The ketogenic diet was first discovered for its ability to mimic fasting. In the early 1920s, researchers found that a low-carb diet led to ketone production, similar to fasting. Since fasting had been used as a treatment for epilepsy since 500 B.C., doctors speculated that a very low-carb diet, which they termed "ketogenic", could be used in the treatment of epilepsy with added benefits of A) being able to sustain someone longer than fasting and, B) it offered greater nutrient intake for growing children.
While early doctors were looking to use keto instead of fasting, more recently, we have found that pairing the two can lead to robust health improvements. Let's look at the similarities between these two diet strategies to see why combining the two can be a great strategy.
Similarities Between Keto and Fasting
When you fast, your body has no choice but to use up the available glucose in your blood. Once that’s used up, it goes for glucose in its storage form (glycogen) from your muscles and liver. As insulin and blood sugar lower and glucose stores become depleted, your body begins to dip into your fat stores as a source of energy. This initiates the production of ketones, energy compounds produced by the breakdown of fat when carbohydrate sources are low.
Similarly, on a ketogenic diet, your body makes shifts that encourage the production of ketones. However, instead of fasting and avoiding food altogether, you only need to cut back on your carbohydrate intake on a ketogenic diet. When carbohydrate intake is low, much like in fasting, your body will first burn through your blood glucose and stored glycogen and then shift to burning fat when carbohydrate stores are depleted.
In this way, fasting and the ketogenic diet produce very similar outcomes.
The primary difference between the two, however, is that when you follow a ketogenic diet, you still get to eat regular meals — you only need to avoid carbohydrates. But what happens when you combine these two strategies for ketones production?
The Benefits of Combining Fasting and Keto
If ketone production is your goal, then it makes sense that anything you could do to encourage ketosis would be supportive. Combining fasting with a ketogenic diet gives your metabolism an extra push in the direction of more ketone production and can make both fasting and ketosis more enjoyable.
Aside from the individual health benefits keto and fasting can provide, combining the two offers its own unique list of benefits.
Supports Greater Ketone Production
When you follow a ketogenic diet and include fasting, you're doubling your body's efforts in producing ketones. While a ketogenic diet on its own will promote the generation of ketones, adding in fasting pushes production even further by keeping your fat cells in a state of breakdown for longer periods of time.
Enhanced ketone production may have particularly beneficial effects on your brain due to the way your brain chooses fuel sources. Research shows that your brain will take up ketones for energy in proportion to their availability in your blood. In other words, increasing your blood ketone levels means more energy to your brain and all of its cognitive functions.
Remaining fasted for an extended period yields benefits that you may not experience during a shorter fast. One of those benefits is something called autophagy. Autophagy is your body’s way of cleaning out old or damaged cells and regenerating new ones.
The longer you stay in a fasted state, the more autophagy will do the work to dispose of old cells. When you combine keto and fasting, you extend the "fasted" state even when you eat because you're still producing ketones, which stimulates autophagy. And more autophagy means increased anti-aging benefits like removing toxic proteins from cells, protecting you from age-related degenerative disease, and much more. 
May Help You Get Into Ketosis Faster
The only thing between you and a ketogenic state is the number of carbohydrates you currently have stored in your body or traveling through your blood. After a meal, your blood sugar slowly declines over the following hours as your cells take up the glucose. What's not used for energy is stored as glycogen.
As long as your body has glycogen to pull from, there is no need to initiate ketone production. Therefore, the faster you burn through those carbs, the faster you'll get into ketosis.
Many people experience a period of "keto flu" as their body taps their remaining glucose and starts to use ketones as fuel. During this time, your body isn't fully adapted to ketones, which means that your energy can lag, and you may experience symptoms like brain fog, fatigue, muscle weakness, and so on.
When you fast, no new energy is coming in from food which means that your body only has one option — to burn up stored fuel. This assists in clearing away any stored carbs and pushing your body into ketosis faster than if you were providing fuel in the form of food.
Promotes Fat Loss
Many people that try the ketogenic diet are looking for some help with weight loss. While keto on its own can promote satiety and weight loss, adding fasting to the mix can help you push through a plateau if you've been struggling to get the pounds off.
Animal research shows that fasting can enhance thermogenesis, which is the process of burning calories to produce heat. If weight loss is your goal, fasting not only helps you tap into fat stores but also enhances the rate at which those fat stores are burned.
May Help Preserve Muscle Mass
One of the downsides to losing weight is that, along with fat, muscle mass often becomes a target for breakdown. As a result, many people find that as they lose weight, their metabolism also drops. This is because muscle is the most energy-hungry tissue in your body, and when you lose muscle, your body's overall energy needs decline.
But fasting may provide a weight loss strategy that also helps to preserve lean muscle mass. This means that as you lose fat with fasting, you're able to retain lean muscle and therefore keep your metabolism higher than it would be on a regular calorie-restricted diet.
What Would Combining A Keto Diet and Intermittent Fasting Look Like?
Combining intermittent fasting with a ketogenic diet couldn't be more simple. All you have to do is choose what type of fasting window you want to aim for and keep your same (keto) diet, just limit food intake to those hours.
For instance, if you want to try a 16-hour fast, 8-hour feeding window, you could just finish your last meal around 7 pm and then wait to have breakfast or lunch until 11 am the following day. The key is to find your sweet spot so that when you go to break your fast, you aren't starving. If you find that you're ravenous by 11 am the following day, this will likely lead to overeating, and it may incite cravings for foods that would kick you out of ketosis like bread, pastries, and so on.
Therefore, if you want to maintain your ketogenic diet while fasting, it's crucial that you know your body and know your limits. Start slow, and increase your fasting window as you see fit.
While fasting is not something you have to do on keto, it is certainly something that can make things easier. When you fast, it helps to shift your body into ketosis, assisting in burning away your glucose stores and tapping into fat stores.
Fasting may also improve your weight loss efforts while helping you maintain lean body mass – crucial for a healthy metabolism.
If you're new to the ketogenic diet, adding in some intermittent fasting will likely help you see the results you're looking for in a shorter time. Keep in mind that fasting doesn't have to be strict; you can listen to your body and take it day by day. There are a lot of benefits you can get from a fast as short as 14 hours.