How Many Calories Break a Fast? - An In-Depth Guide

As a general rule of thumb, most experts recommend staying under 50 calories to remain fasted, but no studies are confirming the amount. It does, however, depend on what you eat, as certain foods are quicker to kick you out of a fasted state.

Fasting in any form has been something that’s gained significant interest over the past few years.

From weight loss and less inflammation to more energy, appetite control, and cognitive health, there’s no shortage of benefits to curbing food intake for a specific number of hours.

But while there are more perks to fasting than you can count on one hand, there’s still something many people are confused about: What breaks a fast?

Some experts say you can have certain drinks or small snacks, while others stick to water only.

In this article, we’re looking through the research to find the answer to how many calories break a fast.

Ready to get started?

The basics of dirty fasting

Before we get into the caloric side of things and what breaks a fast, there’s an informal concept in the fasting world we call dirty fasting.

In most cases, a fast means you abstain from consuming all food and beverages with any caloric value. That means sugar in your tea or coffee is out; bone broth is out; slightly sweetened beverages are out—you get the point.

But in some types of fasting, there are exceptions to the rule, where small amounts of food or drinks are okay to consume and won’t break your fast. While you’re not consuming diet soda pop or a small meal, certain things might get the green light.

We call this dirty fasting—be forewarned, this isn’t a medical term, and there’s no scientific research on its benefits or how it compares to “clean fasting.”

We use this term when discussing consuming some calories during your fasting window. This term differs from traditional fasting, where consuming any calories is prohibited.

Depending on how you practice dirty fasting, you can consume anywhere from 50 to 100 calories daily with foods like coffee with milk/cream or bone broth, but some people may consume less.

Essentially, this is a modified form of alternate-day fasting, whereby you consume a small number of calories, usually around 500, on fasting days ¹.

People who advocate for alternate-day fasting or a modified form suggest that a small number of calories per day will still allow you to achieve the benefits of fasting without cutting calories cold turkey.

It’s also the premise behind other types of fasting diets such as the “fasting-mimicking diet.”

Is dirty fasting effective?

But is dirty fasting as effective as traditional fasting?

As we said, there isn’t much scientific research to go on where dirty fasting is concerned, but any form of fasting that gives your digestive system a break may offer health benefits.

For example, if you’re looking to lose body weight or lose body fat, dirty fasting may be an easier approach to fasting that can promote fat loss.

However, some people choose to use an intermittent fasting plan, thanks to its lengthy list of health benefits, such as promoting autophagy, reducing insulin resistance, regulating blood sugar levels, and more ^{2, 3}.

While adding a splash of milk or sipping on a cup of bone broth isn’t the same as having a full meal, technically speaking, consuming any calories is not a true fast, regardless of how many.

And most experts agree that the only way to ensure you’re in a true fasted state is to abstain from consuming any calories (i.e., you’re drinking water only).

So, while dirty fasting might not be effective to the same extent as traditional fasting, it may offer some benefits, but more research is needed.

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Key takeaway: There’s no scientific evidence to support the efficacy of a modified fasting diet like dirty fasting, so the only way to ensure you are in a fasted state is to abstain from consuming any food or drink with a caloric value.

How many calories will break a fast?

You’ll find hundreds of studies on the benefits of intermittent fasting for all sorts of health-related outcomes—fat burning, weight loss, cognitive health, inflammation, metabolic health, longevity, and more ⁴–many of which are achieved on a keto diet by entering a state of metabolic ketosis.

But while research shows many benefits to fasting, most people still need clarification in one area: How many calories break your fast?

Generally speaking, there’s no clear answer.

Some research suggests you must stick to water-only fasts to reap the benefits, while others suggest that coffee, tea, MCT oil or coconut oil, and even bone broth are acceptable during the fasting window.

The general rule for consuming calories during a fast is to stay under 50. That said, even very few calories will technically break your fast because you’re not in a true “fasted state.”

Some experts suggest that while a small number of calories won’t curb your efforts completely, it can inactivate some of the perks you’ll get from fasting.

Ketosis may remain active with a few calories, but other processes like autophagy might not. It’s thought that any increase in insulin levels will downregulate autophagy, so we avoid consuming food to keep insulin levels low ⁵.

