Almost all fitness-related materials mention that aerobic exercise must exceed a certain duration (e.g., 20 or 30 minutes) to burn fat, citing the reason that before this point, the body prioritizes carbohydrate consumption. Only after this threshold, when carbohydrates are gradually depleted, does the body begin to burn fat. Is this truth or myth?
I. What is Aerobic Exercise? What is Anaerobic Exercise?
Aerobic Exercise: Also known as aerobic metabolism exercise, this refers to physical activity performed with ample oxygen supply to the body.
Anaerobic Exercise: This refers to high-intensity, rapid muscle movements performed in a "oxygen-deprived" state.
The distinction between aerobic and anaerobic exercise isn't based solely on the type of activity, but rather on whether the energy for muscle contraction comes from aerobic or anaerobic metabolism.
II. Differences Between Aerobic and Anaerobic Exercise
III. Is It True That Fat Burning Only Starts After a Certain Duration of Aerobic Exercise? Does Aerobic Exercise Really Begin Burning Fat Only After Glycogen Is Depleted?
First, we need to understand how much glycogen the body contains and how long it can sustain exercise with these reserves.
Carbohydrates in the body primarily come from three sources: blood glucose, muscle glycogen stored in muscles, and liver glycogen stored in the liver.
The average person has about 5 grams of blood glucose, 100 grams of liver glycogen, and 400 grams of muscle glycogen. These three components make up the body's carbohydrate reserves, all capable of providing energy for exercise.
In reality, carbohydrates and fats contribute to energy production simultaneously during any form of exercise, differing only in their relative proportions.
Secondly, exercise intensity determines the ratio of carbohydrate to fat energy contribution. During moderate to low-intensity exercise, fat energy contribution increases while carbohydrate contribution decreases.
During high-intensity exercise, carbohydrate energy contribution increases while fat contribution decreases. Therefore, fat begins to be consumed as soon as exercise commences.
Experiments show that during prolonged aerobic exercise, carbohydrates and fats contribute proportionally to energy supply (roughly 55:45). Women, having higher levels of free fatty acids, initially burn fat at a slightly higher rate, with fat contributing about 60% of energy.
The notion that fat burning only begins after 30 minutes of running stems from a statement in Exercise Physiology: after 30 minutes of exercise, the proportion of energy derived from fat increases while that from carbohydrates decreases.
This viewpoint was likely misinterpreted, evolving from "the proportion of energy from fat increases 30 minutes after exercise begins" to "fat only starts contributing energy 30 minutes after exercise begins."
Therefore, the claim that the body first depletes carbohydrates before mobilizing fat is scientifically unfounded.
IV. Key Considerations for Aerobic Exercise Weight Loss
1. Warm-up and Cool-down for Aerobic Exercise
Incorporate both warm-up and cool-down phases before and after each aerobic session. These stages enhance safety and effectiveness.
Warm-up (Preparation Phase)
Warm-up typically involves low-intensity aerobic activity to gradually prepare your body. This raises body temperature, increases heart rate, and accelerates breathing rhythm. Blood circulation also quickens, delivering oxygen and nutrients to the heart and muscles to prepare for exercise. A key indicator that the warm-up is complete is when you begin to sweat lightly.
A warm-up lasting 5 to 10 minutes is sufficient.
On cold days, extend the warm-up duration and wear additional layers.
Quick tip:
Many people skip warm-ups to save time and jump straight into high-intensity cardio. Doing so increases injury risk because your cardiovascular system and lungs aren't primed, your body temperature is low, and your muscles lack flexibility.
Additionally, exercising after warming up feels better and allows for longer workout sessions.
In other words, exercising without warming up makes you tire more easily.
Cooling Down
Cooling down serves the same purpose as warming up. During exercise, blood circulation accelerates and blood volume increases, especially in the limbs.
Quick tip:
If you stop exercising abruptly, blood may pool in your lower limbs, placing extra strain on your heart.
In severe cases, this can affect blood supply to the brain, potentially causing dizziness or lightheadedness. Therefore, after achieving your workout goals, allow 5–10 minutes for relaxation—gradually reduce intensity and slowly return to a resting state.
2. What Should Women Pay Attention to During Aerobic Exercise?
Wear a supportive sports bra during workouts.
Avoid intense exercise during menstruation.
Women without a regular exercise routine should not begin aerobic workouts during pregnancy. Even those with prior aerobic training should consult a doctor to determine if continuing is safe.
Quick Tips:
Change out of sweaty clothes promptly after exercise to avoid catching a chill, especially in air-conditioned rooms. Perform some stretching exercises before showering.
Regular aerobic exercisers should pay attention to foot care. Trim toenails frequently, as broken nails can tear skin and cause toe infections.
Heavy sweating during hot weather can trap moisture between toes, fostering bacterial growth. Keep feet dry at all times. If blisters form, avoid breaking them.