Researchers in the US showed that exercise minimized weight regain in rats that lost weight on a long term diet and were then allowed to eat freely. The evidence suggests that exercise causes the body to burn fat first and store carbohydrates for later, a process that reduces appetite and signals fullness to the brain, leading to slower weight regain.

The research was done at the University of Colorado in Denver, and is published in the 2 September issue of the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology.

Lead author Dr Paul S MacLean, who works at the University’s Center for Human Nutrition in the Department of Medicine, and also at its Division of Endocrinology, Diabetes, and Metabolism, and colleagues, also showed that exercise stops fat cell numbers from increasing during weight regain, overturning the commonly held view that it has no effect on fat cell numbers.

On the surface, weight gain looks like a simple process: it’s an obvious result of eating more calories than you use. But a closer look reveals a complex process involving brain signals that suppress and arouse the desire to eat. Following periods of weight loss our body metabolism alters to make us put the weight on again quickly and efficiently, and that is where these signals play a key role.

In animals, these signals are more strongly connected to what is going on in their bodies, in humans it is not so straightforward because of other factors like our more complex psychology (how we think about food and our bodies) and lifestyle.

But after dieting, the physiological signals up their hand and play a strong role in controlling food intake. For instance, feeling hungry all the time is a big problem after losing weight with restricted diets, and most people find the urge so strong they end up over-eating and put all the weight back on that they fought so hard to lose. That’s because while they were dieting, their metabolism was changing to make sure the urge to eat got stronger and stronger.

But in the US, by tracking people on the National Weight Control Registry, researchers found that some people were successful at keeping the weight off, and one of the things they had in common was they exercised regularly. So MacLean and colleagues decided to investigate what might be going on in the body to make this possible: how does exercise affect physiology to minimize weight regain (and might this have something to do with these signals)?

For the study they used specially bred obesity-prone rats and kept them sedentary (ie no exercise) for 16 weeks during which they fed them a high fat diet and let them eat as much as they wanted.

After this they put the rats on a diet for two weeks. The diet was a low fat, low calorie diet and the animals lost about 14 per cent of their body weight. Then followed another 8 weeks on a less severe maintenance diet, where they maintained their weight loss. During the total 10 weeks of dieting half the rats exercised regulary on a treadmill (30 minutes a day, 6 days a week) while the other half stayed sedentary (no exercise).

Then followed a final 8 weeks, the “relapse” phase, where the rats were taken off diets altogether and allowed to eat as much low fat food as they wanted. Again, the rats in the exercise group continued to do regular treadmill exercise and the sedentary group remained sendetary.

By taking various measures over these periods, the researchers found that, compared to the sedentary rats the exercising rats:

  • Regained less weight during the “relapse” period.

  • Developed a lower “defended” body weight (the weight that our phyisiology drives us to achieve).

  • Burned more fat earlier in the day, and more carbohydrates later in the day.

  • Accumulated fewer fat cells and less belly fat during the relapse.

  • Had less urge to overeat.

  • Were better at balancing calories consumed with calories burned.

MacLean and colleagues concluded that regimented exercise caused the changes in metabolism that happen during weight loss to have a smaller effect on weight regain.

During feeding, the sedentary rats were burning carbohydrates for energy while their digestion sent the fat in the food to their fat tissue. There are two interesting points about this: first storing fat uses less energy than storing carbohydrates, and also, burning carbohydrates first makes you feel hungrier.

But exercise caused the exercising rats to burn their fat for energy, and save their carbohydrates for later, and this combined to give them lower appetites and fewer calories going to fat storage.

MacLean and colleagues also found that exercise stopped the increase in fat cells that normally happens in the relapse period, as it did in the sedentary rats.

They suggest that in the sedentary rats, once they came off the diets, small new fat cells started appearing early in the relapse period. The appearance of small new fat cells not only contributes to weight regain in their own right, but they also increase storage capacity for even more fat cells in the abdomen, and this may explain why the sedentary rats overshot their previous weight when they relapsed.

There is a commonly held belief that the number of fat cells in the body is determined by our genes, and is not regulated by diet or lifestyle. So this fat cell phenomenon is a new discovery for the team who are now going on to do more work to find out if exercise does indeed stop new fat cells from forming early in the relapse and it is not just a case of existing fat cells getting larger.

“Regular exercise attenuates the metabolic drive to regain weight after long-term weight loss.”
Paul S. MacLean, Janine A. Higgins, Holly R. Wyatt, Edward L. Melanson, Ginger C. Johnson, Matthew R. Jackman, Erin D. Giles, Ian E. Brown, and James O. Hill.
Am J Physiol Regulatory Integrative Comp Physiol, Sep 2009; 297: R793 – R802.
DOI:10.1152/ajpregu.00192.2009

Additional sources: American Physiological Society.

Written by: Catharine Paddock, PhD