What Causes Obesity?
What Causes Obesity?
Experts and laypeople alike once thought that simply taking in more calories than are expended caused a person to become obese. However, scientists have determined that there are many factors—including genetics, the environment, and behavior—that contribute to obesity in varying degrees for different people.
The Role of Genetics
Genes are the part of a DNA molecule that pass hereditary information from parents to their offspring. They reside on wormlike bodies called chromosomes in the center, or nucleus, of each cell. The sequence of genes on each chromosome provides the cell containing those chromosomes with a set of instructions on how to grow and operate. Humans have forty-six chromosomes in each cell. Twenty-three come from the mother and the other twenty-three from the father. The genes on each chromosome also come in pairs, with one copy of every gene from the mother and one from the father.
When a gene or chromosome is damaged, the resulting change is called a mutation. The damage that produces a mutation can involve an entire chromosome, one or more genes, or one or more of the chemicals that make up DNA. Any of this mutated genetic material can be passed to a child if it happens to be part of the sets of chromosomes and genes transmitted from either the mother or father. When this happens, the altered genetic instructions may cause various malfunctions that produce certain diseases or disorders.
Genes and Obesity
Unlike some disorders where abnormal genes directly cause a particular condition, with obesity, the situation is more complex. Genes do not make a person fat or thin. They merely determine which individuals are more or less susceptible to weight gain due to a variety of factors, but the weight gain will not occur unless certain behavioral and environmental conditions are present. Examples of behavioral factors include eating too much and leading an inactive lifestyle. Examples of environmental conditions are having high-fat foods available in profusion and living in a culture that encourages overindulgence.
In some cases, a genetic mutation will lead to behavior that produces obesity; for example, a disease called Prader-Willi syndrome leads a person to consume enormous quantities of food, which results in extreme obesity. First described in 1956 by Andrea Prader and Heinrich Willi, along with Alexis Labhart and Guido Fanconi, this disease affects a baby early in fetal development. The baby has feeding problems from birth on, along with short stature and a degree of mental retardation. Someone with Prader-Willi syndrome also has personality problems and is prone to temper tantrums, particularly if food is withheld. If food is not withheld, though, the individual will eat compulsively and become severely obese.
Other genetic mutations may lead to imbalances in brain chemicals and body hormones that directly influence food intake and therefore may cause obesity. Several abnormalities that can cause such imbalances are Cushing's syndrome, a disease involving overproduction of the hormone cortisol by the adrenal glands located at the base of the kidneys; hyperinsulinism, where malignant or nonmalignant tumors in the pancreas cause too much insulin to be secreted; and hypothyroidism, a condition where the thyroid gland in the neck does not produce enough thyroid hormone to properly regulate weight. The most common of these is hypothyroidism, which can be corrected by giving the affected person doses of artificial thyroid hormone to correct the imbalance. However, doctors say that even this condition is rare and is not responsible for most instances of obesity, though it is common for many obese people to try to blame an underactive thyroid gland for their fatness. Says Dr. Norman B. Ackerman in his book Fat No More, "Through the years I have seen only three morbidly obese patients with thyroid abnormalities."5
A Tendency to Store Fat
Unlike the rare cases of obesity where specific genetic abnormalities underlie the tendency to develop the condition, the majority of cases results from a genetic predisposition to gain weight and store fat. Again, in order for the weight and fat gain to occur, the person must consume too many calories, but the genetic tendency toward obesity may explain why some people are able to easily maintain or lose weight while others are not.
Experts believe that the capacity to store fat evolved in some humans over thousands of years. Until relatively recently, when food became abundant, those whose bodies were efficient at storing fat in times of famine had an advantage over those who did not. They were less likely to succumb to starvation or malnutrition, and having some extra body fat also made it more likely that a woman could bear children, another survival advantage from an evolutionary standpoint.
Indeed, many experts who have argued that obesity is not a result of an individual's weakness for overconsumption of food have pointed to the argument that genetic selection is in fact the responsible variable. According to the Centers for Disease Control and Prevention:
People who are affected with overweight and obesity are often victims of stigmatization and discrimination. It is time to stop blaming the victim. Many obesity researchers believe that people who struggle with their weight are pushing against thousands of years of evolution that has selected [genes] for storing energy as fat in times of plenty for use in times of scarcity.6
The fact that many obese people have family members who are also obese supports the notion that certain genes that have been handed down within a family are at least partially responsible for the condition. While some experts have argued that this may be due to behavioral factors, such as family members sharing a high-fat diet, research on identical twins lends support to the role of genes in producing a tendency toward obesity. Identical twins have identical genes, and studies show that identical twins who do not live together have more similar eating habits and weights than do fraternal twins, who do not share identical genes.
Which Genes Play a Role in Obesity?
Since the mid-1990s, scientists have made progress in identifying some of the genes that contribute to obesity. These genes influence the tendency to gain weight and store fat in a variety of ways. Some influence the amount of certain hormones that regulate food intake and how food is processed in the body. One of the best-known genes of this type is called the ob gene. Ob encodes a hormone known as leptin. Leptin normally serves as a signal from fat cells to the brain, telling the brain that the body has eaten enough. Leptin was discovered by a team of researchers led by Dr. Jeffrey Friedman at Rockefeller University in New York in 1994. Friedman and his colleagues found that the brain believes the body is still hungry in laboratory animals where the ob gene is defective and the animals do not produce leptin. This leads the animals to overeat and become obese.
