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The “fat-burning” zone for weight loss and performance

February 17th, 2010

I exchanged a few e-mails last week with Canadian Running blogger Rebecca Gardiner about weight loss and exercise. It’s a hot topic these days, thanks to Matt Fitzgerald’s recent book Racing Weight and the subsequent media coverage, including this piece by Gina Kolata in the New York Times.

But if you have a little time to spare and you’re looking for a well-informed scientist’s perspective on weight loss, I’d recommend taking a look at Ross Tucker’s series at the Science of Sport blog. (Here’s part 1, part 2A, part 2B, and part 3. The series has been stalled for a few weeks, but may resume soon.) He gives a very basic explanation of the essential facts about losing weight, keeping it simple while acknowledging the complexity that lurks behind many of the statements.

In particular, he takes aim in part 3 at one of my favourite pet peeves, the “fat-burning zone” that encourages people to take it easy during cardio workouts. It’s true, he notes, that you burn about 80% fat (and 20% carbohydrate) when you exercise at low intensity, and those ratios are reversed at high intensity.

So, what you’re probably thinking is that theory that low intensity exercise is better if you want to burn fat is correct. Well, think again. It is true that at low intensity, when you walk, most of your energy comes from fat, and that as you increase the intensity, less and less comes from fat.

But what is missing in this picture is the TOTAL amount of energy.

It turns out that you burn about 50% more fat per hour at moderate intensity than you do at low intensity. So the rationale for a low-intensity fat-burning zone is spurious, unless you have time to exercise for several hours a day. But really, the most important message comes later in the same post, and I hope people don’t miss it:

[P]erhaps most significantly, the key is still to create a calorie deficit, which means that you need not worry too much about whether your energy use is coming from fat or carbs – the key is to create that deficit, because in the long run, the energy will have to be provided and you will achieve similar results regardless.

Another point Tucker makes is that it’s really hard to provide general-purpose weight-loss advice, because there are so many different things that can be going on physiologically. He advises consulting a dietitian to get personalized advice if you’re struggling to lose weight. The corollary that I’d add is that anyone who tells you they have The One True Answer to your weight-loss problems without knowing in detail about your history is kidding themselves.

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