No, one cheat day will not ruin ketosis. In fact, it can actually help you stay on track with your diet plan and prevent feelings of deprivation. While a cheat day may cause some temporary disruption to the metabolic state of ketosis, it shouldn’t have any long-term effects if planned and executed properly. To keep your metabolism running optimally and minimize disruption to the state of ketosis after a cheat meal or snack, try to stick as close as possible to the foods that are already part of your regular meal plan. Avoiding processed carbs is essential when planning a cheat meal since these will cause an immediate spike in blood sugar levels which could affect ketone production. Consuming healthy fats such as avocados, nuts and nut butters along with lean proteins like chicken breast or fish can also be beneficial for maintaining a steady level of energy throughout the day without causing too much disruption to your body's natural state of ketosis.