Determination of Metabolic Rates

Transcription

1 Determination of Metabolic Rates Overview In this exercise, you will determine your basal metabolic rate and compare your average daily energy intake with your average daily energy requirement. You will also determine the metabolic rates of mice using indirect calorimetry and investigate the effects of temperature on the rate of metabolism. Lab Objectives You should be able to: 1. Explain the general equation for metabolism of food. 2. Compare average daily energy intake and average daily energy requirements. Predict whether weight gain or loss will occur. 3. List the parts of the mouse respirometer and the functions of each. 4. Compare and contrast direct and indirect calorimetry. 5. Define the term basal metabolic rate. Explain the process by which it is measured and the conditions affecting the BMR. 6. Distinguish between endotherms and ectotherms in terms of external temperature and the regulation of metabolic rates. 7. Explain the results attained with the different experimental mouse groups concerning oxygen consumption and metabolic rate (at room temperature vs. cold). 8. Describe the hypothalamic-pituitary-thyroid axis. List all chemicals released and the feedback mechanisms operating in this scheme. 9. List the systemic effects of thyroid hormone on the body. Introductory Notes Cellular respiration: process in living organisms that extracts energy from the chemical bonds of organic molecules (food) and converts it into ATP. ATP is used to run cell activities. Equation: Organic Molecules + O 2 CO 2 + H 2 O + ATP + Heat Metabolism: the sum of all chemical reactions occurring inside an organism Calorimetry: the measurement of heat energy released from the body via metabolism calorie = amount of heat energy needed to raise 1 gram H2O, 1 C Direct calorimetry: a technique of measuring heat energy. An organism is placed in a calorimeter chamber; as the organism gives off heat it is transferred to the surrounding water jacket in the walls of the chamber. This T is = energy released. Indirect calorimetry: the measurement of body heat indirectly through the dependency of heat production on oxygen consumption. Based on the principle that: Energy (heat) liberated inside the body = Energy (heat) liberated outside the body (consuming 1 liter O2) (consuming 1 liter O2) 1

2 C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O kcal 6 moles O 2 x 22.4 liters / mole O 2 = liters O kcal / liters O 2 = 5.01 kcal/liter O 2 Average (kcal / liter O2) energy for the primary food groups: Fats (4.7 kcal / liter O2) Proteins (4.6 kcal / liter O2) Carbohydrates (5.1 kcal / liter O2) Combined together as food averages kcal / liter O2 Metabolic Rate: the total energy expenditure per unit time (kcal / hr) Basal Metabolism: the minimum amount of energy the body needs in a resting and fasting state. Basal Metabolic Rate (BMR): the rate of energy expended for basal metabolism. BMR is measured under a set of standard conditions: Standard Conditions 1. No food ingestion 12 hours prior to test 2. Subject mentally, physically relaxed (morning, prone, dark) 3. Room temperature must be comfortable 4. Subject must not have a fever (which elevates BMR) Conditions Affecting the BMR 1. Exercise ---> increases BMR 2. Food ingestion ---> increases BMR 3. Increased sympathetic activity ---> increases BMR 4. Release of thyroxin and growth hormone ---> increases BMR 5. Other Factors: Climate, nutrition, age, sex, body surface area Specific dynamic action (SDA): energy expended to process food (a.k.a diet induced thermogenesis) Total Energy Required = BMR + SDA + energy for physical activity Total Energy Expenditure = Body Heat + [External Work Done + Stored Energy] Effects of Thyroid Hormones Primary effect is stimulation of calorigenesis (heat production) in the body causes increases in catabolism and anabolism of proteins, lipids, and carbohydrates response is reflected by an increase in oxygen consumption in most tissues (except the brain, spleen, and testes) Overall systemic effects: increases in cardiac output, heart rate, stroke volume increase in respiratory rate increase in RBC count promotes normal growth promotes development and maturation of the nervous system 2

3 Hypothalamic- Pituitary- Thyroid Axis Environmental Influence (cold) Thermoreceptors TRH (Thyrotropin Releasing Hormone) (Thyroid Stimulating Hormone) (Tetraiodothyronine and Triiodothyronine) Target Cells Increased Cell Metabolism (heat production) Prior to coming to lab: Before you come to lab it is necessary for you to keep two daily diaries for a period of three days. One will be a physical activity diary to record the energy you expend for physical activity. This is just a portion of your daily energy requirement. You also need energy for basal metabolism and specific dynamic action, which we will calculate in lab. The other diary will be a daily record of the average energy intake as food. 1. To determine your average energy intake keep a written food diary of all food and beverages consumed throughout the day. Make sure your food diary contains a column for recording the energy in Kcal of the food you eat. (See sample data table 1.1) Repeat for days 2 and 3. You can often find Kcal information on packages, calorie books, and online websites. 2. Calculate your total energy intake for each day and then average these values to determine your average total energy intake / day. 3. To determine your daily energy requirements for physical activity, keep a physical activity log for a period of three days. (See sample data table 1.2) 3

