Fat, carbs and the fat-burning zone

0:03 · Same share of fat, very different grams

Resting in the lab, eight cyclists got fifty six per cent of their energy from fat. Then they rode at a light effort, and fat supplied fifty five per cent. Yet the fat they burned went from zero point zero eight grams a minute to zero point six eight. And later in this video, two groups ride at about two hundred watts, and one of them burns far more fat.

0:24 · The fat burn zone on your watch

Your watch, or the console of a gym treadmill, probably has a zone called fat burn. It is easy to read that label as a promise that this is where you burn the most fat. There is something real behind it, but the answer depends on how you count. Count the share of your energy, or count the grams, and you end up at different efforts. This video is about fat as fuel for moving, and losing body fat is a separate subject.

0:49 · Fat or carbs? Always both

First the question in the title: do you burn fat or carbs? You burn both, at every effort you can hold for long. A Dutch and British group showed this in two thousand and one, with eight male cyclists tested after a night without food. They measured them at rest, and then over three half-hour stages of cycling at a light, a moderate and a hard effort. Those efforts were forty four, fifty seven and seventy two per cent of maximal oxygen uptake, the most oxygen the body can use. At rest, fat supplied fifty six per cent of the energy, at the light effort fifty five, and at the moderate effort fifty. With only eight riders, those three cannot be told apart. At the hard effort, fat had fallen to twenty four per cent, and carbohydrate supplied the other seventy six. Nowhere on that scale does one fuel switch off and the other switch on.

1:39 · Where do you burn the most fat?

So where do you burn the most fat? Count the share, and low efforts win. The riders got as large a share of their energy from fat at rest as at any effort on the bike. Count the grams, and low efforts lose. At rest they burned zero point zero eight grams of fat a minute. At the light effort that was zero point six eight, and at the moderate effort zero point seven nine. With eight riders, those two are too close to call. At the hard effort it dropped to zero point five one. As a yardstick for what follows, half a gram a minute is thirty grams an hour. An older study saw the same split within exercise itself. Its riders had the richest mix of fat at a quarter of their maximal oxygen uptake, but burned more fat at sixty five per cent. So a high share of fat and a large amount of fat do not belong to the same effort.

2:29 · The same slice of a bigger pie

How can the same share be so much more fat? Because the whole pie grows with the effort. By our own conversion, the riders used about one point four kilocalories a minute at rest. At the light effort, that was about twelve point two. The same slice of a far bigger pie is far more fat. One detail helps when you read grams. A gram of fat holds about two and a half times the energy of a gram of carbohydrate. That is why, at the moderate effort, zero point seven nine grams of fat matched two point zero four grams of carbohydrate. Above the moderate effort the picture changes. The pie keeps growing, but now the fat slice shrinks, and carbohydrate covers all of the extra work. At the hard effort the riders burned three point nine one grams of carbohydrate a minute, about two hundred and thirty five grams an hour.

3:16 · The whole curve, up to exhaustion

That study stopped at a hard effort the riders could hold for half an hour. A study from two thousand and eighteen took twenty two professional cyclists all the way to exhaustion, ten minutes at a time. Carbohydrate climbed the whole way, from one point four six grams a minute at one hundred and thirty seven watts. At three hundred and seventy two watts it reached six point zero two, about three hundred and sixty grams an hour. Fat rose to a peak of zero point six seven grams a minute at two hundred and thirty nine watts, about forty grams an hour. Then it fell away, to zero point zero two at three hundred and seventy two watts. These are professionals, so the scale is theirs and not yours. What carries over is the shape, and a review describes it in the same terms. Fat burning rises from low to moderate efforts, and falls when the effort becomes high. Read the far right end with some care, though. Close to maximum, the breath measurements behind these numbers overstate carbohydrate and understate fat. So the data show fat burning fading to almost nothing, but they cannot show that it stops.

4:19 · Where the peak sits, person by person

So where does that peak sit for you? In the study that introduced the test for it, moderately trained cyclists peaked at sixty four per cent of maximal oxygen uptake on average. Individually, the peak sat anywhere between forty two and eighty four per cent. Two larger studies on treadmills landed lower. In three hundred healthy men and women the average was forty eight point three per cent, with individuals between twenty five and seventy seven. In one thousand one hundred and twenty one athletes it was forty nine point three on average. The lowest individual peaked at twenty two point six per cent, and the highest at eighty eight point eight. The peak is also more of a plateau than a point. In the first study's cyclists, fat burning stayed within ten per cent of its highest rate between fifty five and seventy two per cent. Sex is one of the factors that shows up. In the study of three hundred men and women, women peaked at a higher effort than men, fifty two against forty five per cent.

