A ketogenic diet is a high-fat, very low-carbohydrate eating pattern that shifts your body’s primary fuel source from carbohydrates to fat. Most versions limit carbohydrate intake to less than 50 grams per day, which is less than what you’d find in a single plain bagel. This restriction forces your body into a metabolic state called ketosis, where it burns fat for energy instead of its usual preferred fuel, glucose.
The diet was originally developed in 1921 at the Mayo Clinic as a treatment for epilepsy. Russell Wilder, the physician who coined the term “ketogenic diet,” designed it to mimic the metabolic effects of fasting, which had already shown promise in reducing seizures. A century later, the diet has found a much wider audience, primarily among people looking to lose weight or manage blood sugar.
How Ketosis Works
Your body normally breaks down carbohydrates into glucose, which fuels your brain, muscles, and organs. When carbohydrates drop low enough, your liver begins breaking down fatty acids and converting them into molecules called ketone bodies. These ketones circulate through your bloodstream and serve as an alternative energy source, particularly for the brain, which can’t burn fat directly.
Nutritional ketosis is typically defined as having blood ketone levels between 0.5 and 3 mmol/L. Most people reach this state within a few days of restricting carbohydrates to under 50 grams daily, though some people need to go as low as 20 grams to get there. The threshold varies based on your activity level, metabolism, and how much protein you eat, since excess protein can also be converted into glucose.
What You Eat on Keto
The bulk of your calories come from fat and protein. Meat, poultry, fish, shellfish, and eggs form the core of most ketogenic meals, and you can eat these until comfortably full. Vegetables are part of the diet but in controlled amounts: leafy greens, spinach, cucumber, celery, cauliflower, broccoli, asparagus, and squash are all common choices because they’re low in carbohydrates. Cheese, avocados, butter, cream, oils, and olives round out the fat intake.
What you avoid is more straightforward: bread, pasta, rice, potatoes, most fruits, sugary foods, and anything starchy. Even foods that seem healthy, like beans, lentils, and whole grains, contain enough carbohydrates to push you out of ketosis. This is the trade-off that makes keto effective but also difficult to sustain for many people.
Why It Affects Appetite Differently
One of the more interesting effects of ketosis is what it does to hunger. Normally, when you lose weight through dieting, your body fights back. It ramps up production of ghrelin, a hormone that drives hunger, making you want to eat more and regain the lost weight. This is one of the main reasons conventional diets fail long term.
Ketogenic diets appear to suppress or completely prevent this ghrelin spike. People in ketosis consistently report less hunger than people losing the same amount of weight on other diets. The mechanism isn’t fully understood, but the result is practical: many people on keto find it easier to eat less without feeling deprived, at least while they remain in ketosis.
Blood Sugar and Type 2 Diabetes
Because the diet virtually eliminates carbohydrates, it has a direct and measurable effect on blood sugar. For people with type 2 diabetes, this translates to lower fasting glucose, less insulin resistance, and reduced HbA1c levels (the marker that reflects average blood sugar over two to three months). Studies lasting six to twelve months have found significant decreases in both HbA1c and fasting insulin compared to control groups eating standard diets.
The logic is straightforward: fewer carbohydrates coming in means less glucose entering the bloodstream, which means the body needs less insulin to manage it. Over time, this can reduce the cycle of high insulin and insulin resistance that drives type 2 diabetes. Some people are able to lower or stop their diabetes medications, though this requires close medical supervision since dropping medications without adjusting for a new diet can cause dangerous blood sugar swings.
The “Keto Flu” Transition Period
The first week or two on a ketogenic diet can feel rough. Somewhere between day two and day seven, many people experience what’s commonly called keto flu: headache, fatigue, brain fog, irritability, nausea, difficulty sleeping, and constipation. These symptoms happen because your body is adjusting to burning fat instead of glucose, and because the sharp drop in carbohydrates causes you to shed water and electrolytes quickly.
Keto flu is temporary. Staying well hydrated, eating enough salt, and including plenty of vegetables helps most people get through it within a week. Eating more frequently during the transition can also ease symptoms. Once your body fully adapts to using ketones as fuel, energy levels typically stabilize and many people report feeling more mentally clear than before they started.
Potential Risks of Long-Term Use
The most commonly reported long-term concern is kidney stones, particularly uric acid stones and calcium oxalate stones. A ketogenic diet temporarily raises uric acid concentrations by 25% to 50%, with levels peaking around two to four weeks in. These levels generally return to baseline by about eight weeks, but for people already prone to kidney stones, this window of elevated uric acid can be a trigger.
The diet’s effect on cholesterol is more variable and harder to predict. Some people see improvements in their lipid profiles, particularly in triglycerides and HDL cholesterol. Others experience a significant rise in LDL cholesterol, especially if their fat intake leans heavily toward saturated sources like butter, cream, and red meat. The restrictive nature of the diet can also make it hard to get enough fiber, certain vitamins, and minerals without careful planning.
Variations for Athletes and Flexibility
The standard ketogenic diet works well for sedentary or moderately active people, but athletes and people doing intense exercise sometimes need modifications. Two common variations address this.
A targeted ketogenic diet adds carbohydrates specifically around workouts. You eat carbs before exercise to fuel performance and again within 45 minutes afterward to support recovery, then return to keto eating the rest of the day. A cyclical ketogenic diet takes a different approach: you follow a standard keto plan for five or six days, then eat 100 to 150 grams of carbohydrates on one or two “refeed” days per week. Both variations are designed to support high-intensity training while maintaining the benefits of ketosis during the rest of the week.
For most people exploring keto for general health or weight loss, the standard version is the place to start. The athletic variations add complexity and are primarily useful for people whose exercise demands genuinely outpace what a standard ketogenic diet can fuel.