What is the keto diet?
First, a quick refresh: Eating keto-style means your meals are high in fat (usually 70%-80% of daily calories), low carb (about 5%-7% of daily calories), and contain moderate protein. When you eat this way, your body goes into ketosis, which means “it becomes super efficient at burning fat for energy,” says Alicia Galvin, R.D., a registered dietitian in Texas. This way of noshing also reduces blood sugar and insulin levels to further promote weight loss and management.
What is intermittent fasting?
Simply put, IF is when you eat all of your meals within a certain window of time. There are a few different models. The most common type is 16:8, meaning you eat over the course of 8 hours during the day and fast for 16 hours overnight. You can also follow a pattern where you fast for an entire day once or twice a week, or where you eat normally for five days and severely restrict your caloric intake for two days (typically 600-700 calories). Regardless of the eating/fasting schedule, IF is supposed to help you lose or keep off weight by naturally helping you control the amount of calories you eat, and it’s thought to also reduce insulin, the fat-storing hormone.
Should I combine the keto diet and intermittent fasting?
You can, but you shouldn’t. For one, there has been no research on combining these two diet fads, so experts can’t be sure exactly how your body will respond if you follow both simultaneously. Then there’s the fact that following a super restrictive eating plan like the keto diet on its own means you may already be missing out on necessary vitamins, minerals, and other essential nutrients your body needs to do its job. “Improperly following a ketogenic diet could result in you eating too much fat and meat and not enough vegetables, which won’t trigger ketosis at all,” says Mark Sherwood, N.D., of the Functional Medicine Institute in Tulsa, Oklahoma.
Add in another layer of eating restriction with IF and you’ll end up putting a lot of stress on your body. “Combining these two diets can be very hard on the body and cause you to overly restrict calories and put you at risk for nutrient deficiencies, as well as mess with your adrenal system, which regulates important hormones like cortisol,” says Galvin.
The bottom line:
Don’t combine keto and intermittent fasting, but if you do want to try one, opt for IF, which comes with fewer risks. The keto diet can be especially dangerous for people with prediabetes and type 1 or 2 diabetes, conditions in which blood sugar and insulin regulation is critical, and it can cause electrolyte imbalances that may trigger weird side effects like headache, nausea, fatigue, dry mouth, brain fog, bad breath, and flu-like symptoms known as “keto flu.”
Plus, if done properly, intermittent fasting can actually trigger the ketosis strived for in keto. “It’s actually normal to get into ketosis during the night on a normal calorie or timed restricted feeding schedule like intermittent fasting, regardless of the macronutrients consumed,” says Steven Gundry, M.D., medical director at The International Heart and Lung Institute Center for Restorative Medicine and author of several books including the upcoming book The Energy Paradox.