Metabolism Lab:
Metabolic rate is the overall rate at which metabolic reactions use energy. The basal metabolic rate (BMR) is the amount of energy usage under fasting, restful conditions. Typically, BMR is determined by measuring the amount of O2 consumption per kilocalorie of food metabolized. However, the measurement of O2 consumption requires specialized equipment. We know that metabolic rate depends on the type of energy producing nutrient that is being used and on the body surface area, age, and sex. Standardized tables, such as the one below have been developed to estimate the average BMR based on age and gender.
Normal Standards of Basal Metabolic Rate
Age Cal/M2/Hr
Male Female
6 53 50.6
10 48.5 45.9
16 45 38.9
17-19 42.7 36.7
20-21 41.1 36.2
22-23 40.8 36
24-30 40 35.9
31-40 38.7 35.7
41-50 37.4 35
51-60 36.4 33.6
61-70 34.8 32.3
Additionally, we can estimate the average BMR based on the following formulas:
Basal Metabolic Rate Calculations
Age Equation
Males
22.7 x body wt (in kg) + 495
17.5 x body wt (in kg) + 651
15.3 x body wt (in kg) + 679
11.6 x body wt (in kg) + 879
> 60 13.5 x body wt (in kg) + 487
Females
22.5 x body wt (in kg) + 499
12.2 x body wt (in kg) + 746
14.7 x body wt (in kg) + 496
8.7 x body wt (in kg) + 829
> 60 10.5 x body wt (in kg) + 596
A sample calculation based on a 154 lb male, age 20 would be:
154 lbs/2.2 = 70 kg
15.3 x 70 + 679 = 1750 kcal/day
EXERCISE 1: Calculation of average BMR and Daily Caloric Needs:
Calculate your estimated BMR using the above formula.
It is best to recognize that this is a table based on statistical analysis and that each individual will have variations depending on their own body composition. If you have low thyroid hormone, your values would be less than these predicted values, for example. Muscle requires more energy than fat to sustain, therefore a person weighing 200 lbs who is lean and musculature will have a higher BMR than someone who weighs 200 lbs due to excess fat. Most people tend to look at these values within a range of plus or minus 10%, which takes in much more of the population. To determine the range find 10% of your predicted BMR. Now subtract that number from the BMR to get one side of the range and then add it to BMR to get the high side of the range.
1. Your calculated BMR____________Your range of BMR ________________________________
Of course, this is based on resting values. A basically sedentary person (a typist) would add 15% increase to their BMR to determine the amount of caloric intake they need per day. Light activites (like a teacher) add 25-30% to your basic nutrient requirements. Moderate activity ( like a nurse) adds 30-45% and heavy work (like a laborer) adds 75 to 85% to your daily energy requirement. Remember, these are approximations, and may not truly capture your individual energy requirements.
Determine the percentage category your activity level places you in. Determine that percent of your BMR and add it to your BMR. For example, if our 154 lb male laborer above had a BMR of 1750 Kcal./day, 75% of that would be 1,312.5 kcal day.
1750kcal/day x .75 (75%) = 1,312.5 kcal/day
Which would mean he would need to ingest:
1750 (BMR) + 1,312.5 (additional) = 3,062.5 kcals/day to maintain his body weight. We could again use our 10% figure to determine his range by adding and subtracting 306 to and from 3,062.5 such that
3062.5 + 306 = 3368.5
3062.5-306=2,756.5
Making his daily intake range between 2,756.5 and 3,368.5.
Once again, his body composition and specific things can push him to the top or bottom of the range, depending on his particular circumstances. Since he is a roofer and tends to work in the hot sun we could predict he would be at the top of the predicted range (elevated temperature elevates metabolic rate!).
Calculate your daily intake range based on your activity level.
2. Your estimated daily caloric range (+/- 10%)_______________________________
Your actual average daily intake (from your diet recording)__________________
One thing that we also know is that there is a relationship between heart rate (HR) and O2 consumption. There are other factors, such as temperature, hormonal influences, and level of activity and/or stress, etc. that can affect metabolic rate. But for our purposes, let’s first look at the relationship between O2 consumption and HR. This relationship varies between men and women and between individuals, depending on their fitness level. Below are data that depict the average relationship between HR and O2 consumption for females and males.
Figure 1.
