you do aerobics, the proportion of fat energy supply can reach more than 50% from the beginning, which is 10%-15% higher than if you do aerobics after a meal, the fat energy supply ratio is higher, and the efficiency of fat removal is higher. ； The opposing faction said that although fat accounts for a high proportion of energy supply, the total consumption is similar to that after a meal, and the energy supply proportion of protein in the fasting state will be 1-2 percentage points higher than after a meal. Long-term exercise will easily lead to muscle loss, and it is not friendly to people who need to gain muscle. I have tried both methods myself. When I was brushing my body fat last year, I climbed stairs for 30 minutes on an empty stomach in the morning, and stepped on the elliptical machine for 20 minutes after doing strength training in the evening. In two months, I lost body fat at a similar rate. I just occasionally felt dizzy 10 minutes before climbing stairs on an empty stomach. Later, I took a sip of electrolyte water mixed with glucose before going out. To put it bluntly, the most important thing is to suit yourself. If you don’t need to gain muscle and want to save time and quickly clean fat, you can do aerobics on an empty stomach. If you still train strength and want to keep the muscles you have worked hard to build, it is safer to do aerobics 1-2 hours after a meal.

Many people worry that they will lose muscle by doing aerobics. In fact, there is really no need to worry. Under normal circumstances, as long as you are not hungry for two or three days and run long distances, or do high-intensity aerobics for more than 2 hours in a row, the energy supply ratio of protein will basically not exceed 10%. This consumption can be fully replenished by the eggs and milk you eat daily, and it will not be the turn to break down your muscles. The runners around me who run 10 kilometers three times a week have higher muscle mass than many people who only do strength training without doing aerobics, and they have not lost any muscle at all.

Finally, to be honest, we ordinary people do not need to worry about the order of energy supply and whether the first 30 minutes are useful when doing aerobics. If you can stand on the treadmill and move, you will already beat 90% of people who lie down and watch short videos. If you want to increase the efficiency of fat brushing, keep your heart rate in the 60%-70% range of (220-age), and insist on it for more than 30 minutes each time. If you really can’t squeeze out time, even taking two stops away from get off work and walking for 20 minutes is still useful.