The usual reason given for people gaining an excess amount of weight is that they eat too much and/or exercise too little. That reflects one of the basic laws of thermodynamics—I forget which one. The amount of energy you put into a system minus the energy you take out has to be stored somewhere—i.e., as fat.

This formulation—true though it is—does not entirely explain ; some people seem to eat more than fat people and exercise no more than these same fat people, and yet they are not fat. Chalking this fact up to the general perversity of the universe is not sufficient explanation; other factors must come into play. I mention below some of the ideas thoughtful people have proposed to explain why fat people become fat:

1.

It is thought that some people are destined to gain weight because they have inherited “fat genes” from their parents. There can be no doubt that people vary in their genetic makeup and some people are inclined, because of genetic reasons, to become tall, or broad-shouldered, or squat, or fat. It is known, for instance that height is controlled by a number of different genes, probably around twenty. Nevertheless, height is affected also by diet.

Fat people tend to come from fat families. That doesn’t necessarily mean, however, that they are fat for genetic reasons (at least not exclusively). They could have learned fat-making behaviors, i.e. eating habits, from their parents when they were growing up. Because of studies of identical separated at birth, however, it can be said, certainly, that there is indeed a strong genetic component to body weight. This is not to say that someone so constructed genetically is doomed to become fat.

I once saw a woman in her mid-forties who, by this theory, should have been fat, but wasn’t. She had two brothers, each of whom weighed over four hundred pounds; and she had a grown child who weighed over three hundred and fifty pounds. Plainly, she had whatever genes it took to get fat. I asked her one day, “How is it that you’re not fat?” She explained that she thought it had to do with the fact that she ran for an hour and a half every day before she went to the gym.

2. Diet

Remember that law of thermodynamics. The amount of energy that goes into a machine (the human body, for instance) has to balance the amount that goes out; or the extra energy that goes in (food) has to be stored somehow (fat). So, for everyone, no matter what that person’s genetic vulnerability, either lowering the food intake or increasing the energy output (exercise) will lower the energy stored as fat. Eating less and less or exercising more and more will, by this theory, cause weight loss.

Our modern diet, however, seems to be designed to pack as many calories as possible within the human capacity to eat. High caloric foods seem to be inherently more attractive than other foods for reasons that probably have to do with survival in conditions that human beings found themselves in most of the time throughout our history. But no longer.

Some examples: Among the Chinese American population, some Chinese people who were born in China subsisted on a relatively calorie poor diet while living there and were, for the most part, thin. In some cases, however, their children—once they came to this country and began eating like the rest of the Americans—became fat in higher numbers and began developing all those illnesses associated with obesity.

There is an Indian tribe that lives in an arid part of the West and has lived there for many generations. (Apparently their land was so unproductive, none of the other tribes were inclined to go to war to evict them.) They survived very well on a very low-calorie diet. When the white men came, however, bringing their accustomed diet with them, the Indians became fat. Now over 90 percent of them are very fat and approximately the same number suffer from diabetes. They had evolved over many generations to live on a sparse diet. Probably the rest of us are similarly not quite evolved enough to cope with our modern diet.

3. Exercise

A lack of exercise can be one contributing factor to obesity—and we are getting better and better at not getting enough exercise. There are exceptions, of course. A whole lot of people are running around, literally, or playing sports, including group sports, at an advanced age. This never used to happen. When I was a kid, if I saw someone running through the streets of Manhattan, I knew they were running for a bus.

Now, there are a minority who are very active physically. But a greater number are less active than people used to be. Television (or the internet) usually gets blamed. Before that there was radio. When telephones first came in, most people thought the device would never catch on. If they wanted to talk to someone, they said, they could just walk over to that person’s house. Nowadays, if people really have to go somewhere, they drive rather than walk, use an elevator rather than climb stairs and are, in general, passive rather than active.

Over the long run, physical activity has an effect on maintaining proper weight that may be at least as important as a proper diet.

4. Eating Habits

Improper eating habits, learned while growing up, are thought to contribute to excessive weight gain over the course of a lifetime. The problem is there is no agreement about what constitutes "proper" or "improper" eating habits.