4 4. Use the weight scale on the nomogram (figure 1.1) to convert your body weight to kilograms (kg). Place this value on your data sheet. 5. Consult table 1.3 Energy Cost for Activities to fill in the energy cost factor column of your activity diary. 6. Fill in the last column of your table 1.2 for total Kcal expended using a calculator and this formula: Time spent (minutes) X Energy cost factor X Body weight (kg) = total energy expended for activity Example: If you walked for 30 minutes and you weigh 77.3kg your calculation would be: 30 minutes X light exercise X 77.3 kg = Kcal 7. To determine your basal metabolic rate, use the nomogram (figure 1.1) to determine your body surface area. Using a ruler, draw a straight line from your height to your weight (mass). The point where that line crosses the middle column shows your surface area in square meters (m 2 ). Example: a person who is 6 ft tall and weighs 170 lbs has a body surface area of 1.99 m Use table 1.4 Basal Metabolic Rate Constants to find your BMR constant for your age and sex. Multiply your surface area by this factor to calculate your BMR / hour. Example: a 17 year-old male has a BMR constant of 41.5 Kcal / m 2 hr. If his surface area is 1.99 m 2, his BMR is 82.6 Kcal/hr. 9. To determine your daily BMR, multiply your hourly rate by 24 to obtain the total number of Kcal you need for BMR / day. Example: if the BMR is 82.6 Kcal/hr, then the daily BMR rate is 82.6 X 24 = 1,982 Kcal/day. 10. To calculate the average amount of energy required for daily specific dynamic action (SDA), add the energy for daily BMR and the average energy required for daily physical activity (from page 6). Multiply the total by 10% to obtain an estimated daily SDA. Example: If 1,982 Kcal/ day are required for BMR and 1,044 Kcal/day are required for physical activity, than an average SDA would be ( ) x 0.1 = Kcal 11. Total the amounts you have calculated for average daily physical activity, daily BMR, and average daily SDA. This is your average energy required per day. 12. Compare your average daily energy intake with your average daily energy required. Answer the application questions on your data sheet. Indirect Calorimetry Introductory Notes: Humans and mice are endotherms. That means they generate most of their heat from inside their own body to maintain a stable body temperature. Cell metabolism generates the heat that keeps these animals warm. During this exercise you will monitor the rate of metabolism in a mouse. In addition, you will investigate the influence of environmental temperature on the mouse s metabolism by comparing a mouse in a warm environment with a mouse in a cold environment. In comparison an ectotherm gets its heat from the environment. The body temperature of an ectotherm is warm when the environment is warm, and the body is cooler when the environment is cooler. Examples of ectotherms are fish, reptiles, and frogs. 4

5 Experimental Procedures: 1. Obtain a metabolism chamber (respirometer apparatus) and mouse from the storage carts. 2. Cover the bottom of the metabolism chamber with a thin layer of soda lime. Soda lime will absorb all expired carbon dioxide produced and allow for the measurement of oxygen consumption only. Safety: Do not touch this caustic material or allow the experimental animal to come into contact it. 3. Weigh the wire cage on the scale. Record on your data sheet. 4. Carefully get your mouse into the wire cage. Then weigh the cage with the mouse in it to determine the weight of your mouse. Safety: Do not grab the mice or clasp them in your hands. This will scare your mouse and they may bite or scratch to defend themselves. 5. Put the caged mouse into the metabolism chamber and seal the cork tightly. Make sure the inside thermometer is positioned so that it can easily be monitored. 6. Allow your mouse to acclimate in the chamber for 10 minutes. Be sure to vent the chamber every 5 minutes during this equilibration period as well as between each trial. 7. Wet the inside of the 5 ml pipette with soapy water to prevent the bubble from popping during the lab. 8. Insert the pipette into the hole of the rubber stopper. Place a drop of soap on your finger and wipe it on the open end of the pipette to create a bubble sealing the pipette. 9. Begin measuring the time it takes for the bubble to move down the pipette 5 ml. Record the time in seconds on your data sheet. 10. Repeat the O2 consumption measurements 4 more times. Vent the chamber and wet the pipette between trials. 11. Record the ambient temperature (after the 2nd trial) and barometric pressure (supplied by instructor). These values are required to correct O2 volumes to standard conditions for determining metabolic rates. 12. After completing room temperature trials, calculate the metabolic rate of your mouse by following the calculation steps on your data sheet. 13. Now repeat your experiment with your mouse and chamber packed in ice. 14. Obtain a bucket of ice and a cold metabolism chamber from your instructor. 15. Move your wire cage with mouse and soda lime from the room temperature chamber into the cold chamber. 16. Pack your metabolism chamber into the ice bucket. Allow your mouse to acclimate in the chamber for 10 minutes. The cold temperature will not harm the mouse as long as the mouse is removed before 45 minutes. It should not take you that long for our experiment. 17. Perform the oxygen consumption measurements as before for at least 5 total trials. 18. After your trials are complete, carefully return the mouse to its individual cage, recycle the soda lime back into the original container, put ice in the sink, and return chambers to storage cart. Calculation of Metabolic Rates When calculating the metabolic rate, begin by averaging the trial times (in seconds) for the consumption of 5mls of O2. In the equation below, this value is converted into liters O2/hour, which will be converted into units of kcals/hour (kcal/hr) under standard temperature and pressure conditions (STP). Note: The value in the equation is derived experimentally, and represents the calculated amount of heat energy, in kcals, released from the metabolism of food utilizing 1 liter of O2. The following must be entered into the equation. Equation for calculating metabolic rate: 5

