OK, that doesn’t even make any sense. Although there are people who would like to face the fat head-on. This is easier said than done, though.

It is amazing what humans find controversial. I can see religions and unusual sexual practices causing some agitation, especially in combination. But fat burning exercise? Yet, there is a pretty intense debate around a concept called the “fat-burning zone”.

I think I first heard of this after I bought my first pulse watch back in 2005. It is already 7 years ago, around this time of the year. The watch had three zones, one for light exercise, one for medium and one for hard or intensive exercise. The medium zone more or less corresponds to the so-called “fat-burning zone”, which is generally said to be from 60 to 70% of maximum heart speed.

One thing that has made me wonder from time to time is that our muscles get tired even at moderate intensity of exercise, such as a brisk walk or a slow jog. This was at odds with the explanations I read, that we got tired because we accumulated lactic acid in the muscles (this theory is pretty much discarded now, I believe) or that we got tired because we ran out of glycogen. If we kept going in the fat-burning zone, shouldn’t we be able to run 16 hours a day until we ran out of fat?

Actually, there once was a man who could do that, run most of the day every day. His name was Mensen Ernst, and he was of course a Norwegian. (It is typical Norwegian to be good, as a former Prime Minister in Norway once said.) He was born in 1799, unfortunately, long before modern genetic testing. He seems to not have left any children either, so we will probably never know whether he was some sort of mutant, or whether it was some kind of technique (he was said to run in a different manner than other runners, described as a “loping” run). Or perhaps it mostly came down to practice. But probably not only that, for there has never been anyone like him again, and as far as we know there were none before him either.

The rest of us get tired eventually. So I once again looked at the mystery of the fat-burning zone, and found this controversy. So, based on reading various sides of the issue, and my own personal experience, I’ll try to throw some light on this.

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First, warming up is not a one-step process. For walkers, it is recommended to walk leisurely for 5-10 minutes before speeding up to a brisk pace.  (Longer the older you are, children don’t really need to warm up at all.) However, this does not burn off the sugar and start on the fat. Warming up simply increases the blood flow through the muscles, making them more elastic and active so you avoid minor muscle damage and discomfort that you would otherwise have.

Ten minutes of leisurely walking will not budge your blood sugar or make any noticeable impact on your glycogen storage. It takes about half an hour of energetic walking to do that, less with jogging or running. With practice you should be able to feel for yourself when this shift occurs: Heart rate, breathing and body heat shift to a slightly higher level without a corresponding increase in speed or elevation. The difference in pulse is something like 10% in my experience. Not dramatic, but noticeable if you keep track. This is the point where you enter the actual fat-burning zone, no matter what your heart rate monitor may have been telling you for the last half hour. (The exact time varies – for me it was 25 minutes a year ago, but is now up to 35, probably because I store more glycogen in my muscles after almost a year of frequent, long walks.)

So when one article claims that you burn 85% fat in the “fat-burning zone” of moderate exercise, and another claim 50%, you have to ask: How long have you exercised before you start measuring? The higher number may be rather optimistic, but it may well be true if you don’t start measuring until the actual zone shift has happened.  If you measure before that, half and half sounds more likely.

However, even if half your calories come from sugar, you don’t burn equal amounts of fat and sugar. Fat contains twice as much energy, so you still burn two grams of carbs for one gram of fat. That means that even should you burn 85% fat, you don’t actually do that in terms of body weight.  85% fat calories would correspond to 59% fat weight. The rest comes mainly from glycogen in your liver, and there is only so much liver in a human.

Luckily at this point you can accept a bottle of cold, fresh Pepsi cola from your attractive friend, and continue exercising. (Imagine TV ad here.) Yeah, that would help you lose weight for sure. ^_^ But seriously, if you are planning to keep walking or jogging for hours, you should add sugar and water from time to time. For a one-hour stretch, it is not necessary. Your liver typically stores enough glycogen for one day or so of normal activity. (Jogging is not normal activity for most of us, so it will deplete faster, but not in an hour or two if you are reasonably healthy.)

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 So to sum it up: The fat-burning zone is real, but it burns only a little more than half fat and the rest sugar or excess protein. Also it only kicks in after around half an hour of brisk walking. Also, if you exercise harder, you will burn just as much fat per minute or more, but also more sugar and you will tire much faster.

If you have plenty of time, you can keep going much longer in the “fat-burning zone” (moderate exercise), but if time is your limit, you will lose more fat by exercising harder. Speak with your doctor before starting an intensive training program, especially long-distance or high-intensity.