How can we know what our body is using for energy?
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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 CO2 exhaled divided by the amount of O2 inhaled.
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If we measure a person’s gases during a period of time we can calculate a few things. For example, say that during one hour a person consumed 15 liters of oxygen and expired 12 liters of carbon dioxide; we can first calculate their RER for that hour:
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RER = 12/15 = 0.80 |
We can find the RER of 0.80 on the table below 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 O2 consumed (15 L) by the Caloric Value for 1 liter of oxygen for an RER = 0.80. Their metabolic rate would be:
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15 × 4.801 = 72 Cal/hour |
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Respiratory Energy Ratio (RER) |
O2 Calories/Liter |
CO2 Calories/Liter |
Carbohydrate (%) |
Fat (%) |
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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 |
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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 |
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0.97 |
5.010 |
5.165 |
90.4 |
9.58 |
|
0.98 |
5.022 |
5.124 |
93.6 |
6.37 |
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0.99 |
5.035 |
5.085 |
96.8 |
3.18 |
|
100 |
5.047 |
5.047 |
100 |
0 |
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