Is it better to eat less or exercise more – take advantage of the “fat-burning” zone
To lose weight, is it better to eat less or exercise more?
A calorie is a calorie is a calorie, according to the old-school, keep-it-simple school of nutritional thinking. But as we’ve seen, good health is a little more complicated than just matching the calories you eat to the calories you burn. A study published in 2010 in the journal Medicine & Science in Sports & Exercise suggests that your body can tell the difference between a calorie burned through exercise and a calorie avoided through dieting—and both types turn out to be important.
Researchers at Louisiana State University recruited 36 moderately overweight volunteers and divided them into three groups. One group was the control and stayed exactly the same throughout the six-month study. A second group cut their calorie intake by 25 percent, while the third group cut calories by 12.5 percent and increased their calories burned by an equivalent amount through physical activity. That meant both intervention groups had the same total “calorie deficit”—one through diet alone, and the other through a 50-50 mix of diet and exercise.
As expected, the two intervention groups lost exactly the same amount of weight—a fairly impressive 10 percent of their starting weight. They also lost about 25 percent of their total body fat and 25 percent of their abdominal fat, again with no difference between the two groups. This confirms that the amount of weight you lose is a function of calorie deficit, whether you create the deficit through diet or exercise. But when the researchers took a closer look, they found some important differences between the two groups. Only the diet-plus-exercise group had significant improvements in insulin sensitivity, LDL cholesterol, and diastolic blood pressure—crucial risk factors for heart disease and diabetes, but changes you can’t measure by looking in the mirror or stepping on a scale.
This research sheds new light on an ongoing “fitness v. fatness” debate. While some researchers believe that weight and body mass index are the simplest measure of cardiovascular risk, others—most prominently Steven Blair of the University of South Carolina—argue that aerobic fitness is more important than body shape. It’s difficult to tease apart the effects because, in general, those who are fattest are the least fit and vice versa. The Louisiana State results support Blair’s position by showing that there are some key health benefits that you can’t get just by being skinny: you have to exercise too.
But that’s not the final word. Two other risk factors measured in the Louisiana State study—systolic blood pressure and HDL cholesterol—remained identical between the two experiment groups. That suggests that they depend on weight rather than aerobic fitness, so that “fat but fit” people may still be missing out on some health benefits. The challenging but inevitable conclusion is that both diet and exercise are important to optimize your health, and you can’t ignore either of them.
How can I take advantage of the “fat-burning” zone?
If you’re looking to shed fat, wouldn’t it be nice if you could choose to selectively burn fat instead of wasting your time burning carbohydrates? That’s the idea behind the famous “fat-burning” zone touted by exercise equipment makers and fitness gurus. Not only that, but the key to staying in the fat-burning zone is not to push too hard—a welcome message indeed. Unfortunately, there are flaws of both logic and physiology behind these claims.
Let’s start with the kernel of truth. You do indeed burn a mix of fat and carbohydrate when you exercise, and the exact proportion varies with your exercise intensity. When you go for a leisurely walk, you might burn 85 percent fat and 15 percent carbohydrate. If you pick up the pace to a jog, you’ll start burning more carbohydrate. The harder you go, the more carbohydrate you burn, until you’re burning about 70 percent carbohydrate and 30 percent fat at the highest intensities. The cross-over point, where you get half your energy from each source, occurs at about 60 percent of your maximum intensity (though it can vary considerably from person to person and increases as you get fitter).
On the surface, it sounds like low-intensity is indeed the way to go for fat-burning. But this ignores the question of total calories burned. If you go for a leisurely walk and burn 100 calories, it’s true that 85 of them will be provided by fat. But it’s a lot better to go for a moderate run and burn 500 calories in the same amount of time, with 250 from fat. (Exercise at the highest intensities has many benefits for health and fitness, but it’s hard to sustain for long periods of time—so from a fat-burning perspective, moderate-intensity exercise is the most effective.)
But there’s a more fundamental problem with the fat-burning idea, relating to how your body recovers after physical activity. If you burn mostly carbohydrates during a workout, the calories you consume in the hours after the workout will be used to replenish your depleted carbohydrate stores. If you manage to rely more on fat during the workout, on the other hand, your carbohydrate stores will remain full. As a result, whatever calories you consume afterwards will be stored directly as fat, undoing your fat-burning efforts.
This phenomenon was demonstrated in 2010 by researchers at the Garvan Institute of Medical Research in Australia. In mice that were genetically altered to burn fat instead of carbohydrate, unburned carbohydrates were simply converted into fat for storage. The total “energy balance” remained unchanged, and the mice gained or lost the same amount of weight as normal mice under the same conditions. The results serve as a warning not to waste your time and money on pills that claim they enhance fat-burning, according to Greg Cooney, the study’s senior author. “Our data urges a correction in people’s concept of a magic bullet—something that will miraculously make them thin while they sit on the couch watching television,” he said.
So far, the only technique that reliably boosts the proportion of fat you burn is—you guessed it—exercise. After a few months of training, studies have found that you will indeed burn more fat at a given level of intensity than you did when you were out of shape.
Of course, while that may help you stay fueled in the late stages of running a marathon, it doesn’t really matter for weight-loss purposes. The only thing that counts is how many calories you burned to get there.