The keto diet works by drastically cutting carbohydrates so your body switches from burning glucose to burning stored fat for fuel. When carbs drop low enough (typically under 20 to 50 grams per day), your liver begins converting fatty acids into molecules called ketone bodies, which your brain, muscles, and organs use as their primary energy source. This metabolic state is called ketosis, and it’s the engine behind the diet’s effects on weight loss, appetite, and energy.
What Happens When You Cut Carbs
Your body normally runs on glucose, which comes from carbohydrates. You store a backup supply of glucose as glycogen in your liver and muscles, but those reserves are limited. When you restrict carbs to roughly 20 to 50 grams a day, your glycogen stores deplete within a couple of days, and your body needs a new fuel source.
That’s where fat comes in. As glycogen drops, insulin levels fall. Insulin is the hormone that normally tells your fat cells to hold onto their stored energy. With less insulin circulating, fat cells release fatty acids into the bloodstream at a much higher rate. Your liver picks up those fatty acids and begins converting them into ketone bodies through a process called ketogenesis. This whole transition typically takes two to four days, though it can stretch to a week or longer depending on your activity level, metabolism, and how strictly you limit carbs.
How Your Liver Makes Ketones
Ketogenesis happens inside the mitochondria of liver cells and involves a tightly controlled chain of events. First, long-chain fatty acids need to be shuttled from outside the mitochondria to the inside using a transport system that acts like a molecular shuttle bus. This transport step is one of the key control points: when insulin is high (as it is on a normal carb-rich diet), it blocks the shuttle, keeping fatty acids out of the mitochondria. When insulin drops on keto, the shuttle opens up.
Once inside the mitochondria, fatty acids are broken down into smaller units called acetyl-CoA. Specialized enzymes then condense acetyl-CoA into two main ketone bodies: acetoacetate and beta-hydroxybutyrate. A third ketone, acetone, is produced in smaller amounts and is mostly exhaled through the lungs, which is why some people notice a distinct fruity or metallic breath smell in early ketosis. The entire process is suppressed whenever insulin rises, which is why even a moderate spike in carbohydrate intake can temporarily knock you out of ketosis.
How Ketones Power Your Body
Ketone bodies are small, water-soluble molecules, which gives them an advantage that regular fat molecules don’t have: they can cross the blood-brain barrier. Fatty acids cannot. This means ketones serve as the brain’s primary backup fuel when glucose is scarce. Your heart, kidneys, and skeletal muscles also readily use ketones for energy.
When ketones enter a cell, beta-hydroxybutyrate is converted back into acetoacetate and then into acetyl-CoA, which feeds directly into the same energy-production cycle (the citric acid cycle) that glucose normally fuels. The end result is ATP, the molecule every cell uses as energy currency. So the fundamental output is the same. Your cells are still making ATP; they’re just using a different raw material to get there.
Why This Leads to Fat Loss
The primary reason people lose weight on keto comes down to a sustained shift in where energy comes from. With insulin levels consistently low, fat breakdown (lipolysis) runs at a much higher rate than on a standard diet. Your fat cells are essentially unlocked, steadily releasing fatty acids to be burned or converted into ketones. This is the opposite of what happens after a carb-heavy meal, when elevated insulin actively promotes fat storage and blocks fat release.
There’s also an appetite effect. Many people on keto report feeling less hungry, which naturally reduces calorie intake without deliberate portion control. Ketones themselves appear to have a mild appetite-suppressing effect, and the high fat content of meals increases satiety. The combination of elevated fat burning and reduced hunger is what makes the diet effective for weight loss in practice, not any metabolic magic that defies basic energy balance.
Nutritional ketosis is defined as blood ketone levels between 0.5 and 3 mmol/L. This is the range associated with active fat burning for fuel. It’s worth noting this is completely different from diabetic ketoacidosis, a dangerous condition where ketone levels soar far higher in people whose bodies can’t produce insulin.
The Standard Keto Macronutrient Split
According to Harvard’s School of Public Health, a typical ketogenic diet gets 70 to 80 percent of daily calories from fat, 10 to 20 percent from protein, and just 5 to 10 percent from carbohydrates. On a 2,000-calorie diet, that 5 to 10 percent translates to roughly 25 to 50 grams of carbs per day.
In practical terms, this means building meals around oils, butter, avocados, nuts, fatty fish, cheese, eggs, and meat while avoiding bread, pasta, rice, sugar, most fruits, and starchy vegetables. Even foods that seem healthy on other diets, like beans, oats, or bananas, contain enough carbs to push you out of ketosis. Leafy greens, broccoli, cauliflower, and berries in small portions tend to be the main plant foods that fit.
The “Keto Flu” Transition Period
During the first week or so, many people experience a cluster of symptoms commonly called the keto flu. This typically includes headaches, fatigue, muscle aches, nausea, brain fog, and constipation. These symptoms are largely driven by two things: the abrupt shift in fuel sources as your body adapts, and a rapid loss of water and electrolytes.
When glycogen stores empty out, your body releases a significant amount of water (glycogen is stored with water bound to it). That water loss pulls sodium, potassium, and magnesium along with it. The resulting electrolyte imbalance is behind much of the achiness, cramping, and fatigue. Adding extra salt to food, eating potassium-rich foods like avocados and spinach, and staying well-hydrated can ease the transition. Most keto flu symptoms resolve within a week or two as your body becomes more efficient at producing and using ketones.
Uses Beyond Weight Loss
The ketogenic diet was originally developed in the 1920s as a treatment for epilepsy, and it’s still used today for children and adults with seizures that don’t respond to medication. The mechanisms are surprisingly complex. Ketones alter how mitochondria function inside neurons: they improve ATP production efficiency, reduce the buildup of damaging reactive oxygen species, and widen the electrical gradients across cell membranes, making neurons less likely to fire uncontrollably.
Ketones also appear to reduce levels of glutamate, the brain’s primary excitatory chemical messenger, by interfering with the transporters that load it into nerve terminals. At the same time, the diet enhances calming signaling pathways and boosts production of brain-derived neurotrophic factor, a protein that supports neuron health. These combined effects help stabilize the electrical activity in the brain that triggers seizures. Researchers are also exploring whether these same neuroprotective properties could benefit people with other neurological conditions, though evidence outside epilepsy is still early.
Who Should Avoid Keto
Keto is not safe for everyone. People with conditions affecting the pancreas, liver, thyroid, or gallbladder should avoid it, as the high fat intake and metabolic shifts can worsen these conditions. The gallbladder, for example, is responsible for processing fat, so anyone with gallbladder disease or who has had it removed may struggle to handle the fat load. Liver and pancreatic conditions can be directly aggravated by the increased demand for fat metabolism and bile production.
People with certain rare genetic enzyme deficiencies that impair fat metabolism are also unable to safely follow the diet, since their bodies can’t properly break down fatty acids or produce ketones. Pregnant or breastfeeding women, people with a history of eating disorders, and those on insulin or blood sugar-lowering medications need medical guidance before attempting keto, as the rapid changes in blood sugar and insulin levels can create serious complications.