5:17 · Is the zone on a watch wrong?

So is the zone on a watch wrong? Not wrong, but it is a band, and people are a spread. Fitbit defines its fat burn zone in two ways. On the devices it lists by name, the zone runs from forty to fifty nine per cent of your heart rate reserve. That reserve is the gap between your resting and your maximum heart rate. On some of those devices the band is called fat burn, on others the moderate zone. On all its other devices, the zone runs from fifty to sixty nine per cent of maximum heart rate. Fitbit gives the example of a forty five year old with a resting heart rate of seventy four. For that person, by our conversion, the first version runs from sixty five to seventy seven per cent of maximum. With a lower resting heart rate, the band sits lower. Now the measured peaks, in the same unit. Three hundred men and women peaked at sixty one point five per cent of maximum heart rate on average. Individually, they ranged from forty one to ninety one. The moderately trained cyclists peaked at seventy four, with individuals from fifty four to ninety two. As drawn here, each version of the band catches one of those two averages and misses the other. Neither version can catch the spread. Fitbit also has to estimate your maximum heart rate first, with the common formula of two hundred and twenty minus your age. So as a pointer to where your own peak sits, the zone is a rough guess.

6:40 · What training does to the mix

That is what the mix is, and where it peaks. Now the two things that move it, training and time, starting with the gap we promised. The classic description of the fuel mix, from nineteen ninety four, treats it as a tug of war. Effort pulls toward carbohydrate, and endurance training pulls toward fat. Studies that follow the same people through a training programme find more fat in the mix afterwards, at the same power. Here are the two groups from the start of this video. The same study of professionals also tested twenty moderately active men on the same protocol. At about one hundred and thirty five watts, the professionals burned zero point five zero grams of fat a minute. The moderately active men burned zero point three eight. At about two hundred watts, it was zero point six five against zero point one eight. That is roughly thirty nine against eleven grams an hour. Part of that gap is simple. By our arithmetic, two hundred watts is a little over half of what the professionals reached at the end of the test. For the moderately active men it was about four fifths of their maximum. The same power is a lighter effort for the trained rider, and a lighter effort comes with more fat in the mix. Two cautions belong with that comparison. These are two different groups, each measured once, and the professionals were also younger, so training is not the only difference. And at the same fraction of your own maximum, the effect of training is less clear. One study of seven trained and seven untrained men found a larger share of fat in the trained men, but only at easy efforts.

8:13 · Long sessions: the mix drifts

Does the mix change during a long session? It does, but slowly and gradually. Seven trained men set out to cycle for four hours at one steady, moderate effort. Five of them completed it, and two stopped after three and a half hours. The researchers followed the mix through the breath, the ratio of carbon dioxide breathed out to oxygen taken in. That ratio falls as fat takes a larger share. It fell from zero point eight nine after half an hour to zero point eight three after two and a half hours, and stayed low. An older study of two hours at a moderate effort found something different. There, the totals from fat and from carbohydrate did not change, and only where the fuel came from shifted. So the drift toward fat is slow, and one study did not see it within two hours.

9:00 · Where each fuel comes from, and the small tank

Where does each fuel come from? Some is delivered by the blood, and some is stored in the working muscle itself. At the light effort, one form of fat carried by the blood supplied thirty one per cent of the energy. Muscle glycogen, the carbohydrate stored in the muscle, supplied thirty five. At the hard effort, those two were fifteen and fifty eight per cent. So the harder you go, the more of the work is paid for from glycogen inside the working muscle. And that store inside the muscle is small. On average the body holds about five hundred grams of glycogen in the muscles and about eighty grams in the liver. Together that is about four per cent of its fuel stores. Even someone with seven to fourteen per cent body fat carries more than thirty thousand kilocalories of fat. That mismatch is why carbohydrate is the fuel endurance athletes think about running out of. One caution about all of this, though. The detailed fuel numbers here come from small groups of men on stationary bikes. Where the peak sits also comes from two large treadmill studies of men and women.

10:02 · Three questions to take with you

So the fat-burning zone hides three questions. Where is the share of fat highest? At low efforts, and it is as high at rest as anywhere. Where do you burn the most fat per minute? At a moderate effort, on a broad plateau that sits in a different place from person to person. And what carries a hard effort? Carbohydrate, most of it from a small store inside the muscle. None of this says which effort makes for better training, and that is a different question. A large share of fat and a large amount of fat are different things. The whole piece, with all eighteen sources, is on enduranceproof dot com. Two pieces build on this one, how much carbohydrate to take in on a long effort, and whether easy training has effects of its own.