The Science of Fat

Last Editorial Review: 11/16/2004

Nature, nurture, or just plain hard work?

By Holly Wyatt, MD
WebMD Weight Loss Clinic - Feature

Reviewed By Charlotte Grayson, MD

There is an ongoing debate among researchers who study the causes of obesity. It's the old "nature vs. nurture" question: When people who are obese lose weight and then regain it - sometimes over and over again - what or who is to blame? Does genetic inheritance doom us to the boom-bust cycle of diet, relapse, diet? Or is it a failure to make and maintain the necessary changes in one's diet and exercise routine?

When people ask me, "Why are we facing this obesity epidemic?" my answer is that it's really a combination of factors. Physiology definitely plays a role, as does genetics, and there are undoubtedly certain people who are predisposed by their physiology - by the way their specific bodies work - to gain weight more quickly than others, or have trouble losing weight and keeping it off.

Bodies at Rest

Our basic physiology - the way the human body functions - hasn't changed over the last 50 to 100 years, yet during that time we have seen an alarming increase in obesity in our society. What has changed is our environment. Once we were a nation largely of farmers and laborers; now we are a nation of office and industrial workers whose lives have been made easier - and less active - by technology. In addition, millions of us live in neighborhoods that are designed for drivers, not walkers. We park as close as possible to the mall, take the elevator instead of the stairs to go up one or two flights, and drive to the corner store rather than walk because we're in a hurry.

And while we are less active as a society than we once were, we still tend to eat large amounts of high-calorie foods. That's where we begin to get into trouble.

People with certain physiologies who are placed into this new, less active environment will become obese. Had they lived in the 19th century, say, they might never have had the chance to get overweight. They would have expended more calories just going about their daily business, and probably wouldn't have had access to as much high-calorie food.

Yet there are other people who will eat a modern diet and not exercise, and these fortunate individuals, perhaps because of their physiologies, will not have to struggle with their weight.

In other words, as we have discovered over the last two decades of research into obesity and metabolism, obesity is not simply a matter of willpower or laziness, but also a product of our bodies, our genes, and our way of life.

We're still not sure to what degree physiology and genetics contribute to differences among people in their ability to maintain a healthy weight, but evidence suggests that metabolism, which frequently gets the blame for excess poundage, may not be as important as a factor as once thought.

People frequently come into my office and say, "I'm obese because my metabolism's slow," but I think that's not necessarily true. To understand why, it's important to understand what we mean when we talk about metabolism.

On the most basic level, the term metabolism refers to how our bodies absorb, store, and use energy from foods. In the field of obesity research, we find it helpful to focus on a more specific concept, which we call the "resting metabolic rate," or RMR.

"Evidence suggests that metabolism, which frequently gets the blame for excess poundage, may not be as important as a factor as once thought."

The resting metabolic rate can be thought of as the pilot light for your body furnace: if you were lying awake in bed but not moving, the RMR would be the total number of calories it takes to keep your muscles working, kidneys and liver filtering, and heart pumping. Scientific studies, casual observation, and our own experience tell us that metabolic rate can vary from one person to the next.

The biggest predictor of resting metabolic rate is lean tissue such as muscle and organs. In other words, the more lean tissue you have, the higher your metabolic rate will be, because lean tissue requires more energy than fat. In fact, we can predict about 80% of someone's metabolic rate by knowing how much lean tissue they have. Age and gender play a bit of a role, but lean tissue is by far the best indicator of RMR.

When it comes to obesity, however, the importance of the resting metabolic rate may be overestimated. There have been many studies of obesity and RMR, both in people who manage to lose a great deal of weight and keep it off, and in those who fail. But there has been little hard evidence that RMR plays a role in determining whether someone of "normal" weight will become obese, or that a low RMR was a contributing factor to obesity in the first place among overweight people.

No "Magic Bullet"

Although we're learning more about the science of obesity and metabolism with every passing day, what we've learned thus far doesn't change the fact that there is still no magic bullet for treating obesity, no special exercise that will work for everyone.

Losing weight and keeping it off may be more difficult for some people than for others, but no matter what type of physiology nature has saddled us with, eating less and exercising more is still the best way to achieving and maintaining a healthy body weight.

That's a discovery we can all live with.

Published Feb. 3, 2004.

Holly Wyatt, MD, is an assistant professor of medicine and medical director of the Centers for Obesity Research and Education at the University of Colorado Health Sciences Center in Denver.

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