# Respiratory Quotient Substrate Calculator (RQ: Fat vs Carbohydrate)

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.

> Canonical page: https://elysiatools.com/en/tools/respiratory-quotient-substrate

- **Category:** Health

- **Keywords:** respiratory quotient, rq, substrate utilization, fat oxidation, carbohydrate oxidation, non-protein rq, lusk table, caloric equivalent, kcal per liter oxygen, metabolism, nutrition, indirect calorimetry

## 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.

## Inputs

- **Respiratory Quotient (RQ)** (number): Respiratory quotient (V̇CO₂/V̇O₂ at the cell, or a steady-state resting RER). Range 0.60–1.50.
- **Decimal Places** (number)

## 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

- Enter the measured respiratory quotient (RQ) or resting steady-state RER value between 0.60 and 1.50.
- Optionally adjust the decimal precision used for output rounding.
- The 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.
- It 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.

## Frequently asked questions

### 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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