What is a mass gainer?

In one line

A mass gainer is a powdered supplement that mixes carbohydrate and protein into a high-calorie drink, taken to gain body weight.

What is a mass gainer?
The powder people buy because they 'can't gain weight'
At the gym you hear people say they take a gainer because they 'can't gain weight'. In the supplement aisle it sits next to protein powder. What is in a gainer, why do people drink it, and what does it change?
In one line
A high-calorie carbohydrate and protein powder
A gainer is a powdered supplement that mixes carbohydrate and protein so you can take in a lot of calories at once. People drink it to gain body weight, and in English it is also called a 'weight gain powder'.
Background: calorie surplus
Eat more than you burn and weight goes up
When the calories you eat and drink are more than the calories your body uses, your weight goes up. This state of leftover energy is called a 'calorie surplus'. People drink gainers to create that surplus easily.
Where calories come from
Carbohydrate and protein give 4 kcal per 1 g
The calories in food come from carbohydrate, protein and fat. Carbohydrate and protein each give about 4 kcal per 1 g, and fat gives about 9 kcal.
What is inside
In one study's gainer, over three times more carbohydrate
The gainer used in one study had more than three times as much carbohydrate as protein in a day's serving. Since both give 4 kcal per 1 g, about 70% of its calories came from carbohydrate. The International Society of Sports Nutrition also calls these 'high-calorie carbohydrate/protein supplements'.
Maltodextrin
A starch powder that is barely sweet
Starch is a carbohydrate made of many small sugars strung together in a long chain; bread, potatoes and corn are full of it. Maltodextrin is a white powder made by cutting the starch of corn, potatoes or rice into smaller pieces, and it is barely sweet. It is used as a carbohydrate ingredient in gym supplements.
Why drink it as a powder
When meals alone are not enough
People who train hard find it hard to get all the calories they need from meals alone. So the International Society of Sports Nutrition describes eating four to six meals a day plus snacks, or filling the gap conveniently with high-calorie carbohydrate/protein supplements. It is a way to cope when eating more food is hard.
What a study found
8 weeks of strength training, about 3 kg gained
People who drank a gainer of about 2,000 kcal a day on top of their usual meals and did strength training for 8 weeks gained about 3 kg on average. That was more than the group that drank nothing. A group that drank a carbohydrate drink with the same calories gained a similar amount of weight.
A common misconception
Does a gainer also make you stronger?
Not in that study. The heaviest weight people could lift once went up in all three groups over 8 weeks of strength training, with no difference between those who drank the gainer and those who drank nothing. Gaining more weight did not mean gaining more strength.
Summary
A calorie powder for gaining weight
A gainer is a powdered supplement that mixes carbohydrate and protein into a high-calorie drink, taken to gain body weight. In a study where people drank it with strength training, they gained more weight but not more strength. Related terms: calorie surplus, bulking, maltodextrin.