Causes of Obesity

Many adults achieve an energy balance in which caloric intake matches energy expenditure. Body fat does not change very much under these conditions. 

Excessive body fat could be related to eating more calories or to small energy expenditure, or both. 

Energy expenditure refers to the calories spent for body functions, physical activity, digestion, and food metabolism. Both heredity and the environment play a part in obesity and, therefore, there is no single approach to treatment. 

Overeating, differences in metabolism, AGING, genetic predisposition, and excessive food consumption during early childhood have been implicated.

Overeating Clearly the prolonged consumption of excessive calories, when energy intake exceeds energy expenditure, leads to obesity. 

Energy expenditure refers to the calories spent for body functions, physical activity, digestion, and food metabolism. Body fat can be reduced only when energy expenditure exceeds caloric intake. 

The body adapts to excessive food consumption— whether excessive PROTEIN, CARBOHYDRATE, fat, or ALCOHOL—by storing the surplus calories as body fat. Many reports have suggested that obese people eat the same, or sometimes less than nonobese people. 

Using new research methods based on ingesting double-labeled water, that is, water containing a “heavy” form of oxygen (O18) and “heavy” hydrogen (deuterium), investigators have demonstrated that, on the average, obese people generally eat more, but they habitually underreport their food consumption.

Differences in Energy Expenditure

Although obese people are generally less active than nonobese people, they tend to use the same amounts of energy because they weigh more. 

Sedentary lifestylescontribute to obesity. 

About 70 percent of adult Americans fail to exercise 20 minutes or more three times a week as recommended. Most people will lose weight if such an exercise program is coupled with consuming no more than 1,500 calories daily.

 Individuals who exercise regularly, or who exercise before and after a highcalorie meal, lose more energy as heat after eating than those who do not exercise. Differences in Metabolism This picture is unclear. Very rarely do glandular imbalances lead to obesity. 

Cushing’s syndrome, excessive production of glucocorticoids, a form of adrenal hormone, is an example of hormone imbalance that can promote obesity.

 Obese people do not have unusually slow metabolisms. When resting metabolic rates are compared based upon the muscle/bone mass, there is not a significant difference between metabolic rates of nonobese and obese individuals.

 Formerly obese individuals preferentially store fat rather than burn it, and studies suggest that overweight and obese people tend to eat more fat and less carbohydrate. 

In general, the body consumes calories more slowly after weight is lost, and it burns calories more rapidly when weight is gained, for fat as well as for thin people. 

One hypothesis contends that people adjust their metabolism to maintain a “set point” weight. Thus someone who has lost significant amounts of fat (10 percent of their body weight) will burn fewer calories when exercising than someone who has maintained his or her weight without a weight-loss program.

Apparently, the body adjusts its metabolism by altering the efficiency of muscles in burning calories. 

Recently, a type of prostaglandin has been shown to act as a hormone to trigger the production of fat cells from immature cells. 

Aging In the United States, both men and women tend to become fatter with increasing age. This could be due to a decreased metabolic rate (a lower BASAL METABOLIC RATE) and a sedentary lifestyle coupled with an easy access to high-calorie food.

Meal Frequency

The frequency of meals and meal composition may be a factor in obesity. 

Eating fewer meals may increase fat deposition, while smaller, more frequent meals, with more food at breakfast and less at supper, may promote weight loss. 

TV Watching Excessive TV watching correlates with overeating. Reduced physical activity, lowered metabolic rates, as well as visual cues to eating high-fat snack foods and drinking alcoholic beverages, contribute to the increased prevalence of overweight. 

Dietary fat, which provides nine calories per gram, is more fattening than either protein or carbohydrate, which provide four calories per gram. 

Fat calories in food differ from calories in carbohydrate: Fat in food is more easily converted to body fat than is carbohydrate.

Inheritance One broad generalization can be made: Obesity persists over a life span. 

Fat children tend to become fat adults, suggesting a predisposition to being overweight. Early adulthood is an important period for the development of lifelong patterns. 

The question remains, to what degree is obesity the product of genetics? Studies with twins suggest that between 50 percent and 70 percent of the variability in relative body weight represents genetic variability. 

Current research focuses on locating specific obesity genes. Genes influence both metabolism and behavior. 

Many genes regulate hunger and satiety. A flurry of recent research has yielded impartial genetic discoveries: gene OB causes fat cells to produce a satiety protein called lepin. 

A gene then codes for the receptor of this hormone in the brain. 

Still another gene codes for a hormone-producing enzyme (carboxypeptidase E). 

A gene that codes for a neurotransmitter receptor (binding site; Beta3-adrenergic receptor) for norepinephrine is also implicated in maintaining weight. 

Mutations of these genes can increase the risk of obesity and diabetes in lab animals and possibly in people. There will be more to add to this unfinished story as more discoveries are made.