Does strength training burn glycogen or fat?

Let me give you something accurate first: Both glycogen and fat will be consumed during strength training. There is no absolute situation of "burning only one type". The energy supply ratio of the two will be adjusted at any time according to your training intensity, training duration, and even your recent dietary status. There is no black or white answer at all.

I was also confused by this problem when I first entered the gym. The old coach I followed at that time opened his mouth and said that strength training relies entirely on glycogen for energy. If you want to lose fat, you have to finish running and then train strength. I foolishly followed it for half a month. I practiced squatting and fell twice due to hypoglycemia. Later I realized that this was not the case at all.

If you press heavy weights, such as deadlifting 90% of your maximum weight, and you have to rest for 3 minutes after each pull, the energy supply will be almost all phosphate and glycogen, and fat will not even account for 10%. After all, in this kind of explosive output work, the speed of fat decomposition and energy supply cannot keep up. It is difficult for a clever woman to make a meal without rice. On the day I tried to PR in the 180kg deadlift last year, I gnawed two large bananas before training. After the training, my blood sugar dropped by 2 points, which clearly showed that glycogen was supplying energy.

But if you do the kind of low-weight, high-volume circuit training, such as holding 5kg dumbbells to do shoulder presses, curls, and lunges, with only a few dozen seconds of rest between groups, and the training lasts for an hour, the proportion of fat energy supply can climb to 30% or even higher. I had previously measured during the preparation period that when doing this kind of moderate-intensity strength training on an empty stomach, the fat energy supply reached a maximum of 42%. Of course, this was related to my body fat rate not being low at the time and having adapted to a low-carbohydrate diet for three weeks. A newbie who is new to fitness would definitely not be able to reach this number.

In fact, there is no completely unified conclusion on this issue in the academic world. Traditional exercise physiology textbooks still classify strength training as high-intensity exercise that mainly supplies energy through glycolysis. The default is that the proportion of fat energy supply is very low. However, in recent years, many controlled experiments on trainers have also shown that for people with a certain training foundation, in moderate-intensity and long-term strength training, the proportion of fat energy supply is much higher than previously estimated, and can even be equal to that of moderate-intensity aerobics. The practical side of the fitness circle is even more straightforward. If you look at the bodybuilders preparing for competitions, which one does not do crazy volume training two months before the competition? If strength training really doesn't burn fat, why are they going to all the trouble?

And many people have missed the most critical point: the bulk of fat burning in strength training is not at the moment of training, but in the dozen or so hours after training. This is what we often call EPOC (excess post-exercise oxygen consumption). During exercise, you may burn mostly glycogen, but after exercise, the body needs to repair muscle fibers and replenish depleted glycogen reserves, all of which require additional energy consumption. At this time, the energy supply may be mainly fat. I have read a study before. The EPOC effect after heavy strength training can last for 24 hours, and the total fat consumption can account for more than 60%. Isn't this better than running on a treadmill for 40 minutes?

Of course, many coaches feel that it is completely pointless to dwell on this. To put it bluntly, the essence of weight loss is a calorie gap. It doesn’t matter whether you burn glycogen or fat during training. As long as your total daily consumption is greater than your intake, sooner or later your body will use fat reserves to fill the hole. Even if you burn all the glycogen during strength training today, the carbohydrates you eat later will first make up for the glycogen pit, and the calories that were originally going to be turned into fat will have no place to store. Isn't this still equivalent to losing fat?

To be honest, I have been involved in fitness for so many years, and I have seen too many people worry about this little detail. Sometimes they are worried that training for strength will not burn fat, and sometimes they are worried that they will burn glycogen and lose muscle. In fact, it is completely unnecessary. If you like to charge heavy weights, do it well. If you want to lose fat, control your diet and add enough volume. If you are struggling with the energy supply ratio, it is better to do two more sets of deadlifts.