When the researchers gave the leptin-deficient animals leptin artificially, the animals lost weight. Doctors initially thought that a solution for obese people was just around the corner, but it was not that simple. Scientists found that most obese people are not leptin deficient, so it appears that, at least in humans, other genes and hormones as well play an important role in obesity.
One of the other genes discovered by scientists in the late twentieth century is MC4R, which stands for melanocortin 4 receptor gene. MC4R governs a type of receptor in the area of the brain known as the hypothalamus. This area plays a role in controlling appetite. Studies have shown that over 5 percent of severely obese people have mutations in the MC4R gene. These mutations lead to too few MC4R receptors, which in turn leads to overeating. Those who inherit a copy of the defective gene from both parents showed higher degrees of obesity than did those with only one copy of the gene.
Researchers found that all the people they tested who had an MC4R mutation exhibited binge-eating behavior, a condition in which the person sporadically and uncontrollably consumes huge amounts of food. Although knowing that this binge eating is probably caused by a genetic defect rather than by a weakness of character is comforting to many of the affected persons, this knowledge still does not fix the problem. Experts say that someday doctors may be able to target therapy at fixing the defective gene, thereby alleviating the cause. More research is being carried out on this gene to determine its role in obesity and methods of fixing it.
Other Modes of Gene Action
Other genes influence obesity in a variety of ways. One such mode of action is by encouraging or discouraging the formation of fat cells. Researchers at two research centers, for example, discovered two genes that prevent immature cells from developing into fat cells. Investigators at the University of Texas Southwestern Medical Center in Dallas found that a gene called insig-1 blocks the formation of new fat cells. They hypothesize that the gene is less active in some people and in some areas of the body, thus explaining why some individuals tend to gain fat in certain areas. At Rockefeller University in New York, another team of scientists found that a gene called foxa-2 acts in a similar manner. The researchers say that someday these genes may be artificially introduced to prevent fat-cell formation as a method of combating obesity.
Other genes regulate how much fuel the muscles use to perform various tasks. People whose muscles tend to use more fuel do not gain weight as easily as do those whose muscles use less fuel to accomplish the same task. By the same token, other genes influence how many calories the body burns just to stay alive. Depending on the particular individual, breathing and other essential functions use up a widely varying range of calories. One researcher found that this level of calorie expenditure can vary from about one thousand to three thousand calories per day.
Still other genes determine the effects of exercise on a given individual. The same amount of exercise can affect oxygen usage, muscle size, and a host of other factors that in turn affect calorie burning. So individuals whose genes give them a tendency not to expend as much energy on a given amount of exercise will be more likely to retain excess weight.
Besides these genetic influences, scientists have found that some genes determine whether or not an obese person develops complications from obesity. Some extremely overweight individuals have normal blood pressure, blood sugar, and heart function. Experts believe such people possess a set of genes that protects them from these common complications.
Environmental and Behavioral Influences
Genes that predispose people to obesity or its complications, as stated before, do not on their own make someone fat or have complications. In order to become obese, an individual must take in more calories than his or her body expends, and this behavior in turn is caused by a variety of factors.
Medical experts say that one of the primary reasons for the dramatic increase in the number of obese people since the mid-1900s is that the environment is much more conducive to causing weight gain in susceptible individuals. Many of the environmental changes began with the advent of modern tools, vehicles, and other technology that has increased convenience foods—such as prepackaged meals, chips, and sweets—and decreased physical activity. "Modernization, the growth of industry and technology was introduced over 50 years ago in the Western world. Modernization has led to an abundance of food (particularly high calorie intake) and a decrease in overall physical activity, contributing to increased rates of obesity,"7 notes the American Obesity Association.
People now lead much more sedentary lives than in past eras of history. Many people drive their cars everywhere, sit and watch television for hours each day, and sit in front of computers or video games. Modern labor-saving devices, such as dishwashers, clothes dryers and washers, and microwaves, have led to a decrease in physical energy expended in the home. Many schools in the United States do not require adequate amounts of physical education, according to obesity experts. And in the workplace, many people now have jobs that involve sitting at a desk all day.
Modern convenience and prepackaged foods also have contributed to the obesity epidemic. Experts say that the vast number of women working full-time outside the home is one force behind the increased use of high-calorie convenience foods and the subsequent rise in obesity. Many families use large quantities of these high-fat, high-sugar items, and in addition, many people eat frequently at fast-food restaurants that specialize in high-calorie foods. Several obese people have directly blamed certain fast-food chains like McDonald's for their condition, stating in lawsuits that the chain is responsible for the person's obesity. As of October 2003, whether these lawsuits will actually yield settlements or be thrown out of court remains to be seen. The fast-food chains maintain that they do not force anyone to eat their food and are therefore not responsible for the choices that obese or nonobese people make.
