What is a low-carbohydrate diet?

In one line

A low-carbohydrate diet is a way of eating that cuts carbohydrate well below usual levels and fills the gap mainly with protein and fat.

What is a low-carbohydrate diet?
Is every cut in rice 'low carb'?
People who diet often say they are 'going low carb'. Someone who eats half a bowl of rice and someone who has almost quit rice and bread both use the same words. So what exactly does a low-carbohydrate diet cut, and by how much?
In one line
A diet that cuts carbohydrate well below usual
A low-carbohydrate diet means eating much less of carbohydrate-rich foods such as rice, bread, noodles and sweets than usual. The calories that are cut are filled mainly with protein and fat. In Korean it is often shortened to 'jeotan' (low carb).
Background
A usual diet gets about half its calories from carbohydrate
Carbohydrates such as rice and bread turn into 'glucose' in the body, the main fuel for cells. US nutrition standards set 45-65% of daily calories from carbohydrate as the suitable range. A low-carbohydrate diet eats less than this range.
The carbohydrate recommendation
130g a day, based on the glucose the brain uses
US nutrition standards set the daily recommended carbohydrate intake for adults at 130g. It is set as the minimum needed to supply the glucose the brain uses, so it is lower than the suitable range on the previous card. Even when people eat less than this, the body releases carbohydrate stored in the liver and makes fuel from fat and protein to supply the brain.
How low counts as 'low carb'?
The common line is under 130g a day
There is no single official cut-off for a low-carbohydrate diet. The line used most often in studies is under 130g a day, which is about 26% of calories for someone eating 2,000kcal a day. In other words, it means eating less than the recommendation on the previous card.
The lowest level
Cut to 20-50g a day and it becomes a ketogenic diet
Cutting carbohydrate further, to 20-50g a day, is called a 'very low-carbohydrate diet'. At this level the liver starts making a fuel called 'ketones' from fat, and a diet aimed at this state is a ketogenic diet. So a ketogenic diet is the lowest level of low-carbohydrate eating.
What do people eat?
More vegetables, meat, fish and eggs; less rice, bread and potatoes
A low-carbohydrate diet centres on low-carbohydrate vegetables such as leafy greens, broccoli and cabbage, plus meat, fish, eggs, cheese and nuts. Fats such as butter, cooking oil and avocado also go up. Carbohydrate-rich foods such as rice, bread, noodles, potatoes and snacks go down.
Where is it used?
Blood sugar control in type 2 diabetes
Type 2 diabetes is a disease in which glucose in the blood, or blood sugar, stays too high. The low-carbohydrate diet is one of the most studied eating plans for controlling blood sugar in this disease. The more carbohydrate was cut, the more blood sugar fell over a few months, but after more than 1 year there was no difference from diets with more carbohydrate.
What fills the gap?
Filling it with plant foods went with lower death risk
Pooled observational studies that followed people's usual diets for years found that people eating under 40% of calories from carbohydrate had about a 20% higher risk of death than people eating a moderate amount. But it depended on what replaced the carbohydrate. Replacing it with animal fat and protein, such as meat, went with higher risk, while replacing it with plant foods such as vegetables and nuts went with about 18% lower risk.
Background: glycogen
The carbohydrate store holds water
The body stores carbohydrate it does not use right away as 'glycogen' in the liver and muscles. Each 1g of glycogen is stored with about 3g of water. When carbohydrate is cut, the body uses up glycogen, and the water stored with it leaves too.
A common mistake
Is the weight lost in the first days all fat?
No. In a ten-day experiment that gave people with high body weight two diets with the same, very low number of calories, the diet with almost no carbohydrate lost weight about 1.7 times faster. But most of the extra loss was water, and the amount of fat lost per day was almost the same as on the diet with mixed carbohydrate.
Summary
Cut carbohydrate, fill with protein and fat
A low-carbohydrate diet cuts carbohydrate well below usual and fills the gap mainly with protein and fat; its lowest level is the ketogenic diet. In a ten-day experiment with almost no carbohydrate, the extra weight lost was mostly water. Related terms: ketogenic diet, glycogen, macronutrients.