What Is Ketosis? Benefits & the Ketoacidosis Risk

What Is Ketosis, at the Cellular Level?

Ketosis is what happens when your liver starts turning fat into ketone bodies because glucose and stored glycogen are running low. Your body prefers glucose as fuel under normal conditions and keeps a reserve of it stored as glycogen in the liver and muscles. Once that reserve drops — typically after 12 to 24 hours without carbohydrate, sooner with vigorous exercise — the liver begins breaking down fatty acids into three related molecules: acetoacetate, beta-hydroxybutyrate (BHB), and acetone.

Together these are called ketone bodies, and cells throughout the body — including the heart, muscles, kidneys, and to a large extent the brain — can burn them for energy almost as easily as they burn glucose. Beta-hydroxybutyrate is the main ketone your body actually uses for fuel and the one most home test devices measure. Acetoacetate is the one urine strips detect. Acetone is largely a byproduct cleared through breath and urine — it's the molecule behind the fruity, nail-polish-remover smell some people notice on their breath during deeper ketosis. This whole process, called ketogenesis, happens almost entirely in the liver.

Does the Brain Need Carbohydrates to Function?

The brain runs mostly on glucose day to day, but it can adapt to run largely on ketones when carbohydrate intake stays low. Under normal, carb-containing conditions, the brain uses roughly 120 grams of glucose a day and gets nearly all its energy that way. During extended fasting or a sustained low-carb diet, the brain adapts over one to several weeks and can derive an estimated two-thirds of its energy from ketone bodies, with the remainder still coming from glucose.

That remaining glucose need doesn't require eating carbohydrate. The liver can manufacture the glucose the brain and red blood cells still require through a process called gluconeogenesis, using amino acids from protein and glycerol from fat breakdown as raw material. This is a well-established, evolutionarily old backup system — part of what let humans survive extended famines and lean hunting seasons long before agriculture existed.

Reported Benefits of Nutritional Ketosis

Research links nutritional ketosis to several outcomes, though results vary by person and study design. The most consistently reported benefit is appetite reduction — many people report feeling less hungry on a ketogenic or very-low-carb diet, which researchers attribute partly to ketone bodies' own appetite-suppressing effects and partly to steadier blood sugar. That reduced hunger is one reason ketogenic diets often produce weight loss without an explicit focus on counting calories.

Ketogenic and very-low-carb diets have also shown blood-sugar-lowering effects in people with insulin resistance and type 2 diabetes, and some studies report improved triglycerides and HDL cholesterol. The oldest, best-established medical use predates the weight-loss trend by a century, though: physicians have used ketogenic diets since the 1920s to treat drug-resistant epilepsy, particularly in children, and it remains a recognized therapy today under medical supervision. None of this makes ketosis automatically right for a given person — the size of the benefit and the tradeoffs depend on individual health status, which is why the next distinction matters so much.

Nutritional Ketosis vs. Diabetic Ketoacidosis

Nutritional ketosis and diabetic ketoacidosis (DKA) both involve elevated ketones, but one is a normal metabolic adaptation and the other is a medical emergency. The difference comes down to insulin. In nutritional ketosis, you still have enough working insulin to keep ketone production within a controlled, moderate range — blood ketones typically stay under about 3 mmol/L, blood sugar stays in a normal range, and blood pH stays normal. Insulin acts like a brake on ketone production, and in a person without diabetes, or with well-managed diabetes, that brake keeps working.

DKA happens when there isn't enough insulin to apply that brake — most often in type 1 diabetes, less commonly in advanced type 2 diabetes. Without insulin regulating the process, ketones and blood glucose climb together, sometimes to extreme levels, and the blood becomes acidic. That combination — high ketones, high blood sugar, and blood acidity — is what makes DKA dangerous; it can cause vomiting, severe dehydration, and confusion, and can be life-threatening without emergency treatment.

