Image: Jonathan Bell
John D McHugh  /  AP
Jonathan Bell is seen in the kitchen of his home in North London, on Aug. 12. The 32-year-old says he has an emotional attachment to food.
updated 9/13/2004 11:15:06 AM ET 2004-09-13T15:15:06

At 43, Anita Sanders is a sylph who can eat as much as she likes without having to exercise like a fiend.

“I can’t say I work hard on trying to stay slim,” she admits, adding that she is passionate about food. She puts her slimness down to lucky genes and nervous energy.

“I eat whatever I want but I feel full quite quickly, and I’m not one of those people who runs after the guy carrying the canape tray as he passes by.”

Sanders is in a shrinking minority of people who follow a Western lifestyle. The norm is to be fat. In most of western Europe, easily more than half the adults are overweight, and in the United States two-thirds are fat.

What, then, explains the ability of Sanders and others like her to stay thin in such an environment?

The answer is complex and not the same for everyone. Apart from upbringing and environment, scientists are discovering, genes and biology are more important than previously believed.

“The people who are not increasing their weight have something special — they are resistant to obesity,” said Philippe Froguel, an obesity exert and chair of genomic medicine at Imperial College in London.

Environment, upbringing affect lifestyle
Experts are investigating couch potato genes, stop-eating genes, can’t-resist genes, and even the possibility of a party platter gene — which turns people into opportunistic eaters, who eat whenever they are offered food.

“If there is such a thing, I most definitely have that party platter gene,” said Jonathan Bell, a 32-year-old information technology specialist who struggles with his weight. “I’m one of those who lives to eat. I’d like to say that I can pass those things up, but my body craves it.”

Bell, who lives in London, confesses an emotional attachment to food. His mother would offer chocolate in exchange for good behavior when he was a child, and that habit has stuck with him.

It also takes a lot of eating before he feels satisfied — a stop-eating trait that experts say is genetically based.

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“I’d love to have that thing where I eat something and five minutes later I’m full, but I don’t get that. I always get overfull when I eat because it doesn’t click in in time,” said Bell, who is almost 40 pounds overweight.

What about genes?
Each person has a unique profile of genes, biology and lifestyle; a slight tweak in any of those influences could make the difference between fat and thin, experts say. Where they disagree is over how powerful each individual factor is alone.

“There are about 340 genes involved in weight control. Most of them increase the likelihood of your being fat, but there are actually genes that protect against being fat,” said Dr. Stephan Rossner, head of the obesity unit and professor of health behavior research at the Karolinska Institute in Stockholm, Sweden.

“You can have muscles that react just a little slower; a mental setting that makes you just a little more prone to use food for comfort or a body temperature that is just one-tenth of a centigrade higher,” Rossner said. “If you have 20 of these factors all going in one direction — but still each and every one of them is normal — that can explain why people who live in the same environment weigh different amounts.”

While the laws of physics — which dictate that you cannot get fat unless you eat more calories than you burn — are inviolable, a person’s genetic makeup influences the decisions made about eating and exercising, experts say.

There are certainly lean people who can eat whatever they want, but in many cases, like Sanders, they don’t want much.

Different thresholds
“What we have found with our (obesity) resistant people is that they do have a greater satiety than the susceptible ones,” said Dr. John Blundell, chair of psychobiology at the human appetite research unit at Leeds University in England. “When they are offered the same amounts of food at different times, they eat less and it suppresses their hunger more. Their appetite system seems to operate more sensitively."

Eating increases the levels of the calming brain chemical dopamine. Brain scans have indicated that obese people have a lower concentration of dopamine receptors in their brains than lean people do.

“It’s possible that some people need to eat more to get the same level of pleasure,” said Dr. Kishore Gadde, director of obesity clinical trials at Duke University Medical Center in North Carolina.

There is also some evidence that fatty high-fat foods might dull the appetite control signals in the brain.

It is unknown whether the appetite threshold is raised by a lifetime of overeating or whether the overeating is due to the threshold being higher. However, scientists believe it could be linked to chemicals in the gut that are stimulated by fatty food and signal the brain when the belly is full.

