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Many runners tell each other that, “To burn fat, you must run slowly, whereas running fast burns carbs,” which leads to more effective weight loss. However, the human body never operates on a rigid on-off switch.

When you run, the body always uses a combination of fat and carbohydrates. Fat is a massive energy storehouse that provides long-lasting energy, but it is mobilized relatively slowly.

Carbs, on the other hand, exist primarily as glycogen stored in the muscles and liver. While limited, they supply energy much faster when the body needs to step up its output.

During low-intensity exercise, your body burns more fat, but as you speed up, it is forced to increase its use of carbohydrates. This transition happens continuously, rather than abruptly “shutting off fat burn to turn on carb burn.”

It is worth nothing that the greatest value of running lies not just in the amount of fat burned, but in the metabolic process and building of endurance. On the other hand, during high-speed runs or competitions, if glycogen stores drop significantly and the body’s ability to maintain blood glucose does not meet demand, runners can become exhausted and have difficulty concentrating.

To run effectively and protect your health, you need to balance exercise intensity with proper nutrition. Build a workout plan with clear goals, alternating rest intervals and light runs for recovery, instead of assuming that running to complete exhaustion will help you progress faster.

When exercising continuously for 60 to 90 minutes or longer, it is crucial to replenish your body with carbohydrates. Specifically, during a 1- to 2.5-hour run, the body needs approximately 30-60g of carbohydrates per hour; this figure rises to 60-90g per hour if the run extends beyond 2.5 to 3 hours, depending on intensity and absorption capacity.

It is recommended that each person should engage in moderate exercise for at least 150 minutes per week. You can walk, jog, go to the gym, or do yoga to improve your health, depending on your personal preferences and physical condition.

The ideal time for working out is between 4 p.m. and 7 p.m., which aids in weight loss and muscle toning, whereas time after 9 p.m. should be reserved for rest.

If you are in good health or have years of training experience, you should still listen to your body to choose a suitable time frame. Whenever you feel tired, drained, or short of breath, stop and rest rather than pushing through.

However, you should avoid late-night training sessions over an extended period, even if you are healthy with no underlying conditions.

Many cases of overexertion or working out through the night have led to muscle atrophy and loss, long-term mobility impairment, and even irreversible joint conditions. Additionally, overtraining is likely to cause injury, insomnia, restless sleep, and have negative effects on your heart.

Dr. Le Thi Huong Giang

PhD, Head of Dietetics and Nutrition Department, 19-8 Hospital (Ministry of Public Security)