Behavioral explanations of obesity can be placed into two categories: excessive caloric intake (overeating) and lack of physical activity (hypokinesis).
The basic laws of nature require that calories must be consumed before energy can be stored as excess weight. The body cannot make energy on its own. For the obese and overweight therefore, caloric intake is an important issue.
Do obese people eat more than normal-weight people? This has been a controversial question and researchers are divided in their answer. Numerous studies show that obese people eat no more and sometimes eat less than normal-weight people. These researchers would suggest that blaming obesity on a lack of willpower is inaccurate and an over simplification of the facts. They maintain that the body of the obese person is simply more efficient at converting calories to adipose cells for reasons beyond his or her control, such as genetic predisposition or higher set point for fatness. Stated another way, obese people may not eat more than normal weight people; they just consume more calories than are required by their bodies.
Other researchers claim that obese people do eat more than normal-weight people; the problem is a discrepancy in accurately reporting food intake. In one study obese subjects underestimated food intake by about 50% and overestimated physical activity by 33%. In another study obese women were found to expend and therefore eat about 400 to 500 calories more on average each day than normal-weight women.
Whereas researchers are divided on the issue of overeating, they tend to agree that the abundance of food high in fat and calories is a major factor in the prevalence of obesity in the United States. In his review of the literature on obesity-a review that included 284 citations from the scientific literature Knifeman stated: “Of all the current theories attempting to explain the epidemic of obesity in most Western societies, the high dietary fat intake hypothesis is most widely accepted by experts. Overweight people tend to eat a higher-fat diet than people of normal weight. Ounce for ounce, fat yields more than double the number of calories as protein or carbohydrates . This energy difference partially explains the relationship between dietary fat intake and weight gain. It is not a complete explanation because in studies that hold the total number of calories constant and more calories are consumed than needed, a person eating a high-fat diet tends to store more excess calories as body fat than someone eating a lower-fat diet. Your body is more sensitive to excess carbohydrates than excess fat. Studies conducted in laboratory-type settings (for example, in a respiratory chamber where oxygen is measured, burning calories requires oxygen, more to burn fat, less to burn carbohydrates) show that once the glycogen stores are filled, the body burns carbohydrates first and then stores the fat. We use fat for energy, especially between meals. The brain uses exclusively glucose for energy, but other tissues use fatty acids. Right after meals, all tissues burn glucose.
Excess carbohydrates are converted to glycogen in the liver and muscle. The body can only store a limited amount of glycogen. This is not true for fat. Fat cells are distributed throughout the body. Whereas carbohydrate storage is carefully regulated, fat storage is not, allowing a high degree of expansion. After a large meal, a bit of glucose and amino acids are left over, some of which are used to make fatty acids. This is why the American population has an increasing incidence of obesity on diets high in carbohydrates and excessive in calories. The body’s capacity to make and store fat may have served a useful purpose in earlier times when humans faced the constant threat of famines and an energy reserve meant survival. But now with the sedentary lifestyles of most Americans, glycogen stores are rarely exhausted. The reality for too many people is that they burn carbohydrates and store fat.
Dietary fat also has less of a thermogenic effect than does carbohydrate or protein and can thus be easily stored as adipose tissue. The thermogenic effect of food (TEF) represents the amount of energy required by the body to digest, absorb, metabolize, and store nutrients. The body expends only 3 calories of energy to process 100 calories from fat compared with 25 calories to process 100 calories from carbohydrates. Between 7% and 10% of total body energy goes to support the TEF. For example, after a meal containing 1000 calories, the body uses 70 to 100 calories just to process the meal. The TEF helps to explain why studies consistently show that when adults and children overeat and consume high amounts of dietary fat, they tend to gain weight. Conversely, when the intake of dietary fat is low and the intake of carbohydrate and fiber is high from nutrient-dense foods, desirable body weight is more readily achieved. Cross cultural comparisons confirm the dietary fat-obesity relationship. For example, Americans eat more than twice as much fat as the Chinese, yet the Chinese consume 20% more calories and there is little obesity in China.
Because the energy in carbohydrate stores (glycogen) is very small in relation to that in fat stores, the glycogen can be depleted quickly and frequently. The glucose requirement of the brain, other cells of the nervous system, and a few other tissues provides a stimulus for carbohydrate ingestion. The higher the proportion of fat in the diet, the more total food must be consumed to obtain enough carbohydrates to fill glycogen stores in the muscle and liver.
The high-fat theory of obesity leads to an important question: can a person lose weight by simply focusing on fat calories? In highly controlled studies in which food intake is carefully monitored, researchers found that subjects whose diets contained no more than 20% fat calories lost weight even when there were no restrictions on how much food was eaten. But was the weight loss the result of eating less fat or less calories? The answer seems to be that both fat and calories matter. In controlled studies, when people concentrate on eating less fat they also
Dietary fat promotes fat storage in the body. The body expends only 3 calories to convert an extra 100 calories of dietary fat to storage fat. Fat storage is accelerated if dietary fat is combined with simple sugar in the same meal, such as a hamburger with a sugared cola drink. Sugar triggers the release of extra insulin. Insulin activates fat cell enzymes that promote movement of fat from the blood to the fat cells. On the other hand, the ingestion of an extra 100 calories of complex carbohydrates requires that the body expend 25 calories to convert it to fat for storage. The conversion process for carbohydrates is more costly than that for fats and provides another reason for reducing fat consumption and increasing carbohydrate consumption.
consume fewer calories and lose weight. This is due to the consumption of mainly nutrient dense foods. Does the same thing happen in real life? Apparently not. Studies of the eating habits of both men and women who consume low-fat products reveal that they often compensate by eating more snack and dessert foods high in sugar but low-fat so the use of lowfat foods becomes an excuse to eat more. The idea that people can eat as much as they want as long as it’s fat free appears to be more fantasy than fact. Although some researchers believe that fat is the main culprit associated with obesity; other experts emphasize that calories count. This helps to explain why Americans are getting heavier with each passing year, even though they are consuming a lower proportion of fat. Increased consumption of low-fat foods by itself does not result in weight loss. Whereas eating low-fat foods is good for health and weight control, eating them in buffet-sized portions isn’t.
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