6 NAME LAB SECTION Data Collection and Follow Up Questions for Determination of Metabolic Rates Table 1.1: Energy Intake Diary Type of Food Portion size Kcal / serving Total Kcal Example :Bread Two slices 70/ slice 140 *use additional paper as needed. Day 1 total energy intake = Kcal Day 2 total energy intake = Kcal Day 3 total energy intake = Kcal Average energy intake / day Kcal Table 1.2: Average Daily Energy Required for Physical Activity Time of day Activity Time spent (min.) Cost factor (chart) (Kcal/kg min) Example:9:00am Walked my dog 30 minutes Total Energy Expended (Kcal) *use additional paper as needed. Time spent (minutes) X Energy cost factor X Body weight (kg) = total energy expended for activity Example: 30 minutes X light exercise X 77.3 kg = Kcal Body weight = kg Day 1 total energy expended = Kcal Day 2 total energy expended = Kcal Day 3 total energy expended = Kcal Average energy expended for physical activity / day Kcal 6

7 Table 1.3: Energy Cost for Activities (exclusive of basal metabolism and specific dynamic action) Type of Activity Energy Cost (Kcal/kg min.) Type of Activity Energy Cost (Kcal/kg min.) Sitting or standing still -studying, writing, typing, TV watching, eating, talking Heavy exercise - fast dancing, walking uphill, jogging, fast swimming Very light activity -driving, walking slow Light exercise - housework, walking moderate speed, carrying books / boxes Moderate exercise -fast walking, slow dancing, slow bicycling, golf Severe exercise Tennis, racquetball, running, aerobics, soccer Very severe exercise - boxing, wrestling, racing, rowing, basketball Table 1.4 Basal Metabolic Rate Constants (Kcal/m 2 hr) Age Male Female Age Male Female

8 Your body surface area = m 2 Your calculated hourly basal metabolic rate (BMR) = Kcal/hr Your calculated daily basal metabolic rate (BMR) = Kcal/day Your average specific dynamic action (SDA) / day = Kcal Calculating average daily energy requirements: Average physical activity / day BMR / day Average SDA / day Total average energy expended / day Kcal + Kcal + Kcal = Kcal 8

9 Figure 1.1 Nomogram to estimate body surface area from height and weight. A straight line is drawn from the subject s height to the subject s weight (mass). The point at which the line intersects the middle graph is the subject s body surface area in m 2. 9

10 Review Questions: 1. List the three energy requirements that determine your total energy requirements for the day. 2. Why is energy needed when the body is at rest? 3. SDA refers to the energy needed for what activity? 4. Compare your average daily energy intake and your average daily energy expended. If your two figures are within 20% of each other, you will most likely neither lose nor gain weight. If the two figures are not within this range are you apt to lose weight or gain weight? Explain why. *NOTE: 1 lb of fat represents 3,500 Kcal. Therefore, if you want to lose 1 lb of fat, you must either reduce your intake by 3,500 Kcal / unit time (week, month, year) or increase your activity by 3,500 Kcal / unit time. Assuming the same amount of physical activity you could lose 1 lb per week by reducing your Kcal intake by 500 Kcal per day or gain 1 lb per week by increasing your Kcal intake by 500 Kcal per day. 10

11 Data Collection and Follow Up Questions for Indirect Calorimetry Room Temperature: Trial Times (sec): Mouse s Weight (g) Chamber Temp ( C) BP (mmhg) # # Average Calculation: (5) (3600) ( )Bp (273) (4.825) = kcal/hr, STP (1000) ( )Tm (760) ( )Tp Divide the kcal/hour value by the mouse s weight = kcal / hr / g Packed in Ice: Trial Times (sec): Chamber Temp ( C) BP (mmhg) # # Average Calculation: (5) (3600) ( )Bp (273) (4.825) = kcal/hr, STP (1000) ( )Tm (760) ( )Tp Divide the kcal/hour value by the mouse s weight = kcal / hr / g Calculate % change (Room Temp vs. Packed in Ice) Show your work: Graph the metabolic rate values in your experiment. Draw a line between the two values. Metabolic Rate (kcal / hr / g) Room Temp Cold Temp Environmental Temperature (K) 11

12 Review Questions: 1. How would a decrease in temperature affect your metabolic rate? Why does this occur? 2. Based on the definitions of endotherm and ectotherm in your introductory notes predict the effect decreasing temperature would have on an ectotherm s metabolic rate. Explain your reasoning for your prediction. 3. What factors or conditions other than temperature may alter the rate of metabolism? 4. Describe the role of the hypothalamus pituitary thyroid axis in maintaining body temperature and metabolic rate. 12