Dirty fasting is a newer approach to intermittent fasting that has emerged.

Within the past few years, fasting has become a phenomenon. Intermittent fasting was Google's most researched diet in 2019, and its popularity continues to grow. One common way to practice intermittent fasting is through time-restricted eating, which limits the amount of time in a day you can eat. There's now a newer type of time-restricted eating that's catching on: dirty fasting. Here's my take on the trend, and how to determine if dirty fasting may be right for you.

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What is dirty fasting?

To understand dirty fasting, you first must understand 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 last option, referred to 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, only beverages with zero or very minimal calories—like water, black coffee or tea, and unsweetened herbal tea—are consumed during fasting hours, which has become known as "clean fasting." Research shows that time-restricted eating, or what's thought of as clean fasting, may offer benefits that include cardiovascular protection, reductions in blood pressure and blood sugar, and weight loss.

But a new approach has emerged in which more calories, or certain foods, are allowed during the fasting window. The goal? To disrupt the concept of fasting as little as possible but while achieving similar benefits as a clean fast. The practice is referred to as "dirty fasting." There is no human clinical research on the health benefits of dirty fasting, but some people who do it claim that the approach does provide similar benefits as clean fasting.

How does dirty fasting work?

Fasting has long been understood as the absence of calories. But there's an emerging concept that redefines what it means to achieve a state of physiologic or molecular fasting—basically, when your cells aren't impacted in the same way they likely are during a "fed" state—that may allow for dirty fasting to still be considered a form of "fasting."

In a nutshell, when you're fasting, your calorie and carbohydrate availability is low, which causes your insulin levels to drop. As a result, the hormones glucagon and epinephrine, which trigger the release of stored fat from fat cells, rise. Some of that fat travels to the liver, where it gets converted to ketones and is released back into the bloodstream. These ketones become an energy source for the brain, in place of glucose, its typical fuel. Some would argue that if glucose and insulin remain low and ketone levels remain elevated, physiological or molecular fasting is maintained. And this fasting state can potentially be achieved even with the limited intake of calories that a dirty fast allows.

But there's a whole lot we don't know about dirty fasting, and there is very limited 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.  

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 fine, as long as it's less than 100 calories. Other sites only sanction high-fat foods, which don't immediately spike insulin. Some allow artificial sweeteners because they're zero calories, but research shows these sweeteners may increase insulin levels during a fast, even when tasted and not swallowed.

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

That study is the only published clinical human research available that has looked at the impact of calorie consumption during the fasting window. Without more research on how various foods, macronutrients, ingredients, and/or calorie levels precisely impact the body during a dirty fast, there are no hard and fast science-backed rules about exactly what you can green light during fasting hours. Most of the suggestions offered for dirty fasting are based on theory, not clinical research. Note: despite what you may see online, urine ketone strips aren't an accurate way to measure a fasting state alone, since you can be in ketosis while being in a "fed" state (a keto diet, for instance).

Is dirty fasting effective?

Many people who are interested in dirty fasting seek the health benefits of time-restricted eating but with the flexibility to be able to eat or drink something with calories during the fasting window. 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, I've had plenty of clients who've told me that time-restricted eating helps them prevent overeating, maintain a consistent eating schedule, and eat more mindfully but that they really miss having a nut milk latte in the morning. That latte wouldn't be allowed on a traditional clean fasting protocol, but dirty fasting may allow it.

The one human study available on dirty fasting was able to shed some light on the diet's effectiveness. That randomized, controlled trail was conducted in 105 adults who were assigned to either water only, a traditional breakfast, or a commercially available bar called Fast Bar (which, full disclosure, I do consult for L-Nutra, the company that makes this bar and led the study) after a 15-hour overnight fast. Participants had their blood glucose and ketone levels measured every hour for four hours after consuming each of the options. Researchers found that the Fast Bar group had glucose levels comparable to that of the water-only group throughout the hours after the meal and ketone levels that 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 reduced desire to eat compared to the water only group. Scientists say the results indicate that one Fast Bar consumed during the fasting window does not interfere with physiological fasting and could be used to help facilitate the practice of time-restricted eating.

Fast Bars were designed to support intermittent fasters who want to fast without completely foregoing food. Essentially, this means they could support dirty fasting. Each bar provides about 200 calories, with a relatively low protein content of 5 grams, a low glycemic index, about 6 grams of net carbs, and 17 grams of fat. The ingredients include nuts, seeds, vegetable fiber, and honey.

But the Fast Bar study begs the question: would a similarly comprised bar, or a comparable combination of foods, also work for a dirty fast? And what foods or calorie level could also result in essentially maintaining low glucose and higher ketone levels during fasting hours? The truth is, it's unknown.

Nevertheless, regardless of what research says (or doesn't say) about the physiological effects of dirty fasting, there can be psychological or behavioral benefits to it. For example, maybe being able to nibble on something in the morning prevents over-splurging later in the day. Or maybe just knowing what you can eat or drink helps you get through the tail end of a fasting window, even if you don't wind up needing the food or drink. 

Bottom line

One of the appeals of time-restricted eating is that, in addition to weight loss, the practice may offer bonus health benefits. However, experts point out that the quality of what you eat during non-fasted hours (or the small dirty fasting allowances) matters—a lot. That is, nutrition is still key. Be sure to build in a variety of veggies, 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 routine that's sustainable long term. 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 healthier compromise for you, it may be the best approach for your lifestyle and relationship with food.

Cynthia Sass, MPH, RD, is Health's contributing nutrition editor, a New York Times best-selling author, and a private practice performance nutritionist who has consulted for five professional sports teams.

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