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Metabolic Effects of Obesity

Updated on February 17, 2012

My Observations

I have been a personal trainer for over 19 years now, and I have worked with many different body types during that time. I have worked with people who were already in shape when they came to me and just wanted to push their physique over the top, along with individuals who were out of shape and wanted expert guidance in their fitness endeavors. Needless to say, I have also worked with people who were obese (over 35% body fat). Of every type that I worked with, those that were obese had the most difficult time losing fat and getting fit. Most of the time, they would quit once the exercise program became challenging and uncomfortable. Other times, they would see quick fat loss for about the first 10% of their body weight, and then hit a real plateau...and eventually quit. I had two guys stick it out and lose over 100 pounds each and keep it off. Even after they lost the weight and got much fitter, they never developed the "ripped ab" look everybody seems to covet. I know from working with different personality types that some people prefer the feeling of exercise and actually like the labored breathing and burning sensations in their muscles from hard intense exercise, while a good many do not. Clearly motivation and mindset is a factor to success, but I must admit, I always wondered if there was something more going on. Do genetics play a role? How about permanent changes on a cellular level once someone got overweight for a long period of time? These were important questions to address.

The Latest Research

Research is showing that it is much more difficult for obese people (35% body fat and above) to lose fat and keep it off than people who are overweight (30% body fat and under). The body goes through metabolic changes to keep the body from losing fat. When obese people do lose weight, there was significant change in the circulating hormones that regulate hunger. Ghreling, the hormone that tells the body it is hungry was 20% higher, while peptide yy which signals the body that it is full was lower. Leptin, which suppresses appetite and increases metabolism was lowered as well. You can read about it at The New England Journal of Medicine. The body's metabolic rate slows down, and at the same time the appetite increases. It's like the body does not want to stay at the new weight level. For example, someone in a recent study who went from 230 pounds down to 190 pounds needed 2,300 calories a day to maintain the new weight, meanwhile, someone who already weighed 190 pounds needed 2,600 calories a day to maintain the same amount of weight. That is a 300 calories a day difference. To put it in perspective, that equals 2,100 calories a week, or 10,400 extra calories a month.

Loss of Fast Twitch Fibers

Part of the reason the body has a lower metabolic rate is from the fact that the dieter lost muscle. Here is how it is explained:

Scientists are still learning why a weight-reduced body behaves so differently from a similar-size body that has not dieted. Muscle biopsies taken before, during and after weight loss show that once a person drops weight, their muscle fibers undergo a transformation, making them more like highly efficient “slow twitch” muscle fibers. A result is that after losing weight, your muscles burn 20 to 25 percent fewer calories during everyday activity and moderate aerobic exercise than those of a person who is naturally at the same weight. That means a dieter who thinks she is burning 200 calories during a brisk half-hour walk is probably using closer to 150 to 160 calories.

Excerpt from The Fat Trap.

This does not surprise me. After reading this whole article, the person writing was describing her struggles with body fat along with another couple who struggled with their weight as well. They all exercise very regularly and track both their diet and calories burned. They have it down to a science, but they do not do strength training. This is common among exercisers who want to lose fat (especially obese people). The assumption that aerobic exercise does a better job of burning fat leads most exercisers to embark on an aerobic exercise program and neglect the strength training aspect.

For clarity, let me describe the difference between fast and slow twitch fibers. Slow twitch fibers produce low levels of force and can go for a long time. Fast twitch fibers produce a large amount of force and burn out quickly. They work with the anaerobic system and use glycogen for energy, whereby the slow twitch fibers use glycogen and fat. Fast twitch fibers require more energy overall to work, and the fast twitch fibers are the ones that change shape. When an individual goes on a diet and does not strength train, the person will lose the fast twitch fibers, not the slow twitch. Strength training grows the fast twitch fibers, and raises the metabolic rate. I will come back to the effects of strength training and weight loss later, but I want to address other challenges of obesity first.

