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Since obesity clearly is an important risk factor for diabetes, researchers are working on developing effective methods for helping heavy people lose weight safely and permanently, and for preventing obesity in people with a genetic inclination to gain weight. To better understand why certain people have a tendency to be overweight and how their bodies react to various diets, NIH researchers are measuring many aspects of energy expenditure using precise and highly sophisticated technology.
The most accurate methods of determining energy expenditure involve continuous measurements of heat output (direct calorimetry) or exhaled gas exchange (indirect calorimetry) in people confined to metabolic chambers. A metabolic chamber is a small room a person can live in for a 24 hour period, while metabolic rate is measured during meals, sleep, and light activities. Scientists measure the heat released from a person's body to determine how much energy each activity has burned for that person. In indirect calorimetry, researchers measure oxygen consumption, carbon dioxide production and nitrogen excretion to calculate a ratio that reflects energy expenditure.
Both measurements indicate whether a person has a "slow" or "fast" metabolic rate. A person with a lower metabolic rate burns calories slower and is at higher risk for gaining weight than people with higher metabolic rates. Above: staff prepare lunch for research volunteers.
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Since living in a chamber for a day or two restricts normal activity, researchers have developed methods to measure energy expenditure in free-living situations. They include monitoring heart rate and maintaining an activity diary. Using a person's activity diary, researchers can compute energy expenditure during various activities.
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A form of indirect calorimetry based on the elimination of deuterium and oxygen (18) from urine, the doubly labeled water technique measures the turnover of hydrogen and oxygen into water and carbon dioxide; energy expenditure is calculated from the difference. This method of determining energy expenditure is useful because it enables researchers to measure total carbon dioxide production over a long period of time—from five to 20 days—and yet only requires periodic sampling of urine. People being tested can continue their normal routines because the method does not require special arrangements or devices.
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Scientists have known for many years that there is a close correlation between the energy burned at rest (resting metabolic rate, or RMR) and body size. A low RMR is a risk factor for weight gain. In their search for the possible mechanisms underlying the variability in RMR among people, NIDDK researchers have explored the effects of gender, physical training, age, muscle metabolism, sympathetic nervous activity, and body temperature on RMR. In a large number of Caucasian volunteers, scientists found that females had a lower RMR than males because of the effect of sex hormones on metabolic rate. Other studies have demonstrated that diet, physical fitness and body temperature also influence RMR. Scientists also know that aging reduces RMR slightly.
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The Dexa Machine has replaced underwater weighing as the gold standard in measuring body composition because of its high degree of accuracy. It uses very low doses of radiation—about the amount a person would receive on a plane flight from Phoenix to Denver. A computer calculates the body's absorption of radiation and determines body density—the ratio of fat to muscle mass.
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The sympathetic nervous system, which controls the blood flow and blood pressure in skeletal muscle, plays a role in human obesity through its effect on energy expenditure and food intake. One method used to assess this effect is to measure fasting muscle sympathetic nerve activity in the peroneal nerve and its relationship to energy expenditure and body composition. Researchers have found that sympathetic nervous activity is related to a person's metabolic rate and that reduced sympathetic nervous activity indicates a tendency to gain weight. Pima Indians have reduced sympathetic nervous activity compared to Caucasians of similar age, body weight and body composition.
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Studies have shown that when overweight people are asked to record their daily food intake in a diary they tend to underestimate and record only two-thirds of what they actually ate. Researchers think this tendency to underestimate caloric intake may be part of the reason heavy people overeat. Therefore, NIDDK developed a special vending machine to accurately measure a person's food intake in a day. This computerized, automated food selection device calculates the person's caloric and fat intake and meal duration. Items in the vending machine include entrees such as chili, spaghetti and meat balls, chicken chimichangas, cheese pizza and macaroni and cheese, and snack items, including peanuts, sunflower seeds and french fries. Study volunteers may select foods of their choice but they are required to eat only from the vending machine for a set period of time.
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The hyperinsulinemia euglycemic clamp is used to measure a person's insulin sensitivity. People with low insulin sensitivity are considered insulin resistant and are more likely to develop diabetes. People with a high degree of insulin sensitivity are less likely to develop diabetes.
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