Ketosis is a metabolic state where your body switches from burning carbohydrates to burning stored fat as its primary fuel source. When carbohydrate intake drops low enough, typically below 50 grams per day, your liver begins converting fatty acids into molecules called ketone bodies. These ketones then circulate through your bloodstream and fuel your cells in place of glucose.
How Your Body Enters Ketosis
Under normal circumstances, your body runs on glucose, which comes from the carbohydrates you eat. Your muscles and liver store a backup supply of glucose in a form called glycogen. When you drastically cut carbs, your body burns through those glycogen stores first. Once they’re depleted, it needs an alternative energy source, and that’s when ketosis kicks in.
Your liver takes fatty acids from stored body fat (or dietary fat) and breaks them down into ketone bodies. These small, water-soluble molecules travel through the blood and are taken up by cells throughout the body, including brain cells, which can’t burn fat directly but can run on ketones. The process is a built-in survival mechanism. It evolved to keep your brain and organs functioning during periods of fasting or food scarcity.
For most people eating between 20 and 50 grams of carbohydrates per day, it takes two to four days to enter ketosis. Some people take a week or longer, especially if they were eating a high-carb diet beforehand, since their glycogen stores take more time to deplete.
The Three Ketone Bodies
Your liver produces three types of ketone molecules, each with a slightly different role. The first, acetoacetate, is the starting point. Your liver makes it from fatty acid fragments, and most of it is then converted into a second molecule, beta-hydroxybutyrate, which is the main ketone transported through the bloodstream to fuel your tissues. Beta-hydroxybutyrate is what blood ketone meters measure.
The third ketone, acetone, is a byproduct. It forms when acetoacetate breaks down spontaneously, and your body can’t use it for energy. Instead, acetone is expelled through your breath and urine. It’s the reason many people in ketosis notice a distinct fruity or metallic smell on their breath.
How Ketones Compare to Glucose as Fuel
Ketones are actually more energy-dense than glucose on a gram-for-gram basis. When cells convert 100 grams of glucose into usable energy (ATP), they produce about 8.7 kilograms of ATP. The same weight of beta-hydroxybutyrate yields roughly 10.5 kilograms of ATP, about 20% more. Acetoacetate falls in between at 9.4 kilograms. This efficiency is one reason researchers have been interested in whether ketosis offers metabolic advantages beyond simple weight loss.
Measuring Ketone Levels
Ketosis is defined by having a blood ketone level of at least 0.5 mmol/L. You can measure this with an at-home blood ketone meter, which works similarly to a blood glucose monitor using a finger prick. Urine test strips are a cheaper option but become less reliable over time as your body gets better at using ketones instead of excreting them.
The ranges break down like this:
- Below 0.6 mmol/L: Normal, not in ketosis
- 0.6 to 1.5 mmol/L: Light ketosis
- 1.6 to 3.0 mmol/L: Deeper ketosis
- Above 3.0 mmol/L: Potentially dangerous, requires medical attention
Most people following a ketogenic diet aim for the 0.5 to 3.0 mmol/L range. Levels above 3.0 are not a goal and can signal a problem, especially in people with diabetes.
Ketosis vs. Ketoacidosis
These two terms sound nearly identical but describe very different situations. Nutritional ketosis is a controlled, low-level production of ketones that your body regulates through insulin. Ketoacidosis is a medical emergency where ketone levels climb to five to ten times higher than normal ketosis, making the blood dangerously acidic.
Ketoacidosis occurs almost exclusively in people with Type 1 diabetes. Without enough insulin, the body can’t regulate ketone production, and ketones accumulate unchecked. It’s usually triggered by missed insulin doses, illness, or hormonal imbalances. For people with a functioning pancreas, the body’s own insulin response keeps ketone levels in a safe range during nutritional ketosis. The two states share a name but not a mechanism.
Medical Uses of Ketosis
The most established medical application of ketosis is treating epilepsy, particularly in children who don’t respond well to medication. The ketogenic diet has been used for this purpose since the 1920s, making it one of the oldest epilepsy treatments still in use. Johns Hopkins has treated more than 1,500 children through its ketogenic diet program for seizures over nearly a century.
Researchers don’t fully understand why ketosis reduces seizure activity, but the effect is well documented. The high-fat, very-low-carb diet remains a standard option when anti-seizure medications aren’t enough on their own. Beyond epilepsy, researchers are investigating whether ketosis may have benefits for other neurological conditions, though the evidence in those areas is still developing.
What It Feels Like to Enter Ketosis
The transition into ketosis is often uncomfortable. During the first few days, as your body depletes its glycogen and adjusts to burning fat, you may experience fatigue, headaches, irritability, brain fog, and nausea. This cluster of symptoms is commonly called the “keto flu.” It’s temporary, typically lasting a few days to a week, and is largely driven by fluid and electrolyte shifts that happen when glycogen stores empty out (glycogen holds water, so losing it means losing fluid too).
Once your body adapts, many people report stable energy levels, reduced hunger between meals, and improved mental clarity. The fruity or acetone-like breath is another reliable sign. Some people also notice a change in the smell of their urine. These signals, along with ketone testing, can help confirm that you’ve entered and are maintaining ketosis.
Who Should Be Cautious
Ketosis is generally safe for healthy adults, but it’s not appropriate for everyone. People with Type 1 diabetes face a real risk of ketoacidosis. People with Type 2 diabetes on insulin or certain blood sugar medications can experience dangerous drops in blood sugar. Pregnant or breastfeeding women, people with pancreatic insufficiency, and those with a history of eating disorders should approach ketosis with caution or avoid it entirely. Anyone with liver or kidney disease needs medical guidance, since both organs play central roles in producing and clearing ketones.