Putting your body into ketosis—causing it to switch from burning carbs to burning fat for fuel—is a major metabolic change. Some people don’t react well, while others thrive on it and feel more energy. “Low-carb and ketogenic diets aren’t for everyone, so it’s important to listen to your body,” says Alissa Rumsey, R.D., C.S.C.S., of the Academy of Nutrition and Dietetics. “If after a few weeks you still feel worse than you did before you started, you should stop and reconsider if it’s the right diet for you.” Here’s what she says you should expect when going keto.

The first few days on keto

You may feel low on energy, lethargic, and dizzy. Some people suffer from headaches or flulike symptoms during the first few days or weeks due to the depletion of muscle glycogen and the lack of glucose. This is the hard part, because you have to avoid eating carbohydrates to halt the cycle of glucose-based metabolism, even though your glycogen stores are dwindling and your cravings are probably increasing. Your body is used to burning glucose, not fat, for fuel, which is why—for the first few weeks, while your body adapts—your energy levels may be down.

The first week on keto

You’ll likely see a pretty rapid weight loss in the first week, though this will be from water depletion. Glycogen is attached to water in our bodies, so when you lose glycogen, you also lose water—as much as four-plus pounds of just fluid. Your workouts may also suffer during the first few weeks; many people report a loss of strength and endurance until their bodies become more efficient at using fats for fuel.

The second week on keto

Your body will still be adapting to using fat for fuel, but toward the end of the second week and into the third week, you may start to see increasing energy levels. As glycogen becomes depleted, your body will start producing ketones—some of which are excreted in the urine, so you can now start measuring the levels with keto test strips (available at most pharmacies) to see if you’re at a low enough level of carbohydrate restriction.

3–4 weeks on keto

At this point, if you’ve been keeping your total carbs to less than 50g per day, you should be in ketosis. Getting down to 10% body fat, or even 7%—a truly shredded physique—is totally doable, depending on your starting percentage. Your energy levels should start to improve, though it can take several months for your body to fully adapt to using fat for fuel. The desire to eat regularly throughout the day to sustain mental clarity will have faded, and your muscles will have access to a near-limitless supply of energy.