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Both diet and exercise help you lose weight, but it's the kind of support you have as you diet that makes the critical difference.

Which matters more when it comes to losing weight, diet or exercise? Let's take a common sense look at the most frequently asked question about losing weight.

Dieting Makes a Critical Difference in Weight Loss

There is a lot to be said for the theory behind dieting, for simply restricting calories, to lose weight. It's a matter of math. Cut out 3500 calories, lose a pound of fat. (Or cut out 7700 kcal, lose a kilogram of fat, for those who prefer the metric system.) The principle is very straightforward. Actual calorie counting isn't.

Let's say you weigh 220 pounds (100 kilos). You have a penchant for potato chips. You can't eat just one, as the old marketing slogan used to say. You always eat the whole bag of potato chips, the whole 1400 calories of chips. Being health-minded, however, you vow to burn those calories off on the elliptical machine in your basement.

Since you weigh 220 pounds, you aren't going to do a high-intensity workout, but that's OK, since slow workouts burn fat, right? You just have to exercise long enough to burn off those tasty potato chips you love to eat.

How long does it take someone who weighs 220 pounds to burn off 1400 calories at a leisurely pace on an elliptical machine? The calorie meters on elliptical machines are notoriously unreliable. How you set the tension on the machine makes a big difference, as does whether you do interval training, short bursts of intense physical activity in the middle of a longer routine. However, if you weight 220 pounds, you can burn off 1400 calories in two hours or less even if you use the easiest setting and even if you work out very slowly.

That's two hours of exercise every time you eat a bag of potato chips, or two chunks of cake, or four pieces of fried chicken, without rewarding yourself with even more food after you do the workout, and without "forgetting" to complete the entire exercise routine. If you like to eat, you need to exercise, a lot, several hours a day, to lose weight, without compensating for exercise by eating more. But let's suppose you are really disciplined.

We Need to Eat Fewer Calories to Lose Weight, But How Do We Really Know the Calories in Food?

Let's suppose you are a disciplined dieter who isn't going to eat a whole pie or a whole bag of chips (or at least not two) or a bucket of fried chicken and French fries. You know you are eating too much, but you know you can count calories or— can you?

Calorie counting involves websites, databases, handbooks, and math even after you measure your food. Eyeballing what you eat doesn't really work. You really need to use measuring cups and scales to know how much food you are really eating. Once you measure your food, you need to look up the calorie content and do the calculations that tell you the number of calories in it. 

Let's suppose you are dedicated and smart and you can do the intellectual work of monitoring your calories. Your non-dieting friends and family patiently wait to eat while you are measuring your food (fat chance, most people would say). Then the problem is that all those websites, databases, and handbooks can be 25 to 35 percent off. The moisture content of your food makes a large difference in the number of calories it contains. Labels aren't always accurate. You could be eating too many calories to lose weight or too few to avoid hunger despite all your efforts.

Getting Around the Calorie-Counting Problem

There are a couple of ways to get around the problem of counting calories. One is to go for one of the macronutrient-restricted diets, a diet that restricts protein or a diet that restricts fat. The problem with those kinds of diets is that they tend to be unsustainable. 

Another way to get around the calorie-counting problem is to let a diet center do the calorie-counting for you. Jenny Craig, Weight Watchers, and Nutrisystem all promise to count your calories so you don't have to. If you are extremely overweight, this may be the right way to go, provided you can stick to the program. When the instructions for frozen weight loss dinners say you can supplement the frozen dinner with a salad, for instance, they don't mean a bacon-and-fried-chicken Cobb salad with extra cheese and three cups of salad dressing plus a Twinkie. Supervised weight loss programs seem to have a lot going for them, or do they?

Continue reading after recommendations

  • Ball SD, Bolhofner A. Comparison of a Commercial Weight Loss Program to a Fitness Center. Journal of Exercise Physiology Online. 11(3). June 2008.
  • Heshka S, Greenway F, Anderson JW, Atkinson RL, Hill JO, Phinney SD, Miller-Kovach K, Xavier Pi-Sunyer F. Self-help weight loss versus a structured commercial program after 26 weeks: a randomized controlled study. Am J Med. 2000 Sep. 109(4):282-7. PMID: 10996578.
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