# Respiratory exchange ratio

Respiratory exchange ratio (RER) is the ratio of carbon dioxide produced to oxygen consumed, used in labs to infer carbohydrate versus fat use.

Values near 0.70 at rest suggest mostly fat oxidation, near 0.85 a mix, and near 1.00 or higher heavy carbohydrate reliance during hard exercise. Consumer watches do not measure RER; heart rate zones are a practical proxy.

Also called: RER.

Related: [Zone 2](https://revns.com/dictionary/zone-2.md), [VO2 max](https://revns.com/dictionary/vo2-max.md)

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Canonical: https://revns.com/dictionary/respiratory-exchange-ratio
Index of everything: https://revns.com/llms.txt
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