At this point, you probably know all about the keto diet—if you’re not currently on it, you probably know someone who’s tried it, or maybe you’ve dabbled in the high-fat, low-carb diet just to see what all the fuss is about. And if you’re super into diet trends, you might even know about intermittent fasting (IF), a method of eating that requires you to fast for a set number of uninterrupted hours per day.
But here’s one trend you may not know a lot about: doing IF and keto at the same time. Whoa. As if one diet wasn’t hard enough, are there really people out there tackling two diets at once?
Yup. And keto proponents claim good reasons for doing it, too—some of which could be legit.
“Practicing IF while on the keto diet may help the body reach ketosis faster,” says registered dietitian Danielle Schaub, culinary and nutrition manager for Territory Foods. “Because both IF and the keto diet are based on the beneficial shift toward burning body fat instead of carbs for energy, similar health goals can be accomplished by both.”
Now for the really important questions: How do you actually do IF and keto together? Does it help you lose more weight? Do you end up with double the side effects? And, finally, is it really a good idea? Here’s everything you need to know about combining keto and intermittent fasting.
First, what exactly is keto?
Keto is a high fat, low-carb diet often mistaken for a high protein diet because so many of its popular recipes include bacon and other types of animal meat, but the focus really is on fat. Some people even do a vegetarian version of keto, chowing down on healthy fat sources like olive or coconut oil, nuts and seeds, avocados, and dairy products (while avoiding greasy cheeseburgers and sirloin steaks).
The goal of the keto diet is to eat meals that are 60 to 75 percent fat, 15 to 30 percent protein, and 5 to 10 percent carbohydrates; doing so puts you into a state of ketosis where you can burn ketones from fat instead of glucose from carbs, speeding up your metabolism and increasing the potential for weight loss (according to the diet’s proponents).
Ok, so what’s the deal with intermittent fasting?
IF is a pattern of eating where you restrict foods and sweetened beverages for an uninterrupted period of time every day (usually 16 hours) and then eat only during the remaining 8 hours.
“You can pick your windows but we encourage people to eat when the sun is up and fast when the sun is down,” says registered dietitian Amy Shapiro, founder of Real Nutrition. “During fasting hours you can drink water, black coffee or plain tea, but nothing that provides calories or requires any sort of digestion or shift in hormones.”
The theory behind IF is that depriving your body of calories for an extended period of time forces it into a temporary state of starvation and slowed metabolism. This makes your fat cells burn glucose for energy and can lead to improved weight loss results.
There are several different ways to schedule your fasting times, but the 16:8 version is the most commonly used.
How can you combine keto and intermittent fasting?
It’s actually not that hard if you’ve already got a good handle on keto, since that diet has a stronger learning curve in terms of what’s allowed. Technically IF isn’t even a “diet” in the traditional sense, says Brigid Titgemeier, RDN, a functional medicine dietitian and the founder of BeingBrigid.
“Intermittent fasting can be paired with any kind of diet because it simply refers to the number of hours that you fast,” she explains. “Combining keto with intermittent fasting means adhering to the parameters of a ketogenic diet and eating within a condensed window of time.”
For example, Titgemeier lays out the following schedule for eating within the parameters of both diets (assuming a 16-hour fast between 6 p.m. and 10 a.m.).
7 a.m.: Green tea or black coffee
10 a.m.: Keto macadamia bread topped with ricotta cheese and sauerkraut
12 p.m.: 4-6 ounces of wild salmon served with a bed of sautéed greens and ½ avocado
3 p.m.: Grainless granola with unsweetened almond yogurt
5:30 p.m.: Shrimp bowl with 4-6 ounces of shrimp, cauliflower rice and sunflower seeds
7 p.m.: Herbal tea
Is keto better than IF (or vice versa)?
It depends on your nutrition goals, says Shapiro, but she doesn’t recommend keto for the majority of people because of its restrictive nature (there are some people with health conditions, like epilepsy, who may benefit from a ketogenic diet).
IF, though, is a different story: “I recommend 12 to 14 hour IF for most individuals because it helps to break mindless eating patterns, encourages balanced meals, and allows for proper digestion, better sleep, and [better] hormone balance,” Shapiro says.
Schaub agrees, saying that “in general, intermittent fasting is safer and more in line with a natural way of eating, [and it] can be practiced for long periods of time.”
In other words, IF is a more flexible way of eating that often fits organically into a person’s lifestyle (which also makes it easier to stick with long-term). Keto, on the other hand, may be too restrictive to be sustainable—or healthy. Because it’s a medical diet, it carries risks of nutritional deficiencies as well as liver and kidney problems, per Harvard Health.
Can combining keto and intermittent fasting help me lose weight?
There’s some solid consensus among the RDs: combining keto with IF does help you reach ketosis faster, which may lead to additional weight loss.
“IF paired with keto intensifies the effects of the ketogenic diet, since they both increase the ketones in the blood,” says Titgemeier. Since the time it takes to enter into ketosis differs from person to person (leaving you suffering from the keto flu in the meantime), anything that can make that happen sooner is appealing to keto dieters…even if it means piling yet another diet on top of the one they’re already doing.
Additionally, Shapiro says that grouping your meals into a shorter window of non-fasting may allow your body to burn its energy stores instead of continuing to build them up—something which could cause more fat loss than if you did only keto or IF.
“Eating heavy meals and then letting your body digest will allow for weight loss and hormone balance as well as time for our bodies to use the nutrients we are ingesting for energy,” she explains.
Are there any side effects to combining keto and intermittent fasting?
WH has previously reported on the many gag-inducing side effects of going keto (constipation and diarrhea and hair loss, oh my!), and the potential downsides of IF, which include low blood sugar, fatigue, nausea, and being just plain hangry all the time.
So do you end up with double the side effects when you combine the diets? In theory, yes. Putting the two together doesn’t do anything to counteract the side effects of either diet, so you could wind up with the undesirable combination of nausea and constipation (or hair loss and hanger, or…well, you get the idea).
More importantly, though, the biggest downside to pairing keto with IF is the sheer level of commitment involved—and the high potential for failure.
“It’s very difficult to maintain such a narrow way of eating,” says Schaub. “Eating an extremely high-fat, low-carb diet itself is difficult to do while maintaining a social life and flexibility in your schedule; layer in a restricted eating window of eight hours, and the ability to sustain it becomes that much tougher.”
The one plus side? Schaub does say that IF could become less uncomfortable when coupled with keto: “The ketogenic diet is very satiating—eating fat leads to less hunger overall, which makes restricting your eating window significantly easier.”
Finally, while IF is a more natural way of eating, keto is decidedly stricter/not for everyone (and definitely not for kids, pregnant women, people with a disordered history of eating, high-intensity athletes, or people with certain health conditions).
“Consult a dietitian or other healthcare practitioner if you are considering following a ketogenic diet paired with intermittent fasting,” says Titgemeier. “You may want to start with intermittent fasting first to see how you feel before going full force with both intermittent fasting and keto.”
The bottom line: There may be additional weight loss benefits to doing both the keto and intermittent fasting diets together instead of alone…but it may be hard to stick with both long enough to reap any benefits.