The curve for females is a little steeper than for men, meaning that females need to generate a slightly higher HR in order to consume a given amount of oxygen compared to men. This is because females tend to have smaller lung and hearts, less blood volume, fewer RBC’s, etc. than men. Also remember that in the respiratory lab, your rate of breathing increased as you exercised. This is because your body requires more oxygen to produce more energy that is needed during exercise. Do you remember your estimated tidal volume from the respiratory lab?_____________________________________________
The amount of energy expended at rest and during increased periods of activity is also related to oxygen consumption and can be added to the graph above to estimate the kcals expended based on HR. As you know, different nutrients release different amounts of energy. For example, metabolism of one gram of carbohydrates releases 4.1 kcals, one gram of carbohydrate releases 4.1 kcals, and one gram of fat releases 9 kcals. What this all means is that fat releases twice as much energy as carbohydrate or protein when you burn one gram of it. However, a more relevant relationship is that between O2 consumption and energy expenditure. At rest, when one liter of O2 is consumed, carbohydrates release 5.05 kcals, fats release 4.74 kcals, and protens release 4.46 kcals. Most people have a mixed diet, which averages the release of 4.825 kcals per liter of O2 consumed. Someone on a low carb diet would release less energy for a given activity, since carbohydrates release the greatest number of kcals per liter of O2 consumed.
Confused yet? Burning one gram of fat releases twice as much energy as burning one gram of carbohydrate and yet consuming one liter of O2 releases more energy from carbohydrate than from fat! What that means is that carbs are easier to burn- in other words, it takes more O2 to burn one gram of fat than one gram of carb, even though one gram of fat has the potential to release more energy than one gram of carbohydrate. The fact that fat stores more energy per gram is one reason why the body prefers to store energy as fat. The other reason is that carbohydrates are stored primarily as glycogen, and glycogen is stored in a water matrix, which takes up space. The liver and muscle are the primary storage sites for glycogen and their physical capacity to store glycogen is limited. Our ability to store fat is virtually unlimited!
The amount of energy released during periods of activity increases to an average of 5.0 kcal per liter of O2 consumed in a mixed diet. The relationship between kcals and HR (based on O2 consumption at a given HR) is shown below:
Figure 2
The energy used during various activities is important to individuals on a weight control program and again, is based on oxygen consumption. In order to loose one pound of fat you have to burn 3500 kcal more than you consumed. On the other hand, if you consume 3500 kcal more than you burn, you would gain a pound. You could estimate your BMR and then, based on your HR during periods of activity, predict your daily energy expenditure, which could then be compared with your daily caloric intake. The amount of energy burned during an activity is related to the intensity (which is often gauged by HR) and the duration (time spent) of the activity. There are then two ways to increase your caloric “burn” – either increase the intensity of your activity level (i.e. work at a higher HR) or increase the duration of time you spend engaging in that activity. As the intensity is increased, however, our ability to continue that activity for long periods of time diminishes, resulting in a trade off between intensity and duration. For example, you could use 100 kcal by running a fast mile or you could jog that mile, taking a longer period of time at a comfortable pace, and still burn a similar amount of kcals. Comparing running with jogging, you could jog for a longer period of time and for a further distance without becoming exhausted. In general, we say that an untrained male can work at an intensity level that uses 11.6 kcals for somewhere around an hour. Refer back to the HR vs Kcal graph for males and you can see that this amount of energy usage correlates with a HR between 140 and 150 bpm. Training, of course, increases the ability to perform work at higher intensities for a longer duration. This is because a trained individual does not have as great an increase in HR for a given level of intensity (O2 consumption) compared to the untrained individual. Also, age is important, because as we age the maximum HR we can attain is reduced.
The graph depicting HR and O2 consumption can also be used to estimate an individuals maximal cardio-respiratory endurance or maximal oxygen consumption (VO2 max). VO2 max is roughly defined as the maximal amount of O2 an individual can take in, transport and use per minute during maximal exertion. This value can be approximated from the heart rate-O2 consumption graph if you use the formula 220-age to predict the maximum HR in an individual. Draw a line from the predicted maximum HR on the Y axis across the graph (to the right) until you reach the HR-O2 consumption line on the graph. Then draw a line down to the X axis and read the O2 consumption for that HR value. You must remember that the graphs you draw are based on the average values for men and women. Individuals will have their own line, depending on their fitness level, but again, most people do not have the equipment to determine their own line, so we are stuck with the average values. VO2max is typically expressed as mlO2/kg/min. This can be calculated by dividing mlO2 /min used by a persons body weight in kg (2.2kg/lb). Also, remember that the data are graphed as liters of O2, so you would multiply the number of liters of O2 consumed by 1000 to get ml of O2 consumed.
Exercise 2. Determine your max HR, VO2 max, training HR, and energy usage.
Based on your age and gender what is your predicted VO2max?
Determine the max HR by subtracting 220-age.
= 200 BPM maximum predicted HR
Your max HR ________________
Then find the maximum HR on the graph for your gender and read down to the X-axis to determine the liters of O2 used at that HR.
Ex: Amt O2 used at 200 BPM in a female = 2.8 l/min
Your O2 consumption at max HR ___________________
Then convert liters to ml.
Multiply your O2 consumption above by 1000
Your O2 consumption at max HR in mls/min ________________
Then determine body weight in kg
Divide your body weight in lbs by 2.2.kg/lb
Your body weight in kg _____________________________
Then determine mlO2/kg/min by dividing the ml O2/min by body weight in kg
Your VO2 max in mlO2/kg/min_____________________________
The highest VO2 max reported in the literature for men is 94 mlO2/kg/min, a value obtained from a Swedish cross country skier, and for women is 79 mlO2/kg/min (Joan Benoit).
