The keto diet shifts your body’s primary fuel source from carbohydrates to fat. By cutting carbs to roughly 5–10% of daily calories and replacing them with fat (70–80%), you force your liver to break down fatty acids into molecules called ketones, which your brain, muscles, and organs burn for energy instead of glucose. This metabolic state, called ketosis, triggers a cascade of changes throughout your body, from how you store fat to how hungry you feel.
How Your Body Enters Ketosis
A standard ketogenic diet gets about 70–80% of calories from fat, 10–20% from protein, and just 5–10% from carbohydrates. For most people, that means eating fewer than 20 to 50 grams of carbs per day. When carb intake drops this low, your body burns through its stored glucose (glycogen) within a day or two. Once those reserves are depleted, your liver starts converting fatty acids into two main ketone molecules: acetoacetate and beta-hydroxybutyrate.
This conversion happens inside the liver’s mitochondria. Insulin levels fall because there’s little sugar entering the bloodstream, and glucagon rises, which together flip the metabolic switch toward fat burning. The ketones produced are water-soluble organic acids that pass freely through cell membranes and into the bloodstream, where they travel to organs that need fuel. Your liver makes the ketones but can’t use them itself; they’re exported entirely for other tissues.
What Happens to Your Brain
Your brain normally runs almost entirely on glucose. Under typical conditions, ketones contribute less than 5% of its energy. But during sustained ketosis, that number climbs dramatically. After several weeks of carb restriction or fasting, ketones can supply close to 60% of the brain’s fuel, effectively replacing glucose as the dominant energy source.
Ketones cross into the brain through specialized transporters in the blood-brain barrier. Unlike glucose, which requires energy (ATP) to be converted into usable fuel, beta-hydroxybutyrate and acetoacetate can be oxidized without that extra step. This makes them a somewhat more efficient fuel molecule. One notable detail: ketone uptake in the brain depends on how much is circulating in your blood rather than on how active your neurons are, which is the opposite of how glucose delivery works.
Why It Suppresses Appetite
One of the most consistent effects people notice on keto is reduced hunger, and the research backs this up. Ketogenic diets suppress the rise in ghrelin, the hormone that signals hunger to your brain. Normally, when you lose weight on any diet, ghrelin levels climb as your body tries to defend its previous weight. On a ketogenic diet, that compensatory hunger spike is significantly blunted.
The effect appears to be dose-dependent. People with higher blood levels of beta-hydroxybutyrate report less hunger and show greater release of satiety-related hormones. Studies using ketone supplements (not a full keto diet) also reduce appetite, which suggests the ketones themselves are directly involved rather than something else about the diet. This appetite suppression is one reason people find it easier to eat fewer total calories on keto without feeling deprived, which drives much of the weight loss.
Effects on Blood Sugar and Insulin
Cutting carbs naturally lowers blood sugar and reduces the amount of insulin your body needs to produce after meals. In the short term, this can be beneficial, particularly for people who struggle with blood sugar spikes. Fasting insulin levels typically drop on a ketogenic diet.
The longer-term picture is more complicated. Animal research has found that after eight weeks on a ketogenic diet, rats developed decreased sensitivity to insulin and impaired glucose tolerance. When these animals were given a glucose challenge, their bodies had to produce significantly more insulin to handle it compared to animals eating a normal diet. In response to a high-carbohydrate meal, their insulin response was exaggerated. These changes reversed quickly once the diet was stopped, suggesting they reflect a temporary metabolic adaptation rather than lasting damage. The practical takeaway: your body may become less efficient at processing carbs while you’re on keto, which matters if you suddenly reintroduce them.
Medical Uses Beyond Weight Loss
The ketogenic diet was originally developed in the 1920s as a treatment for epilepsy, and it remains a legitimate medical therapy. In a prospective study of 136 children with drug-resistant epilepsy, those placed on a ketogenic diet showed a significantly higher rate of seizure reduction (50% or greater) at six months compared to controls. Newer research in mice has found that the fiber content of ketogenic formulas plays a direct role in seizure protection, with fiber-supplemented versions outperforming fiber-deficient ones.
Researchers are also investigating ketogenic diets for neurodegenerative conditions, based on the idea that providing the brain with an alternative fuel could help compensate for impaired glucose metabolism seen in diseases like Alzheimer’s. In healthy middle-aged adults, even a short infusion of beta-hydroxybutyrate reduced the brain’s glucose consumption by about 14% while total oxygen use stayed the same, confirming the brain can switch fuels rapidly.
Effects on Physical Performance
How keto affects exercise depends heavily on the type of activity. For strength and power, the impact appears minimal. A study of elite artistic gymnasts found no significant change in strength performance on a ketogenic diet. For endurance exercise, results are mixed and often disappointing. A study of elite race walkers found that a low-carb, high-fat diet impaired both exercise economy and performance after intensified training. A pilot study of New Zealand endurance athletes reported improved body composition and wellbeing but no improvement in endurance capacity.
The underlying issue is that high-intensity and explosive activities rely on glucose broken down quickly through anaerobic pathways. Fat and ketones burn more slowly and require oxygen, making them adequate for low-to-moderate intensity efforts but insufficient for sprints, heavy lifts, or sustained high-output performance. Most athletes who stay on keto long-term either train at moderate intensities or strategically add carbs around workouts.
The “Keto Flu” Transition Period
Most people experience some combination of headaches, fatigue, nausea, irritability, and brain fog during the first week. This cluster of symptoms, commonly called “keto flu,” peaks within the first seven days and steadily fades over the first month. Among people who reported resolution times online, the median was about four and a half days, though some experienced symptoms for up to 30 days.
The primary culprit is electrolyte loss. When insulin drops, your kidneys excrete more sodium, which pulls water, potassium, and magnesium along with it. The most commonly recommended remedies reflect this: increasing sodium intake was the top suggestion among keto forum users, followed by general electrolyte supplementation, magnesium, and potassium. Staying well-hydrated and salting your food liberally during the first two weeks helps most people get through it faster.
Potential Long-Term Risks
Kidney stones are one of the better-documented long-term risks. A meta-analysis of 36 studies covering nearly 2,800 patients on ketogenic diets found an overall kidney stone incidence of 5.9% over an average follow-up of about 3.7 years. Adults had a slightly higher rate (7.9%) than children (5.8%). Nearly half of the stones were uric acid stones, which makes sense given that high protein intake and ketone production both increase uric acid levels. Calcium-based stones accounted for about 37%.
Gut health is another concern. Because most high-fiber foods (whole grains, beans, many fruits) are restricted on keto, fiber intake tends to drop significantly. Research in mice shows that fiber-deficient ketogenic formulas alter the gut microbiome differently than fiber-containing versions, shifting bacterial communities toward pathways involved in degradation rather than fermentation. The fiber content of the diet directly affected microbial diversity and evenness. For people on long-term keto, prioritizing low-carb fiber sources like leafy greens, nuts, seeds, and avocados becomes important for maintaining a healthy gut.
LDL cholesterol often rises on ketogenic diets, particularly in people who consume large amounts of saturated fat. The degree varies widely between individuals, and whether this increase translates to greater cardiovascular risk on a keto diet specifically remains debated. Periodic lipid panels are a reasonable way to monitor your response.