Fat fasting is a dieting technique used by people who want to achieve quick fat loss.
It works by raising your blood levels of molecules called ketones and pushing your body into ketosis, mimicking the biological effects of fasting.
People who use fat fasting claim it’s useful for breaking weight loss plateaus, getting back into ketosis after a cheat day, and losing a few pounds quickly, without hunger or cravings.
Still, you may wonder whether this technique is healthy.
This article explores what fat fasting is and whether it’s good for your health.
A fat fast is a high-fat, low-calorie diet that typically lasts 2–5 days.
During this time it’s recommended to eat 1,000–1,200 calories per day, 80–90% of which should come from fat.
Though not technically a fast, this approach mimics the biological effects of abstaining from food by putting your body into the biological state of ketosis (1Trusted Source).
In ketosis, your body uses fat, rather than carbs, as its main energy source. During this process, your liver breaks down fatty acids into molecules called ketones, which can be used to fuel your body (2Trusted Source).
Ketosis occurs during times when glucose, your body’s main source of energy, isn’t available, such as during periods of starvation or when your carb intake is very low (1Trusted Source, 3Trusted Source).
The time it takes to achieve ketosis can vary considerably, but if you’re following a ketogenic diet you can typically expect to reach this state between days 2 and 6 (4Trusted Source).
Fat fasting is designed to get you into ketosis quickly or to boost ketone levels if you have already achieved ketosis by restricting both your calorie and carb intake.
It’s usually used by people on a ketogenic diet who want to break through an ongoing weight loss plateau or by those wanting to get back into ketosis after a cheat day, on which the rules of a low-carb diet are relaxed and you eat foods that are high in carbs.
Others implement a fat fast to lose a few pounds quickly.
A fat fast is very low in calories and high in fat. It’s designed to create a calorie deficit, which is needed for weight loss, while quickly depleting your body’s carb stores so you move into ketosis and burn more fat.
Thus, if you adhere to this protocol strictly for 2–5 days, you may enter ketosis and begin burning fat as your primary source of fuel, particularly if you’re already on a very-low-carb diet.
If you’re already following a low-carb or ketogenic diet, you may also find that a fat fast boosts your ketone levels, as your body burns more fat to meet your body’s energy needs.
In theory, the combination of a calorie deficit and lack of dietary carbs during a fat fast could lead you to burn more fat (1Trusted Source, 5Trusted Source).
Nonetheless, a fat fast only lasts a few days, so large shifts on the scale can’t be explained by fat loss alone.
The loss of your body’s carb stores also leads to a loss of water, which is stored alongside glycogen, the stored form of glucose. This gives the illusion of fat loss (6Trusted Source, 7Trusted Source, 8Trusted Source, 9Trusted Source).
In fact, if you’re not already keto adapted or you’re doing a fat fast after a cheat day, a lot of the weight lost during a fat fast is likely water weight.
This weight will return as soon as you begin eating carbs again and replace your body’s glycogen stores.