Fat and training intensity in practice

You burn fat during a workout even when you are not breathless or drenched in sweat. The question is more about which intensity makes your body use more fat and when total energy expenditure really starts to add up.

Why does fat matter during training?

This question often comes up when the goal is to maintain body weight or gradually lose weight. The thinking is simple: if training burns fat, it should also help improve body composition. In practice, the picture is a little more complicated.

Your body actually uses fat at every training intensity. Even gentle walking, cycling and light aerobics draw some of their energy from fat stores. The difference lies in the proportion of energy that comes from fat and the total amount of energy you expend.

An old rule says it takes about 20 minutes to settle into a predominantly aerobic zone. After 20 minutes of training, more fat than carbohydrate is used for energy. According to Smart Exercise, the shift towards greater fat use can begin after just 12 minutes of exercise. The longer the workout lasts, the greater the role of fat stores.

This brings us to a familiar debate: is a longer, gentler workout better, or a shorter, harder effort? The honest answer is that both have their place. If your goal is fat burning, a high percentage of energy coming from fat is not enough on its own. Your total energy expenditure matters too.

How is fat burned at different intensities?

In simple terms, aerobic training can be divided into three intensity levels: low, moderate and high. Low intensity is roughly equivalent to a gentle walk. You can keep it up for a very long time, but you burn few calories per minute.

At moderate intensity, you can still talk to a training partner, but singing becomes difficult. On average, you can sustain this pace for 3 to 4 hours. This is often considered the most favorable zone for using fat.

High-intensity aerobic training feels quite different. Talking is difficult, breathing is rapid, and you can sustain the effort for about 30 to 45 minutes. Energy expenditure per minute is higher, but the proportion supplied by carbohydrates also increases.

Training at 75% of your maximum heart rate burns an average of 0.5 to 1 grams of fat per minute. The 0.5 grams applies to a 45 kg individual, while someone weighing 90 kg burns about 1 gram. These figures sound high, but the context matters: they were measured in people who already had good aerobic fitness.

Once an aerobic workout goes beyond 60 minutes, fat use for energy increases by a further 10% or so. This does not mean everyone should immediately start exercising for an hour. If your fitness is poor, it is wiser to start with shorter workouts and gradually increase your training volume.

Fat, calories and practical choices

At low intensity, most of the calories you burn may come from fat—as much as 85 to 90%. That sounds good, but total calorie expenditure remains low, at about 60% of what you would burn during moderate-intensity exercise. The simple truth is that a percentage is not the same as an amount.

At high intensity, the pattern is reversed. The contribution from fat is about 65% of that used during moderate-intensity exercise. More of the remaining energy comes from carbohydrates. This also means more lactate, faster fatigue and more rapid depletion of carbohydrate stores.

So what actually happens? A trained person is better able to use fat as fuel. Even at lower intensities, an untrained person uses more carbohydrates than a trained person. This is why you should not compare yourself with someone who has been running, cycling or rowing regularly for years.

Meal timing also changes the picture. Aerobic training immediately or shortly after eating favors carbohydrate use over fat use. The same applies when you consume carbohydrates during training. This is not a bad thing, especially during longer or harder workouts, but the proportion of energy supplied by fat may decrease.

The original figures were measured on a cycle ergometer. Running and rowing may increase expenditure because they engage more muscles. The effect depends on body mass, and particularly muscle mass. During training, rowers may record figures as much as 40% higher than those mentioned earlier.

Practical tips for fat burning

• If you are a beginner, choose a pace at which you can still talk. This is a simple sign that the intensity is not too high.
• If you want to do longer aerobic workouts, increase the duration gradually. 20 minutes is a good initial target, not a magic threshold.
• If you train harder, expect to tire sooner. 30 to 45 minutes can already be a substantial workout.
• For weight loss, training and nutrition work together. Aerobic training alone will not do the job if your overall daily diet works against you.
• Do not confuse the percentage of energy supplied by fat with total energy expenditure. A gentle workout may draw a greater proportion of its energy from fat, but a moderate pace may result in higher total expenditure.

Translator’s note: 0.5 1.0 grams of fat per minute—those seem like astonishingly high figures to me, don’t they?

FAQ: fat and training

Does fat burning only start after 20 minutes?

No. Your body uses fat before that, too. Around 20 minutes is more a point at which fat may begin to account for a noticeably larger share of the energy you use when training in the aerobic zone.

Is low intensity always best?

Not always. At low intensity, fat may account for a large share of the energy you use, but your total calorie expenditure may be low. For many people, a moderate pace they can maintain consistently works best.

Does fasted training burn more fat?

It may increase fat use during training, but that does not automatically mean better weight loss. Your overall diet throughout the day and consistency in training still matter.

Translated by: Janar Rückenberg

Source: Fats

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