Sluggish Nervous System May Help Explain Obesity

A study has found that the fatter people are, the more lethargic is their sympathetic nervous system, the built-in regulator that switches the body into high gear during emergencies and exertion.

The research supports the growing belief that fat people don’t always get that way simply because they are gluttonous eaters. Instead, they may have an inborn tendency to put on weight and keep it on.

“We are saying that body fat or weight is somehow regulated, just like blood pressure is,” said Dr. Hugh H. Peterson, a researcher at the University of Louisville.

“It may be that something is wrong” when people are overweight, Peterson said. “One arm of the regulatory system is impaired.”

Slower Metabolisms Found

Further evidence of Peterson’s theory comes from recent studies showing that fat people often have slower metabolisms. In many cases, it seems, they are chubby because their bodies are more fuel efficient. They burn up fewer calories doing the same things as slender folks.

Peterson’s study provides one explanation of why fat people’s metabolic rates are slow. It suggests that an out-of-kilter nervous system helps keep this internal thermostat turned down.

“It’s another piece of the puzzle that’s been unlocked,” commented Dr. Jules Hirsch, an obesity expert at Rockefeller University.

Breathing, heart rate, contraction of the pupils and a host of other involuntary bodily movements are controlled by the autonomic nervous system. This system has two main branches, the sympathetic system and the parasympathetic system, which balance each other.

In general, the sympathetic system speeds things up, often in response to an emergency or exercise. The parasympathetic system does just the opposite, slowing down the heart and other organs.

Prompts Hormone Production

The sympathetic system prompts production of adrenaline and other hormones. Among other things, these natural chemicals signal the body’s fat cells to give up some of their cargo so it can be burned as fuel.

Peterson’s report, published in the New England Journal of Medicine, measured adrenaline levels, heart rates and other factors under the control of the nervous system in 56 healthy men of varying sizes. It showed that the fatter they were, the less active their sympathetic nervous systems.

Other scientists have tried but failed to find such differences in fat people. Peterson said his findings should be replicated by other scientists, and he said the possible meaning of his findings is speculative.

“We have not proved that a disturbed autonomic nervous system maintains or causes obesity or causes some of the health hazards of obesity,” he said. “But certainly it’s titillating to think that might be the case.”

Peterson found that the parasympathetic system, the other half of the internal control machinery, is also turned down in fat people. Under his theory, an active parasympathetic system might help make people fat, while a lethargic one would keep them skinny.

However, he theorizes that fat people have slow parasympathetic systems as well to balance their sluggish sympathetic systems.

“They got fat and stayed fat because their sympathetic nervous system tone was turned down,” he said. “As a response to that, they were able to turn down their parasympathetic nervous systems to reach a kind of equilibrium. That’s quite speculative, but that’s the way we think about it.”

Exciting Findings

Dr. Elliott Danford of the University of Vermont, who has also looked for links between the nervous system and fat, called Peterson’s findings exciting and said they further erode the idea that obesity results solely from overeating.

Another authority in the field, Dr. Lewis Landsberg of Boston’s Beth Israel Hospital, noted that about 15% of the body’s metabolic rate is under the control of the sympathetic nervous system.

“Their findings are consistent with recent evidence that indicates there is a biological difference between obese and lean,” he said. “This difference involves energy expenditure, and the sympathetic nervous system is important in regulating energy expenditure. What they find shows diminished sympathetic activity, and that works into the overall equation.”

Peterson said it is possible that fat people’s lethargic nervous systems are a result, rather than the cause, of their weight problems, but he said this is unlikely.

“It would seem rather illogical that a fat person would have acquired a state by virtue of becoming fat where he would be set to be fatter,” he said. “It seems to me rather unlikely that the depression in sympathetic nervous system tone is going to be explained as a result of being obese.”