If you’re like many of the women athletes I’ve coached in the past, you’ve probably spent time gathering information on the use of aerobic exercise as part of a weight maintenance plan. You may have come across two contradictory suggestions for how hard you should exercise. One approach argues that the harder you exercise, the more total calories you expend in the time spent exercising—so the harder you exercise, the faster you’ll lose weight.
On the other side we have an approach focusing on the “fat burning zone.” This approach recognizes that the lower the intensity of effort, the higher the proportion of the expended calories coming from fat. In this sense, easy walking might be the ultimate exercise, because in everyone but the totally unfit, easy walking maximizes fat expenditure as a fraction of total calories expended. Fat calories are the ones that matter as you reduce your body fat, get a trim figure, or lose weight. Easy walking doesn’t use many calories though…
So what’s the answer? Easy exercise or hard? As with so much in life, the truth lies somewhere in the middle. Even ignoring the fact that if you exercise too hard you’ll be worn out and quit sooner, the harder you exercise, the greater the proportion of your expended calories coming from carbohydrate. In fact, if you exercise at or above your upper ventilatory threshold (the speed or heart rate at which you begin to breath hard enough that chatting is no longer possible) you use virtually no fat at all, but almost exclusively carbohydrate. Now, you can’t exercise hard unless you have full or nearly full carbohydrate stores, so between hard workouts you have to replace any carbohydrate calories expended, or you won’t be able to exercise hard again. This means that exercise near the upper ventilatory threshold is virtually useless in a weight control plan.
Exercise near the upper ventilatory threshold is actually worse than useless in many cases. If you exercise long enough and hard enough to deplete glycogen stores, your blood sugar levels drop and you get hungry. Unfortunately appetite is not so finely tuned that it shuts off when you have consumed exactly the number of calories that are needed to replace carbohydrate used during exercise. Most people, if they do not willfully restrain themselves, will eat more than is directly needed for replenishment, a few minutes to hours after a very hard workout. This is true even for people who lose their appetite after hard exercise; they just take a little longer to overeat. In addition, the harder the exercise, the more willpower required for willful restraint. By stimulating appetite, hard workouts make it harder, rather than easier, to lose weight.
So what to do? How do you pick the right intensity? It turns out that there is an ideal range of exercise intensity for each person, which elicits the optimal balance of maximum total caloric expenditure and highest possible percentage of calories drawn from fat. The fitter one is, the higher the speed at which one can run or ride or swim or row or cross-country ski in the optimal zone. Aerobic training actually boosts your ability to use fat as a fuel at higher exercise intensities, so there is no good formula based just on speed or age or maximum heart rate for everyone.
The optimal intensity can be anywhere from about 55 percent of maximum heart rate in a mostly sedentary individual, to 75–80 percent of maximum heart rate in a very fit individual. If you know your lactate threshold, the maximal fat-burning/minimal appetite-boosting zone ends about 10 to 15 beats below it. Laboratory tests can precisely pinpoint your optimal intensity, which is why competitive athletes have themselves tested (though your range will change depending on each specific exercise mode and as you become fitter, so if you want to pinpoint your zone precisely you need to revisit the lab frequently).
