Like HowStuffWorks on Facebook!

Does exercise really help with weight loss?

How Exercise Affects Our Bodies and Our Eating

One of the first things to realize about exercising is that while it's great for our bodies, it doesn't torch as many calories as we think it does. Here are some examples of the (not-all-that-many) calories burned through various physical activities. (Figures are based on a 150-pound or 68-kilogram person exercising 30 minutes) [source: My Fitness Pal].

  • Biking at a leisurely pace (less than 10 miles or 16 kilometers per hour) = 136 calories
  • Walking briskly (3.5 miles per hour) = 129 calories
  • Strength training/weightlifting = 102 calories
  • Running (10-minute miles) = 340 calories
  • Hiking (without a backpack) = 204 calories

So if you're thinking your brisk 30-minute walks will cause a sensational slim-down, think again. You're only burning another 129 calories each time you head out the door, which is about the number of calories in a small chocolate chip cookie — the cookie you probably ate to reward yourself for exercising.

That's another problem, researchers say. Many of us feel our daily gym session entitles us to a whipped-cream-topped latte, extra-large. Or a burger and fries. Some even believe that if they exercise daily, no matter what the exercise or intensity level, they can eat anything they want the rest of the day. This kind of thinking can lead exercisers to ingest more than they burn off, with, ironically, weight gain as the result.

Another potential pitfall to exercising, especially if you're doing more intense and/or lengthy workouts, is that it can make you hungry. This is especially true for women, whose bodies are programmed to store fat for reproductive purposes. And researchers increasingly believe that after we exercise, we're so hungry we end up eating more calories than we burned off during our workout. Perhaps you know exercising doesn't mean you can eat three brownies after dinner, but if you finish a rather grueling hour-long run (8-minute pace) and your stomach's rumbling, you might quickly down a 200-calorie energy bar. Seems innocent, and it might be. But keep in mind that doing so immediately reduces your overall calorie burn from 850 to 650 [source: My Fitness Pal].

Finally, some of us spend so much energy working out that we slow down the rest of the day. Maybe we take the elevator at work instead of the stairs, or skip walking the dog. In a worst-case scenario, this subsequent lethargy could mean we actually burn fewer calories overall on the days we exercise.

With all this in mind, it's not surprising that groups like the Mayo Clinic say taking in fewer calories helps you lose weight more easily than physical activity alone. In fact, experts say 80 percent of weight loss is due to diet and only 20 percent is due to exercise [sources: John, CBS News].