When your body goes into ketosis, it switches from burning glucose (sugar) as its primary fuel to burning fat instead. Your liver breaks down fatty acids and converts them into molecules called ketone bodies, which then travel through your bloodstream to power your brain, muscles, and other organs. This shift typically begins within 24 to 72 hours of significantly cutting carbohydrates or fasting, once your stored glucose runs out.
How Your Body Makes the Switch
Your body normally stores glucose in a form called glycogen, packed into your liver and muscles. When you stop eating carbohydrates, those glycogen stores deplete within roughly 24 hours. Once they’re gone, your body needs another fuel source, and it turns to fat.
Fatty acids from your fat stores travel to the liver, where they’re broken down and converted into three types of ketone bodies. Two of them are your body’s main alternative fuels. The third, acetone, is a byproduct that your body mostly exhales through your lungs, which is why many people notice a distinct fruity or metallic smell on their breath during ketosis.
Blood ketone levels typically reach about 0.6 mmol/L by day three of a very low-carb diet, which crosses the threshold into what’s considered nutritional ketosis (anything above 0.5 mmol/L). Levels continue rising over the following one to two weeks and can keep climbing for several weeks as your body becomes more efficient at producing and using ketones.
The First Few Days: Water Weight and “Keto Flu”
The most noticeable early change is rapid weight loss, but most of it is water. Every gram of glycogen in your body is stored alongside roughly 3 to 4 grams of water. When you burn through your glycogen reserves, all that bound water gets released and excreted. This can mean several pounds lost in just a few days, which is encouraging but doesn’t represent fat loss yet.
At the same time, your kidneys start flushing out more sodium than usual, a process called the natriuresis of fasting. When sodium drops, your body activates a hormone called aldosterone to hold onto whatever sodium remains. The trade-off is that aldosterone forces your kidneys to dump extra potassium. This double loss of sodium and potassium is the main driver behind what people call “keto flu”: headaches, fatigue, dizziness, muscle cramps, irritability, and brain fog. These symptoms usually peak within the first week and fade as your body adapts, especially if you make a point of getting enough salt and potassium-rich foods.
How Your Brain Adapts
Your brain is one of the most energy-hungry organs in your body, and it was long thought to run exclusively on glucose. That’s not quite true. During the first few days of ketosis, your brain still relies heavily on glucose, which your liver produces in small amounts from protein and other non-carb sources. But over several weeks of sustained ketosis, your brain gradually shifts to using ketone bodies for a larger share of its energy.
After five to six weeks of sustained ketosis, ketones can supply close to 60% of your brain’s energy needs, with glucose covering the rest. This adaptation is one reason many people report improved mental clarity once they push past the initial foggy period. Your brain doesn’t abandon glucose entirely, but it becomes far less dependent on it.
Changes at the Cellular Level
Ketosis doesn’t just change what fuel your cells burn. It also changes how they behave. One of the main ketone bodies acts as a signaling molecule, not just an energy source. It activates pathways that encourage your cells to build more mitochondria (the structures inside cells that generate energy) and to burn fatty acids more efficiently. It also dials down certain inflammatory processes, which is one reason ketogenic diets are being studied for conditions involving chronic inflammation.
These cellular changes take time to develop. The first week or two of ketosis is largely a period of adjustment, where your body is still learning to run on fat. Full adaptation, sometimes called “fat adaptation,” generally takes three to six weeks. During this period, your muscles become better at using fatty acids directly, your liver gets more efficient at producing ketones, and your brain upregulates the transporters it needs to pull ketones from the bloodstream.
What You’ll Notice Physically
Beyond the scale dropping and the initial flu-like symptoms, ketosis produces several recognizable signs:
- Fruity or chemical-smelling breath. Acetone, the least abundant ketone body, forms spontaneously and gets exhaled through your lungs. Some people describe the smell as nail polish remover.
- Reduced hunger. Many people find their appetite noticeably decreases once they’re consistently in ketosis. Studies on ketogenic diets have found that participants report less hunger even while eating fewer calories, though the exact mechanism isn’t fully understood.
- Increased thirst and urination. The sodium and water loss in the early phase means you’ll likely urinate more frequently and feel thirstier than usual.
- Temporary drop in exercise performance. During the first one to three weeks, you may feel weaker or more fatigued during workouts, particularly high-intensity efforts. This improves as your muscles adapt to burning fat.
Nutritional Ketosis vs. Ketoacidosis
There’s a critical distinction between ketosis from a low-carb diet and a dangerous condition called diabetic ketoacidosis (DKA). In nutritional ketosis, blood ketone levels typically stay between 0.5 and 3.0 mmol/L. Your body has functioning feedback loops: insulin, even at low levels, keeps ketone production in check.
Ketoacidosis is a different situation entirely. It occurs primarily in people with type 1 diabetes (and occasionally type 2) whose bodies produce little or no insulin. Without insulin to act as a brake, ketone production spirals out of control, blood becomes dangerously acidic, and ketone levels can soar well above 3.0 mmol/L. This is a medical emergency. For people with normal insulin function, the body self-regulates and ketoacidosis is extremely unlikely.
Who Should Be Cautious
Ketosis is generally safe for healthy adults, but certain groups face higher risks. People with type 1 diabetes are the most obvious concern, since their inability to produce insulin makes ketoacidosis a real danger. Women with gestational diabetes are typically advised to avoid diets that significantly raise ketone levels. People taking certain diabetes medications that affect how the kidneys handle glucose may also be at increased risk of ketone buildup, even with relatively normal blood sugar readings.
If you have any form of diabetes, kidney disease, or a history of eating disorders, ketosis is something to approach with medical guidance rather than on your own.