Carb fasting glossary
Plain-language definitions for every term used across the carb fasting hub — metabolic flexibility, the fat switch, glycogen, insulin, ketosis, CGM, and more.
- Metabolic flexibility
- The body’s ability to switch fuel sources — burning carbohydrate when it’s available and fat when it isn’t. Rebuilding it is the entire goal of carb fasting.
- Fat-adapted
- A state in which the body efficiently uses stored and dietary fat for fuel, reached over days to weeks of repeated low-carb periods.
- Fat switch
- A plain-language name for the metabolic tipping point: when carbs are scarce and glycogen runs low, the body flips from storing fat to burning it.
- Glycogen
- The body’s stored form of carbohydrate, held in the liver and muscles. When glycogen is full the body has little reason to burn fat; depleting it on fast days is what flips the fat switch.
- Insulin
- The hormone that moves glucose out of the blood and into cells for use or storage. Frequent carb intake keeps insulin elevated, which favors fat storage over fat burning.
- Insulin resistance
- When cells respond poorly to insulin, so the body produces more of it. It’s associated with weight gain and is a precursor to prediabetes and type 2 diabetes.
- Ketosis (nutritional)
- A normal metabolic state in which, with carbohydrate low, the liver makes ketones from fat as an alternative fuel. Distinct from the dangerous diabetic ketoacidosis.
- Ketones
- Fat-derived molecules the liver produces when carbohydrate is scarce, used as fuel by the brain and muscles.
- Postprandial glucose
- Blood sugar measured after a meal. Large post-meal spikes drive the insulin surge, crash, and cravings that carb fasting is designed to blunt.
- Glycemic variability
- How much blood sugar swings up and down across a day. Steadier glucose generally means steadier energy and fewer cravings.
- HbA1c
- A blood test reflecting average blood sugar over roughly three months — a core measure for prediabetes and diabetes.
- Continuous glucose monitor (CGM)
- A small wearable sensor that tracks blood glucose in real time, useful for seeing how specific foods and fast days affect your curve.
- Fasting insulin / HOMA-IR
- A fasting blood measure (and a derived index) used to estimate insulin resistance — often improved by lowering carbohydrate exposure.
- Carb cycling
- Rotating high, medium, and low carbohydrate days, usually timed around training and tracked with macros. Carb fasting is its simpler, binary cousin.
- Time-restricted eating
- Limiting the hours of the day in which you eat (for example, an 8-hour window). It changes when you eat; carb fasting changes what you eat on certain days.
Want the bigger picture? Start with what carb fasting is, or read the science.