What Is Fat Fasting, and Is It Good for You?

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 (1).

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 (2).

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 (1, 3).

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 (4).

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.

Summary

Fat fasting is a short-term, low-calorie diet that mimics the effects of fasting by putting your body into ketosis. People on a fat fast eat around 1,000–1,200 calories a day, 80–90% of which come from fat.

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 (1, 5).

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 (6, 7, 8, 9).

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.

Summary

Fat fasts lead to a calorie deficit and may help you reach ketosis quicker. Still, as this approach is very short term, much of the weight lost is likely to be water weight.

Fat fasting is low in calories, protein, and micronutrients essential for good health. So it should not be recommended as a long-term diet plan.

High intakes of fat on diets like the ketogenic diet have been shown to promote weight loss and improve some health markers like blood sugar (10, 11).

However, fat fasting is higher in fat and lower in carbs and protein than a standard ketogenic diet, which typically contains 65-90% fat, 10-30% protein, and under 5% carbs. Plus, its effects on health are not well studied.

Many questions about fat fasting remain unanswered, including what risks are associated with using this method and what types of fat may be optimal when eating this way.

If you decide to try fat fasting, only follow this plan for 2–5 days, as this diet is lacking in many important nutrients, including protein, micronutrients, and fiber.

Summary

A fat fast is lacking in important micronutrients and protein and thus may have detrimental effects on your health if you follow it long term. Therefore, it’s not recommended to stay on this diet for longer than 5 days.

As a fat fast requires you to get most of your calories from fat, you need to choose foods that have a very high fat content.

This means that foods higher in protein and carbs are restricted.

Foods to eat

During a fat fast, your diet is quite limited. Foods you can eat include:

• High-fat meats and fish: bacon, sardines, and salmon
• Eggs: whole eggs and egg yolks
• Oils and high-fat spreads: coconut oil, mayonnaise, olive oil, and avocado oil
• Low-carb vegetables and high-fat fruits: avocados, olives, and non-starchy vegetables like kale, spinach, and zucchini that have been cooked in fat.
• High-fat nuts and nut butters: macadamia nuts, macadamia nut butter, etc.
• High-fat dairy: butter, cream cheese, heavy cream, and high-fat cheese like brie
• High-fat, nondairy products: full-fat coconut milk and coconut cream
• Drinks: water, tea, coffee, and sparkling water

High-fat foods should make up most of your intake so that the proportion of fat in your diet remains at 80–90%.

High-protein foods like meat, chicken, and fish, which are normally consumed in moderate amounts on a ketogenic diet, are not often included during a fat fast, as they can push your protein intake up too high.

Still, adding a very small amount of meat to your fat fast can be helpful from a flavor perspective, as long as you use it sparingly or focus on high-fat options.

Foods to avoid

Foods that are high in carbs and protein and low in fat are limited during a fat fast to ensure you’re getting the majority of your calories from fat.

Foods you need to avoid include:

• Cereals and grains: breads, pasta, crackers, cereal, oats, rice, etc.
• Beans and pulses: lentils, black beans, butter beans, etc.
• Most fruits and vegetables: avoid all except those listed above
• Low-fat dairy foods: skim milk, low-fat cheese, low-fat yogurt, etc.
• Low-fat meats and fish: chicken, beef, lamb, cod, etc.
• Cakes and confectionaries: sweets, biscuits, cakes, pastries, etc.
• Sweet drinks: juice, energy drinks, sweetened coffee, etc.

Summary

A fat fast includes only foods with a very high fat content to ensure you’re getting most of your calories from fat. Foods that are high in carbs and protein and low in fat are limited.

Given that very little is known about the safety or efficacy of fat fasting, it’s not often used in evidence-based nutrition therapy.

In fact, a fat fast is typically only used by people who are already following a ketogenic diet and experiencing a weight loss plateau that has been ongoing for several weeks.

Some people also use fat fasting to get back into ketosis after a cheat day, though this is unnecessary. If you have a cheat day, the best course of action is to return to your regular dietary pattern.

In general, the extreme restriction of a fat fast is unnecessary for most people. What’s more, for some people, including those on medications or with a health condition, it could be risky.

Summary

Fat fasting is primarily used by people who are already following a very-low-carb ketogenic diet and are struggling to lose weight. This technique is unnecessary for most people and could even be risky for some.

Fat fasting is not risk-free, and some people may experience side effects.

If you’re not following a ketogenic diet before doing a fat fast, you may experience the keto flu.

This is a set of symptoms that some people notice when they dramatically reduce their carb intake at the beginning of a low-carb diet.

Symptoms of the keto flu can last from a few days to a few weeks. Though a fat fast is a short-term, high-fat diet, it’s possible that you could experience the following (12):

• headaches
• nausea
• diarrhea
• constipation
• dizziness
• fatigue
• low exercise tolerance

Fat fasting is also extremely restrictive, and it’s not recommended to follow this eating pattern for more than 5 days. A longer duration may put you at risk of muscle wasting due to the lack of protein and calories (13).

Muscle wasting occurs when your body breaks down your muscle fibers to meet its needs for protein and energy, which your diet doesn’t provide.

The lack of variety in the diet may also lead to nutrient deficiencies if you follow it long term.

To avoid these potential side effects, only follow a fat fast for the short period recommended. You may also want to consider taking a multivitamin.

Though you may experience some side effects, being in ketosis is generally considered safe (14, 15).

Still, if you become unwell at any point during a fat fast, stop the diet and contact your healthcare provider.

In some rare instances, very-low-carb diets have been linked to the dangerous condition ketoacidosis, in which ketone levels become so high that they cause serious health problems (16, 17, 18, 19).

Therefore, people who are taking medications or have a condition like heart disease or diabetes should seek advice from their healthcare provider before beginning a fat fast and avoid doing this diet without medical supervision.

The technique is also not recommended for women who are pregnant or breastfeeding or for people considered cholesterol hyper-responders, as the diet may lead to a significant increase in cholesterol levels in susceptible individuals (16, 20).

As there is no research on the safety and efficacy of fat fasting, the potential risks associated with this eating pattern are unknown. Additionally, it’s unclear whether fat fasting provides any benefits for weight or overall health.

Thus, it may be safer to forgo fat fasting until more research is done.

Summary

The risks of fat fasting are not well known. If you have a medical condition or take medications, you should avoid fat fasting. Additionally, this diet shouldn’t be used by women who are pregnant or breastfeeding.

A fat fast lasts 2–5 days, during which you eat 1,000–1,200 calories per day, 80–90% of which comes from fat.

It’s primarily used by people experiencing a weight loss plateau on a low-carb diet.

Still, research is lacking, and it’s unknown whether this highly restrictive method is effective or safe.