Meat is harder to digest and stays in the gut longer, causing micro inflammations in your body which can be insignificant, but adds up if you have other inflammation habits, like working out. Its important to eat a small amount of fibrous vegetables when going keto, and taking a meal of the week to be composed when in solids, of exclusively fibrous vegetables. A high dose of vit C in the same day also helps your body self-regulate. Keto flu is exactly that; your body gets too busy fighting gut inflammation and lets the lungs get slightly overwhelmed. It also helps to keep a variety of protein sources. Fish, red meat, poultry and dairy cycled will always keep your body well-maintained.