Cutting carbs and fueling the body with fat is a popular diet strategy called the ketogenic diet, or keto for short. But if you’ve already found weight loss success using this approach, you may be thinking of taking things up a notch and combining keto with intermittent fasting to bust through a plateau or improve your results. Is this something you should try?
The short answer is maybe, though you should know up front that there is a lack of rigorous research on this combined approach, and it hasn’t been proven to work for weight loss. Experts say it could make sense, though the lack of research means you may want to think twice before delving into this eating approach.
Combining the two diets gained popularity when intermittent fasting expert Jason Fung, MD, author of The Obesity Code, recommended using keto as a foundation for fasting, explains Lori Shemek, Ph.D., a nutrition and weight loss expert in Dallas and the author of How to Fight FATflammation. Celebrities including Halle Berry and Jennifer Aniston have been reported to use both diets together, says Dr. Shemek.
Let’s talk about what each diet is.
What Are the Basics of the Ketogenic Diet?
Scientists originally designed a keto diet in the 1920s to help control seizures in children with epilepsy, research shows. This version called the classic ketogenic diet or the long-chain triglyceride diet, specifies eating 3 to 4 grams (g) of fat for every 1 g of carbs and protein, according to the Epilepsy Foundation.
The version of keto many are using for weight loss today is a little different in that it is a high-fat, moderate-protein, and very low-carb plan. Fat makes up about 80 percent of the daily calories, and you’ll aim to consume between 20 g and 50 g of net carbs (carbs minus fiber) each day, depending on your personal needs.
A typical keto diet food list ditches most carbs, and even healthy foods such as fiber-rich whole grains and most fruits while prioritizing fats, such as avocados, olive oil, grass-fed beef, and even bacon occasionally.
The idea of this eating plan is to transition your body from one that burns glucose (or carbs) for fuel to one that relies on fat for energy. This state is called ketosis or being keto-adapted.
While the keto diet appears to lead to quick weight loss in the short term, critics say that much of that is water weight and point out that long-term research (more than two years) on people using keto for weight loss is lacking. Some available research shows that keto is no more effective than other diets for weight loss.
It is also important to note that many of the claims that keto can treat health conditions other than epilepsy — such as cancer, high blood pressure, polycystic ovary syndrome, diabetes, and neurological diseases like Alzheimer’s disease — need more research, per a review.
How Does Intermittent Fasting Work?
Intermittent fasting is a diet approach in which you plan specific times when you are not eating. There are several ways to do intermittent fasting, which some people simply call IF.
For instance, some people may follow the popular 5:2 IF, for which five days are normal eating and two are extremely low-calorie (around 500) fast days. Other plans call for not eating for a 24-hour period, and still, others involve what is known as time-restricted eating, such as eating for 8 hours and fasting for 16 each day.
When it comes to IF, exciting research is emerging, including the role that the approach may play in treating obesity and insulin resistance (the hallmark of type 2 diabetes), according to a review. Yet the authors of that review concluded that there’s a lack of high-quality evidence on the health effects of IF and that it’s still unclear which type of IF is best to follow.
In addition, IF can be an effective way to lose weight — in fact, it’s been shown to be just as effective as traditional calorie restriction. For people who have obesity, when it came to losing weight or body fat or improving metabolic risk factors after one year, combining intermittent fasting with calorie restriction was just as good as simply restricting calories, according to a randomized trial. The IF group consumed 1,200 to 1,800 calories between 8 a.M. And 4 p.M., while the calorie-restricted group had the same number of calories but without the time limit. The takeaway? There are numerous ways to lose weight, and finding one that suits your preferences and eating style — which may or may not be IF — is key to creating lasting weight loss.
One major limitation is that many of the studies of IF (for weight loss or otherwise) have been in animals, not humans, or have been short-term.
Why Combining Intermittent Fasting With Keto Has Become Popular for Weight Loss
Doing keto or IF can help with weight loss in the short term, though each diet is very restrictive, so they certainly aren’t for everyone. But how about combining them? Could two be better than one?
First, in some experts’ view, it makes sense to pair the two approaches. The keto diet increases levels of ketones in the body; during times of fasting, ketones are also increased. “The brain will rely less on glucose for energy when in a state of nutritional ketosis. Therefore, the transition into a fasted (ketogenic) state eventually becomes seamless after eating low carb or ketogenic for a few weeks,” says Dominic D’Agostino, Ph.D., associate professor at the University of South Florida in Tampa and founder of KetoNutrition.Org.
This is a strategy that practitioners counsel patients on at the Cleveland Clinic’s Functional Ketogenics Program in Ohio. “Adding intermittent fasting can take things up to the next level,” says Logan Kwasnicka, a functional medicine practitioner formerly at the Cleveland Clinic. That next level may be overcoming a weight loss plateau, as people may eat fewer calories when doing IF. It can also be a natural progression from a keto diet for those who feel satiated eating so much fat (ketosis may also decrease appetite) and aren’t bothered by shrinking their eating window.
Who Should Try an Intermittent Fasting Keto Approach?
Anyone who has been on keto for more than two weeks and would like to add IF may do so — with the okay from their healthcare team. Notably, it may not be appropriate for people who are following keto as part of a plan to address prediabetes or diabetes. Kwasnicka says, “Asking these patients not to eat for a significant amount of time can be dangerous.” Also, if you have chronic kidney disease or a history of eating disorders, are undergoing active cancer treatment, or are pregnant or breastfeeding, it’s unlikely that you’re a good candidate for this combined diet plan. Even individual diets (keto and IF) may not be recommended for these people. Check with your healthcare team.
If you’re currently following the keto diet and are happy and feel good with the way you’re eating, you do not have to add IF, Kwasnicka notes.
