Energy Metabolism PDF

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ENERGY METABOLISM DEFINITION OF KEY TERMS IN ENERGY METABOLISM Adenosine tri-phosphate (ATP) This is a high energy compound involved in the transfer of energy within the cell. Basal energy Expenditure (BEE) The measurement of basal metabolic rate; usually expressed as kilocalories per 24 hours (Kcal/24hrs) or (kcal/day) Basal Metabolic Rare (BMR) The energy needed to sustain the metabolic activities of cells and to maintain the circulatory, respiratory gastrointestinal and renal process. Basal Metabolism The amount of energy required to carry on vital processes when the body is at rest. Caloric Density It is the number of calories in a unit of weight of a specific food Energy Metabolism It is all the chemical changes that result in the release of energy in the body Energy regulation It is the process by which energy intake and energy expenditure are balanced in the body Hyperthyroidism It is the excessive secretion of thyroid gland resulting in an increased basal metabolic rates. Hypothyroidism This is the deficiency of thyroid secretion resulting in a lowered basal metabolic rate Krebs cycle (Tri-carboxylic acid cycle) This is a set of complex chemical reactions within the cell that produces energy through the oxidation of carbohydrates, proteins and fat. Nutrient Density This is the nutritional content of a food in relation to its caloric value usually determined in terms of specific nutrients. Catabolism

This is the breakdown of complex molecules into simplex ones with the release of energy Anabolism This the process through which the body forms new tissue INTRODUCTION TO ENERGY METABOLISM Energy metabolism is the general term for all the chemical processes carried out by the cells of the body, chief among these processes is the oxidation (combustion or burning) of food which produces energy. This process is analogous tocar engine burning petrol to produce the energy that makes it run. In most forms of combustion, be it in the car or in the human, heat is produced as well as energy. Energy is defined as the capacity to do work. The ultimate source of all energy in living things (plants) is the sun, through the process of photosynthesis. Humans obtain energy by consuming plants and flesh of animals. The energy provided by macronutrients is locked in chemical bonds within the food and is only released when food is metabolized. Energy must be supplied regularly to meet the body’s need for survival. All forms of energy can be converted into heat energy. It is possible to measure the heat produced burning a litre of petrol, for example. Food energy can also be measured and is expressed as heat energy. The unit of measurement used has been largely calorie (cal) or kilocalorie (kcal) which is 1000 times the small calorie used in physics, but this measure is increasingly being replaced by the joule (J) or kilo joule (KJ). The Kilocalorie is defined as the heat necessary to raise the temperature of 1 litre of water from 14.5 to 15.5 degrees centigrade. Whereas the kilocalorie is a unit of heat, the joule is truly a unit of energy. The human body requires energy for all body functions, including work, the maintenance of body temperature and the continuous action of the heart and lungs. In children, energy is essential for growth. Energy is also needed for breakdown, repair and building of tissues. These are metabolic processes. The rate at which these functions are carried out while the body is at rest is the basal metabolic rate (BMR). BASAL METABOLIC RATE BMR for an individual is usually defined as the amount of energy expressed in kilocalories or Mega joules (MJ per day) expressed when the person is at complete rest both physical (i.e. lying down) and psychological. It can also be expresses as kilocalories per hour or per kilogram of weight. BMR provides the energy required by the body for maintenance of temperature; for the work of the body organs such as the beating of heartand the muscles working for normal, at rest, breathing and for functioning of other organs such the liver, kidneys and brains. BMR varies from individual to individual and the important general factors influencing BMR are the person’s weight, age and state of health. BMR is also influenced by the person’s body composition for example the amount of muscle and adipose tissues and therefore the amounts of

