Fructooligosaccharides (FOS) are made from short chain fructose (fruit sugars). They’re carbohydrates that occur naturally in plants such as chicory root, leeks, asparagus, bananas, garlic, and onion. They aren’t absorbed by the body, so won’t increase blood sugar levels or hamper keto diets. They're often used as a low-cal sweetener. They may also help the body protect itself from unhealthy bacteria, and may be a good source of dietary fiber.
Resources: Health Line