Your body normally uses glycogen for energy (stored sugars from your diet that’s stored in your liver and muscles). When you deprive your body of carbohydrates and these sugars are not present to use as energy, your body begins producing ketones as a form of energy.
To get into this state – which involves a period of often unpleasant transition – you need to consume a high-fat, moderate protein, and low-carbohydrate diet. To get and remain in ketosis, most people need to limit their carbohydrates to 50 grams per day.
A ketogenic diet can be beneficial and effective for someone who needs to lose fat (if you’re trying to bulk up or put on weight, it’s definitely not recommended as carbohydrates are an essential part of healthy weight gain).
Evian always recommends well-rounded eating as a first step towards fat loss – cutting out processed carbs, sweets and other extra/empty calories and replacing them with nutritionally dense whole foods in appropriate amounts. But for those having trouble losing the last few pounds – or who have reached a fat-loss plateau – it can be an effective short-term measure to reach a specific goal.