The idea of “fat burning” has become something of a cliché these days. I’m not surprised. It sounds logical that if you want to lose weight you should burn up that body fat that makes you fat . . . right?
In a way that’s correct, but what has been grossly misunderstood — even by people who should know better — is that you don’t have to ‘target’ fat burning during exercise with low to medium effort because, as the mythology goes, that is the ‘fat-burning’ zone.
At this point I should declare that an article I wrote a few years back for About.com Weight Training is the number one article for “fat burn” as a Google search term — out of nearly 25,000,000 articles indexed by Google (US). The reason it ranks so high, is, I suspect, because it was one of the first popular articles, if not the first, to describe what really happens with your metabolism as you exercise in relation to the use of fat as a fuel.
How Fat Burning Works During Exercise
I won’t repeat it all here — you can follow the link — but the nub of the matter is that, yes, low-intensity exercise burns a greater percentage of fat than high-intensity exercise, but as you increase the intensity, and the percentage of fat used falls, it’s still possible to use more fat as fuel than you did at lower intensity because the total energy expenditure is greater — for a similar duration of exercise. But that’s not the end of it . . .
How Fat Burning Works After Exercise
Now here’s the thing most people don’t consider. Even allowing for that little calculation above on how much fat you use to fuel any activity, energy use and fat burning needs to be seen as a continuum over an extended period. Twenty-four hours is a nice round number. You need to consider what happens after you stop exercising as well as when you are exercising. Here’s one thing that happens when you exercise at higher intensities, say above 75% of maximum heart rate. As you use glucose for energy at higher intensities, your blood, liver and muscle glucose (glycogen) falls somewhat — and of course this is tricky for endurance athletes like marathoners because glucose is a more empowering fuel up to 2 hours or so of racing than fat. But then, when you stop exercising and you have low blood and muscle glucose, insulin is low, the hormone glucagon is rising and so is hormone-sensitive lipase, a hormone that promotes the break-up of fat triglycerides to free fatty acids for fuel. Fat burning is prioritised, even though eventually you will replenish glucose stores with food and you will start to burn a little more glucose and less fat. It’s a cyclical process. You could even prolong this fat burning phase by eating low-glycemic index foods after your workout.
What You Need to Do to Burn Fat
The main point here is that you don’t have to worry about fat-burning zones and exercising at a mythical fat-burning intensity for weight loss. It all gets sorted out over a 24-hour period as the priority fuels fat and glucose wax and wane according to physical activity intensity and food intake. All you need to be concerned about for weight loss is energy expenditure, and, of course, creating an energy deficit, which means not pigging out after your workout and destroying all your hard work.