What is carb cycling?

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Carb cycling is a way of eating that alternates high-carbohydrate days and low-carbohydrate days to match each day's training.

What is carb cycling?
'Today is a high-carb day'
People who manage their diet sometimes say 'today is a high-carb day, tomorrow is a low-carb day'. It means changing the amount of carbohydrate from day to day. This way of eating is called 'carb cycling'. What exactly changes, and how?
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Alternating high days and low days
Carb cycling changes the amount of carbohydrate from day to day, alternating days of eating more and days of eating less. Days of eating more are called 'high-carb days' and days of eating less 'low-carb days'. The amount is usually set to match that day's training.
Background: glycogen
The carbohydrate store used during exercise
The body stores carbohydrate it does not use right away as 'glycogen' in the liver and muscles. Muscles draw on this glycogen as fuel during exercise. Eating less carbohydrate leaves this store less full.
How are days split?
More on heavy training days, less on light days
Carb cycling usually means eating more carbohydrate on days with a lot of training and less on rest days or light training days. The idea is to supply only as much fuel as that day's training needs.
How it differs from a low-carbohydrate diet
High days are built in
A low-carbohydrate diet keeps carbohydrate low every day. Carb cycling puts high days between the low days. So carbohydrate does not stay low all week long.
A similar term in sports nutrition
'Carbohydrate periodization'
Sports nutrition uses the term 'carbohydrate periodization'. It means eating plenty of carbohydrate for hard training and competition, while doing some training sessions on purpose with low carbohydrate. It has mainly been studied in long endurance sports such as distance running and cycling.
A common mistake
Is it a proven, scientific method?
Carb cycling is often presented as an advanced, scientific diet, but almost no studies have compared a diet by this name with other diets. Even the similar idea of carbohydrate periodization did not improve race results more than training with plenty of carbohydrate all the time, in people with long training histories in running or cycling.
Summary
Alternating carbohydrate amounts to match training
Carb cycling alternates days of eating more and less carbohydrate to match each day's training. Almost no studies have tested a diet by this name, and the similar carbohydrate periodization did not improve race results in trained endurance athletes more than training with plenty of carbohydrate all the time. Related terms: glycogen, low-carbohydrate diet, refeed.