Energy Balance and Body Composition

Transcription

1 Energy Balance and Body Composition

2 THE ECONOMICS OF FEASTING

3 THE ECONOMICS OF FEASTING Everyone knows that when people consume more energy than they expend, much of the excess is stored as body fat. Fat can be made from an excess of any energyyielding nutrient. The fat cells of the adipose tissue enlarge as they fill with fat, as Figure 6-1 shows.

4 Excess Carbohydrate Surplus carbohydrate (glucose) is first stored as glycogen in the liver and muscles, but the glycogen-storing cells have limited capacity. Once glycogen stores are filled, most of the additional carbohydrate is burned for energy, displacing the body s use of fat for energy and allowing body fat to accumulate. Thus, excess carbohydrate can contribute to obesity. Excess Fat Surplus dietary fat contributes easily to the body s fat stores. After a meal, fat is routed to the body s adipose tissue, where it is stored until needed for energy. Thus, excess fat from food easily adds to body fat. Excess Protein Surplus protein may also contribute to body fat. If not needed to build body protein (as in response to physical activity) or to meet energy needs, amino acids will lose their nitrogens and be converted, through intermediates, to triglycerides. These, too, swell the fat cells and add to body weight. Figure 6-2 shows the metabolic events of feasting.

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6 Review Notes Too little physical activity encourages body fat accumulation. Any food can make you fat if you eat too much of it. A net excess of energy is almost all stored in the body as fat in adipose (fat) tissue. Alcohol both delivers kcalories and encourages storage of body fat. Fat from food is particularly easy for the body to store as adipose tissue. If not needed to build body protein or to meet energy needs, excess protein can be converted to fat. In short, excess energy intake from carbohydrate, fat, protein, and alcohol leads to storage of body fat.

7 Energy Deficit The body s top priority is to meet the energy needs for this ongoing cellular activity. Its normal way of doing so is by periodic refueling, that is, by eating several times a day. When food is not available, the body uses fuel reserves from its own tissues. If people voluntarily choose not to eat, we say they are fasting; if they have no choice (as in a famine), we say they are starving. The body, however, makes no distinction between the two metabolically, fasting and starvation are identical. In either case, the body is forced to switch to a wasting metabolism, drawing on its stores of carbohydrate and fat and, within a day or so, on its vital protein tissues as well.

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9 Glycogen Used First As fasting begins, glucose from the liver s glycogen stores and fatty acids from the body s adipose tissue flow into the cells to fuel their work. Within a day, liver glycogen is exhausted, and most of the glucose is used up. Low blood glucose concentrations serve as a signal to promote further fat breakdown. Glucose Needed for the Brain At this point, a few hours into a fast, most of the cells depend on fatty acids to continue providing fuel. But the nervous system (brain and nerves) and red blood cells cannot use fatty acids; they still need glucose. Even if other energy sources are available, glucose has to be present to permit the brain s energy-metabolizing machinery to work. (+) Normally, the nervous system consumes a little more than half of the total glucose used each day about 400 to 600 kcalories worth.

10 Protein Breakdown and Ketosis Because fat stores cannot provide the glucose needed by the brain and nerves, body protein tissues (such as liver and muscle) always break down to some extent during fasting. In the first few days of a fast, body protein provides about 90 percent of the needed glucose, and glycerol provides about 10 percent. If body protein losses were to continue at this rate, death would ensue within about three weeks. As the fast continues, however, the body finds a way to use its fat to fuel the brain. It adapts by condensing together fragments derived from fatty acids to produce ketone bodies, which can serve as fuel for some brain cells. Ketone body production rises until, after several weeks of fasting, it is meeting much of the nervous system s energy needs. Still, many areas of the brain rely exclusively on glucose, and body protein continues to be sacrificed to produce it.

11 Figure 6-3 shows the metabolic events that occur during fasting.

12 Energy balance If a person maintains a healthy weight over time, the person is in energy balance.

13 Energy requirements Are defined as the dietary energy intake that is required to maintain energy balance in a healthy person of a defined age, gender, weight, height, and level of physical activity consistent with good health.

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16 Energy expenditure Energy expenditure is the amount of energy (or calories) that a person needs to carry out a physical function such as breathing, circulating blood, digesting food, or physical movement. Your total daily energy expenditure (TDEE) is the total number of calories you burn each day. Basal metabolism: the energy needed to maintain life when a person is at complete digestive, physical, and emotional rest. Basal metabolism is normally the largest part of a person s daily energy expenditure. Voluntary activities: the component of a person s daily energy expenditure that involves conscious and deliberate muscular work walking, lifting, climbing, and other physical activities. Voluntary activities normally require less energy in a day than basal metabolism does.

17 Basal metabolic rate (BMR): the rate of energy use for metabolism under specified conditions: after a 12-hour fast and restful sleep, without any physical activity or emotional excitement, and in a comfortable setting. It is usually expressed as kcalories per kilogram of body weight per hour. Resting metabolic rate (RMR): a measure of the energy use of a person at rest in a comfortable setting similar to the BMR but with less strict criteria for recent food intake and physical activity. Consequently, the RMR is slightly higher than the BMR. Thermic effect of food: an estimation of the energy required to process food (digest, absorb, transport, metabolize, and store ingested nutrients)

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23 Measuring human energy expenditure: Units: Calories: the amount of heat energy required to raise the temp. of 1ml of water at 15 º C by 1 º C Kcal=1000cal Method for measuring human energy expenditure: Direct Calorimeter. In Direct Calorimeter. Doubly labeled water.

24 Direct calorimeter A method for measuring the amount of energy expended by monitoring the rate at which a person loses heat from the body to the environment when placed inside a structure large enough to permit moderate amounts of activity. Direct calorimetry provides no information on the kind of fuel being oxidized. Disadvantageous of Direct calorimetry: Limited method Physical activity within chamber is limited It is high cost Complex

25 Indirect calorimeter: A method for estimating energy production by measuring oxygen consumption and carbon dioxide production rather than by directly measuring heat transfer typically takes 30 min to 1h to complete. Procedure: The person usually breathes into mouth piece or ventilated hood through which his or her expired gases are collected. Data are obtained from Indirect calorimetry in a form that permits calculation of the respiratory quotient= RQ= moles CO2expired / moles O2 consumed This determination is converted into kcal. Of heat produced per m2 of the body surface per hour and is extrapolated to energy expenditure in 24hs RQ for : carbohydrate= 1 Protein = 0.82 Fat= 0.7 Advantageous of Indirect calorimetry : Mobility ;Low equipment cost

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27 Calculating food energy: Total energy available from a food is measured with Bomb calorimeter. Bomb calorimeter: Consist of 1. closed container in which a weighted food sample, 2. ignited with an electric spark, is burned in an oxygenated atmosphere. 3. The container is immersed in a known volume of water and rise the temp. Of the water after igniting the food is used to calculate the heat energy generated.

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