Wednesday, June 01, 2011
8842: Weighing In On Obesity.
From USA TODAY…
Genetics drives obesity; so don’t judge
By David Linden
Why can’t that obese man just eat less and exercise more? He lacks willpower, surely.
It’s a value judgment that is made countless times a day, in our lives and in the news media. But these attitudes are destructive, cruel and scientifically wrong.
Willpower is a fine thing, but the best intentions of this man’s conscious mind — indeed, all of ours — must struggle against tens of thousands of years of evolutionary history. Simply put, our bodies contain appetite-control circuits that make it very hard to lose a lot of weight and keep it off.
Body fat secretes a hormone called leptin, and this hormone is carried by circulating blood and passes into the brain to reduce appetite. When we lose weight, less body fat means that less leptin gets to the brain. This causes a strong subconscious drive to eat and makes foods — particularly fatty and sweet foods — more pleasurable when consumed. The more weight that is lost, the stronger this drive will be.
While moderate weight loss can be maintained through willful monitoring of food intake and exercise, and dramatic weight loss can be achieved temporarily, it is very difficult for most people to maintain an extreme loss of weight over the long term because of this leptin-feedback system.
Even liposuction is only a temporary fix: Removal of fat from the body reduces levels of leptin, thereby increasing appetite. This is the sad yet unavoidable truth that the multibillion dollar weight-loss industry — from diet book authors to weight-loss reality shows to manufactured “diet foods” — doesn’t want you to know.
Unfair and cruel stereotyping
The ugly flip side of the notion that dramatic sustained weight loss is within everyone’s grasp is the idea that if you’re overweight, it’s just because you’re just a lazy slug.
Depicting an overweight character in a TV sitcom or a film is an easy shorthand: This person is sloppy, unsexy and lacking in self-control. Not like us.
Here’s the root of the problem: Evolution is slow, but cultures and technology can change quickly. For most of our human history we rarely had access to sweet or fatty foods. We belonged to hunter-gatherer societies and burned a lot of energy in everyday tasks. In that distant past, it made sense to have an appetite-control system in the brain that made eating those sweet, fatty foods highly pleasurable. This behavior was useful to pack on the pounds when these energy-rich foods were available so that you wouldn’t starve during the next protracted famine.
Today, when we try to lose large amounts of weight and keep it off, we are fighting against an evolutionary history geared to a food landscape that no longer exists. Our appetites are calibrated to a diet of roots and shoots and very little meats or sweets — not the McDonald’s Extra Value Meal and a 64-ounce soda.
But why, given access to unlimited calories, as is the case for most of us North Americans, will only some people become obese? Is there evidence for a genetic component, or is it all the result of environmental factors?
For a significant fraction of the world’s population, environmental concerns are overriding: If you don’t have access to sufficient nutrition, you can’t become obese. Likewise, many cultural factors as well as aspects of an individual’s life history also come into play. Stress also has an important role in appetite that is produced by stress hormone action in the brain.
The role of genetics
However, surprisingly, data from adopted twin studies indicate that in the United States, about 80% of the variation in body weight is determined by genes. Again, that’s about the same degree of heritability as a characteristic such as height, and much greater than that for other conditions that we now clearly regard as running in families, including breast cancer, schizophrenia and heart disease. Yet, we don’t typically call a heart disease sufferer a weak-willed loser.
Brain-imaging studies of obese patients indicate that a genetically determined alteration in the brain’s pleasure circuitry makes them crave food more while getting less pleasure from eating than those in a lean control group.
The idea that eating is an entirely conscious and voluntary behavior is deeply rooted in our culture. When we show compassion to the overweight, we must confront the difficult truth that we are not pure creatures of free will. We are — all of us — subject to powerful subconscious forces that influence our behavior.
It’s not just that fat guy.
David J. Linden is a professor of neuroscience at The Johns Hopkins University School of Medicine and the author of the new book The Compass of Pleasure.