Another group of genes are involved with physical activity.

“There are large segments of the population who seem to have an inherited tendency to be real couch potatoes,” said Dr. Claude Bouchard, director of the Pennington Biomedical Research Center at Baton Rouge in Louisiana. “We have several studies to support this contention and we have candidate genes.”

Norma Savage, a 58-year-old retired real estate saleswoman who constantly struggles with her weight, believes she might fit that description.

She always gets the willpower to reach her goal weight, but the fat creeps back on and she has to start again. She knows that exercise would help keep her weight down, but she simply cannot bring herself to do it.

“No way. It’s a dead no-no,” she said. “It’s like a block. If there’s a gene for repulsion to exercise, I’ve more than likely got it.”

Differences in genes and biology are also thought to result in variations in the amount of energy the body uses just to keep its basic functions going. Normally, about two-thirds of calorie-burning is spent on basic metabolism to maintain the body.

But scientists suspect that maintenance may be more expensive for some people than others.

Studies by Bouchard have shown that when several sets of twins were given 1,000 more calories a day than needed, the weight gain varied enormously from one set of twins to another but was almost identical within the twin pairs, even though they had all overeaten by exactly the same amount and were equally inactive.

The same happened when they were put on a diet. Twin brothers lost almost the same amount, but the weight loss differed wildly from one set of twins to another.

Once the body has been fat, it wants to stay fat, experts say, and that’s why most people find it so difficult to lose weight. There are scores of hormones, brain signals and other chemicals at work striving to keep the body fat. The system is backed up all over the place, which makes it difficult to come up with a single drug that will succeed in tricking the body.

Starting early
Evidence is also emerging that conditions in the womb and infant-feeding habits could also play a role in determining how fat someone turns out to be.

Studies are suggesting that insufficient nutrition during pregnancy could program a baby to prepare itself for food rationing in the outside world and could have long-term effects on brain structure and the functioning of the body.

Also, breast-fed babies seem to be less likely to become fat than formula-fed babies, but some studies indicate it is the overfeeding of babies that sets them up for the battle of the bulge. Formula has more calories than breast milk and parents tend to want their babies to finish a bottle.

But however strong the genetic or biological influence, almost nobody is doomed by their genes, experts say.

There is a “massive learnt behavior component” to the obesity issue, said Andrew Prentice, an international nutrition expert at the London School of Hygiene and Tropical Medicine.

Factors such as education, money and the way people are brought up to eat and play also matter. There are many people who succeed in fending off weight gain by working like hell not to get fat, so it’s clearly not hopeless, Prentice said.

But genetics experts say even the ability to work hard at weight control is genetically influenced.

“It is very likely that some of the behaviors we ordinarily attribute to individuals having a choice about are genetically driven,” said Dr. Rudolph Leibel, an obesity expert at Columbia University in New York. “What we describe as willpower, or behavior in the general sense, is genetically driven.”

Also, portion size is something that certainly is learned and can be unlearned. Studies have indicated that overweight people underestimate what they eat by about 49 percent.

“We are not saying that they are lying. It’s just that their perception is different,” said Rossner of the Karolinska Institute. “The portions that are labeled normal by a fat person would be regarded as a big portion by a thin person.”

It’s also partly a question of circumstance, said Dr. James Hill, director of the Center for Human Nutrition at the University of Colorado Health Sciences Center. Some people just happen to be in the right set of friends, the right job, the right city. Those also are influences that can be challenged.

“At the end of the day, we have to tell people to try harder,” Froguel said. “But we have to recognize that things are more difficult for some people than for others and it’s not because they are weaker by definition.”

And whether those who are lean now will forever be able to resist obesity is unclear.

This resistance is certainly not infinite. It can be overwhelmed, said Bouchard.

“Will they still remain lean when they are fully exposed to the obesogenic environment?” he asks. “We don’t know. Can they resist an even more obesogenic environment? We don’t know.”

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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