What Is Dirty Fasting—and How Is It Different From Clean Fasting?

Keto, Whole30, Mediterranean. You may have heard of all the popular diet trends. In particular, one of those dieting techniques is intermittent fasting.

Intermittent fasting involves periods of little to no eating. For example, a person may fast for 24 hours every other day. Then, on non-fasting days, a person may eat up to 125% of their average caloric intake.

One common way to practice intermittent fasting is through time-restricted eating. With time-restricted eating, you may limit how much time you can eat in a day. 

Dirty fasting is a specific type of time-restricted eating that allows you to consume about 100 calories during your daily fasting periods. 

Here's what you need to know about dirty fasting, how it works, and whether it's an effective weight-loss technique. 

What Is Time-Restricted Eating?

Time-restricted eating requires limiting the hours you eat within a day to a 12-, 10-, or even eight-hour window.

The latter option, also known as the 16:8 intermittent fasting diet, involves a 16-hour fast followed by an eight-hour eating window. For example, a 16:8 protocol can involve fasting between 8 p.m. and noon and eating between noon and 8 p.m.

Traditionally, during fasting hours, you only consume beverages with zero or minimal calories—like water, black coffee or tea, and unsweetened herbal tea. That type of fasting is also known as "clean fasting."

In a 2021 study published in Frontiers in Endocrinology, researchers explain that the science behind time-restricted eating relates to the circadian clock. They write that our body's circadian clock is tightly connected to our body's metabolism, and meal timing is an essential factor in metabolism.

So, does time-restricted eating work? Some evidence shows that "clean fasting may offer benefits that include:

• Improved heart health
• Reduced blood pressure and blood sugar
• Weight loss

Other research has found that early time-restricted eating—specifically, an eight-hour eating window from 7 a.m. to 3 p.m.—may be effective for losing weight and body fat.

What Is Dirty Fasting?

Dirty fasting is a different take on time-restricted in which more calories, or certain foods, are allowed during the otherwise clean fasting window. 

The goal is to disrupt the concept of fasting as little as possible while achieving similar benefits as a clean fast. 

How Does Dirty Fasting Work?

Fasting is known as the absence of calories. But some evidence suggests that diets like time-restricting eating have redefined what it means to achieve a state of physiologic or molecular fasting. 

Basically, if cells don't react as they likely do during a "fed" state, dirty fasting may still be considered a form of "fasting."

When you're fasting, your caloric and carbohydrate availability is low, which causes your insulin levels to drop. As a result, the hormones glucagon and epinephrine rise. Those hormones trigger the release of stored fat from fat cells.

Some of that fat travels to the liver. There, the fat converts to ketones and releases back into the bloodstream. Those ketones become an energy source for the brain in place of glucose, its typical fuel.

Physiological or molecular fasting is maintained if glucose and insulin levels remain low while ketone levels remain high. And people can possibly achieve that fasting state even with the limited caloric intake that a dirty fast allows.

What Are the Rules of Dirty Fasting?

The rules of dirty fasting differ depending on who you ask. 

Some websites that promote dirty fasting say that any food or beverage during the fasting hours is OK, as long as it's less than 100 calories. In contrast, other sources only sanction high-fat foods, which don't immediately spike insulin.

Some may allow artificial sweeteners because they're zero calories. Still, a 2017 study in Physiology and Behavior showed that those sweeteners might increase insulin levels during a fast, even when tasted and not swallowed.

Other followers of dirty fasting allow higher-protein foods, like bone broth or collagen, during the fasting window. In contrast, one 2021 study in the journal Nutrients pointed out that a lower protein intake is more effective at not triggering metabolic pathways in the body that sense the availability of nutrients.

But keep in mind: Most of the suggestions offered for dirty fasting are based on theory, not clinical research. 

Without more research on how various foods, macronutrients, ingredients, and caloric intake impacts the body during dirty fasting, as of November 2022, there are no science-backed rules about what you can eat during fasting hours.

Is Dirty Fasting Effective?

Many people interested in dirty fasting seek the health benefits of time-restricted eating but the flexibility to eat or drink something with caloric value during the fasting window. Some proponents of dirty fasting say that flexibility helps them stay on track with their fasting routine because they're not as limited or as hungry.

For example, many clients have told me that time-restricted eating helps them prevent overeating, maintain a consistent eating schedule, and eat more mindfully. Still, they miss having a nut milk latte in the morning. Usually, a traditional clean fasting protocol wouldn't allow that latte. But dirty fasting may allow some flexibility to enjoy it. 

Some evidence suggests that there's some effectiveness to dirty fasting. For example, in the 2021 Nutrients study, researchers assigned 105 adults either water, a traditional breakfast, or a commercially available bar called Fast Bar after a 15-hour overnight fast. Full disclosure: I consult for L-Nutra, the company that makes this bar and funded the study.

Fast Bars, made from nuts, seeds, vegetable fiber, and honey, support intermittent fasters who want to fast without eliminating food. One Fast Bar provides:

• About 200 calories
• A relatively low protein content of five grams
• A low glycemic index
• About six grams of net carbohydrates
• 17 grams of fat

Participants had their blood glucose and ketone levels measured every hour for four hours after consuming each option. The researchers found that the Fast Bar group had glucose levels comparable to that of the water-only group throughout the hours after the meal. Also, their ketone levels were similar to the water-only group two or more hours after the meal.

In contrast, the breakfast meal spiked glucose and reduced ketones. The Fast Bar eaters also experienced high self-rated levels of fullness and a decreased desire to eat compared to the water-only group.

According to the researchers, the results indicated that one Fast Bar consumed during the fasting window does not interfere with physiological fasting. So, followers of intermittent fasting may use the Fast Bar to help facilitate the practice of time-restricted eating.

But the study begs the question: Would a similarly comprised bar, or an equivalent combination of foods, also work for dirty fasting? Also, what foods or caloric intake could also result in maintaining low glucose and high ketone levels during fasting hours?

The truth: There's a lot we don't know about dirty fasting. And there is minimal research to support some of the theories espoused online about health or weight loss outcomes associated with dirty fasting. 

In short, much more research is needed to understand the best way to practice dirty fasting and its possible benefits.

Nevertheless, regardless of what research does or doesn't say about the physiological effects of dirty fasting, there can be psychological or behavioral benefits. 

For example, being able to nibble on something in the morning prevents over-splurging later in the day. Or knowing what you can eat or drink helps you get through the tail end of a fasting window, even if you don't need the food or drink.

A Quick Review

One of the appeals of time-restricted eating is that, in addition to weight loss, the practice may offer bonus health benefits. But experts point out that the quality of what you eat during non-fasted hours (or the small dirty fasting windows) matters. 

Nutrition is still key. Be sure to build in a variety of vegetables, fruits, and other whole foods daily to optimize your intake of vitamins, minerals, fiber, antioxidants, and macronutrients.

For both weight management and health, the ultimate goal is to settle into a sustainable long-term routine. Intermittent fasting or time-restricted eating is about striking a balance between restricting and nourishing, not just the former. 

If dirty fasting feels like a healthy compromise, it may be the best approach for your lifestyle and relationship with food.