Finishing eating everything on your plate used to be considered good; now it is considered bad. You should stop eating when you are no longer hungry. Do not eat that last potato just because it is sitting there on your plate. It is often recommended to dieters that they get in the habit of purposely leaving some food uneaten on their plates.

5. Television, Social Media, and More

Another reason given for gaining weight is too much TV. The amount of time children watch television has been shown to correlate with weight; the more they watch, the heavier they get. Possibly the effect is through a lack of exercise. Someone sitting inert on a couch is not outside playing ball. Or perhaps the food commercials on television that are designed to make people hungry actually do make them hungry.

6. Sleep

People who sleep less eat more. Maybe they have nothing better to do. Maybe there are chemical changes that take place in the brain to compensate for inadequate sleep. There is an center in the brain, and it is known to be affected by circumstances, including, possibly, sleep deprivation. (The appetite center in the brain is close to the area that controls —which may say something, but I don’t know what.)

7. Certain Drugs

Some , for example, can cause weight gain. Over the years, through my practice, I have converted a few chronically , thin women into chubby—but cheery—women, (although they are not often cheery about being chubby). There are a number of other drugs that act similarly.

8. Poverty

Poverty correlates with obesity. There are probably two reasons. For some living in low income areas, there is little access to fresh fruit and vegetables. Processed foods are more fattening. Secondly, healthy foods are more expensive—so, in poor communities, eating fried, processed, or other unhealthy foods may make economic sense.

9.

The usual “glandular conditions” that are mentioned often by lay people as an explanation for obesity include low thyroid and high cortisone levels. The effects they produce on weight, however, are relatively minor and inconsistent.

There are other hormones, however, secreted in the stomach that are known to affect appetite one way or another. Strategies are being developed to control weight using them, but the results so far have been disappointing. The stomach bypass operations that are used currently to treat morbid obesity are known to affect these hormones, and it may be partially through that mechanism that these operations work.

10. Bacteria

There are a great many human cells in the body, but ten times that many bacteria take up residence in each of us, particularly in our intestines. These communities of bacteria vary from one person to the next. They help us to digest our food, and some are more efficient at that task than others. Therefore, some people, given the same amount of food, absorb more calories than others. So, it is true that a particular person can eat very little—even less than other people—and still gain weight.

A study was conducted in which a number of people ate the same number of calories and exercised to the same extent; yet there was a perceptible difference in the change of weight each person experienced. There are three possible reasons why this could happen.

First, for some reason, some people may be better at absorbing the calories from their food than others, perhaps because of those bacteria which assist digestion. Secondly, some people are more active when they are resting (not exercising) than others. Probably both of these explanations are true. The third reason, an innate difference in metabolism, may result in a somewhat higher body temperature; but the mechanism of this higher metabolism may still come down to moving imperceptibly more than other people.

11. Bacteria + Hormones

Bacteria affect weight in a second way: they seem to affect the hormones that the stomach secretes to regulate weight. A common cold virus, adenovirus-36, has been linked to obesity, perhaps because it affects the number of fat cells in the body.

12. The Greater Availability of Food

Over the last 50 years, changing agricultural policies have encouraged more planting of food which then becomes more available. When food becomes cheaper, people eat more. Sugar may be especially important. Some people date the obesity epidemic to the widespread availability of sugar.

13. Timing

It seems that college students gain an average of one to three pounds during their freshman year. Similarly, men gain a few pounds the first year after they marry. A strategy for avoiding weight gain immediately suggests itself: don’t go to college and don’t marry.

On top of the reasons listed above, obesity has also been linked to:



not enough protein in the diet

too much fat in the diet

too much carbohydrate in the diet

an overly warm house

too much light

not enough light

pollution

It is evident, therefore, that there are many causes of obesity. They overlap with each other. Perhaps there is a genetic predisposition to have a particular community of intestinal bacteria; and that might in turn affect those hormones that control appetite in a particular way. Recent evidence suggests that exercise changes the effect of the “fat genes.” The inclination to exercise itself might be controlled genetically.

What matters, I think, is the fact that excessive weight is not simply a failure of . It is not a moral failing. has to be approached in a practical way—not with finger wagging.

Excerpted from The Stuff Yourself Diet © Fredric Neuman and Warren Goodman