proteins and fat in the body. In broad terms, bigger people with more muscles and larger body organ have a higher BMR than smaller people. Elderly people tend to have lower BMR than they had when they were young and females tend to have lower BMR than males even on a per kilogram body weight basis. There are exceptions, however to all these generalizations. Energy requirements According to “WHO” energy requirement of an individual is the level of energy intake from food that will balance energy expenditure when the individual has a body size, body composition, and level of physical activity, consistent with long term good health, and that will allow for the maintenance of economically necessary and society desirable activity. The body has a unique ability to shift the fuel mixture of carbohydrates, proteins and fats to meet energy needs. Consumption of too little or too much energy over time may results in the body weight changes. Body weight is a reflection of adequacy or inadequacy of energy, but is not a reliable indicator of macronutrient or micronutrient adequacy. CONVERTING FOOD TO ENERGY Energy metabolism refers to the chemical changes that results in the release of energy (how the body gets and uses energy from macronutrients after absorption). The light energy of the sun is converted to chemical energy in food. The body has the ability to transform food energy from one form to another. The chemical energy of food can be converted to mechanical energy, electrical heat energy and other forms of chemical energy from the body. The carbohydrates, fats and proteins in food are the nutrients that yield energy. Alcohol derived from glucose also supplies energy. Vitamins, minerals and water do not supply energy. Energy metabolism is a catabolic process. Normally catabolism and anabolism are in balance, except in periods of childhood, pregnancy, lactation and convalescence when anabolism is higher than catabolism. Both processes take place simultaneously. The energy that results from breakdown of nutrients in the body is either converted to heat, which maintains the body temperature or temporarily held by a phosphorus compound, adenosine triphosphate (ATP),to be released as needed. ATP provides energy for all work performed by the cell. FORMS OF ENERGY 1. The energy from ATP Some of this is converted to various forms of energy for different uses by the body. They include: a) Potential Energy: This is energy which can be changed to other forms to accomplish the work of the body e.g. mechanical energy for muscle contraction. This energy is

continuously available in the body from small amounts of glycogen in the muscle and liver and the sizable fat deposits and cellular mass itself. b) Osmotic energy: This is the energy used to maintain transport of fluid in the body. c) Electrical energy: The energy is used in the transmission of nerve impulses d) Chemical energy: This energy is used in thesynthesis of new compounds. e) Thermal energy: The energy needed for the regulation of heat in the body. 2. Energy from food The approximate amounts of energy from different foods are; Carbohydrate

which provides 4Kcal/g

Protein

provides 4Kcal/g

Fat

provides 9Kcal/g

Alcohol (produced from fermentation of glucose) provides 7Kcal/g has more than twice the calorie value of protein and carbohydrates. The energy value of a food can be estimated if the composition of the food is known since the value depends on the proteins, carbohydrates and fat content the food contains. For example: a) ½ Cup of bakes macaroni and cheese contains 8g protein, 11g fat and 20g carbohydrates. Therefore energy content of this food will be: Protein

8g x 4 = 32

Fat

11g x 9 = 99

Carbohydrates

20g x 4 = 80

Total Calories

211 Kcal

b) 1 slice of bread with 1 tablespoon peanut butter contains 16gcarbohydrates, 7g protein and 9g fat Therefore the energy content will be: Carbohydrates

16g x 4 = 64

Proteins

7g x 4 = 28

Fat

9g x 9 = 81

Total calories

173 Kcal

FACTORS INFLUENCING ENERGY NEEDS Total energy needs are based on three factors: these are also known as components of energy expenditure. 1. Basal Metabolism (BM) or resting energy expenditure (REE) 2. Voluntary physical activity 3. The thermic effect of food; - increment in energy expenditure above resting metabolic rate (RMR) due to the cost of processing food for storage and use.

