Intermittent fasting and the ketogenic diet are two popular eating strategies that can both be beneficial for weight loss, metabolic health, and more. You might be wondering if combining them will help you reap even more rewards or just be too much of a good thing.

Intermittent fasting (IF) is an umbrella term that describes the many ways you can go with and without food. (We have covered the ins and outs of IF in previous articles, and we recommend you start here if you're new to the concept of intermittent fasting.)

Fasting for a certain number of hours each day can flip the "metabolic switch1" from a carbohydrate-dominated state to a fat- or ketone-burning state. Fasting can also be a weight loss tool since it can help you reduce your overall energy intake2.

The most common form of IF is time-restricted eating (TRE), which will be the focus of this article. TRE limits daily food intake to a fixed window of each day, typically ~8 hours. The eating window usually aligns with daylight. This leaves a ~16-hour fasting window that happens when the sun is set.

This pattern of eating is thought to align with our natural internal clocks and support our circadian systems3 to maximize the timing of biological processes. For example, glucose tolerance (blood sugar response to carbohydrates) is better in the morning4 than it is at night.

As nutrition scientist Courtney Peterson, Ph.D., says on the mindbodygreen podcast, "We've actually known for about the past 50 years that your blood sugar control in most people is best in the morning, and it gets worse as the day progresses."

Speaking of the history of fasting…this is exactly what led to the introduction of what's known today as the ketogenic diet.

This high-fat, very low-carbohydrate diet mimics human fasting metabolism and was used medically in the early 1900s to manage epileptic seizures5 as well as Type 1 diabetes6 before the advent of insulin.

Keto is a way of eating that restricts carbohydrates to the point that our body needs to vet out an alternative fuel source to feed our glucose-hungry brain. (Unlike most of our tissues, the brain cannot burn fats for fuel.) That's where ketones come in.

When we lower carbohydrate intake, and by virtue, blood sugar and insulin levels, to the point that puts the brain at risk of an energy deficit, the liver starts converting fats (either from our diet or body fat stores if in a caloric deficit) into ketones.

This is what defines a keto diet: an elevation of ketones. It turns out that the brain loves ketones so much that it uses them even in the face of impaired energy metabolism. Ketones do a lot of other extraordinary things in the body that scientists are still unpacking. 

Today, the ketogenic diet is being researched in various clinical settings for its effect on cancer7, neurodegenerative diseases8, and most notably, Type 2 diabetes9.

We can think of fasting and keto as two peas in a pod. We derive energy during fasting and following a keto diet in the same way: from fats and ketones.

If intermittent fasting is going to be paired with any sort of diet, keto is ideal given the energy density of keto foods lends itself to reduced meal frequency. 

If you go cold turkey into a fast from a high-carbohydrate diet, you will most likely experience some symptoms of low energy, low blood sugar, and hunger as your body transitions into ketosis. It would be like stepping out of a sauna into the winter air.

Beginning a fast off of a keto diet, however, is more seamless. It's less of a shock to the system since your metabolism is already happily running on fats and ketones. It's more like coming out of a sauna into warm sunlight. So, if you are new to IF, a keto diet can help smooth the transition.

Conversely, fasting can be a great tool to support your ketogenic diet. It can both kick-start ketosis (the state where your body uses broken-down fat for energy instead of glucose from carbs) and help you stay in ketosis for longer periods of time.

The foods you eat on a keto diet tend to be more energy dense, and therefore you may need to eat less frequently on this plan. Unless you are trying to force multiple meals into your IF schedule, you typically end up removing snacks and/or a meal from your usual eating patterns while intermittent fasting too.

So, if you are eating a ketogenic diet and find you only really need two meals a day (maybe you aren't hungry throughout the morning, or you no longer desire that snack after dinner), well, now you're well on your way to time-restricted eating.

Keto and fasting are not magic weight loss solutions. However, they certainly can support an overall energy deficit when properly combined.

Whether or not the pair supports weight loss will come down to the individual since adherence to any diet is the best predictor of weight loss. In other words, if you can stick to a keto diet while intermittent fasting, then you will likely find success! However, if your intermittent fasting schedule leads you to binge on a tub of keto ice cream at the end of the day, then this strategy is not for you.

As of now, no research trials have studied using a ketogenic diet alongside time-restricted feeding for weight loss. However, given the promise of both interventions, it is likely that those studies will eventually be done,

It's worth noting that there is one case study12 of a woman with Type 2 diabetes who was able to dramatically improve glycemic control by sticking to a ketogenic diet and utilizing prolonged fasts (24- to 42-hour fasts 3 days/week). She maintained these results using daily 16-hour fasts.

If you want to limit your eating window, go easy on processed foods13 as they tend to be easier to overeat. This goes for keto foods too: Just because a highly processed cookie is technically a "keto" snack does not make it any less processed!

There is some research showing that TRE could lead to muscle loss14 when prescribed without exercise. However, engaging in regular strength training and ensuring adequate protein intake (1.2 to 1.6g/kg body weight) will likely mitigate this.

When paired with resistance training, adequate protein intake will be important for maintaining or building muscle15 in the face of long durations (e.g., 16 hours) without food. Protein will also help with appetite management if you find yourself unable to stick to your IF schedule because you're absolutely ravenous. Protein helps us stay full for longer, so use this to your advantage! 

First and foremost, eat when the sun is out. This is a foolproof way to stick to time-restricted eating, as it was designed by its originator, Satchidananda Panda, Ph.D.

His research protocol suggests avoiding eating at least two to three hours before your regular bedtime, eating after the sun has risen, and eating within a 12-hour window.

Within this framework, you can adjust your exact eating window based on your schedule, lifestyle, and preference. Maybe you skip breakfast and eat from noon to 8 p.m. or skip dinner and eat from 8 a.m. to 4 p.m., for example. Here is more information about getting started on a 16-hour or 18-hour fast.

There is research to suggest if you are trying to optimize for metabolic health and glycemic control, then eating earlier in the day (i.e., early TRE) is best.

You can (and should) definitely still exercise in a fasted state17, given you're not in the gym for hours on end. The ideal timing of that exercise will ultimately come down to your individual needs and preferences.

If your goals are to maximize muscle gains, for example, you'll want to position your eating window to begin when you complete your workout. You could also train within your eating window. When compared to eating over the course of the day, resistance training paired with TRE18 (when training takes place in the eating window) leads to similar gains in muscle. Though, there is also a study19 that found training in the fasted state may increase our muscle's response to protein intake post-exercise.

Option 1: Eggs cooked how you like with a side of avocado and sauteed spinach. 

Option 1: Salmon (or sardines) salad with chopped veg, cheese, avocado, seeds, and an olive-oil-based dressing. Add a boiled egg or two for some extra protein.  

Option 1: Grilled fish or steak (in moderation) with steamed nonstarchy vegetable with olive oil or butter.

Option 1: Heavy whipping cream (whipped) with a few berries.

If you are getting adequate nutrition and calories during your eating window, then there really is no risk to the simple advice to eat when the sun is up (< 12 hours).

Keto and TRE, both separate and paired, are not recommended for pregnant and breastfeeding women, and you should seek professional advice from your doctor if you are interested in either from a health perspective depending on your current health status.