The keto diet is a high-fat, very-low-carbohydrate eating pattern that forces your body to burn fat for fuel instead of its usual energy source, glucose. A typical day on keto means getting 70 to 80% of your calories from fat, 10 to 20% from protein, and just 5 to 10% from carbohydrates. For most people, that works out to roughly 20 to 50 grams of carbs per day, a dramatic drop from the 200 to 300 grams in a standard diet.
How Ketosis Works
Your body normally runs on glucose, which comes from carbohydrates. When you cut carbs severely, your glucose stores run out within a day or two. Your body then shifts to its backup fuel system: it begins breaking down stored fat into fatty acids, which travel to the liver. The liver converts those fatty acids into molecules called ketone bodies, and these ketones become the primary energy source for your brain, heart, and muscles.
This metabolic state is called ketosis. You can confirm you’re in it by measuring the level of ketones in your blood. Nutritional ketosis typically shows blood ketone levels between 0.5 and 3 mg/dL. That range is safe and intentional. It’s completely different from diabetic ketoacidosis, a dangerous condition in people with type 1 diabetes where ketone levels spike five to ten times higher because the body can’t produce insulin to regulate the process.
What You Eat on Keto
The backbone of a keto meal plan is fat-rich whole foods. Staples include meat, fish, eggs, cheese, avocados, nuts, seeds, olive oil, and coconut oil. Full-fat dairy like butter and cream are common ingredients. For vegetables, you stick to nonstarchy options with fewer than 8 grams of net carbs per cup: think broccoli, cauliflower, zucchini, spinach, green beans, and bell peppers.
Net carbs are what matter on keto, and the calculation is simple: total carbohydrates minus fiber. Fiber doesn’t raise blood sugar, so it doesn’t count against your daily limit. A cup of broccoli, for example, has about 6 grams of total carbs but 2.4 grams of fiber, giving it roughly 3.6 net carbs.
Foods that are off the table include bread, pasta, rice, most fruit, sugary drinks, potatoes, and anything made with refined sugar or flour. Even some foods that seem healthy, like bananas or oatmeal, are too carb-heavy to fit within the daily limit.
The “Keto Flu” Transition
Many people feel noticeably lousy during the first week or two of keto. This cluster of symptoms, commonly called the keto flu, can include headache, brain fog, fatigue, irritability, nausea, difficulty sleeping, and constipation. The exact cause isn’t fully understood. It may relate to the sudden drop in carbohydrates, shifts in gut bacteria, or the body’s adjustment period as it ramps up its fat-burning machinery.
The symptoms are temporary for most people and tend to fade within one to two weeks. Staying well hydrated and making sure you’re getting enough sodium, potassium, and magnesium can help ease the transition, since low-carb diets cause your kidneys to flush out more water and electrolytes than usual.
Weight Loss Results
Weight loss is the most common reason people try keto, and the evidence shows it works, though the advantage over other diets is modest in the long run. A meta-analysis of randomized controlled trials published in the British Journal of Nutrition found that people on a very-low-carb ketogenic diet lost about 0.91 kg (roughly 2 pounds) more than those on a low-fat diet over the same period. That’s a real but small difference.
Where keto often shines is in the short term. People frequently lose several pounds in the first week, though much of that initial drop is water weight rather than fat. The diet’s high fat and protein content also tends to reduce appetite, which can make it easier to eat fewer calories without feeling constantly hungry. Over months, however, the gap between keto and other calorie-controlled diets narrows considerably.
Effects on Blood Sugar and Insulin
For people with type 2 diabetes or prediabetes, the keto diet can produce significant improvements in blood sugar control. A meta-analysis of studies involving type 2 diabetics found that a ketogenic diet lowered fasting blood glucose by about 1.29 mmol/L and reduced HbA1c (a three-month average of blood sugar) by roughly 1.07 percentage points. To put that in perspective, some diabetes medications produce similar reductions.
One striking short-term study showed that just six days on a low-carb ketogenic diet reduced liver fat by 30% in people with fatty liver disease, while fasting insulin dropped by 53% and a key measure of insulin resistance fell by 57%. The diet also led to a significantly higher number of participants being able to reduce their diabetes medications compared to control groups on other diets.
These blood sugar benefits likely come from two directions. First, eating fewer carbs directly means less glucose entering the bloodstream. Second, the diet appears to improve insulin sensitivity on its own, making the body more efficient at using whatever insulin it produces. That said, one meta-analysis found that after 12 months, the differences between keto and higher-carb diets for blood sugar markers leveled out, suggesting the advantage may diminish over time.
Other Health Markers
The keto diet affects cholesterol in a mixed way. The same meta-analysis that confirmed its weight loss advantage found that keto raised HDL cholesterol (the protective kind) but also raised LDL cholesterol (the kind linked to heart disease). It lowered triglycerides and diastolic blood pressure modestly. Whether the overall cardiovascular trade-off is favorable or not depends on your individual health profile, which is why people with heart disease risk factors should approach keto with medical guidance.
Medical Origins in Epilepsy
The ketogenic diet wasn’t invented for weight loss. It was created in 1921 at the Mayo Clinic as a treatment for epilepsy in children and adults. Doctors noticed that fasting reduced seizures, and the ketogenic diet was designed to mimic fasting’s metabolic effects while still allowing patients to eat. It remains a recognized therapy for drug-resistant epilepsy today, particularly in children.
How to Know If You’re in Ketosis
There are three ways to test for ketosis, each measuring a different ketone molecule. Blood meters test for beta-hydroxybutyrate, the most abundant ketone in circulation, and are the most accurate option. Urine test strips measure acetoacetate, which is convenient and cheap but less reliable. Urine results reflect an average since your last bathroom visit rather than your current state, and they can be misleading because the reading lags behind what’s actually happening in your blood. Breath analyzers measure acetone (a byproduct of ketone breakdown) in exhaled air, but studies show inconsistent accuracy, especially in children.
For most people simply tracking their diet, urine strips are fine as a rough indicator during the first few weeks. If you want precise feedback, particularly for managing a health condition, a blood ketone meter is the more reliable choice.