Hunger and Eating Disorders

C. George Boeree

The Biology of Hunger

We usually first become aware of the fact that we are hungry when we feel "hunger pangs," which are just our stomach contractions.  For many people, this is a strong incentive to eat, but it is not, physiologically, the most significant indication of hunger.

More important is the level of glucose (blood sugar) in the blood.  Most of the food you eat gets converted to glucose, much of which is converted by the liver into fat for later use.  When the levels of glucose are low, the liver sends signals to the hypothalamus - specifically, the lateral hypothalamus - that levels are low.  The hypothalamus in turn triggers whatever habits you have accumulated relating to food seeking and consumption.

Another portion of the hypothalamus (the paraventricular hypothalamus) actually tells you more specifically which foods you need, and seems to be responsible for many of our "cravings."

The feeling that it is time to stop eating is called satiety.  Again, the first indicators may be the distension of the stomach and the intestines -- that full or even bloated feeling we all know from Thanksgiving dinner.

There is also a certain hormone called CCK (Cholecystokinin) that is released when food begins to move from the stomach to the intestines and that signals the hypothalamus (this time, the ventromedial hypothalamus) that it's time to stop eating.

And there is a hormone released by the fat cells themselves called leptin that decreases appetite via the hypothalamus.

I'm sure you've all talked about one person having a better metabolism than another.  Some people just seem to burn calories as quick as they eat them, while others gain weight just by looking at food.  This is called the set point hypothesis.  It suggests that everyone has a certain metabolic set point, a certain weight that your body is geared towards, which is determined by your metabolism, or the rate at which you burn calories.  Different people have different set points, and it is believed that these set points can change depending on a number of factors, including eating patterns and exercise.

The Psychology of Hunger

Hunger is not, of course, entirely a physical process.  For one thing, the cultural and even individually learned preferences and eating habits can make a difference.  For example, some of us eat regular meals and rarely snack, while others just nibble throughout the day.  Every culture has its collection of foods that are preferred and those that are avoided.  Many people like the burned flesh of large herbivores (i.e. a steak); others prefer raw squid; others still prefer to graze on a variety of vegetation....

Our culture and upbringing also provide us with various beliefs and attitudes about food and eating in general, and our personal memories can influence our eating behaviors as well.  Some of us grow up with  the idea that we should never waste food, for example, and many of us have particular attachments to what are sometimes called "comfort foods."

Eating is a social thing in human beings and can give one a sense of love and belonging.  It has been suggested that for some people, food is a "substitute" for the love they crave.  Also, some foods - chocolate and ice cream come to mind - seem to reduce anxiety and stress for many of us.

One of the strongest learning experiences both humans and animals have is called taste aversion:  If we get sick soon after eating something, we can develop a instant dislike for that food for the rest of our lives!  Children often say they are "allergic" to one food or another when this happens.

Eating Disorders

As is the case with anything as important as eating, human beings have developed a number of eating disorders.  One is called bulimia nervosa, and consists of a pattern of "binging" and "purging" - periods of sometimes extreme overeating followed by periods of vomiting or the use of laxatives.

Bulimics are usually obsessed with maintaining or reducing their weight.  They tend to suffer from depression, anxiety, poor self-esteem, and poor impulse control.  They tend to come from families with a history of emotional problems such as depression, as well as families with obesity problems.

Anorexia nervosa is another eating disorder which involves dieting to the point of starvation.  The "rule of thumb" is that you are seriously underweight if you are more than 15% below your ideal weight.  Anorexics often use vomiting and laxatives, just like the bulimics.  They have an intense fear of being fat and are obsessed with being thin.  They often have a distorted body image, meaning that when they look in the mirror, they tend to see someone overweight, when others see them as walking skeletons.  Anorexics often come from very competitive, demanding families, and are often perfectionists with a strong need to control all aspects of their lives.

Physiologically, anorexia has been linked to abnormal levels of the neurotransmitter seratonin, which is involved in eating regulation.  Twin research suggests that there may be a genetic aspect to anorexia as well.

Most anorexics and bulimics are young women, including from 1 to 4% of high school and college girls.  It may be that there are physiological aspects of female adolescence contributing to the problem, but we should note that 10% of teenages with anorexia or bulimia are boys.  But a significant root of these disorders is likely social:  In our society at this point in history, the standards of beauty tend to emphasize thinness, and women in particular tend to be judged on the basis of beauty, sometimes to the exclusion of all  else.  Certainly, if you look at many magazines for young women, or advertisement directed at them, you would think that looks are everything, and that fat is the kiss of death for self-worth!

It is interesting to note that, whereas the average American woman is 5 foot 4 inches and weighs 142 pounds, the average model is 5 foot 9 inches and weights 110 pounds.  If Barbie, that childhood ideal of feminine beauty, were full size, her figure would read 36-18-33!

It is interesting that cultures with standards of beauty that have more respect for a woman's personality or other traits, and cultures that appreciate heavier women, have little or no trouble with bulimia or anorexia.


For all the suffering that bulimia and anorexia are responsible for, another eating disorder causes far more:  Obesity.  The "rule of thumb" is that you are obese if you are more than 35% over your ideal weight. By that standard, about 21% of Americans are obese.  Europeans and others with "slimmer" populations shouldn't gloat too much, however:  This tendency is actually world-wide!

Physiologically, obesity is strongly associated with diseases such as diabetes, high blood pressure, heart disease, and some cancers.  Psychologically, the toll is great as well, and obesity is associated with depression.  Even sociologically, obese people face considerable discrimination, from childhood teasing to denial of employment in adulthood.  And unlike other kinds of discrimination, people actually consider obesity the fat person's own fault!

Genetics is a major cause of obesity, and somewhere between 40 and 70 percent of the variation in body mass seems to be genetic.  Our ancestors that passed those genes down to us didn't get fat, mainly because they didn't have as much easily available food as we do, and because they had to work harder and walk further to get by.

But learning is also a major factor, including childhood eating patterns and a sedentary lifestyle.  Our culture doesn't help at all, in that our food and snack industry spends billions of dollars every year encouraging us (including children) to eat food filled with sugar and fat.  Often the same companies then make billions (33 billion dollars in 1996) selling us weight loss programs and products!

Most people attempt to deal with obesity with dieting. In fact 80% of all American women diet, and 25% of men.  50% of girls below the age of 18 have dieted!  Unfortunately, although dieting often works in the short-run and for small amounts, it usually fails in the long-run for the people who are obese.

Dieting is made even more difficult by the way in which "set point" works: When you diet, your body thinks you are starving, and so readjusts your metabolism to be more efficient, thereby causing you to need less food to maintain your body, and making it even more difficult to lose weight. Although you do need to eat more than you burn up to get fat, once you are fat, you don't have to eat much at all!

Diet and exercise are, of course, the only hope, but the failure rate is so great - over 95% - that doctors usually focus on treating the diseases that result from obesity rather than dealing with the obesity itself.  One might want to keep in mind that weight training, which increases muscle mass, helps:  Muscles use up more calories even in a resting state than other tissues.  There is also some hope for the future in medical research, including research on the effects of leptin and possible genetic interventions.

© Copyright 2003, 2009, C. George Boeree