Evaluating the efficacy and mechanisms of a ketogenic diet as adjunctive treatment for people with treatment-resistant depression: a protocol for a randomised controlled trial by Gao, M, Kirk, M, Lash, E, Knight, H, Michalopoulou, M, Guess, N, Browning, M, Weich, S, Burnet, P, Jebb, SA, Stevens, R, Aveyard, P