One important, real exception: a class of diabetes medications called SGLT2 inhibitors (drugs such as empagliflozin, canagliflozin, and dapagliflozin) can trigger a form of DKA where blood sugar looks deceptively normal, sometimes called euglycemic DKA. Combining an SGLT2 inhibitor with a ketogenic or very-low-carb diet raises this risk further. Anyone taking an SGLT2 inhibitor, anyone with type 1 diabetes, and anyone with diabetes generally should talk with their doctor before starting a ketogenic approach, given this risk.

Warning signs of DKA include excessive thirst, frequent urination, nausea or vomiting, abdominal pain, deep or rapid breathing, fruity-smelling breath, and confusion. These are not symptoms of ordinary nutritional ketosis. Anyone experiencing them — especially alongside high blood glucose readings or a diabetes diagnosis — should seek emergency medical care rather than assume it is a normal keto adjustment.

Tips for Reaching Nutritional Ketosis

Most people reach nutritional ketosis by restricting carbohydrate enough that the liver switches to fat-burning for fuel. Clinical and dietary research on ketogenic diets commonly describes roughly 20 to 50 grams of net carbohydrate a day as the level that triggers ketosis for most people, though the exact threshold varies by individual, activity level, and metabolic history. Replacing those carbohydrate calories with fat, rather than protein, is what distinguishes a ketogenic diet from a general low-carb diet.

A few practical factors affect how quickly ketosis develops. Depleting glycogen stores faster — through fasting, intermittent fasting, or exercise — tends to speed the transition. Adequate sodium, potassium, and magnesium matter too: as insulin drops, the kidneys excrete more sodium and water, which can otherwise cause the fatigue and headaches described further below. Staying well hydrated and salting food adequately are commonly recommended, non-prescriptive starting points; anyone with kidney disease, heart failure, or a blood pressure medication should check with a doctor before deliberately increasing sodium intake.

How Protein Affects Ketosis

Protein has a smaller, more nuanced effect on ketosis than carbohydrate does, despite a common claim that too much protein knocks you out of ketosis. The liver can convert some amino acids into glucose through gluconeogenesis, and that process is often blamed for interrupting ketone production when protein intake is high. In practice, gluconeogenesis is demand-driven — regulated by hormones and the body's actual need for glucose — rather than a simple one-to-one conversion of every extra gram of protein eaten.

For most people, moderate protein intake doesn't meaningfully block ketosis, and adequate protein matters for preserving muscle mass on a ketogenic diet, especially during weight loss. Ketogenic diet plans commonly land protein in a moderate range — enough to support muscle, not so much that it becomes the dominant macronutrient — while fat supplies most daily calories. If ketone readings dip after a very high-protein meal, that's more likely a temporary blip than evidence that protein itself is broadly incompatible with ketosis.

What to Eat on a Keto Diet: A Beginner's Framework

A typical ketogenic macro split works out to roughly 70-75% of calories from fat, 20-25% from protein, and 5-10% from carbohydrate — the exact ratio matters less than staying under the individual carb threshold described above. In practice, that means building meals around a fat source, a moderate portion of protein, and low-carb vegetables, rather than counting every macro at every meal.

Foods that fit most keto plans: eggs, fatty fish, poultry, meat, full-fat dairy, olive and avocado oil, nuts and seeds in moderation, and non-starchy vegetables like leafy greens, broccoli, cauliflower, zucchini, and peppers. Foods to limit or avoid: grains, bread, pasta, rice, most fruit (berries in small portions are the common exception), starchy vegetables like potatoes and corn, and added sugar in any form. Many beginners are surprised by how many everyday condiments and sauces carry hidden sugar; reading labels for the first few weeks catches most of these.

A simple way to start a day's planning: pick a protein and fat source for each meal first (eggs and avocado, salmon and olive oil, chicken thighs and cheese), then add a non-starchy vegetable, then check that nothing on the plate is a hidden carb source. This "protein and fat first" approach is easier to sustain for most beginners than trying to hit an exact gram target from day one.