To determine your Kcal burned per minute at VO2 max you will need to take your number obtained in #5 above and divide it by 1000 to convert it into LO2/kg/min.
My Liters/kg body weight of O2 consumed at VO2 max is____________________________
The average diet (a mixed diet) is estimated to burn approximately 4.825 Kcal/L of O2. So multiply your number above by 4.825
Your kcal burned per kg/minute at VO2 max______________________
Multiply this number by your body weight in Kg to determine your total Kcal burned per minute
Total Kcal burned per minute___________________________
Compare the value you calculated above to the Kcal burned at that HR from figure 2. Are they similar?
Most people don’t actually exercise at VO2 max and indeed a training rate based on HR is around 80-85% max HR in most normal, healthy adults.
Multiply max HR x .85 to determine your training HR.
Your training HR ______________________________
How many L O2 are you using at this HR? (look at Figure 1)_______________
Now multiply this by 4.825 to get Kcal burned per minute at your training HR.
My Kcal burned at 85% maximum HR is _______________________________
What was your average caloric intake per day? ______________________________
Multiply the average by 7 to compute your estimated weekly caloric intake
My average weekly caloric intake__________________________________________
Now multiply your estimated daily caloric needs (from exercise 1) by 7 to get your estimated weekly caloric needs. _____________________________________________________
Based on the above (your intake vs estimated needs), should you be gaining or losing weight?
8. In order to lose one pound, you need to take in 3500kcal less than you expend. If you take your Kcal/min used at your training and divide that into 3500, you can determine the number of minutes you need to exercise per week in order to lose one pound. (3500/kcalburned per minute at training HR).
Assuming no change in diet, how many minutes would you have to exercise per week in order to lose 1 lb (3500kcal)?___________________________________
Maybe your results help explain why signing up for an aerobics class twice per week will not really help you lose weight quickly unless you also modify your eating habits. Most people on a weight control program combine dietary changes (lowered consumption) with an exercise program to achieve a balanced, healthy weight loss. Don’t discount the aerobics class, however, since it increases cardiovascular fitness, keeps joints mobile, tones muscle, and is loads of fun!
How might you alter your diet to enhance weight loss or weight gain?
Use of Energy during Activity
A person uses different types of nutrients during activity, depending on the duration of that activity. For example, the body has three systems of energy production:
Stored ATP/creatine phosphate
anaerobic glycolysis
aerobic metabolism (complete oxidation of glucose, fat, and protein)
As a person begins to exercise, the muscles use ATP that is stored in the tissue. After about 3 seconds, this is all used up. Then the body uses creatine phosphate to readily replenish ATP, lasting for about 8-10 seconds. Once all of this stored energy is used, the body shifts into anaerobic glycolysis, which last for about 1 to 2 minutes. Following this, the body derives its energy from aerobic metabolism, which lasts as long as the nutrients (carbs, fats, proteins) last. During aerobic exercise, the body first uses carbohydrates as its primary source of energy. This is one reason why athletes practice “carbohydrate loading” prior to an athletic event, as they wish to maximize their glycogen stores (remember from above also that one L of O2 burns more grams of carbohydrate than any other nutrient). Pure carbohydrate usage lasts until all the stored glycogen has been converted to glucose, or about 15 minutes. Then the body shifts into a fat burning mode, which lasts for a duration of about 90 minutes. If the activity persists longer than 90 minutes, the body shifts into a protein burning state. You can see the importance of proteins for their many jobs other than energy production by noting that proteins are used as a last resort for energy production. Now, if you think back to the renal lab where we compared fluid intake of water and Gatorade, we saw that both were able to rapidly increase the plasma volume, but Gatorade offers an additional advantage for activities that persist for a long duration by providing the individual with extra glucose. This is why athletes generally drink Gatorade or some other sports drink during athletic events. If the intensity of the exercise exceeds the max O2 consumption (VO2max), the body must shift some of its energy production back to anaerobic metabolism to make up the difference in the need for energy and the availability of oxygen. This can cause an increase in the production of lactic acid, which leads to fatigue, and presumably contributes to sore muscles (there is much debate in the literature on this subject).
Once exercise has ceased, the body needs to replace O2 that is typically stored in the lungs, body fluids, hemoglobin, and muscle. This is typically about 2 liters of O2 and requires about 4 minutes to replenish the stores that were used during the workout. Also known as the O2 debt, the individual will continue heavy breathing during this period even though the exercise has ceased. Longer periods of exercise will require a longer time to replace the O2 debt. You should have noticed in the respiratory lab that it took your breathing rate longer to recover following exercise than any of the other activities! In addition, the body will replace the ATP, creatine phosphate, and glycogen that were used during the exercise bout and this takes about one hour.
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