The Role of Exercise in Weight Loss


Want to lose weight?  The tried and true advice has always been “Exercise and eat right.”  Hard to argue with that.  Burn calories via exercise and eat less and/or eat better quality food.  Done and done.  Recently however, the role of exercise in this process has been questioned.  Time magazine went so far as to tell us Why Exercise Won’t Make You ThinOther research has suggested that the eating part of the equation is more powerful than the exercise part.  Further, from experience as a personal trainer, I’ve seen many a gym member tell themselves “Hey, I’m exercising.  I can eat whatever I want!”  I have yet to see anyone succeed following that route.  So what if any role does exercise play in shaping our physique?

The New York Times has weighed in with a very interesting, nuanced and informative article on the topic.  Weighing the Evidence on Exercise tells us among other things that exercise alone may not make you lean, but that it will likely help keep you thin if and when you get there.  Further, it seems exercise has a different effect on the appetites of men vs. women.

“When you look at the results in the National Weight Control Registry,” Harvard researcher Barry Braun says, “you see over and over that exercise is one constant among people who’ve maintained their weight loss.”

The article cites two studies to this effect  One by the American College of Sports Medicine demonstrated that appetite was blunted in men who ran on a treadmill for 1.5 hrs.  In another study by Harvard researchers reported in the American Journal of Physiology, men and women walked on treadmills and their appetites were monitored.  The men showed similar results to the ACSM study.  The women however showed increased appetite.  It seems that female physiology is very favorably given over to storing energy (as fat–eeeeeeech!!!)

The Harvard study found other very valuable information which I’ll get to in a moment but first, more about who was studied.  The weight-change history of 34,000 women was tracked for 13 years.  (The large sample size and length of the study are two strengths.)  The average age of the women at the start of the study was 54 years.  Now the important information as reported by the Times:

“During that time, the women gained, on average, six pounds. Some packed on considerably more. But a small subset gained far less, coming close to maintaining the body size with which they started the study. Those were the women who reported exercising almost every day for an hour or so. (emphasis is mine) The exercise involved was not strenuous.”

Finally, the Times article discusses a study from the University of Colorado.  In this study, rats were fattened and allowed to be sedentary for a time.  The rats’ diets were then switched to low-calorie fare and half of them were made to run on treadmills for about 30 minutes a day.  (Strangely, Jillian Michaels was not involved.)  The results of the study as reported by the Times are as follows:

“Then the fun began. For the final eight weeks of the experiment, the rats were allowed to relapse, to eat as much food as they wanted. The rats that had not been running on the treadmill fell upon the food eagerly. Most regained the weight they lost and then some.”

But the exercising rats metabolized calories differently. They tended to burn fat immediately after their meals, while the sedentary rats’ bodies preferentially burned carbohydrates and sent the fat off to be stored in fat cells. The running rats’ bodies, meanwhile, also produced signals suggesting that they were satiated and didn’t need more kibble. Although the treadmill exercisers regained some weight, their relapses were not as extreme. Exercise ‘re-established the homeostatic steady state between intake and expenditure to defend a lower body weight,’ the study authors concluded. Running had remade the rats’ bodies so that they ate less. (emphasis is mine)

To reinforce the message from all this, Science Daily reports on another study not mentioned in the Times article: Diet Alone Will Not Likely Lead to Significant Weight Loss, Study Suggests.  Here,  Oregon Health & Science University studied monkeys that were fed a high-fat diet for several years.  They then were placed on a low-fat diet for a month.  The important information is this:

“Surprisingly, there was no significant weight loss at the end of the month,” explained OHSU post-doctoral fellow Elinor Sullivan, Ph.D. “However, there was a significant change in the activity levels for these monkeys. Naturally occurring levels of physical activity for the animals began to diminish soon after the reduced-calorie diet began. When caloric intake was further reduced in a second month, physical activity in the monkeys diminished even further.”

So what’s the take-home message from all this?  First, as a personal trainer, I find it highly interesting  the actual role that exercise seems to play in weight loss.  It’s not so much a direct influence as it is something that changes our bodies over the long haul, and sets the stage for us to stay trim.  Beyond that, think of this evidence in light of research (here, and here for instance) showing that exercise alone won’t do the trick when it comes to weight loss and we have a very familiar message: To lose weight we must eat right and exercise.

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