Your body makes ketones when it runs low on carbohydrates and starts burning fat for fuel instead. This happens in the liver, where fatty acids are broken down into three types of ketone molecules that your brain, heart, and muscles can use for energy. You can trigger this process through diet, fasting, or specific supplements, and each method raises blood ketone levels on a different timeline and to a different degree.
How Your Liver Produces Ketones
Ketone production starts when your insulin levels drop low enough for fat cells to release stored fatty acids into the bloodstream. Those fatty acids travel to the liver, where cells break them apart through a process called beta-oxidation. The fragments that result are then assembled into ketone bodies through a series of enzymatic steps. The key rate-limiting enzyme in this chain is activated by glucagon, the hormone your pancreas releases when blood sugar is low. This is why carbohydrate restriction is so central to ketone production: fewer carbs means less insulin, more glucagon, and a green light for the liver to convert fat into ketones.
The liver produces three ketone molecules. Beta-hydroxybutyrate (BHB) is the most abundant and the one measured by blood ketone meters. Acetoacetate is the second, and acetone (the one responsible for the metallic or fruity breath some people notice) is the third. Once released into the bloodstream, these molecules are taken up by tissues throughout the body and converted back into usable energy. The liver itself cannot use the ketones it makes, so it exports virtually all of them.
The Ketogenic Diet Approach
The most reliable way to make ketones is to eat a ketogenic diet. The standard macronutrient breakdown is roughly 70 to 80 percent of daily calories from fat, 10 to 20 percent from protein, and 5 to 10 percent from carbohydrates. On a 2,000-calorie diet, that translates to about 165 grams of fat, 75 grams of protein, and 40 grams of carbohydrates per day. Most people need to stay below 50 grams of total carbs daily to sustain ketone production, and some need to go as low as 20 grams.
Protein matters more than people expect. Eating too much protein can actually prevent ketosis because your body converts excess amino acids into glucose. The goal is enough protein to preserve muscle mass without overshooting into a range that suppresses ketone production. This is the main distinction between a ketogenic diet and a generic “low-carb, high-protein” diet.
Most people enter measurable ketosis within two to four days of carbohydrate restriction, though the transition period varies. Blood BHB levels between 0.5 and 3.0 mmol/L indicate nutritional ketosis. This range is well below the 15 to 25 mmol/L levels seen in diabetic ketoacidosis, a dangerous condition that occurs almost exclusively in people with type 1 diabetes or severe insulin deficiency. For people with normal insulin function, the body self-regulates and keeps ketone levels within a safe range.
Fasting as a Ketone Trigger
Fasting is the fastest way to start making ketones because it depletes your liver’s glycogen (stored carbohydrate) within roughly 12 to 24 hours. Once glycogen stores are low, the liver shifts to ketone production. Even an overnight fast of 12 to 16 hours can produce a mild rise in BHB levels, which is why intermittent fasting and time-restricted eating are sometimes used alongside a ketogenic diet to deepen ketosis.
Extended fasting (24 hours or more) produces higher ketone levels, with starvation-level ketosis reaching 5 to 10 mmol/L. This is still below the threshold for ketoacidosis in healthy individuals, but prolonged fasting carries its own risks, including muscle loss and nutrient deficiencies, so it is not a practical long-term strategy for most people.
MCT Oil and C8 Supplements
Medium-chain triglyceride (MCT) oil offers a way to boost ketone production without strict carbohydrate restriction. MCTs bypass normal fat digestion and travel directly to the liver, where they are rapidly converted into ketones. Not all MCTs are equally effective, though. Caprylic acid (C8), an eight-carbon fatty acid, is roughly three times more ketogenic than capric acid (C10) and six times more ketogenic than lauric acid (C12), which is the predominant fat in coconut oil.
In studies, around 20 grams of C8 taken without carbohydrates raised blood BHB from a baseline of about 0.1 mmol/L to roughly 1.0 mmol/L over six hours, a steady climb rather than a quick spike. However, combining C8 with a carbohydrate-rich meal significantly blunted this effect, cutting ketone production by roughly half. The relationship between C8 dose, carb intake, and resulting ketone levels is not straightforward: simply taking more MCT oil does not proportionally raise ketones if carbohydrate intake remains high. For people who want the benefits of mild ketosis without a full ketogenic diet, taking C8 MCT oil on an empty stomach or alongside a low-carb meal gives the best results.
Start with a small dose (5 to 10 grams) and increase gradually. MCT oil often causes digestive discomfort, including cramping and diarrhea, when introduced too quickly.
Exogenous Ketone Supplements
Exogenous ketones are pre-made ketone molecules you drink, skipping the need for your liver to produce them. They come in two main forms: ketone salts and ketone esters.
- Ketone salts are BHB bound to a mineral like sodium, potassium, or calcium. They raise blood BHB moderately and are widely available as powdered supplements.
- Ketone monoesters are a more concentrated form that raises BHB significantly higher than salts. A meta-analysis found that exogenous ketones overall raised blood BHB by about 2.0 mmol/L above placebo levels for up to five hours, with monoesters producing a significantly greater effect than salts. Monoesters also lowered blood glucose more than salts did.
The trade-off is practical. Ketone esters are expensive and taste notoriously unpleasant. Ketone salts are cheaper and more palatable but less potent. Neither type produces the same metabolic state as making your own ketones through diet or fasting, because your liver’s fat-burning machinery is not engaged in the same way. Exogenous ketones are most useful as a short-term boost, for instance before exercise or cognitive tasks, rather than as a replacement for dietary ketosis.
Exercise and Ketone Production
Physical activity accelerates ketone production by burning through glycogen stores faster and increasing demand for fatty acid oxidation. Moderate-intensity exercise performed in a fasted state or while following a low-carb diet can noticeably raise blood ketone levels. High-intensity exercise has a more complicated effect: it can temporarily suppress ketones because your muscles are burning them so rapidly, but levels often rebound higher afterward.
If you are transitioning into ketosis, exercise during the first few days may feel harder than usual because your body has not yet adapted to using ketones efficiently. This adaptation period typically takes one to two weeks.
Managing the Transition Period
The shift into ketone production triggers a temporary increase in sodium and potassium excretion through the kidneys. This effect is strongest during days one through four and typically subsides within about two weeks. During this window, you lose more water and electrolytes than normal, which explains the fatigue, headaches, dizziness, and brain fog commonly called “keto flu.”
The fix is straightforward: replace what you are losing. Drinking one to two cups of broth or bouillon daily adds 1 to 2 grams of sodium, which is generally enough to offset the increased losses. For potassium, aiming for about 4 grams per day from foods like avocados, nuts, seeds, and leafy greens covers the gap. These are not permanent requirements. Once your kidneys adjust to ketosis after the first couple of weeks, the excessive electrolyte flushing stops and your normal dietary intake is usually sufficient.
Tracking Your Ketone Levels
Three methods exist for measuring whether your body is actually producing ketones. Blood meters that test BHB from a finger prick are the most accurate and give a precise reading in mmol/L. Urine strips detect acetoacetate and are cheap, but they become less reliable over time because your body gets better at using ketones instead of excreting them. Breath meters measure acetone and are reusable but less precise than blood testing.
For most people, blood BHB between 0.5 and 1.5 mmol/L represents light to moderate ketosis and is the range most easily sustained through diet. Levels above 1.5 mmol/L are common during extended fasting or very strict carbohydrate restriction. If you are not interested in buying a meter, consistent signs like reduced appetite, increased mental clarity after the adaptation period, and the faint fruity or metallic smell on your breath are reasonable indicators that ketone production is underway.