Does Dirty Fasting Actually Work? Here's What To Know

From 16:8 to alternate-day fasting, it's hard to keep up with all of the different types of intermittent fasting (IF) out there. Since IF has become so popular, people have come up with a few methods to make fasting a bit easier.

Enter dirty fasting—a term for a modified type of IF that allows for some calorie intake during the fasting window. Although this take on IF is more pleasant and easier to adhere to than strict fasting, there's no hard evidence to back its effectiveness.

To understand dirty fasting, you'll need to know the basics of intermittent fasting (IF). IF is a term for eating patterns1 that involve regularly occurring periods of little to no calorie intake cycled with periods of normal energy intake. 

IF methods that allow for some calorie intake during the fasting window are referred to as "modified" fasting. For example, people following traditional alternate-day fasting (ADF) eat food every other day and completely abstain from calories on fasting days. People who follow modified ADF eat a very small amount of calories—typically under 500—during their fasting days and then eat normally the following day.

Dirty fasting is a term used to describe consuming some calories during a fasting window. While dirty fasting doesn't have a set definition, it could be considered a modified type of fasting because people who use this method typically consume around 100 calories during their fasting window 

Grant Tinsley, Ph.D., a professor at Texas Tech University and intermittent fasting researcher, thinks it's important to separate dirty fasting from traditional fasting methods. "If we stick to the definition of fasting being no calories consumed in a given period of time, is 'dirty fasting' really fasting, or is it just eating a very small number of calories in a given time period? Where is the line here?" Tinsley tells mindbodygreen. "If 100 calories during a fasting period is dirty fasting, what is 150 calories? 200 calories? And so on."

There's plenty of evidence to back up the health benefits of traditional fasting methods like ADF and time-restricted feeding (TRF), which usually involves completely abstaining from calorie intake for 12 to 21 hours per day2.

These IF methods have been shown to boost fat loss3, improve blood lipid levels4, and reduce blood pressure5. 

Modified types of fasting like modified ADF have also been linked to some benefits, like weight loss6 and improvements in fatty liver disease6. 

However, because dirty fasting has no set definition and people following this method may take in varying numbers of calories, it's unclear if dirty fasting offers the same benefits as other types of fasting with set rules. 

That said, if you're significantly cutting calories using any method, whether it be continuous energy restriction or IF, it's likely to result in weight loss. Plus, some studies show7 that both fasting and calorie restriction, in general, can stimulate autophagy, or the self-cleaning of cells, which may slow aging and increase longevity. 

"Some of the effects of dirty fasting could be the same as traditional IF methods, but some of the effects of fasting may not be present," says Tinsley. "If dirty fasting causes someone to consume fewer calories than they would during their normal eating pattern, it's producing the same effect of many other fasting programs that help someone achieve an energy deficit. This, in turn, could allow for some of the downstream effects of an energy deficit, such as fat loss and a possible improvement in some clinical health markers," Tinsley tells mindbodygreen. 

Yet, Tinsley explains, since calories are consumed during the fasting window, it's possible the degree of energy restriction—and subsequent health benefits like weight loss—would be smaller than if complete calorie restriction was used.

Before we dive into more details about dirty fasting, it's important to mention that fasting is inappropriate8 for some people. In general, children, pregnant and breastfeeding women, those taking certain medications, people with eating disorders, and people who are underweight should not fast. 

Also, there's no evidence that dirty fasting is any more effective for boosting weight loss and improving metabolic health than traditional energy restriction methods like simply eating less at meals.

But, if you're interested in trying out IF, dirty fasting could help you ease into IF without having to completely abstain from calories during the fasting window. Whether or not you use traditional IF or dirty fasting, if you're new to IF, it's best to start with a shorter fasting window so you can see how your body reacts to going for long periods with limited calorie intake. 

Most people using this method aim to consume around 100 calories during the fasting window and stick to foods and drinks that are low in carbs and higher in fat. This is because higher-carb foods and drinks like fruit, soda, juice, and grains have more of an impact on blood sugar and certain hormones, like insulin.

Dirty fasting is a nontraditional type of fasting that allows for a small amount of calories to be consumed during fasting windows. There are no set rules for dirty fasting, but some experts categorize dirty fasting as a type of modified fasting.