Common Beginner Mistakes on a Keto Diet

The most frequent beginner mistake is under-eating fat while also cutting carbs, which leaves too few total calories and drives the fatigue and irritability often wrongly blamed entirely on "keto flu." Fat needs to actively replace the calories carbohydrate used to provide, not just disappear alongside the carbs.

A close second is under-salting. Beginners coming from a general low-salt mindset often keep sodium low exactly when the body's insulin-driven sodium loss (described above) means it needs more, not less — this single fix resolves a large share of early headache and fatigue complaints. Third, many beginners chase a specific ketone number instead of tracking how they feel and whether weight and other markers are trending the way they expect; a lower ketone reading is not automatically a failure, since individual variation in the nutritional-ketosis range is normal.

Finally, treating keto as a short-term crash diet rather than a sustained eating pattern leads to the most common failure mode: rapid early weight loss (much of it water weight from glycogen and its associated water), followed by reintroducing carbohydrate all at once, which reverses the water-weight loss quickly and reads, misleadingly, as "keto doesn't work." A gradual, planned reintroduction if and when someone chooses to come off keto avoids that whiplash.

What Counts as "Optimal" Ketosis, and How Do You Measure It?

There is no single universal number that defines "optimal" ketosis — what counts as optimal depends on why someone is pursuing it. For general fat-burning and appetite benefits, many low-carb researchers describe blood ketone (BHB) levels of roughly 0.5 to 3.0 mmol/L as the nutritional ketosis range, with some describing 1.5 to 3.0 mmol/L as a deeper, more stable band. Therapeutic ketogenic diets used for epilepsy are managed under medical supervision with their own closely monitored targets — a reminder that more ketones isn't automatically better; it depends on the goal.

Three methods exist for measuring ketones, each with tradeoffs.

Beyond a meter, some people notice subjective signs of ketosis: reduced appetite, a metallic taste or fruity-smelling breath from acetone, and, for some, an initial dip in energy before adapting. These signs are suggestive but not reliable enough to confirm ketosis on their own — a blood or urine reading is the only way to know with any precision.

Side Effects, the "Keto Flu," and Who Should Be Cautious

The most common early side effect of starting a ketogenic diet is a cluster of symptoms nicknamed the "keto flu" — fatigue, headache, irritability, nausea, and muscle cramps that typically show up in the first few days and resolve within one to two weeks. Researchers generally attribute this to the fluid and electrolyte shifts that happen as insulin drops and the kidneys excrete more sodium and water, not to the ketones themselves. Adequate salt, fluids, and electrolytes are commonly recommended to ease these symptoms, alongside patience while the body adapts.

Other reported side effects include constipation, bad breath, and, in some people, a rise in LDL cholesterol — worth discussing with a doctor if you have existing cardiovascular risk factors. None of these are signs of ketoacidosis; they are common, generally manageable adjustments to a lower-carb way of eating.

Is ketosis safe for everyone? No single approach fits every person, and several groups should talk to a doctor before trying a ketogenic diet rather than starting on their own: anyone with type 1 diabetes or on insulin, anyone taking an SGLT2 inhibitor, anyone who is pregnant or breastfeeding, anyone with a history of an eating disorder, and anyone with kidney disease, gallbladder disease, pancreatitis, or a rare metabolic disorder affecting fat breakdown. Children being considered for a therapeutic ketogenic diet for epilepsy should only do so under the direct supervision of a physician and dietitian, never as a self-directed diet. For general evidence-based background, the American Diabetes Association and the National Institute of Diabetes and Digestive and Kidney Diseases are useful starting points alongside your own clinician.

If you're exploring ketosis specifically to address insulin resistance or prediabetes, see our guide on how to reverse insulin resistance, and consider tracking your fasting insulin alongside ketone readings for a fuller picture of what's changing.