The Right Way to Start an Intermittent Fasting Keto Diet
At the Cleveland Clinic, practitioners do not advise people to start both keto and IF at the same time. “It’s a huge shock to your system to switch from glucose as fuel to ketones, and implementing IF is a significant change,” says Kwasnicka. For that reason, people start with keto, then after being on the diet for a couple weeks to months, they might consider adding IF.
It’s also important to choose the correct timing. For their patients, Kwasnicka suggested a 12- to 16-hour fast. For many people, not eating 12 hours a day (say overnight from 7 p.M. To 7 a.M.) is a natural habit anyway and doesn’t require skipping meals.
To start, consider delaying breakfast (start with an hour and then slowly extend the time) to get your body accustomed to going long stretches without eating, suggests Shemek. When you’ve adjusted to your new eating pattern, reintroduce breakfast earlier in the day and extend your overnight fasting time, as eating breakfast not only leads to better cognition but also improves metabolism and insulin sensitivity, according to a study (PDF). As for the length of time to stay on keto-IF, she recommends no more than six months, then transitioning to a more standard low-carb diet or other recommended healthy eating plan.
A Sample Menu for Keto and Intermittent Fasting
If you’ve got the green light from your healthcare team and want to try this combined approach, you might be wondering what you’ll eat (and when). Your instincts are right: This diet is all about timing.
Here’s what Shemek says three days on the plan might look like when done with time-restricted eating in a 16-hour fast, 8-hour feed pattern. This approach isn’t the only way to do IF — there are many ways to fast. For example, you can also do a 12- or 14-hour daily fast.
In this plan, you’ll see that snacks are optional. Also keep in mind that your carb, protein, and fat need to stay in ketosis depending on your individual health. Working with a registered dietitian familiar with keto-IF can help you determine those ratios.
10 a.M. Black coffee and scrambled eggs topped with avocado slices; water all morning
1 p.M. Large leafy green salad topped with 2 tablespoons (tbsp) of olive oil, vinegar, and 3 ounces (oz) grilled salmon
3 p.M. ¼ cup macadamia nuts (optional)
5:30 p.M. Chicken leg (with skin), ¼ cup of cooked wild rice, and 2 cups of zucchini cooked with olive oil
10 a.M. Plain hot tea and a keto-friendly smoothie; water all morning
1 p.M. 3 oz grilled chicken breast, half a plate filled with broccoli and cauliflower drizzled with 1 tbsp olive oil, and a whole avocado
3 p.M. Unsweetened coconut chips (optional)
5:30 p.M. 3 oz seared tuna cooked in olive oil on a bed of Asian coleslaw, topped with a drizzle of olive oil and sesame seeds
10 a.M. Black coffee with keto chia pudding; water all morning
1 p.M. Three-egg omelet stuffed with half a pepper and spinach (1 cup) cooked in 1 tbsp olive oil, topped with half an avocado, and ½ cup sliced tomatoes on the side
3 p.M. Olives (optional)
5:30 p.M. Large kale salad (3 cups) with 3 oz shrimp drizzled with 2 tbsp of olive oil and vinegar of your choice
The Possible Health Benefits of a Keto Intermittent Fasting Diet
There is very scant research on the health effects of doing keto and IF together, but it’s clear that ketone levels increase when the plans are combined. This may help accelerate weight loss, says Dr. D’Agostino. Still, everyone responds differently, so this may not be true across the board, he says.
Shemek has all of her clients use keto or the IF and keto combo. “My clients typically all have prediabetes and are overweight,” she says. “Once they see and feel what eating in this way does for them — including regulating their blood sugar levels — they are able to stay on an IF and keto plan easily. The success of the combined approach reinforces their commitment.”
One interesting way that low-carb IF diets are being studied outside of weight loss is for cognitive health. Richard Isaacson, MD, director of the Alzheimer’s Prevention Clinic at Florida Atlantic University in Boca Raton, says that doing a low-carb, time-restricted IF may be beneficial for “calming down insulin pathways and letting the brain benefit from cleaner burning [ketone] fuel,” which may offer benefits for people with Alzheimer’s disease.
In fact, Dr. Isaacson personally does an 8-hour feed, 16-hour fast four or five days a week. This strategy, he says, may also help limit fat accumulation around your waist. “A bigger belly [may] mean a smaller memory center in the brain,” he says. Not to mention, more visceral (belly) fat is associated with a higher risk of chronic ailments such as heart disease and type 2 diabetes, according to research.
Are There Any Known Health Risks of Doing Keto and Intermittent Fasting Together?
As for long-term risks, “people with seizure disorders (what keto was developed to treat) have been on the ketogenic diet for decades and demonstrate excellent health,” D’Agostino says. Though, like any diet, it all depends on what foods you’re eating. A keto diet rich in bacon and butter is different from one rich in avocado and olive oil, and poorly planned keto can lead to nutritional deficiencies. Because adding IF may make you cut calories too drastically, you may also lose too much weight or lean mass (muscle) if you take the restriction to the extreme. To maintain muscle mass, he recommends consuming 1 g per kilogram of body weight in protein every day.
The Bottom Line on Combining the Keto Diet With Intermittent Fasting
The keto diet is a restrictive high-fat diet, and intermittent fasting restricts the number of hours you will be eating. There is a lack of research on combining the two, so it’s unclear exactly what you’re getting into if you try them together.
If you decide to give the diets a go, know that they are extremely restrictive, so it may be difficult to stick with the low-carb count and confined eating window. (Even children who follow keto to help control epilepsy have a hard time adhering to the plan.)
Before deciding to follow keto and IF together, be sure to consult your healthcare team. Your provider can help you determine whether this combined diet plan is a good fit for you, and then they can adjust any meds you’re on to help increase your chances for success in the safest way possible.