Long story short, if you want to activate autophagy, a water-only fast is your best way.

However, there are debates about whether coffee breaks a fast, but no clear evidence to support either position.

If you’re going to consume coffee on a fast, ensure it’s organic black coffee with nothing added. Aside from that, keep your fast as clean as possible with filtered water and herbal teas.

Key takeaway: Most studies on the beneficial effects of fasting are done with zero calorie intake, so it’s assumed that any calories can break your fast.

However, some research suggests that certain mechanisms behind fasting's benefits, like ketosis, remain activated when specific macronutrients are consumed, while others are deactivated.

What can you eat while fasting?

An all-or-nothing approach isn’t always suitable for everyone, and it’s not always sustainable—but there are plenty of benefits from modifying your fasting approach. Certain foods can maintain many of the benefits of fasting and keep you sane.

If you consume calories during a fast, keep them small and avoid anything that will cause an insulin response.

One mechanism we know remains active with food intake is ketosis, which allows you to consume less than 50 grams of carbs per day while staying in a ketotic state ⁶.

Some effects, however, may vary based on whether you’re following a water-only fast or a modified low-calorie version.

So, what can you eat if you’re going to consume calories? Let’s weigh in on a few options:

Coffee and tea

Even if avoiding calories, many people consume coffee and tea on a fast—but will they break a fast?

In their pure state, coffee and tea have minimal calories, so theoretically, they shouldn’t reduce any fasting benefits, but they could slightly reduce autophagy.

That said, rodent studies suggest the opposite. A 2014 study published in Cell Cycle found that coffee actually induced autophagy in mice to a greater extent than with no calories at all ⁷. But whether this applies to humans isn’t clear.

Coffee also works as an appetite suppressant, which may benefit people who find it hard to do a water-only fast. And some studies even show that coffee can amplify some of fasting's benefits, such as improving insulin sensitivity ⁸. 

So, good news: If you want coffee during a fast, go for it—just keep it clean.

Healthy fats

Healthy fats are another grey area with fasting. Although most fats contain more than 50 calories per serving, there’s also something called a “fat fast,” whereby you consume calories in the form of pure fat.

You’ve also likely heard of Bulletproof Coffee (coffee + MCT oil + grass fed butter) for keeping you in a fasted state until lunch.

A fat fast will technically break a fast because it reduces autophagy, but it can be effective to curb hunger while maintaining many other benefits of fasting.

Consuming fats during a fast can also benefit hormone balance, as it can keep insulin and cortisol levels lower than on a water-only fast. This is especially true for anyone dealing with adrenal or thyroid issues.

Bone broth

Another beverage that’s often given the green light during a fast is bone broth. Again, this will break a fast, but depending on the end goals, it could be a brilliant addition to your fasting protocol.

Why? It’s loaded with nutrients and can help restore electrolytes, especially during a prolonged fast.

Consuming water alone, especially for longer-duration fasts, can lead to electrolyte imbalances, leading to potential complications like low blood pressure, confusion, nausea, muscle spasms, and fatigue.

While you could also add mineral salt to your water, bone broth is a rich source of gelatin and the amino acid glutamine, which are hugely beneficial for gut health.

However, some studies show that glutamine activates mTOR, which inhibits autophagy, so you don’t want to go nuts with broth ⁹.

Final thoughts

Fasting in any sense of the imagination can be tricky, but its health benefits far outweigh the hunger pangs that come with avoiding food.

If you want to maintain proper nutrition but aren’t thrilled with the potential of breaking your fast, certain supplements can fill that gap—but not all supplements are fast-friendly.

Here are our picks for the best fast-worthy supplements:

• Performance Lab NutriGenesis Multi to supply the essential nutrients during fasting periods
• Performance Lab Prebiotic to support and balance gut bacteria for optimal weight loss, fat burn, and overall health
• Performance Lab Caffeine+ to increase energy during your fasting window without breaking your fast

This stack is an ultra-clean blend of supplements that can be consumed during fasting periods to help you get through the fatigue and grogginess that a fast can bring—all while keeping you in a fasted state.