How Do We Measure Metabolic Rate

Our body works very hard to maintain its temperature at around 37°C (98.6°F). This means that excess heat generated by chemical reactions in cells must be dissipated. Because this dissipated heat is a direct indicator of our metabolism, we can use an insulated chamber sensitive to temperature change to determine how much heat we produce (energy expenditure). This method of estimating metabolic rate is often referred to as direct calorimetry. Calorimetry literally means "heat measurement." However, since the operational expense for this scientific tool is overwhelming, facilities designed to perform direct calorimetry may be found at only a handful of universities and research institutions.

One alternative method can be employed to assess metabolic rate called indirect calorimetry. Because ATP is generated from the combustion of energy molecules which requires oxygen and produces carbon dioxide, it is possible to estimate energy expenditure based upon these gauges. Representative chemical reactions for the combustion of carbohydrates, protein, and fat are shown below. You see that oxygen is used as a react-ant for each reaction while carbon dioxide is a product. Utilizing math-ematic equations we can estimate the amount of heat produced in a given period of time based upon the amount of oxygen inhaled or the amount of carbon dioxide expired. As it turns out, indirect calorimetry is not only a very accurate indicator of metabolism, but it also gives us an idea of the mixture of energy substances our body is using during that time.

Carbohydrate:

166 Energy Metabolism and Body Weight Triglyceride (fat):

2C57H110O6 + 163O2 ^ 114CO2 + 110H20 Protein:

C72H112N2O22S + 77O2 ^ 63CO2 + 38H2O + SO3 + 9CO(NH2)2.

Based on the amount of oxygen used during a period of time, researchers can estimate the amount of energy used or more commonly calories burned. For instance, we can use 4.8 calories burned per liter of oxygen used to estimate calorie needs. If a man uses 20 liters of oxygen an hour (360 liters/day) this would translate to around 96 calories/hour or 2,300 calories daily.

How Can We Know What Our Body Is Using for Energy?

Based on the chemical reactions shown above, we can calculate what researchers call the respiratory exchange ratio (RER) (or respiratory quotient (RQ)) for a given time period. RER is equal to the amount of carbon dioxide exhaled divided by the amount of oxygen inhaled.

RER = CO2/O2

  • RER of glucose 6CO2/6O2 = 1.0
  • RER for the triglyceride 114CO2/163O2 = 0.70
  • RER for the protein 63CO2/77O2 = 0.82

If we measure a person's gases during a period of time we can calculate a few things. For example, say that during 1 hour a person consumed 15 liters of oxygen and expired 12 liters of carbon dioxide; we can first calculate their RQ for that hour:

We can find the RER of 0.80 on Table 8.3 and follow it over to the calorie source columns. At an RER of 0.80 this individual would be using approximately 33 percent carbohydrates and 66 percent fat to fuel his or her metabolism. We will assume that the contribution from amino acids toward energy production during that time is minimal. This is a fair assumption for a healthy person not engaged in prolonged fasting or endurance exercise during this time. Furthermore, we can estimate metabolic rate by multiplying the amount of oxygen consumed (15 liters) by the caloric value for 1 liter of oxygen for an RER = 0.80. Their metabolic rate would be:

15 x 4.801 = 72 calories/hour

Table 8.3 Thermal Equivalent of O2 and CO2 for Nonprotein Respiratory Quotient

Nonprotein

Caloric Value

Caloric Value

Carbohydrate

Fat

RQ

1 Liter O2

1 Liter CO2

(%)

(%)

0.707

4.686

6.629

0

100.0

0.71

4.690

6.606

1.1

98.9

0.72

4.702

6.531

4.76

95.2

0.73

4.714

6.458

8.4

91.6

0.74

4.727

6.388

12.0

88.0

0.75

4.739

6.319

15.6

84.4

0.76

4.751

6.253

19.2

80.8

0.77

4.640

6.187

22.8

77.2

0.78

4.776

6.123

26.3

73.7

0.79

4.788

6.062

29.9

70.1

0.80

4.801

6.001

33.4

66.6

0.81

4.813

5.942

36.9

63.1

0.82

4.825

5.884

40.3

59.7

0.83

4.838

5.829

43.8

56.2

0.84

4.850

5.774

47.2

52.8

0.85

4.862

5.721

50.7

49.3

0.86

4.875

5.669

54.1

45.9

0.87

4.887

5.617

57.5

42.5

0.88

4.899

5.568

60.8

39.2

0.89

4.911

5.519

64.2

35.8

0.90

4.924

5.471

67.5

32.5

0.91

4.936

5.424

70.8

29.2

0.92

4.948

5.378

74.1

25.9

0.93

4.961

5.333

77.4

22.6

0.94

4.973

5.290

80.7

19.3

0.95

4.985

5.247

84.0

16.0

0.96

4.998

5.205

87.2

12.8

0.97

5.010

5.165

90.4

9.58

0.98

5.022

5.124

93.6

6.37

0.99

5.035

5.085

96.8

3.18

100

5.047

5.047

100

0

0 0

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