Something that is really helpful in the transition with the keto flu has been electrolytes; salt and potassium and magnesium in particular.
And I think there may be some benefit to doing that transitioning to a carnivorous diet from a ketogenic diet as well, although I haven’t been paying attention enough do that idea until recently to know if that would be a universal idea or not, but I think it can’t hurt.
Source: Ohearn, Amber, guest. “The Carnivore Diet for Women (Success Story) – Amber O’Hearn” Biohackers Lab, 16 Feb. 2018. https://www.biohackerslab.com/ep37-amber-ohearn/