© The McGraw-Hill Companies, 2001
The U.S. National Institutes of Health considers obesity a "killer disease," and for good reason. A person who is obese — defined as 20% above "ideal" weight based on population statistics considering age, sex, and build — is at higher risk for diabetes, digestive disorders, heart disease, kidney failure, hypertension, stroke, and cancers of the female reproductive organs and the gallbladder. The body is enormously strained to support the extra weight—miles of blood vessels are needed to nourish the additional pounds. In the United States, obesity is the second leading cause of preventable death, following cigarette smoking.
Obesity refers specifically to extra pounds of fat. The proportion of fat in a human body ranges from 5% to more than 50%, with "normal" for males falling between 12% and 23% and for females between 16% and 28%. An elite athlete may have a body fat level as low as 4%. Fat distribution also affects health. Excess poundage above the waist is linked to increased risk of heart disease, diabetes, hypertension, and lipid disorders. Figure 18A shows how to estimate obesity using a measurement called body mass index.
Both heredity and the environment contribute to obesity. We inherit genes that control metabolism, but the fact that identical twins reared in different households can grow into adults of vastly different weights indicates that environment influences weight too. Studies comparing body mass index between adult identical twins reared apart indicate that weight is about 70% influenced by genes and 30% by the environment. Even the environment before birth can affect body weight later. Individuals who were born at full term, but undernourished as fetuses, are at high risk of obesity, beginning in childhood. Physiological changes that countered starvation in the uterus cause obesity when they persist.
A safe goal for weight loss using dietary restriction and exercise is 1 pound of fat per week. A pound of fat contains 3,500 calories of energy, so that pound can be shed by an appropriate combination of calorie cutting and exercise. This might mean eating 500 calories less per day or exercising off 500 calories each day. Actually more than a pound of weight will drop because water is lost as well as fat.
Dieting should apply to the energy-providing nutrients (carbohydrates, proteins, and fats) but never to the vitamins and minerals. A rule of thumb is to leave the proportion of pro tein calories about the same or slightly increased, cut fat calories in half, and cut carbohydrates by a third. Choose foods that you like and distribute them into three or four balanced meals of 250 to 500 calories each.
Ideally, weight loss can be accomplished by changing diet and exercise habits. However, realistically two-thirds of those who lose weight regain it within five years. Physicians are increasingly regarding obesity as a chronic illness that may require more drastic measures than dieting and exercising.
Physicians recommend drug therapy if the BMI exceeds 30 or if it exceeds 27 and the person also has hypertension, diabetes mellitus, or hyper-lipidemia. Several types of "diet drugs" are no longer in use, including amphetamines, which carried the risk of addiction, and the combination of fenfluramine and phenter-mine, which shed weight but damaged heart valves.
Newer anti-obesity drugs target fat in diverse ways. Tetrahydrolipo-statin, marketed as Orlistat and Xeni-cal, inhibits the function of pancreatic lipase, preventing the digestion and absorption of about a third of dietary fat. The fat is eliminated, causing loose feces. This effect is not disruptive as long as the person follows a low-fat diet. The unpleasantness of the loose feces tends to keep users on the diet!
Another approach to treating obesity is developing drugs that
block the proteins (adipocyte transcription factors) that enable fibroblasts to specialize into fat cells (adipocytes). Yet other experimental strategies alter the homeostatic control of body weight by stimulating production of cholecystokinin or lep-tin (so-called satiety factors) or by blocking the appetite stimulant neuropeptide Y.
that were performed in the 1980s, with only limited success. Patients undergoing early stomach stapling tended to eat too much and burst the staples! More extensive, earlier intestinal bypasses led to malnutrition and liver failure. The combination surgery, however, performed on more than 40,000 people a year in the United States, has led to weight losses of more than 100 pounds, and disappearance of several obesity-associated illnesses in some patients, including hypertension, arthritis, back pain, varicose veins, sleep apnea, and type II diabetes mellitus. ■
Surgery is recommended for people whose BMI exceeds 40 or if it exceeds 35 and medical problems are present. Such procedures are called "bariatric surgery," from the Greek for "weight treatment." Today the most common type of bariatric surgery staples off a portion of the stomach so that it can only hold two tablespoons of food at a time, and bypassing part of the small intestine. This gastric bypass is of two types — 2.5 to 5 feet of duodenum, or 12 to 15 feet, which encompasses part of the jejunum too. Because of the shortened alimentary canal, the individual must drastically lower food intake. Overeating leads to "dumping syndrome," a very unpleasant combination of weakness, nausea, sweating and faintness.
Bariatric surgery targeting the stomach and the small intestine evolved from separate procedures
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.