Why it matters. The carbohydrate-versus-fat fuel debate and the '30-minute fat-burning' myth are global fitness misconceptions, so this evidence-based reframing applies to readers anywhere.
Background. Kyunghyang Shinmun is a major South Korean daily newspaper. The author writes under the pen name Soup-hee (수피) and is one of Korea's best-known exercise columnists, whose 'Standard of Health' (헬스의 정석) books are mainstream bestsellers in the country's large self-directed fitness culture. 'Zone 2' training, referenced in the piece, is a widely popular endurance method built around keeping the heart rate in a moderate aerobic band.
What to watch next. Expect Korean fitness media to keep pushing 'metabolic flexibility' as the framing that supersedes the old cardio-versus-weights either/or.
The takeaway: stop picking sides
A widely read South Korean fitness columnist argues that anyone trying to lose weight or manage a metabolic condition like diabetes should not choose between low-intensity cardio and high-intensity training — they should do both, in rotation, every week. The advice, published in the daily newspaper Kyunghyang Shinmun by an exercise writer known by the pen name Soup-hee, author of the popular Korean fitness book series The Standard of Health, reframes a debate that confuses many gym-goers.
The core idea is that the human body is a hybrid engine. Just as a plug-in hybrid car launches on stored electricity but switches to gasoline for sustained driving, the body draws on two fuels: carbohydrates and body fat. Carbohydrates act like the electric battery — instantly available for bursts of maximum effort, but limited in supply. Fat acts like the gasoline tank — abundant, but slower to mobilize.
When each fuel actually burns
The writer takes aim at a stubborn fitness myth: that fat only starts burning after 30 minutes of exercise. That, he says, is an oversimplification. The point at which fat becomes the dominant fuel depends on both exercise intensity and the individual.
In an all-out sprint, the muscles hit their limit before fat-burning fully kicks in, so the body draws carbohydrates to the maximum and stops there. That does not make such workouts useless for weight loss. High-intensity effort trains muscles to absorb blood sugar far more efficiently — which is precisely why it is so effective at improving insulin resistance and diabetes. It also builds explosive capacity, the kind that lets someone who could barely sprint up one flight of stairs eventually run up ten.
Low-intensity exercise works the opposite way. It can be sustained for long periods and burns a higher proportion of fat — the basis of the trendy “Zone 2” training (steady effort kept in a moderate heart-rate zone). This is where the body’s fat-burning ability improves fastest.
Metabolic flexibility is the real goal
There is an irony the columnist highlights: fat burns well only when carbohydrate metabolism is working as a kind of switch. When insulin resistance blocks carbohydrate use, fat does not burn properly either. Excess body fat worsens insulin resistance, which in turn hampers fat burning — a vicious cycle seen in the metabolic syndrome common among people with obesity. People with low insulin resistance, by contrast, burn both fuels well.
The ability to switch freely between the two fuels as conditions demand is called metabolic flexibility, and the writer calls it essential to building real fitness.
His practical prescription: once or twice a week, do something hard — an all-out sprint, CrossFit, or similar — to train sugar metabolism. On the remaining days, do slow Zone 2 running for weight management. The body is a hybrid that can burn both, he concludes, so there is no reason to fixate on only one.
