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 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.
So 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
Energy in, energy out. The body normally burns a mix of carbohydrate, as glucose, and fat for fuel. How much of either depends on your physical activity and if, or what you have eaten recently. When you use more energy than you take in from food and drink, the body burns stored fat and carbohydr
ates, and then even uses protein, to fuel your everyday activities even if you are not exercising.
That’s what happens when people starve of course; the body starts to eat itself. Depending on your family history — your genetics — and the way you eat and exercise to create this energy deficit, your body may decide to get conservative and drop your metabolic rate to try to hold onto body weight. Some of us seem to have inherited this tendency more than others, the origins of which may be in the early periods of human evolution where ‘feast or famine’ was more or less the norm.
Glucose, fat and protein. Even so, starvation always works eventually and the body starts to break down its own tissue for fuel. Stored carbohydrate called glycogen is quickly used up, then goes the fat stored under the skin and around the internal organs. Protein in muscle is then broken down to create glucose to keep the brain working and you conscious.
Fat and glucose are the body’s two main energy sources. Fat you know well, glucose comes mainly from carbohydrate foods like rice and bread and potatoes and protein is supplied mainly by meat and beans and dairy products. The amino acid building blocks of protein foods can be converted to glucose in emergencies. Your body always burns a mix of fat and glucose except at very high intensities, and the ratio of the fat and glucose in ‘the burn’ varies with intensity and time of exercise.
Fat burning zone. You may have noticed that some bikes and treadmills at the gym have a setting that says ‘fat burning zone’, which implies a setting for intensity or speed. The reason for this is that the body burns a greater percentage of fat at a slow pace (or after about 90 minutes of exercise). The fat burning zone, a low intensity speed zone, is mainly a gimmick, and here is the reason.
Even though you burn more fat going slowly, you still burn some fat at much faster speeds or intensity. It all boils down to how much energy you expend in totality. For example, if you compare exercising at a slow rate that burns 60 percent fat and 40 percent glucose and a higher intensity or duration that burns only 30 percent fat and 70 percent glucose, you may still burn more fat at the higher intensity.
A typical example. Exercise (1) is the slower 60/40 mix and exercise (2) is the faster, 30/70 mix of fat and glucose fuel.
- Walking on a treadmill for 30 minutes — 180 calories used — 108 calories of fat burned
- Running on a treadmill for 30 minutes — 400 calories used — 120 calories of fat burned
You can see from this example that the bottom line really is how much energy you expend — and that is the ultimate fat burning measure. The theoretical fat burning zone is mostly a convenient myth.
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 might 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. #