A ketogenic diet is a high-fat, very low-carbohydrate eating plan designed to shift your body’s primary fuel source from glucose to stored fat. In its classic form, fat makes up 80% to 90% of daily calories, protein accounts for roughly 6% to 15%, and carbohydrates are restricted to just 5% to 10%. That carb limit typically translates to about 20 to 50 grams per day, which is less than what you’d find in a single bagel.
How Your Body Switches Fuel Sources
Under normal circumstances, your body runs on glucose from carbohydrates. When you drastically cut carbs, your glucose reserves (stored as glycogen in your liver and muscles) deplete within a day or two. Your body then begins breaking down stored fat into fatty acids, which travel to the liver and get converted into molecules called ketone bodies. These ketone bodies become your brain’s and muscles’ alternative energy source.
This metabolic state is called ketosis. Nutritional ketosis is defined by blood ketone levels of roughly 0.5 to 3.0 mmol/L. This is a normal, regulated process and completely different from diabetic ketoacidosis, a dangerous condition seen in uncontrolled type 1 diabetes where ketone levels spike 5 to 10 times higher. As long as your body produces insulin normally, it keeps ketone production within a safe range.
What to Eat and What to Avoid
The bulk of your plate on keto comes from fat-rich whole foods. Prioritize fatty fish like salmon, sardines, and mackerel (which are also rich in omega-3 fats), avocados, olive oil, butter, cheese, nuts, seeds, and eggs. Full-fat dairy, meat, and poultry form the protein portion. For vegetables, stick to nonstarchy options with less than 8 grams of net carbs per cup: broccoli, cauliflower, green beans, bell peppers, zucchini, and spinach all fit well.
The restricted list is longer than most people expect. Grains, bread, pasta, rice, potatoes, corn, most fruit, beans, sweetened drinks, and anything with added sugar are off the table. Processed foods are particularly tricky because sugar hides under dozens of names on ingredient labels. Watch for terms like corn syrup, high-fructose corn syrup, rice syrup, molasses, agave, and any ingredient ending in “-ose” (glucose, fructose, maltose, dextrose, sucrose). Words like “glazed,” “candied,” or “caramelized” in a product name also signal added sugar.
Variations of the Keto Diet
The standard ketogenic diet described above isn’t the only version. A high-protein ketogenic diet shifts the ratio to about 65% fat, 30% protein, and 5% carbs. It’s a popular choice for bodybuilders, older adults concerned about muscle loss, or anyone who finds the classic version too restrictive on protein. The higher protein intake helps preserve muscle mass while still keeping carbs low enough to maintain ketosis.
Athletes sometimes follow a targeted approach, adding a small amount of carbs around workouts for quick energy, or a cyclical approach that alternates strict keto days with higher-carb refeeding days. These versions are designed for people with intense training schedules and aren’t necessary for general weight management.
What to Expect in the First Few Weeks
The scale often drops quickly at first. Most people lose somewhere between 2 and 10 pounds in the first one to two weeks, but the majority of that early loss is water, not fat. Your body stores glycogen alongside water at a ratio of about 3 grams of water for every 1 gram of glycogen. As those glycogen stores empty out, the water goes with them.
After that initial drop, weight loss slows to a more sustainable pace. From that point, what you lose is primarily fat, and the rate depends on your overall calorie deficit rather than the diet itself. A deficit of 500 to 750 calories per day typically produces 1 to 1.5 pounds of fat loss per week, which is the range most associated with keeping weight off long term.
The “Keto Flu” and How to Handle It
Somewhere between days two and seven, many people experience a cluster of symptoms nicknamed the keto flu: headache, fatigue, brain fog, irritability, nausea, difficulty sleeping, and constipation. These symptoms reflect your body adjusting to its new fuel source and typically resolve within about a week. Some people feel better than their baseline once the transition is complete.
You don’t need expensive supplements to get through it. Drinking plenty of water is the single most important step, since the water loss from glycogen depletion can leave you dehydrated quickly. Eating more frequently during that first week and including a wide variety of colorful vegetables helps maintain electrolyte balance and energy levels. Adding a pinch of salt to meals and eating potassium-rich foods like avocado and spinach can also ease symptoms.
Medical Uses Beyond Weight Loss
The ketogenic diet was originally developed in the 1920s as a treatment for epilepsy, and it remains one of the most evidence-backed dietary therapies in medicine. It carries the highest level of clinical evidence for reducing seizures, particularly in children who don’t respond to medication. Researchers believe the diet improves how brain cells produce energy and influences the way nerve signals are transmitted.
Beyond epilepsy, ketogenic diets show promise for Alzheimer’s disease and cognitive impairment (also supported by strong clinical evidence), Parkinson’s disease, certain cancers, and several rare metabolic disorders where the body can’t properly process glucose for fuel. For type 2 diabetes, the diet’s ability to lower blood sugar and reduce insulin resistance has made it a subject of growing clinical interest.
Long-Term Risks to Be Aware Of
Cutting out entire food groups for months or years creates real nutritional gaps. Low-carb diets tend to be low in thiamin, folate, vitamins A, E, and B6, plus calcium, magnesium, iron, and potassium. Without a multivitamin or careful food planning, outright deficiencies can develop over time.
Kidney stones are one of the most frequently documented complications, especially in studies of children on long-term keto for epilepsy. The diet creates a more acidic environment in the body, which lowers urinary citrate and pH while increasing calcium in the urine, all of which encourage stone formation. Other potential long-term effects include decreased bone mineral density, elevated cholesterol and triglycerides, and elevated uric acid levels (which can trigger gout in susceptible people).
Dehydration is an ongoing concern, not just a first-week issue. The lower insulin levels on keto cause your kidneys to excrete more sodium and water than usual, which means your fluid and electrolyte needs stay elevated for as long as you follow the diet. People with existing kidney disease face additional risks, since the diet increases the workload on the kidneys through both higher protein intake and the acidic metabolic environment.
Making It Practical
Tracking your macronutrient ratios closely matters more on keto than on most diets, because even a modest overshoot on carbs can pull you out of ketosis. Many people use a food tracking app for at least the first few weeks until they develop an intuitive sense of portion sizes. Reading nutrition labels becomes a daily habit, particularly checking total carbohydrates and scanning ingredient lists for hidden sugars.
Meal prep tends to revolve around a few reliable templates: a protein source cooked in fat, paired with a generous portion of nonstarchy vegetables, topped with cheese, nuts, or an oil-based dressing. Eggs, cheese, and nuts make easy snacks that keep you in range without much planning. Dining out is manageable if you focus on grilled meats or fish, ask for extra vegetables instead of starches, and skip sauces that often contain flour or sugar.
If you want to confirm you’re actually in ketosis, inexpensive urine test strips provide a rough estimate, though they become less accurate over time as your body gets more efficient at using ketones. Blood ketone meters are more precise and measure the same range (0.5 to 3.0 mmol/L) that defines nutritional ketosis.