What Is a “Fat Fast” and Is it Healthy?

What is a “Fat Fast” and For What Health Goals Might That Be Appropriate?

Asked By: Sharon F.

A “Fat Fast” is not a true fast as you can still consume Calories, but those Calories come primarily from fat. Essentially, it’s more of an all-fat diet, which also happens to be ketogenic, meaning it supports the metabolic state of ketosis. 

A “Fat Fast” does have some metabolic similarities to a true fast, however. Similar to fasting, with a “Fat Fast,” the body is forced to burn through its limited supply of stored glucose before relying more heavily on fat for fuel. As we ramp up fat burning, the body makes the “metabolic switch” into ketosis, where the liver converts fat to ketones. 

In contrast to carbohydrates or protein, fat has virtually no impact on blood sugar and insulin levels and therefore doesn’t interfere with fasting metabolism. So, a “Fat Fast” may mimic fasting as far as your metabolism can tell, but the fat you are consuming is displacing the need to burn body fat.

The origin of the “Fat Fast” is unclear, but the concept likely originates with research conducted in the 1950s by a Professor named Dr. Kekwick. In his studies, obese participants were placed on a 1000 Calorie diet composed of 90% fat, 6% protein, and 4% carbohydrates for an average of 5-9 days. Those on this diet lost considerably more body weight compared to high protein or high carbohydrate diets when Calories were matched, and the “Kekwick Diet” was born. 

In 2002, Dr. Atkins repackaged the Kekwick diet into what he referred to as the “The Fat Fast,” which he used clinically as a last resort when weight loss stalled. Dr. Atkins’ Fat Fast is a low Calorie (1,000 Calories per day) diet composed of ~80% fat, followed for up to 5-days, and as far as we can tell, these same principles are applied to the now commonly known “Fat Fast” that you may have seen people buzzing about on the internet.  

The inclusion of small amounts of protein and carbohydrates also means that a Fat Fast is not just straight oil or fats, since that is the only food product that is truly 100% fat. Some foods included on a Fat Fast are avocados, eggs, fatty meats, high-fat nuts (e.g., macadamia), mayo, heavy whipping cream, and of course healthy fats (e.g., grass-fed butter and ghee) and oils (e.g., olive oil, avocado oil, coconut oil).

While a Fat Fast has not been studied for this explicit purpose, the primary reason people use it is to overcome weight loss plateaus. At Zero, we stick to the science and don’t recommend engaging in something just because it’s popular. For this reason, until we have evidence to point to, we do not necessarily suggest a Fat Fast to break through a weight plateau. 

Another proclaimed reason to Fat Fast is to jump-start ketosis. Fasting to achieve ketosis, while fool-proof, can be uncomfortable for some in the beginning stages of fasting. A Fat Fast may provide training wheels when getting started with fasting, as your metabolism becomes better and better at relying on fat and ketones for fuel, but perhaps without feeling the restriction of fasting from the outset. In a similar way that a ketogenic diet can be used to prime our metabolism for a water-only fast, fat fasting can be used. However, we generally recommend easing your way into a fasting routine over time and gaining experience with fasting by working your way up to longer fasts to promote the metabolic adaptations that support your fasts.

While theoretically, a Fat Fast can support you on your fasting journey and may even offer some benefits, it lacks primary evidence to suggest it is a nutritionally sound diet pattern and should be approached similarly to a prolonged fast. If you choose to try it out, we advise against anything longer than 7-days unless medically supervised, and recommend consulting with your health care professional for anything over 3-days.

About the Author: Kristi Storoschuk

Kristi Storoschuk is a science communicator who focuses her research on ketogenic diets, metabolic therapies, and fasting for health optimization. She currently works alongside the world’s leading ketogenic researchers providing scientific education for the mainstream audience. She completed her Bachelor of Science (Honours) at the University of Guelph in Ontario, Canada. Outside of her research, she spends her time doing CrossFit and optimizing her health with an ancestral approach.

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Kristi Storoschuk
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