Energy Balance and Food Intake - Physiology Lecture Notes PDF

Energy balance and control of food intake Mona A Hussain Assistant Prof. In Physiology Department FOM-PSU  Introduction Each cell in the body needs energy to: 1- perform the functions essential for the cell’s own surviv...

Energy balance and control of food intake Mona A Hussain Assistant Prof. In Physiology Department FOM-PSU  Introduction Each cell in the body needs energy to: 1- perform the functions essential for the cell’s own survival (such as active transport and cellular repair) and to 2- carry out its specialized function(such as gland secretion or muscle contraction). All energy used by cells is ultimately provided by food intake.  Introduction According to the first law of thermodynamics, energy can be neither created nor destroyed. but it can be converted from one form to another. ENERGY INPUT : The energy in ingested food constitutes energy input to the body. Chemical energy locked in the bonds that hold the atoms together in nutrient molecules is released when these molecules are broken down in the body. Cells capture a portion of this nutrient energy in the high-energy phosphate bonds of ATP. ENERGY OUTPUT: Energy is either used immediately to perform biological work or stored in the body for later use as needed during periods when food is not being digested and absorbed. Most food energy is ultimately converted into heat in the body.  Energy output Energy output or expenditure by the body falls into two categories: external work and internal work. External work is the energy expended when skeletal muscles are contracted to move external objects or to move the body. Internal work constitutes all other forms of biological energy expenditure that do not accomplish mechanical work outside the body. Internal work encompasses two types of energy-dependent activities: (1) skeletal muscle activity used for purposes other than external work, such as the contractions associated with postural maintenance and shivering; and (2) all the energy-expending activities that must go on all the time just to sustain life. The latter include the work of pumping blood and breathing; the energy required for active transport of critical materials across plasma membranes; and the energy used during synthetic reactions essential for the maintenance, repair, and growth of cellular structures. Not more than 25% of nutrient energy is available for work, either external or internal. The remaining 75% is lost as heat Energy input and output  The metabolic rate The rate at which energy is expended by the body during both external and internal work is known as the metabolic rate: Metabolic rate = energy expenditure/unit of time Because most of the body’s energy expenditure eventually appears as heat, the metabolic rate is normally expressed in terms of the rate of heat production in kilocalories per hour. The basic unit of heat energy is the calorie, which is the amount of heat required to raise the temperature of 1 g of H2O by 1°C. This unit is too small when discussing the human body because of the magnitude of heat involved, so the kilocalorie or Calorie, which is equivalent to 1000 calories, is used  CONDITIONS FOR MEASURING THE BASAL METABOLIC RATE The metabolic rate and, consequently, the amount of heat produced vary depending on a variety of factors, such as exercise, anxiety, shivering, and food intake. Increased skeletal muscle activity is the factor that can increase metabolic rate to the greatest extent. Even slight increases in muscle tone notably elevate the metabolic rate. For this reason, a person’s metabolic rate is determined under standardized basal conditions established to control as many as possible of the variables that can alter metabolic rate. In this way, the metabolic activity necessary to maintain the basic body functions at rest can be determined. The so-called basal metabolic rate (BMR)  BMR The BMR is measured under the following specified conditions: 1. The person should be at physical rest, no exercise for at least 30 minutes to eliminate any contribution of muscular exertion to heat production. 2. The person should be at mental rest to minimize skeletal muscle tone (people “tense up” when they are nervous) and to prevent a rise in epinephrine, a hormone secreted in response to stress that increases metabolic rate. 3. The measurement should be performed at a comfortable room temperature so the person does not shiver. Shivering can markedly increase heat production. 4. The subject should not have eaten any food within 12 hours before the BMR determination to avoid diet-induced thermogenesis  Energy input must equal energy output to maintain a neutral energy balance. Because energy cannot be created or destroyed, energy input must equal energy output, as follows: 1- Neutral energy balance (input equal external and internal energy consumption – constant weight) 2- Positive energy balance (input > external and internal energy consumption – increase weight) 3- Negative energy balance (input < external and internal energy consumption – decrease weight) NB***Regulation of food intake is the most important factor in the long-term maintenance of energy balance and body weight.  Neutral energy balance Theoretically, total body energy content could be maintained at a constant level by: 1- regulating the magnitude of food intake, 2- physical activity, 3- internal work and heat production.  Neutral energy balance NB**Control of food intake to match changing metabolic expenditures is the major means of maintaining a neutral energy balance. NB**The level of physical activity is principally under voluntary control. NB**Mechanisms that alter the degree of internal work and heat production are aimed primarily at regulating body temperature rather than total energy balance. However, after several weeks of eating less or more than the body’s desired amount, small counteracting changes in metabolism may occur. For example, a compensatory increase in the body’s efficiency of energy use in response to underfeeding. Similarly, a compensatory reduction in the efficiency of energy use in response to overfeeding.  Food intake is controlled primarily by the hypothalamus. regulation of food intake is the most important factor in the long- term maintenance of energy balance and body weight. No calorie receptors for energy input or output or content Short-term regulation of food intake, helping to control meal size and frequency (Ghrelin -Hunger hormone- PYY (meal-terminator) LONG-TERM MAINTENANCE OF ENERGY BALANCE (Leptin and insulin)  Obesity Obesity is defined as excessive fat content in the adipose tissue stores; Obesity occurs when, over a period of time: 1- more kilocalories are ingested in food than are used to support the body’s energy needs, and 2- the excessive energy being stored as triglycerides in adipose tissue  Obesity The causes of obesity are many, and some remain obscure. Some factors that may be involved include the following: 1- Disturbances in the leptin-signaling pathway 2- Lack of exercise 3- Differences in extracting energy from food 4- Hereditary tendencies 5- Development of an excessive number of fat cells as a result of overfeeding. 6- Endocrine disorders (Hypothyroidism) 7- An abundance of convenient, highly palatable, energy-dense, relatively inexpensive foods 8- Emotional disturbances 9- Stress increases NPY release by sympathetic nerves 10 - Little sleep (increase ghrelin and decrease leptin) 11- A possible virus link (Cold virus Adenovirus 36 ) 12- Colonic bacteria