1. BASAL METABOLISM Basal metabolism is the energy required to carry out vital body processes at rest. These include:  All activities of the cells and glands  Skeletal muscle tone  Body temperature  Circulation and respiration In active people, basal metabolic needs make up the largest part (2/3) of the total energy needs. The basal metabolic rate (BMR) is the rate at which a person requires energy for the basic body processes. The adult basal metabolic rate is approximately 1 Calorie per kilogram body weight per hour. Men weighing 70kg will require 1680 Kcal per day, arrived at by (70 kg x 1 x 24hrs) (one day) = 1680 Kcal. The normal range of metabolism for normal adult is about 1300 – 1700. This accounts for the largest proportion of the total energy requirement for most people. Factors influencing Basal Metabolism (BMR) a) Age and Growth

Infants and children need more energy for growth and activity than adults. In older persons, the need for energy is sometimes reduced because there is decline in activity and because then BMR is usually lower. b) Body Composition and Surface area Larger people have higher BMR than smaller people but tall, thin people have a higher BMR than short wide people. A tall slender person has more surface area and a higher BMR than a shorter person of the same weight. BMR is also higher in people with morelean mass and little body fat. The more the fat tissue, the lower the BMR and vice versa. The grater the skin area, the greater the amount of heat lost and the higher the BMR c) Activity The more physical work or recreation performed, the more energy is required.

d) Pregnancy Pregnant women require extra energy for development of the foetus and carry its additional weight e) Lactation The lactating mother needs additional energy to produce energy containing milk for the suckling baby. The relatively long duration of breast feeding among women especially the Asians and Africans results in a large proportion of women requiring extra energy f) Climate In warm climates that are in most of the tropics and subtropics, less energy is necessary to keep the body at its normal temperature than in cold climate. g) Sleep When one is sleeping, BMR is 105 lower than when one is awake h) Endocrine Glands activity Secretions of the endocrine glands, especially those of the thyroid and adrenal glands have more influence on basal energy needs than any other single factor. If the thyroid gland is

over-active in producing thyroxine (hyperthyroidism), the BMR is raised by as much as 75% to 100%. A reduction is secretion (Hypothyroidism) results in reduced BMR by 30% to 40%. i) Gender (Sex) BMR is approximately 10% higher in men than in women because of the higher percentage of lean body mass in men and the higher proportion of the adipose tissue in women as well as hormonal differences. j) State of Nutrition During prolonged starvation of periods of inadequate calorie intake the body attempts to a decrease in take by decreasing its metabolic rates.

2. VOLUNTARY PHYSICAL ACTIVITY The second largest proportion of energy expended by the body comes from the physical activities we engage in and the amount of time spent in each activity. Influence by physical activity on total caloric needs depends on the type of activity, the length of time it is performed and size of the person. The larger the body, the more the energy is required for activities that require body movement such as walking. Examples of energy used while performing various activities; Everyday activities

Energy spent (Calories/minute)

Sitting

1.4

Standing

1.7

Washing/dressing

3.5

Walking Slowly

3.0

Walking moderately quickly

5.0

Walking up and down stairs

9.0

Work and recreation Light and Sedentary

Most moderate work, lorry driving, light industrial and carpentry require 2.5 to 4.9cals per minute Moderate Gardening, tennis, dancing, jogging, cycling, mopping, scrubbing and walking moderately fast requires 5.0 to 7.4 Kcal per minute Strenuous Playing squash cross country running, swimming, football and construction work e.t.c. need about 7.5 or more Kcal per minute. A person who engages in extraneous physical activity for prolonged periods of time may require as much as 4000 to 5000 Kcal per day. An average day may involve the followingenergy per activity Sleep 8 Hours of rest in bed at BMR of 1cal/min = 1 x 60 x8 = 480 cal Light work 8 Hours at BMR of 25 cal/min = 25 x 60 x 8 = 1200 cal Others 8hrs of sitting and minor activities at BMR at 2cal/min = 2 x 60 x 8 = 960 cal Total = 2640 calories per day 3. THERMIC EFFECT OF FOOD The third and least proportion of energy expenditure is the energy required for the food and nutrients ingestion, digestion, absorption, transportation, utilization, storage and excretion. The production of heat following a meal s known as the thermic effect of food, calorigenic effect of food or specific effect of food or specific dynamic action or diet induced thermogenesis. Thermic effect is the increase in energy expenditure associated with the consumption...