A ketosis diet, more commonly called a ketogenic or “keto” diet, is a high-fat, very-low-carbohydrate eating pattern designed to shift your body’s primary fuel source from sugar to fat. The typical breakdown is 70 to 80 percent of daily calories from fat, 10 to 20 percent from protein, and only 5 to 10 percent from carbohydrates. That carb limit usually works out to roughly 20 to 50 grams per day, which is less than what you’d find in a single bagel.
How Your Body Enters Ketosis
Under normal conditions, your body runs on glucose from carbohydrates. When you restrict carbs drastically, your liver begins breaking down stored fat into molecules called ketone bodies, which your brain, heart, and muscles can use for energy instead. The most abundant of these is beta-hydroxybutyrate, or BHB, which becomes your body’s main circulating fuel during sustained carb restriction.
Most people enter ketosis within two to four days of keeping carbs below 50 grams daily, though it can take a week or longer depending on your activity level, metabolism, and how much glycogen (stored sugar) your body had to burn through first. You can confirm you’re in ketosis with a blood ketone meter. Normal, non-ketotic blood ketone levels sit below 0.6 mmol/L. Nutritional ketosis generally falls in the range of 0.5 to 3.0 mmol/L, though levels above 1.5 mmol/L warrant caution, particularly for people with diabetes, because they can signal a dangerous condition called diabetic ketoacidosis.
What You Eat on a Keto Diet
The diet centers on naturally high-fat, low-carb foods. Staples include fatty fish like salmon, sardines, and mackerel; meat and poultry (which contain essentially zero carbs); eggs; cheese; butter and ghee; olive oil; and nuts like almonds, macadamia nuts, pecans, and walnuts. Avocados and olives are particularly useful because they’re high in fat while also providing fiber.
Vegetables are part of the diet, but you stick to nonstarchy options. Leafy greens like spinach, kale, arugula, and Swiss chard are go-to choices. Other staples include broccoli, cauliflower, zucchini, asparagus, bell peppers, mushrooms, cucumbers, Brussels sprouts, and green beans. These provide volume and micronutrients without pushing you over your carb limit.
The foods you cut are the ones most people eat in large quantities: bread, pasta, rice, potatoes, sweet potatoes, corn, most fruits, beets, winter squashes like butternut and acorn, and anything with added sugar. Even some vegetables like onions need to be used sparingly because their carbs add up faster than you’d expect.
The “Keto Flu” and Early Side Effects
During the first week or so, many people experience a cluster of symptoms often called the keto flu. This isn’t an actual illness. It’s your body adjusting to running on a completely different fuel source. Common symptoms include nausea, stomach pain, dizziness, sugar cravings, muscle cramps, brain fog, irritability, poor sleep, and either diarrhea or constipation.
Much of this comes down to fluid and electrolyte shifts. When you cut carbs sharply, your kidneys excrete more water and sodium than usual, which pulls potassium and magnesium along with it. Staying well hydrated and deliberately increasing your intake of salt, potassium, and magnesium can prevent or reduce cramps and nausea. Sports drinks or electrolyte supplements are a straightforward way to cover the gap. For most people, the keto flu resolves within a week or two as the body adapts to burning fat efficiently.
Weight Loss and Short-Term Benefits
The primary reason most people try a keto diet is weight loss, and the short-term results can be dramatic. Some of the initial drop is water weight, since your body releases stored water as it burns through glycogen. Beyond that, ketosis appears to reduce appetite for many people, making it easier to eat fewer calories without feeling hungry. The high fat content of meals also promotes satiety.
Some people report improved mental clarity and more stable energy levels once they’re fully adapted, likely because blood sugar stays low and steady rather than spiking and crashing after carb-heavy meals. The American Heart Association’s 2026 dietary guidance acknowledges that some popular weight loss diets, including very-low-carb approaches, can produce short-term benefits but notes that their long-term cardiovascular impact remains uncertain.
Medical Uses Beyond Weight Loss
The ketogenic diet was originally developed as a medical treatment, not a weight loss plan. It has the longest track record in epilepsy, where it’s used for people whose seizures don’t respond well to medication. Johns Hopkins, which opened the world’s first adult epilepsy diet center, recommends dietary therapy for adults who have tried multiple seizure medications with limited success, or who have specific metabolic conditions like glucose transporter type 1 deficiency syndrome.
Researchers are also studying ketogenic diets in the context of type 2 diabetes, since dramatically reducing carbs lowers blood sugar and can reduce the need for insulin. Adults with epilepsy who also have diabetes, obesity, high blood pressure, or obstructive sleep apnea may be considered particularly good candidates for diet-based therapy because it can address several conditions simultaneously.
Long-Term Risks and Nutritional Gaps
Sustaining a keto diet over months or years raises several concerns. The most discussed is cardiovascular risk. Because the diet is high in saturated fat from sources like butter, cheese, and red meat, it’s associated with increases in LDL cholesterol, the type linked to heart disease. Major health guidelines recommend keeping saturated fat below 7 percent of daily calories, a threshold that’s difficult to stay under on a standard ketogenic plan.
Nutrient deficiencies are another real issue. Cutting out most fruits, grains, and legumes means you lose major sources of selenium, magnesium, phosphorus, B vitamins, and vitamin C. Over time, these gaps can affect everything from immune function to bone health. People on long-term keto diets typically need to be more deliberate about eating a wide variety of the allowed vegetables and may benefit from a multivitamin.
Kidney strain is a further consideration. The combination of higher protein intake and the metabolic byproducts of sustained ketosis puts extra demand on the kidneys. For someone with existing kidney issues, this can be particularly problematic. Kidney stones have also been reported at higher rates among long-term keto dieters, especially children on medically supervised ketogenic protocols for epilepsy.
Who It Works Best For
A ketosis diet tends to work well for people who find it easier to cut an entire food group than to moderately reduce calories across the board. If you feel more satisfied eating rich, fatty foods and don’t miss bread or fruit much, the eating pattern may feel sustainable. It also has clear medical applications for drug-resistant epilepsy and is being explored for blood sugar management in type 2 diabetes.
It’s a poor fit if you have existing kidney disease, a history of disordered eating, or difficulty affording the higher-cost foods (quality meats, fish, nuts, avocados) that make the diet nutritionally adequate. People who exercise at high intensities may also find that performance suffers, since explosive, anaerobic efforts rely heavily on glucose as fuel. Most people who try keto for weight loss cycle on and off the diet rather than maintaining it indefinitely, which can work well as long as you transition back to balanced eating rather than simply returning to old habits.