Medical brain notes 44

  Energy Intake and Output Are Balanced Under Steady- State Conditions Intake of carbohydrates, fats, and proteins provides energy that can be used to perform various body functions or stored for later use. Stability of body weight and composition over long periods requires that a person’s energy intake and energy expenditure be balanced. When a person is overfed and energy intake persistently exceeds expenditure, most of the excess energy is stored as fat, and body weight increases; conversely, loss of body mass and starvation occur when energy intake is insufficient to meet the body’s metabolic needs.             Because different foods contain different proportions of proteins, carbohy-drates, fats, minerals, and vitamins, appropriate balances must also be main-tained among these constituents so that all segments of the body’s metabolic systems can be supplied with the requisite materials.

  Dietary Balances Energy Available in Foods The energy liberated from each gram of carbohydrate as it is oxidized to carbon dioxide and water is 4.1 Calories (1 Calorie equals 1 kilocalorie), and that liberated from fat is 9.3 Calories. The energy liberated from metabolism of the average dietary protein as each gram is oxidized to carbon dioxide, water, and urea is 4.35 Calories. Also, these substances vary in the average percentages that are absorbed from the gastrointestinal tract: about 98 per cent of carbohydrate, 95 per cent of fat, and 92 per cent of protein. Therefore, the average physiologically available energy  in each gram of these three foodstuffs is as follows:         Average Americans receive about 15 per cent of their energy from protein, 40 per cent from fat, and 45 per cent from carbohydrate. In most non-Western coun-tries, the quantity of energy derived from carbohydrates far exceeds that derived from both proteins and fat...

  Methods for Determining Metabolic Utilization of Proteins, Carbohydrates, and Fats Nitrogen Excretion Can Be Used to Assess Protein Metabolism. The average protein contains about 16 per cent nitro-gen. During metabolism of the protein, about 90 per cent of this nitrogen is excreted in the urine in the form of urea, uric acid, creatinine, and other less important nitrogen products. The remaining 10 per cent is excreted in the feces. Therefore, the rate of protein breakdown in the body can be estimated by measuring the amount of nitrogen in the urine, then adding 10 per cent for the nitrogen excreted in the feces, and multiplying by 6.25 (i.e., 100/16) to determine the total amount of protein metabolism in grams per day. Thus, excretion of 8 grams of nitrogen in the urine each day means that there has been about 55 grams of protein breakdown. If the daily intake of protein is less than the daily breakdown of protein, the person is said to have a  negative nitrogenbalance,...

  Regulation of Food Intake and Energy Storage Stability of the body’s total mass and composition over long periods requires that energy intake match energy expenditure. Only about 27 per cent of the energy ingested normally reaches the functional systems of the cells, and much of this is eventually converted to heat, which is generated as a result of protein metabolism, muscle activity, and activities of the various organs and tissues of the body. Excess energy intake is stored mainly as fat, whereas a deficit of energy intake causes loss of total body mass until energy expenditure eventually equals energy intake or death occurs.         Although there is considerable variability in the amount of energy storage (i.e., fat mass) in different individuals, maintenance of an adequate energy supply is necessary for survival. Therefore, the body is endowed with powerful physiologic control systems that help maintain adequate energy intake. Deficit...

  Neural Centers Regulate Food Intake The sensation of  hunger  is associated with a craving for food and several other several physiologic effects, such as rhythmical contractions of the stomach and restlessness, which cause the person to search for an adequate food supply. A person’s  appetite  is a desire for food, often of a particular type, and is useful in helping to choose the quality of the food to be eaten. If the quest for food is successful, the feeling of  satiety  occurs. Each of these feelings is influenced by envi-ronmental and cultural factors, as well as by physio-logic controls that influence specific centers of the brain, especially the hypothalamus. The   Hypothalamus Contains Hunger and Satiety  Centers. Several neuronal centers of the hypothalamus partici-pate in the control of food intake. The  lateral nuclei ofthe hypothalamus serve as a feeding center,  and stimu-lation of this area causes an animal to eat v...

  Factors That Regulate Quantity of Food Intake Regulation of the quantity of food intake can be divided into  short-term regulation,  which is concerned primarily with preventing overeating at each meal, and  long-term regulation,  which is concerned primarilywith maintenance of normal quantities of energy stores in the body. Short-Term Regulation of Food Intake When a person is driven by hunger to eat voraciously and rapidly, what turns off the eating when he or she has eaten enough? There has not been enough time for changes in the body’s energy stores to occur, and it takes hours for enough nutritional factors to be absorbed into the blood to cause the necessary inhibi-tion of eating. Yet it is important that the person not overeat and that he or she eat an amount of food that approximates nutritional needs. The following are several types of rapid feedback signals that are impor-tant for these purposes. Gastrointestinal Filling Inhibits Feeding.  When ...

  Obesity Obesity can be defined as an excess of body fat. A sur-rogate marker for body fat content is the body mass index (BMI), which is calculated as: BMI = Weight in kg/Height m 2   In clinical terms, a BMI between 25 and 29.9 kg/m 2  is called overweight, and a BMI greater than 30 kg/m 2  is called obese. BMI is not a direct estimate of adiposity and does not take into account the fact that some indi-viduals have a high BMI due to a large muscle mass. A better way to define obesity is to actually measure the percentage of total body fat. Obesity is usually defined as 25 per cent or greater total body fat in men and 35 per cent or greater in women. Although percentage of body fat can be estimated with various methods, such as measuring skin-fold thickness, bioelectrical impedance, or underwater weighing, these methods are rarely used in clinical practice, where BMI is commonly used to assess obesity. The prevalence of obesity in children and adults in the United ...

  Decreased Physical Activity and Abnormal Feeding Regulation as Causes of Obesity The causes of obesity are complex. Although genes play an important role in determining food intake and energy metabolism, lifestyle and environmental factors may play the dominant role in many obese people. The rapid increase in the prevalence of obesity in the past 20 to 30 years emphasizes the important role of lifestyle and environmental factors, because genetic changes could not have occurred so rapidly. Sedentary Lifestyle Is a Major Cause of Obesity.  Regular phys-ical activity and physical training are known to increase muscle mass and decrease body fat mass, whereas inadequate physical activity is typically associated with decreased muscle mass and increased adiposity. For example, studies have shown a close association between sedentary behaviors, such as prolonged televi-sion watching, and obesity. About 25 to 30 per cent of the energy used each day by the average person goes into mus...

  Treatment of Obesity Treatment of obesity depends on decreasing energy input below energy expenditure and creating a sus-tained negative energy balance until the desired weight loss is achieved. In other words, this means either reduc-ing energy intake or increasing energy expenditure. The current National Institutes of Health (NIH) guidelines recommend a decrease in caloric intake of 500 kilo-calories per day for overweight and moderately obese persons (BMI greater than 25 but less than 35 kg/m 2 ) to achieve a weight loss of approximately 1 pound each week. A more aggressive energy deficit of 500 to 1000 kilocalories per day is recommended for persons with BMIs greater than 35 kg/m 2 . Typically, such an energy deficit, if it can be achieved and sustained, will cause a weight loss of about 1 to 2 pounds per week, or about a 10 per cent weight loss after 6 months. For most people attempting to lose weight, increasing physical activity is also an important component of successful...