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Respiratory Quotient Substrate Calculator (RQ: Fat vs Carbohydrate)

· Last updated：2026-08-21

This tool is for informational purposes only and does not constitute medical, financial, or legal advice; consult a qualified professional for advice.

Interpret an RQ against the Lusk non-protein table: 1.00 carbohydrate, 0.707 fat, interpolated %CHO/%fat and kcal per L O₂. RQ above 1.0 is flagged as non-steady-state CO₂ excess. Not medical advice.

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Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category
Health
Input types
number
Output type
json
Sample coverage
4
API ready
Yes

Overview

The Respiratory Quotient Substrate Calculator determines metabolic substrate utilization from your respiratory quotient (RQ) or steady-state resting respiratory exchange ratio (RER). Using the classical Lusk non-protein table, it interpolates the exact percentage of energy derived from carbohydrate versus fat oxidation, provides the caloric equivalent in kilocalories per liter of oxygen consumed (kcal/L O₂), and flags non-steady-state conditions where RQ exceeds 1.00.

When to use

  • Analyzing resting metabolic rate (RMR) reports and indirect calorimetry test results.
  • Evaluating carbohydrate versus fat oxidation ratios under steady-state nutritional or fasting protocols.
  • Determining caloric equivalents per liter of oxygen (kcal/L O₂) for metabolic energy expenditure calculations.

How it works

  1. 1Enter the measured respiratory quotient (RQ) or resting steady-state RER value between 0.60 and 1.50.
  2. 2Optionally adjust the decimal precision used for output rounding.
  3. 3The tool linearly interpolates substrate percentages between pure fat oxidation (RQ 0.707) and pure carbohydrate oxidation (RQ 1.00) using the Lusk non-protein model.
  4. 4It outputs the carbohydrate percentage, fat percentage, caloric equivalent (kcal/L O₂), nearest single-substrate reference, and flags values exceeding 1.00 as non-steady-state CO₂ excess.

Use cases

  • Exercise physiologists calculating substrate utilization rates from steady-state metabolic cart data.
  • Nutrition researchers assessing shifts toward fat oxidation during ketogenic or fasting interventions.
  • Students and educators verifying manual Lusk table interpolations in exercise science and metabolic biochemistry.

Examples

1. Interpreting Mixed-Diet Resting Metabolism

Sports Nutritionist

Background

A nutritionist measures a client's resting metabolic rate following a standard mixed diet to assess baseline energy partitioning.

Problem

Determine the exact split between fat and carbohydrate oxidation and find the corresponding caloric equivalent for oxygen consumption.

How to use

Enter 0.85 in the Respiratory Quotient (RQ) field and set Decimal Places to 1.

Outcome

The calculator reports 48.8% carbohydrate, 51.2% fat oxidation, and a caloric equivalent of 4.9 kcal per liter of O₂.

2. Assessing Fasting Ketosis Substrate Use

Clinical Metabolic Researcher

Background

A researcher conducts indirect calorimetry on a participant after an extended fast.

Problem

Evaluate whether the measured steady-state RQ reflects near-total reliance on lipid oxidation.

How to use

Enter 0.70 in the Respiratory Quotient (RQ) field and leave Decimal Places at 1.

Outcome

The tool identifies 0% carbohydrate, 100% fat oxidation, and 4.7 kcal per L O₂, categorizing the state as fat-predominant non-protein metabolism.

FAQ

What is the non-protein respiratory quotient?

It is the ratio of carbon dioxide production to oxygen consumption (V̇CO₂/V̇O₂) assuming protein oxidation is negligible, mapping values between 0.707 (pure fat) and 1.00 (pure carbohydrate).

Why does an RQ value above 1.0 indicate non-steady-state CO₂ excess?

An RQ greater than 1.00 typically reflects non-metabolic CO₂ production from bicarbonate buffering of lactic acid, hyperventilation, or lipogenesis rather than standard substrate oxidation.

Can this calculator be used with exercise RER data?

It can interpret RER during submaximal steady-state exercise, but high-intensity efforts above the ventilatory threshold produce excess CO₂ that invalidates substrate proportion formulas.

What caloric equivalent does the calculator return?

It returns the thermal equivalent of oxygen in kcal per liter of O₂ consumed, ranging from 4.686 kcal/L O₂ at pure fat oxidation to 5.047 kcal/L O₂ at pure carbohydrate oxidation.

Does this tool provide medical diagnoses?

No. The calculator is an informational physiology tool based on standard metabolic tables and is not medical advice.

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3. Flagging Hyperventilation or Lactate Buffering

Exercise Physiology Student

Background

A student reviews metabolic cart data recorded during intense physical activity where RER exceeded resting physiological limits.

Problem

Verify why an RQ reading of 1.12 cannot be broken down into standard fat and carbohydrate percentages.

How to use

Enter 1.12 in the Respiratory Quotient (RQ) field.

Outcome

The calculator identifies the reading as non-steady-state CO₂ excess, indicating bicarbonate buffering or hyperventilation rather than a resting substrate mix.