Another reason for the quick weight loss when you start a keto plan is water loss. “Nitrogen is one by-product of breaking down protein for fuel,” says Notte. “Free nitrogen floating around the body is toxic, so the body flushes it out with frequent urination.” This loss of fluids shows up as pounds lost on the scale. But it can also lead to dehydration and an imbalance of electrolytes—the minerals and salts your muscles rely upon. This is why some keto dieters get terrible leg cramps, Notte explains. “The same thing happens with the Atkins diet—keto is just today’s version.” Make sure you know what happens to your body on keto.

This means that to achieve ketosis, you have to limit your carb intake to less than 50 grams per day (most people should reach ketosis within a week of following the diet). To put this in perspective, the low-carb diet you’re most familiar with—the Atkins Diet—recommends about 130 grams of carbs per day. Also for reference, one bagel has about 55 grams of carbs.
When you’re eating the foods that get you there (more on that in a minute), your body can enter a state of ketosis in one to three days, she adds. During the diet, the majority of calories you consume come from fat, with a little protein and very little carbohydrates. Ketosis also happens if you eat a very low-calorie diet — think doctor-supervised, only when medically recommended diets of 600 to 800 total calories.