By John Messmer, MD

Some people just seem to be naturally thin. They seem to be able to eat anything they want and never gain an ounce. Other people claim to eat almost nothing yet they continue to grow larger every year. Why is that?

Habits of thin people vs. overweight people

When thin and overweight people are followed around all day and compared, the thin people use more calories and eat less than the overweight people. Thin people get up and move around more, tend to fidget more and sit less. In addition, while a thin person might occasionally eat a donut, over the course of a day, thin people consume fewer calories than their overweight counterparts. Thin people stop eating when they are full, even if they are eating their favorite food. Overweight people tend to eat all their food.

Every body stores calories differently

When calories and activity are controlled, some people will still be heavier than others because they store calories more efficiently. This used to be a good thing. In the days before fast food and grocery store shelves piled high with fatty and sugary snacks, food could be scarce for a large part of the year. If early people had any chance of survival and having more children, they had to store calories for the times of food scarcity.

Our bodies have the capacity of storing extra calories as fat to use for energy during times when we can not get food. Those who are better at storing calories were more likely to survive and continue to have children. Looking at early representations of women in primitive statues, they tended to be obese by our standards. Women who could gain weight were more likely to produce children, so for primitive human civilizations, big was beautiful. For our early forebears who did not expect to live beyond age 40, that's fine. For anyone who wants to stay healthy and active into old age, big is a hazard.

Body chemicals affect people's dining practices

Those who inherit the tendency not to store calories are thinner because their bodies shut off the biochemical signals that prod us to continue eating once we have eaten a certain amount. On the other hand, the overweight tend to want to continue eating even when they know they have eaten enough. Overweight people tend to derive more pleasure from eating than thin people. They also have difficulty convincing themselves that they can stop.

To make matters worse, one would think that at some point, they would be heavy enough and their bodies would finally send the signal to stop overeating. Unfortunately, in many people that does not seem to be the case. Even after eating a large meal, very overweight people will be hungry later. Research is ongoing as to why this occurs, but it may be that overweight people are less sensitive to a hormone called leptin that is supposed to tell our brains that we are no longer hungry.

A healthy weight can be achieved by anyone

The good news is that these innate signals that drive people to continue eating can be overcome with practice. Every overweight person can achieve a normal, healthy weight by learning to reduce intake and increase energy use. The trick is to get beyond our genetic programming, understanding that wanting to eat does not always mean we should and recognizing that physical activity is the most essential part of caring for our bodies. It is not necessary to be thin, but each of us can be normal weight.