You’ve been keto for a few months now (or longer). You know what you’re doing. You feel good about where you are. You’re fat-adapted. You’ve got a slew of recipes under your belt, your gym performance has normalized, the keto-flu is a distant memory. And now, you’re looking to explore further. The natural next step is intermittent fasting.
But is it the right move?
Does intermittent fasting work if you’re keto?
The short answer is: Yes. Intermittent fasting works really, really well if you’re on a ketogenic diet.
Keto smoothes the fasting transition
One way is that being keto makes the transition into fasting smoother and easier.
Fasting shifts you into fat-burning mode. You have no exogenous food coming in, and your only source of energy is the fat that sits on your frame. Anyone who fasts, whether they’re coming from a high-fat diet or a high-carb diet, will end up burning fat.
Keto is fat-burning mode. You have very little dietary carbohydrate coming in, and your body must deal with the “glucose deficiency” by converting fat into ketone bodies—an alternative fuel source that can power many of the same tissues that normally run on glucose.
Long-term keto is even better for fasting. Since you’ve taken the time to get truly fat-adapted, you’ve built the metabolic machinery—the actual mitochondria, the power plants of your individual cells that convert nutrients into ATP—necessary to burn free fatty acids directly. Your reliance on actual ketones goes down, your ability to burn fat directly goes up, and your ability to seamlessly switch between eating and fasting skyrockets.
The problem with going from a high-carb diet to intermittent fasting is that you have to start all over again each time. You have to go through the process of converting your metabolism from sugar-burning mode to fat-burning mode. That takes time and energy, and it often triggers the “keto flu”—that collection of symptoms ranging from headaches to fatigue to brain fog to irritability.
When you fast on a ketogenic diet, there’s no keto flu because you’re already in ketosis and fat-burning mode. The transition is easy because your body has already made the transition, and you can move on and start reaping the benefits of fasting more rapidly.
Keto speeds up fat loss while fasting
Another way fasting works better when you’re in ketosis is for losing weight, specifically body fat.
A 2013 study compared low-fat dieters on an alternate day fasting schedule with low-carb dieters on an alternate day fasting schedule. Both groups lost weight and improved metabolic health markers, but the low-carbers lost more body fat. They were already in “fat-burning mode.” Fasting just kicked it up a notch. Meanwhile, the high-carb group had to take extra time to start burning body fat, and as a result, they lost less overall.
A more recent study putting low-carbers on a fasting schedule for six months saw their body fat drop, lean mass remain stable, and fasting insulin decrease. However, there was no control group and the low-carb diet was still 30% carbs. I think you’d see better results if you dropped those carbs down even lower to full-on ketogenic status.
Keto keeps appetite down while fasting
A third way keto improves the fasting experience is through appetite reduction. Most fasts fail because the faster gets too hungry, too quickly. In the modern food environment where tens of thousands of delicious calories beckon from all angles at any given time, hunger is difficult to resist. And once you eat, the fast is over.
Ketosis suppresses appetite, setting you up for a successful fast free of the kind of ravenous hunger you can’t ignore. This occurs on a physiological level, with keto actively lowering the increase in hunger hormones that normally occurs with caloric restriction. When people attempt to eat less—or no—food despite wanting more, they butt up against their own physiology. Few win that battle.
Is intermittent fasting healthy if you’re in ketosis?
Okay, so we’ve established that intermittent fasting works when you’re on a long-term ketogenic diet, and that it works even better than it does on a higher-carb diet. You lose weight more easily, your metabolism doesn’t have to “switch” to fat-burning mode because you’re already in it, and you often find yourself forgetting to eat rather than fighting off cravings. But what about the health effects—is fasting a good idea from a health perspective if you’re already in ketosis?
One potential problem people new to fasting worry about is muscle loss. After all, if you’re not eating any food at all, it seems possible that you’ll burn through muscle to make up for the lack of incoming energy. This can happen, but not always.
If you’re coming off a high-carb diet, fasting is more likely to result in muscle loss. Your body still expects, still wants sugar, and it will extract and convert the amino acids found in your muscle tissue to get it. This gives you the glucose you (think you) require, but it results in loss of lean muscle mass. Since lean mass is one of the most important markers of good health, any loss of lean mass is undesirable, unwanted, and unhealthy.
If you’re coming off a long-term ketogenic diet, fasting is less likely to result in muscle loss. Ketones spare muscle tissue by reducing your need for glucose. It turns out that a fair number of tissues that would otherwise run on glucose can run on ketones instead. Being in a ketogenic or low-carb fat-burning state before you fast accentuates this effect.
Stress and cortisol
Another potential issue is stress. Food availability can be a major stressor in the human body, and and if zero food is coming in, that’s a whole of potential stress. Intermittent fasting can increase cortisol, which is the hormone released when the body responds to a stressful situation. How does keto affect the cortisol-stress response to fasting?
Your body induces a stress response to fasting in order to procure more glucose. Cortisol is released to trigger gluconeogenesis in the liver, which creates glucose out of amino acids (usually taken from muscle tissue). Your body perceives the lack of incoming glucose as a stressor, and activates the stress response.
This stress response doesn’t happen to the same extent if your body doesn’t perceive the lack of incoming glucose as a problem. The harmful cortisol spike only occurs if your body needs glucose and can’t get it. What if your body doesn’t feel the need for glucose? What if you’re, say, on a long-term ketogenic diet, have full adaptation to ketones and free fatty acids, and simply don’t need very much glucose?
Your body won’t have the same negative stress response to intermittent fasting, nor will it accumulate all that cortisol.
How to Fast on Keto: Tips to Make it Easier
As I said earlier, keto and fasting go hand in hand, but there are a few key things to keep in mind for making your fast a success.
Take electrolytes—sodium, magnesium, potassium
Drink coffee or tea
Start small—14 to 24 hours
One major reason why people complain about energy and fatigue when intermittent fasting is they’re not eating enough salt and other electrolytes. Try 4.5 grams sodium (about 2 teaspoons of fine salt or a little under 3 teaspoons of kosher salt), 300-400 mg magnesium, and 1-2 grams of potassium each day on top of your normal food.
Drink coffee or tea
Both coffee and tea are non-caloric, so they won’t break the fast. They give your mouth something to do, in case you do get hungry. And they contain caffeine, which increases fat-burning and should make the fast more tolerable and more beneficial.
Keeping active with walks and other types of low-level physical activity will get your mind off any residual hunger and increase fat oxidation. Anything that speeds up your fat utilization will increase ketone production and enhance the beneficial effects of the fast.
Lifting weights staves off any muscle loss that can occur on a fast by sending an anabolic message to your body: that your lean mass is important, that you’re still using it, that you can’t afford to lose it.
If you’re fasting for the first time, aim small and start with a fast lasting 14-24 hours. Women may need special considerations, however, so read this post summing up all the possible interactions between sex and fasting.
To answer the question posed in the title: Yes, it’s probably a good idea to try intermittent fasting while on a long-term ketogenic diet. If nothing else, it’s safe and should be easy to do—and it may offer many benefits.
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.