Unfortunately, anyone who engages in this diet leaves themselves open to a problem that other followers call “keto flu.” While it feels like a sickness, it is extreme fatigue. After all, when someone consistently eats carbohydrates, the body gets used to it as the primary energy source that they can access quickly. Without this natural support, the body grows increasingly tired and thirsty until it finally learns that carbohydrates cannot be its primary energy source. That causes the body to switch over to stored fat, a process called ketosis.