They are increasingly popular eating styles among celebrities like Gwyneth Paltrow, who’s reportedly a fan of keto, and Halle Berry who has posted her love for intermittent fasting on Instagram. Some people are even starting to combine the two — but is that actually effective? And more importantly, is it healthy to do the keto diet and intermittent fasting at the same?
Here’s what you need to know about keto and intermittent fasting, and whether or not you should try them together.
What exactly is the keto diet, anyway?
The ketogenic diet is a macronutrient-restrictive pattern of eating where the majority of the calories you eat come from fat, while the rest comes from a moderate amount of protein and little to no carbs. For the most part, this means saying goodbye to fruit and pasta, and hello to steak and eggs.
This high-fat, low-carb diet encourages the body to use fat for energy instead of sugars. When your body does not have enough carbs to sustain everyday activity, the liver breaks down fat into ketones, which can then be used as energy. This metabolic process is called ketosis — that’s where the phrase “going keto” comes from.
Studies have shown that the keto diet can have many health benefits. It’s been proven to increase weight loss, improve brain function, and reduce blood sugar levels. It has helped many people who have suffered from obesity and diabetes. However, eating in this way can be extremely challenging.
“People don’t usually fashion their plate around fat,” says Kristen Mancinelli, MS, RDN, author of The Ketogenic Diet. “I mean, you really eat 25 grams of carbohydrates a day, mostly coming from non-starchy vegetables like broccoli or lettuce. It’s sort of hard to grasp because it’s not what we’re used to.”
So what’s intermittent fasting?
Unlike keto, intermittent fasting is not a diet. It’s an eating pattern fashioned around certain feeding and fasting periods. One of the most popular forms of intermittent fasting is a 16:8 split, where people fast for 16 hours and only eat during an eight hour window. This usually means not eating after 8 p.m., going to sleep, then having a big lunch at noon the next day. Other fans of intermittent fasting might choose to do an 18:6 split or fast 24 hours every other day.
Intermittent fasting can take many forms and has many benefits. Like keto, people have used it to lose weight, control blood sugar levels, and achieve more clarity. When you’re fasting, the body doesn’t have to release insulin to break down sugar, and resorts to turning fats into ketones. Plus, if you aren’t binging during your feeding window, you’re naturally consuming less calories without having to track it. Some people also use intermittent fasting just to give their digestive systems a chance to reset.
“The reality is the period of time that we’re not eating for most of us in the U.S. and the Western world is so short that your body never gets the opportunity to use any stored fat,” says Mancinelli. “Most people don’t go for six hours without eating.”
Some nutritionists argue that intermittent fasting is an eating pattern that has been practiced for centuries, whether intentionally or unintentionally. Mancinelli points out that humans have lived through seasons of feast and famine, while many cultures include fasting in their practices.
“Perfectly healthy adults can go a few days without eating and be totally fine,” she adds. “Many people say they couldn’t go a day without eating and that they would be lethargic or dizzy. Very rarely is that the case.”
Are there benefits to doing both keto and intermittent fasting?
The idea is that combining the ketogenic diet and intermittent fasting will maximize the time that someone is in ketosis. This could mean more weight loss, less hunger pangs, and more energy.
“It’s more common for someone to start a ketogenic diet and then layer on intermittent fasting,” says Mancinelli. “They realize, ‘Oh, I can get more bang for my buck and I’m not as hungry anymore. It’s not as common, though, for people who start with intermittent fasting to layer on keto because then it looks like a lot more work to eat very specific foods.”
While combining the two might boost results, these styles of eating are not for everyone, whether independently or together.
What are the risks of combining keto and intermittent fasting?
Celebrity trainer and nutrition specialist, Harley Pasternak, M. SC., says he would never recommend either eating patterns, let alone combine them.
“I can’t speak for anyone who would want to combine the two, but I personally would not,” he says. “It sounds like an overly regimented, restricted, and antisocial way to live.”
Intermittent fasting and keto might be tough to sustain when you’re out with friends at dinner or celebrating the holidays. You might be tempted to eat something that you “shouldn’t” eat or munch outside of your feeding window. And if you slip, you might be hard on yourself. For many, keto and intermittent fasting might not be sustainable.
There are even some pitfalls if you were to practice either eating style individually. For example, sometimes new keto dieters complain of getting “the keto flu” during their transition, which can include nausea, headaches, dizziness and stomach pain. A low-carb diet also runs the risk of not providing enough vitamins and nutrients like magnesium, potassium, calcium, and fiber.
“If you look at the healthiest, longest living populations in the world, these are people that tend to eat all food groups,” says Pasternak.
Even Mancinelli, who is a proponent of keto and fasting, thinks that “it’s not entirely necessary” to combine the two to get significant benefits. Nor is it necessarily needed to practice either eating pattern for life — particularly keto. You want to be “metabolically flexible” she says.
“You don’t want to also lose the ability to use glucose for energy because then when you do go back to eating something that’s glucose rich, your blood sugar is going to be fairly high,” says Mancinelli. “Now you’re going to have to develop the mechanisms to use that for energy again. You don’t just want to be in ketosis forever. I just am not clear that I see the benefit of that unless you’re using it as a treatment for some disease.”
Who should avoid trying both?
Combining intermittent fasting and the ketogenic diet are particularly risky for certain populations, according to Mancinelli.
Children under the age of 18 who are still growing, for example, need to be able to eat when they feel like it. Those who are over the age of 65 may have trouble getting enough nutrients and calories to sustain certain body tissues and body mass, might want to avoid any restrictive eating regimens.
This can be true for anyone who has a nutritional deficiency or for someone who always has to take their medication with food. “It’s not that you can’t do intermittent fasting but you need to organize yourself,” says Mancinelli. “That can be a pain.”
Pregnant women who plan on breastfeeding after giving birth, might not want to hop straight into intermittent keto fasting to shed the baby weight. Producing milk requires a lot of energy and so restricting calories might not be the best move.
And if you’ve suffered or are suffering from any eating disorders, venturing into these two eating styles can be very emotionally triggering. It might be best to find other methods to reach your health goals.
What should I do if I’m interested in do keto and intermittent fasting together?
Always consult your doctor or registered dietician before changing up your eating habits. You want to have a solid game plan and know that you are practicing in a way that’s healthy for you.
If you’ve got the green light and don’t know where to start, Mancinelli suggests the easiest route is to give intermittent fasting a go first.
“What I recommend is trying something that is going to fit into your schedule,” she says. “Don’t focus on what’s the optimal number of hours that I should be fasting, but what works in your life. Do an inventory of when you eat and which meals are super important to you and say to yourself, ‘Which one could I cut out?'”
Then later on, you can try to tack on keto if you want and see how you feel. Again, consult your health professional and find a community that can inspire you to stay on track.
Whatever you decide to do, you have to continue to ask yourself, “What do I believe will be sustainable for my lifestyle?” and “Which eating style will have me at my best?” — not just physically, but mentally and emotionally as well.