What is a butter fast?

A butter fast is a type of short-term fasting diet that involves getting a large portion of calories from butter during the fasting period. It was popularized by blogger and author Bulletproof Dave Asprey.

The rules of the butter fast diet are:

Fast for 1-3 days
During the fast, get 25-50% of your total calories from butter
Drink only water, herbal tea, or bone broth during the fast

So on a typical butter fast day, you might have 3-5 tablespoons of butter several times throughout the day, along with water or herbal tea. Some versions allow small amounts of MCT oil as well.

What are the claimed benefits?

Proponents claim that a butter fast can provide these benefits:

Weight loss – By severely restricting calories, a butter fast should lead to weight loss over the 1-3 day period.

Reset hunger hormones – Fasting helps reset hunger-regulating hormones like leptin and ghrelin.
Reduce inflammation – High-quality butter contains butyrate and other anti-inflammatory compounds.
Improve cognition – Ketones produced from fasting can provide an alternative fuel source for the brain.

However, there is limited scientific evidence to back up these claims currently. More research is needed on butter fasts specifically.

Is it healthy?

There are some potential concerns with doing a butter fast:

Very low calorie intake – Consuming so few calories can be difficult to sustain for multiple days.

High fat intake – Getting 50% or more of calories from butter is extremely high fat.
Lack of protein – Not eating adequate protein for several days can lead to muscle loss.
Nutrient deficiencies – Going days without vitamins, minerals from whole foods can be unhealthy.

Refeeding syndrome – Transitioning back to normal diet after fast requires care.

So people considering a butter fast should take precautions:

Only fast 1-3 days maximum

Consult doctor if you have any medical conditions
Stay well hydrated and rest during fast
Carefully transition back to normal diet after fast

Overall, more mainstream fasting methods like intermittent fasting or occasional 24-hour fasts are likely safer options for most people.

How does it work?

Here is an overview of how butter fasting produces effects in the body:

Calorie restriction

Consuming mostly butter leads to a very low calorie intake, since butter is high in fat and low in carbs and protein. This severe calorie restriction forces the body to burn fat stores for energy.

Even consuming 500-800 calories from butter can create a large daily deficit for weight loss. However, such low calorie diets should only be followed with medical guidance.

Ketosis

With carb intake under 50 grams per day on a butter fast, it induces a state of ketosis. This means the liver starts producing ketone bodies that can be used for fuel by the brain.

Ketosis mobilizes fat and can reduce appetite. But it may also cause side effects like headache, fatigue, nausea, etc.

Gut rest

Going days without solid food gives the digestive system a rest. This may help reduce inflammation and improve gut barrier integrity.

Intermittent fasting is thought to provide gut benefits. But longer fasts of 3+ days may disrupt the gut microbiome.

Hormone regulation

Fasting is thought to help reset leptin and ghrelin, hormones that regulate appetite and fat storage. This mayexplain some of the weight loss from butter fasting.

But more research is needed on how a high butter diet in particular affects hormones long-term.

Pros and cons

Here are the main pros and cons of following a butter fast:

Pros:

Rapid weight loss in short period

May help improve cholesterol profile
Can induce ketosis and fat burning
Some potential gut rest and anti-inflammatory effects

Cons:

Very low calorie intake may be unhealthy
High butter intake could increase heart disease risk
Lack of protein and other nutrients

May be unsustainable for more than a few days

Who should avoid it?

These conditions mean you should avoid a butter fast:

History of eating disorders – Fasting may trigger disordered eating.

Diabetes – Rapid changes in blood sugar can be dangerous.
GI issues – Fasting may worsen reflux, ulcers, IBS, etc.
Pregnancy or breastfeeding – Provides inadequate nutrition.

Children or teenagers – Need adequate nutrition for growth.

In general, anyone with a medical condition should consult their doctor before attempting fasting or severely restricting calories.

Foods to eat

The key foods to consume on a classic butter fast include:

Butter – Grass-fed is ideal. Aim for at least 25% of calories from butter.
Bone broth – Provides electrolytes and vitamins.
Herbal tea – Green tea, chamomile, etc. Avoid sweetened tea.

Water – Stay well hydrated, especially if exercising.

Some modified versions of the butter fast may also allow small amounts of MCT oil or coconut oil.

Sample meal plan

Here is a sample 1-day butter fast meal plan:

Breakfast: 2 tbsp butter in coffee

Mid-morning: Herbal tea with 1 tbsp butter

Lunch: Bone broth

Afternoon: 2 tbsp butter melted in hot water or bone broth

Dinner: 3 tbsp butter with herbal tea

This provides around 700 calories, with 75% from fat. Make sure to drink plenty of non-caloric beverages like water, herbal tea, or broth throughout the day.

Exercise on a butter fast

You’ll likely have reduced energy on so few calories, so gentle exercise is best:

Walking – 30-60 minutes per day
Yoga – Hold back on intense flows; focus on gentle stretching

Swimming – Low-intensity laps or water aerobics

Avoid prolonged high intensity or strength training workouts since you need carbs to fuel these. Monitor yourself for signs of low blood sugar like dizziness.

Stay well hydrated by sipping electrolyte drinks before, during, and after exercise. Pay attention to signs of potential electrolyte imbalance.

Transitioning off a butter fast

It’s important to carefully transition back to a normal diet after doing a 1-3 day butter fast. Here are some tips:

Keep calories low-moderate on day 1 back – e.g. 1000 to 1500 calories.
Focus on lean proteins, non-starchy veggies, healthy fats.

Slowly increase calories over 3-7 days until back to maintenance.
Avoid sugar, refined carbs, high fat foods at first.
Stay hydrated and monitor for refeeding syndrome symptoms.

Take it slow to give your body time to adjust to digesting regular food again. Watch for abdominal pain, diarrhea, headache, fatigue or other symptoms.

Results to expect

Here are typical results you can expect from a 1-3 day butter fast:

Weight loss: 0.5 to 2 pounds

Reduced appetite: Lower leptin and ghrelin
Mild ketosis: Ketones around 0.5 to 3 mmol/L
Lower blood sugar: Fasting glucose 60-80 mg/dL

Improved focus: From ketones providing energy

However, these effects are temporary. Without changes to diet long-term, weight loss and appetite reductions won’t last. Talk to a dietitian about sustainable nutrition plans.

Longer term fasting risks

While occasional 24-72 hour fasts are likely safe for most people, extended fasting of 5+ days starts posing risks:

Nutrient deficiencies
Increased stress hormone cortisol
Lean tissue and muscle loss

Headaches, fatigue, dizziness
Electrolyte/mineral imbalances
Dangerously low blood sugar

Severe dehydration
Gallstone formation
Irregular heartbeat

Fasting for longer than 3-5 days should only be done with medical supervision. Children, teens, pregnant women, diabetics, and those with eating disorders should not fast.

Bottom line

Butter fasts involve getting a large portion of calories from butter during a short fast of 1-3 days. This can produce rapid weight loss. However, butter fasts are extremely restrictive and high in saturated fat, so they should be followed cautiously and not for extended periods. More research is needed on their long-term health effects.