Another reason for the increase in obesity is that portion sizes have increased. This applies to restaurants, fast-food places, and at home. The "supersize" meals at fast-food places are particularly large, say medical experts. For example, a supersized order of french fries at McDonald's contains 160 calories and seven grams of fat more than the medium-sized order. Prior to the 1990s, most places did not even offer these supersized options. For at-home consumption, packaged foods like chips and candy bars have also gotten bigger. Doctors suggest carefully reading nutritional information on product packages to see how much fat and calories they contain.
Not only has the preponderance of convenience foods and large portions contributed to the epidemic of obesity, but researchers are finding that the combination of foods eaten is also a contributing factor. For example, studies conducted in the late twentieth century show that laboratory rats fed fat and sugar together gain more body fat than those fed these foods separately. Scientists believe the same thing happens in humans.
Experiments also show that people who eat a high-fat diet gain weight faster than those who eat a high-carbohydrate diet with the same number of calories. And the size and frequency of food intake can also affect weight and fat gain: laboratory animals and people who consume their daily calories in two or three large meals gain more weight than those who nibble throughout the day on the same foods and numbers of calories.
Social and Personal Causes of Obesity
Social and personal variables are other types of environmental forces that can contribute to obesity. Overeating is encouraged in some cultures; for example, the stereotype of the Jewish grandmother or the Greek matriarch from the old country saying, "Eat! Eat!" is not an exaggeration in many instances, and such pressures often result in overeating and subsequent obesity in members of such ethnic groups. Mark, a man whose grandmother tried to force food on everyone who entered her house, says, "That was her way of showing you that she cared, and she was highly insulted if you didn't eat and eat and eat. I had to be rude and tell her that I was not hungry."8
Studies have shown in other cases that some ethnic groups have a genetic predisposition for obesity, and this predisposition, coupled with lifestyle factors, often causes people in these groups to gain weight and fat. People of eastern and southern European ancestry, for instance, have a greater tendency to gain weight than do those of northern European descent. If such individuals eat a high-calorie diet and do little or no exercise, they are likely to gain large amounts of weight.
Other research indicates that some ethnic groups with a genetic predisposition for obesity do not experience widespread obesity or complications from the condition until they begin to live a modern Western lifestyle with high-calorie foods and a sedentary existence. For example, studies have shown that aborigines in Australia who once followed a traditional hunter-gatherer lifestyle with a diet low in fat and high in fiber had little problem with obesity. But once many of these tribespeople switched to a more Western, sedentary lifestyle and diet, their incidence of obesity, high blood pressure, and type 2 diabetes skyrocketed. A similar situation exists with the American Pima Indians. Those Pima living in Mexico with a traditional, high-activity lifestyle and consuming a low-fat diet have little obesity, while Pima living in Arizona who follow a sedentary lifestyle and consume a high-fat diet have a high prevalence of obesity and type 2 diabetes.
Personal and social factors, as well as cultural influences, can contribute to obesity in some people. Eating habits followed during infancy and childhood can trigger lifelong obesity. Excessive eating during these periods results in an increase in the number of fat cells in the body. People with many fat cells have been shown to have difficulty losing weight.
Another common influence on obesity is the presence of personal and social problems. Some people tend to eat more when they are unhappy or feel rejected, and this often turns into a vicious cycle. The overweight individual feels rejected socially because of obesity, which in turn leads to more overeating, which causes the person to become even heavier. Says one obese woman who knows her weight problem has a lot to do with eating for comfort, "Whenever I feel neglected, or get angry, or sad, or … I eat something to feel better."9
Studies on whether or not obese individuals get that way in the first place due to severe emotional illness give mixed results, however. One study by psychiatrist Katherine Halmi found no evidence of a higher incidence of psychiatric illness in obese persons. But other studies have indicated a higher-than-normal incidence of such disorders, especially depression, as causative factors for the obesity. Sometimes the drugs prescribed for psychiatric disorders can contribute to obesity; examples are lithium, some antidepressants, and antipsychotic drugs.
Doctors have identified two specific psychological disorders that cause obesity in some cases. In binge-eating disorder, an individual sporadically loses control and ingests huge quantities of food in a short time. Such people feel extremely distressed after the binge. Unlike bulimics, though, who binge eat and then force themselves to vomit, people with binge-eating disorder do not make themselves throw up and therefore gain weight from their behavior. Experts say that 10–20 percent of individuals who enter weight-reduction programs can attribute their obesity to binge-eating disorder.
Night-eating syndrome is the second psychological disorder that can cause obesity. People with this disorder rarely eat in the morning, but consume large quantities of food at night. Most have trouble sleeping, in large part because of the quantities of food they consume. Experts say that about 10 percent of people seeking to lose weight suffer from night-eating syndrome.
A Variety of Causes
Whether social factors, genes, or certain behaviors are involved in causing obesity, it is apparent that the answer to the question of what causes it is not a simple one. For most obese people, no single factor is responsible for their condition. Most likely it is a combination of a genetic predisposition and a variety of behavioral, social, personal, or cultural influences that lead to the excess fat and weight gain that characterize obesity.