The Intuition: What Your Body "Burns" for Fuel
Think of your body like a hybrid car that can run on petrol, diesel, or even cooking gas. Each fuel burns differently — some need more air, some produce more exhaust. Your cells face the same choice: they can burn carbohydrates, fats, or proteins for energy. The question is — how do we tell which fuel is being used at any given moment?
The answer lies in the Respiratory Quotient (RQ). It is simply the ratio of what comes out (carbon dioxide) to what goes in (oxygen) during respiration.
The Precise Definition
RQ=Volume of O2 consumedVolume of CO2 released
This is a pure number — no units, just a ratio. And its value tells you, at a glance, what kind of substrate the cells are burning.
Why Different Substrates Give Different RQ Values
Carbohydrates (like glucose) have plenty of oxygen already built into their structure. So they need relatively less oxygen to burn completely, and they release an equal volume of carbon dioxide.
C6H12O6+6O2→6CO2+6H2O
Here, 6 volumes of CO₂ are released for 6 volumes of O₂ consumed. RQ = 6/6 = 1.0.
Fats (like palmitic acid) are hydrogen-rich and oxygen-poor. They need a lot more oxygen to burn, and release less CO₂ relative to the oxygen used.
2C16H32O2+46O2→32CO2+32H2O
For one molecule of palmitic acid: 16 CO₂ released for 23 O₂ consumed. RQ = 16/23 ≈ 0.7.
Proteins have a more variable structure, but their RQ typically falls around 0.8–0.9.
What the Numbers Mean
| Substrate | RQ Value | What It Tells You |
|---|
| Carbohydrates | 1.0 | Pure aerobic respiration of glucose |
| Fats | ~0.7 | Fat metabolism (common during fasting or long exercise) |
| Proteins | ~0.8–0.9 | Protein breakdown (rare in normal conditions) |
| Mixed diet | ~0.85 | Typical everyday value |
In exam problems, if you see RQ = 1, the substrate is almost certainly a carbohydrate. If RQ < 1, think fats. If RQ > 1, something unusual is happening — like anaerobic respiration or the use of organic acids.
A Special Case: RQ > 1
When the substrate is an organic acid (like oxalic acid or malic acid) that already contains more oxygen than carbon, the RQ can exceed 1. For example: …