Other Challenges of Obesity

Another problem of obesity is that it seems that the body establishes a new "set point" when it gains weight. In a way, the body gets used to it, and wants to maintain it. This is, in essence, homeostasis. A new equilibrium. Research with mice is finding that overweight mice have about 8 months from the point of excess fat onset to lose the weight and be able to keep it off. After 8 months, it gets much harder to do. For humans, that "set point" period is much longer, perhaps years, but it nevertheless exists.

In addition to the above issues, the body also tends to change the way the brain responds to food. Tara Parker-Pope explains:

Rosenbaum and his colleague Joy Hirsch, a neuroscientist also at Columbia, used functional magnetic resonance imaging to track the brain patterns of people before and after weight loss while they looked at objects like grapes, Gummi Bears, chocolate, broccoli, cellphones and yo-yos. After weight loss, when the dieter looked at food, the scans showed a bigger response in the parts of the brain associated with reward and a lower response in the areas associated with control. This suggests that the body, in order to get back to its pre-diet weight, induces cravings by making the person feel more excited about food and giving him or her less willpower to resist a high-calorie treat.

It goes without saying that once an individual reaches the obese stage of weight gain, the odds of getting back to a healthy weight and maintaining it is much lower.

Back to Strength Training

Before I get to some recommendations, I want to spend a little more time explaining the benefits of strength training. Rebuilding the fast twitch fibers in the body is very important for maintaining a healthy body weight in the long run. As mentioned above, the more fast twitch fibers one has, the higher the metabolism is. There are important hormonal responses as well. The body will produce more human growth hormone (and I suspect testestosterone too), as well as improve insulin sensitivity. I do not know if strength training has an impact on ghrelin, peptide yy, or leptin that regulates appetite and metabolism, but with my experience I suspect that it does. With all things considered, strength training has to be an integral piece of the diet and exercise program for obese people.

A Fitness Strategy

From the latest research and my experiences working with obese people, it is clear to me that losing fat and getting in shape is much tougher for obese people than just about everybody else. The body goes through metabolic changes that make it very difficult to get back to a healthy body weight. The very best advice I can give is to avoid getting obese in the first place. If one is gaining fat from poor eating and exercise habits, he should start making changes as soon as possible. One does not have to be perfect with an exercise and diet routine, but he should try to cut back your portion sizes about 10%, choose healthier alternatives more often, and start moving more. This alone will go a long way to maintaining a good body weight.

As for those who are obese, here are some recommendations. As per my experience working with obese people, the most challenging aspect is the mental piece. They just had no desire to exercise. They did not like the feeling at all. Understanding that, I would recommend to start very slowly. Commit to just taking a walk everyday and focusing on changing one meal a day. For example, start with eating a good nutrient dense, low calorie breakfast every day. The dieter can still eat his regular foods the rest of the day, just cut portion sizes back 10%. Once he gets used to eating a healthy breakfast, he can then concentrate on lunch. Once he gets a better grip of eating healthy lunches, he can concentrate on dinner. In the meantime, every couple of weeks he can cut portion sizes back 5 or 10% until he takes in what he needs to see permanent weight loss. After 4 to 6 months, he can start weight training. He should work with a qualified instructor, start with very light weights and gradually progress over time. By taking a slow but steady pace, the exerciser can associate more pleasurable feelings with exercise and diet. I have learned that if you make sudden, sharp changes in personal habits, there is a backlash from the trainee. There is no need for that. There also seems to be a critical point for obese people in regard to exercise and good nutrition. The two clients that were successful were able to stick to it for six months. Once they reached that point, it became easier for them to make it a part of their routine. All of the others dropped off after 2 to 3 months. The trainee would have a much better success rate if he just commits to 6 months of diet and exercise, and after that point, reasses what he wants to do.

Final Thoughts

Research is finding out that there is a metabolic shift in the body when one becomes obese. These hormonal shifts pose a real challenge for obese people in their quest for health and fitness. For the most part, the standard advice of moving more and eating less and a long term commitment is prudent advice. The only thing I would add is a good strength training regimen. If there is any exercise that can "reset" the metabolic system of the body, this is it.

More Information

If you would like more information, you can e-mail me directly at, or check out our web page at


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