Obesity101.com THE LATEST IN OBESITY RESEARCH AND WEIGHTLOSS DRUG DEVELOPMENT Join our list More information What is obesity? Obesity occurs when we eat more calories than we expend. But this simple statement belies the complex chemistry of the human body. Genetic, metabolic and environmental factors all play a role in energy regulation. And these differences explain why some people gain weight eating almost nothing while others eat constantly and never gain an ounce. While the cause of some obesity has been defined, as in the case of Prader-Willi Syndrome, or in small groups of people found to have a specific genetic mutation, such as leptin deficiency, most obesity is much more complex. Over the past 10 years scientists have uncovered many genes that contribute to feeding behavior and energy regulation, and current research indicates that defects in a number of brain and gut peptides play a role in weight. Researchers believe that most obesity is caused by small differences in food intake. As was found in the Framingham Heart Study, most adults will gain 20 pounds between 25 and 55 if no steps are taken to avoid weight gain. But that 20 pound gain represents a very small calorie excess, perhaps a tenth of a percent per year. Obesity and genetics. Twin and adoptee studies indicate that there is a strong genetic basis for obesity. Studies on large populations of adopted children show that there is no relationship between body weight and adoptive parents, but a close correlation with biological parents. Studies on monozygotic (identical) twins show a much stronger correlation in body weight than between other siblings or dizygotic (fraternal) twins. Researchers vary in their opinion on the weight genetics plays in energy regulation, but genetic factors can account for as much as 80 percent. There are many genes which play a part in weight regulation. Peptides in the brain's hypothalamus and the gut send signals to increase or decrease hunger, depending on energy stores in the body. Brain and gut peptides either increase appetite through the alpha-adrenergic system, or decrease it through the beta-adrenergic system. Leptin and cholecystokinin (CCK) are two regulators of appetite in the brain and gut respectively. Opoids in the brain and insulin in the gut, increase appetite through the alpha-adrenergic system. Only a small number of brain and gut peptides have been identified at present, and new neurotransmitters are discovered frequently and reported in medical journals. Metabolism and obesity. Exercise accounts for 20 to 30 percent of metabolism, digesting food 5 to 10 percent. But differences in a person's resting metabolic expenditure (RME), the amount of energy expended while at rest, accounts for 60 to 75 percent of total energy. RME can vary as much as 20 percent between individuals of the same sex, age and body type, accounting for up to 400 calories per day. Conditions like Cushing's Syndrome and hypothyroidism can also decrease RME, but most obesity is not tied to these diseases. Because RME is such a great portion of energy expenditure, researchers have investigated what increases or decreases it. Calorie restriction and metabolism. Very low calorie diets (VLCD) lower resting metabolism, at least temporarily, and studies on very low calorie (VLC) diets versus low calorie or balanced diets show similar long term weight maintenance. Research has shown that morbidly obese patients develop an energy saving adaption during rapid weight loss, and weight loss with VLC diets can level off quickly. Obesity reduces the secretion of growth hormone in the body, and calorie restriction increases growth hormone. But at least one study on VLC diets in obese subjects showed that severe calorie restriction did not stimulate growth hormone, and suggests that VLC diets in very obese people promotes the retention of body fat and perpetuates obesity. The retention of fat by obese people may be explained by the "thrifty gene theory" proposed in 1962 by geneticist James Neel. According to Neel's theory, populations exposed to alternating periods of feast and famine, developed these "thrifty genes" to store fat in times of plenty so that they would survive times of famine. This feast or famine existence was particularly descriptive of North American Pima Indians, but the theory is being extrapolated to explain high rates of obesity, diabetes and coronary heart disease in other populations as well. Exercise and metabolism. Dietary restriction decreases metabolism, and weight loss reduces the number of calories expended during physical activity. But patients who exercise vigorously are more likely to keep off all or most of the weight they lose. This is why doctors recommend exercise as a mainstay of weight loss and weight maintenance programs. Strength training prevents the loss or increases muscle mass while dieting, and both resistance and aerobic activity increase resting metabolic expenditure. Exercise also helps improve insulin sensitivity, cardiovascular function and lipids. The American College of Sports Medicine recommends an energy expenditure of at least 300 calories a day. Exercise can also improve metabolism in patients on VLC diets. In a 1995 study comparing activity level and weight maintenance among patients on a VLC diet, researchers found that patients with a high level of activity regained substantially less weight than less active patients. Dietary fat and metabolism. Whether or not fat in the diet changes metabolism and body composition is a hotly contested issue. Rodent studies have consistently shown that diets high in fat-derived energy result in obesity, increased body fat content and hyperphagia. But research on dietary fat and human obesity is inconclusive. Some studies indicate that high-fat diets cause obesity, but rural populations eating a Mediterranean Diet, which can be up to 40 percent fat, are not obese, have no cardiovascular disease and very low cancer rates. Environmental factors. Undoubtedly the modern world has added to the prevalence of obesity. In the 1940s a single serving of Coca-cola was 6 oz as opposed to 12 oz today. And the serving size of all meals has increased at the same time that energy expended in daily activities has decreased. It takes genes thousands of years to mutate, but the human environment has changed drastically over just the last two hundred years. Cars, modern appliances, and TV watching all result in less activity. The change from an agrarian economy to a service economy in developed countries also adds to the sedentary lifestyle. 200 years ago exercise wasn't necessary to maintain body weight, just the activities of every day existence expended enough energy to keep most people trim. Obesity was only commonly found among the privileged, and until the late 1900s being plump was considered attractive. Poor diet and periods of famine prevented most women from having much fat on their bodies, and some skinny women were infertile. The adipose woman was a sign of fertility and sensuality, a fact well documented in art. A meta analysis of twin and adoptee studies found that the familial correlation of obesity and genetics was between 20 and 80 percent. Twin studies showed a 50 to 90 percent correlation, adoption studies 20 to 60 percent. A recent adoptee study showed that genetic influence on BMI was unaffected by various environmental conditions associated with obesity. It may seem from these studies that it is pointless to try and change environment in the face of such overwhelming genetic odds. But it is clear from history that environment does play a role in obesity, and that certain groups, like the Pima Indians, who no longer live a traditional lifestyle become obese as a result. One study suggests that the reason for this "apparent inconsistency may be explained by the dual phenomena of the near-universality of access to environments that facilitate reductions in energy expenditure (e.g., TV as a recreational pastime), together with heritable individual differences in the response or utilization of these environments." In other words, we eat more exercise less and our genes haven't caught up yet. Home|Subscribe|Archives|Drug info|Contact|Media & Links|Site index|FAQs|Doctors/meetings|Admin Obesity-news is a publication of Hirsch Communications. 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