Ketosis is a metabolic state in which your body shifts from burning glucose (sugar) as its primary fuel to burning fat, producing molecules called ketone bodies in the process. This shift happens when carbohydrate intake drops low enough, typically below 20 to 50 grams per day, that your body depletes its stored glucose (glycogen) and turns to fat reserves instead. It takes most people 2 to 4 days of very low carb eating to enter ketosis, though some need a week or longer.
How Your Body Produces Ketones
Under normal conditions, your cells run primarily on glucose from the carbohydrates you eat. Your liver stores a backup supply of glucose as glycogen, enough to last roughly a day or two. When carbs are scarce and those glycogen stores run low, your insulin levels drop. That drop signals your fat cells to release fatty acids into the bloodstream.
Your liver then takes those fatty acids and converts them into three types of ketone bodies: acetoacetate, beta-hydroxybutyrate (the most abundant one), and acetone. These ketones travel through your bloodstream and serve as fuel for your brain, heart, muscles, and other organs. Your brain, which can’t burn fat directly, is especially dependent on ketones when glucose is unavailable.
Nutritional Ketosis vs. Ketoacidosis
This is a critical distinction. Nutritional ketosis and diabetic ketoacidosis (DKA) both involve elevated ketones, but they are vastly different conditions. In nutritional ketosis, blood levels of beta-hydroxybutyrate typically reach 0.6 to 5 or 6 millimoles per liter (mmol/L). Your body regulates this process through insulin, keeping ketone production within a safe range and blood pH stable.
Diabetic ketoacidosis, on the other hand, occurs primarily in people with type 1 diabetes (and sometimes type 2) whose bodies produce little or no insulin. Without that brake, ketone levels can skyrocket to 20 to 25 mmol/L, and blood glucose rises dramatically. The massive flood of acidic ketones pushes blood pH down to dangerous levels, between 7.20 and 7.35 (normal is around 7.4). DKA is a medical emergency. Nutritional ketosis in a person with normal insulin function does not carry this risk.
What It Takes to Stay in Ketosis
The most common approach is a very-low-carbohydrate ketogenic diet, which limits carbs to 20 to 50 grams per day, gets 60 to 75% of calories from fat, and fills the rest with moderate protein. For context, a single banana has about 27 grams of carbs, so the restriction is significant.
Stricter therapeutic versions, like the classic 4:1 ketogenic diet used in clinical settings, push fat intake to around 90% of total calories with only about 4% from carbs. More flexible versions like the modified Atkins diet allow roughly 60 to 70% of calories from fat with under 20 grams of carbs daily. The common thread across all of them is keeping carbohydrates low enough to suppress insulin and keep your liver producing ketones.
The “Keto Flu” Transition Period
Many people experience a cluster of unpleasant symptoms during the first few days to a week of entering ketosis, commonly called the keto flu. In one analysis of consumer reports, the most frequently mentioned symptoms were flu-like feelings (reported by about 45% of people), headache (25%), fatigue (18%), nausea (16%), and dizziness (15%). Brain fog, gut discomfort, low energy, feeling faint, and heart palpitations also appeared regularly.
These symptoms are largely tied to fluid and electrolyte shifts. When insulin drops and your body burns through glycogen, you lose a significant amount of water and, with it, sodium, potassium, and magnesium. People who have gone through this transition consistently report that increasing sodium intake, supplementing electrolytes, drinking broth, and ensuring adequate potassium and magnesium helped resolve symptoms faster. Most keto flu symptoms are temporary and fade within a week or two.
How Ketosis Affects Appetite
One of the most noticeable effects of sustained ketosis is reduced hunger. Ketone bodies appear to directly influence the hormones that control appetite. In a controlled study, participants who consumed a drink that raised blood beta-hydroxybutyrate levels to 3.3 mmol/L experienced significantly lower levels of ghrelin, the hormone that drives hunger, for two to four hours afterward. Their self-reported hunger and desire to eat dropped measurably at the 90-minute mark compared to a glucose-matched control drink.
The mechanism seems to involve ketones interacting with receptors on cells in the gut lining, altering the signals those cells send to the brain about hunger and fullness. This appetite-suppressing effect is one reason many people find it easier to eat fewer calories on a ketogenic diet without consciously trying to restrict food intake.
Blood Sugar and Insulin Benefits
Ketosis has shown meaningful effects on blood sugar control, particularly for people with type 2 diabetes. Meta-analyses of clinical trials have found that ketogenic diets reduce HbA1c (a measure of average blood sugar over three months) by about 1 percentage point more than comparison diets. To put that in perspective, many diabetes medications aim for a 0.5 to 1 point reduction.
Insulin sensitivity also improves substantially. One study found that just six days on a very-low-carb diet reduced fasting insulin by 53% and improved a standard measure of insulin resistance (HOMA-IR) by 57% in people with fatty liver disease. Multiple meta-analyses confirm these improvements, and some studies have found that insulin sensitivity improves even when participants don’t lose significant weight, suggesting ketosis itself plays a role beyond simple calorie reduction. The likely explanation involves reduced fat buildup in the liver, lower inflammation, and improved mitochondrial function in liver cells.
Ketosis for Epilepsy
The ketogenic diet was originally developed in the 1920s as a treatment for epilepsy, and it remains one of the most well-established therapeutic uses of ketosis. For children with drug-resistant epilepsy, more than 70% show a positive response to the ketogenic diet, a rate that exceeds the average 50% response seen with many medications for conditions like infantile spasms. In studies of less restrictive approaches like the modified Atkins diet, 65% of children experienced at least a 50% reduction in seizures, and some became seizure-free.
The results are more modest in adults, with 20 to 70% achieving at least a 50% seizure reduction depending on the study. Several mechanisms appear to work together: ketones stabilize brain energy reserves and reduce the excitability of nerve connections, they suppress the release of glutamate (a chemical that excites neurons), and they promote production of GABA (a chemical that calms neural activity). There’s also emerging evidence that ketosis reshapes gut bacteria in ways that further increase GABA levels in the brain.
How to Measure Your Ketone Levels
Three testing methods exist, each measuring a different ketone molecule. Blood meters, which use a finger prick, measure beta-hydroxybutyrate directly. This is the most accurate method, with sensitivity around 98% for detecting elevated ketones. Urine strips detect acetoacetate and are cheap and easy to use, but their specificity is low (50% or below), and they frequently produce false positives. They also become less reliable over time because as your body adapts to ketosis, it produces less acetoacetate and more beta-hydroxybutyrate, so your urine strips may show lighter readings even as you become more deeply ketotic. Breath meters measure acetone and offer a noninvasive option, though they’re generally considered less precise than blood testing.
For most people tracking nutritional ketosis, blood beta-hydroxybutyrate levels between 0.6 and 3.0 mmol/L indicate you’re in ketosis. Levels below 0.6 mmol/L are considered normal (not in ketosis). If you’re not diabetic and following a ketogenic diet, readings in the 0.5 to 3.0 range are typical and expected.