Why You Shouldn't Try Keto With Intermittent Fasting, According to a Dietitian

With all of the knowledge we have on nutrition and weight loss, why is everyone still going through drastic measures to lose weight? Simply put: They want the pounds off and fast.

Both the ketogenic diet (keto, for short) and intermittent fasting (IF) can separately help you drop weight quickly, so pairing them up should cut that time in half, right? Not so fast — you might want to rethink that flawed logic.

The typical American diet is made up of roughly 50 percent carbohydrates, 25 percent protein and 25 percent fat.

The keto diet is essentially a high-fat, moderate-protein and very low-carb diet that encourages 60 percent of one's total caloric intake to come from dietary fat, 30 percent from protein and no more than 10 percent from carbs, according to a March 2019 report in StatPearls.

The fact is, the human body is very efficient at using carbs for energy — in fact, glucose, specifically, is the preferred source of energy. When the body isn't fed any carbs to burn, it starts to burn fat. And those fats are converted to ketone bodies, which are used in place of glucose for the energy you need to function.

Contrary to popular belief, you may actually have a calorie deficit with the keto diet, according to the March 2019 report. That's likely because high-fat foods (think avocado, salmon and nuts) are very filling, which may cause you to eat less.

Another reason why you might lose weight on keto: You're forced to cut out sugar, refined flour and processed foods, which can cause you to pack on the pounds.

Intermittent fasting (IF) is just like it sounds — you fast intermittently, not continuously. The two most popular types of IF are the 5:2 method and the 12- or 16-hour fast.

The 5:2 fasting diet combines five days of normal (aiming for healthy) eating with two days of restricted eating, usually consisting of about 500 to 600 calories.

The 12- or 16-hour fasting diet consists of eating within a certain window every day. With the 12-hour fast, you eat within a time frame, let's say, 6 a.m. to 6 p.m., and fast the remaining 12 hours of the day. The 16-hour IF would work the same, but you'd fast for 16 hours instead of 12. So, for example, you'd limit your food intake to the hours between 9 a.m. and 5 p.m.

Without saying it, IF is a form of calorie restriction, no matter how you try and spin it. There are advantages to letting your body rest without food, but realistically, you shouldn't be eating for 24 hours anyway.

Both the keto diet and intermittent fasting have been followed separately for weight loss. And while researchers have noted weight-loss success with both of these diets, regaining the weight later on is very common.

In fact, weight loss peaks around 5 months with the keto diet, according to a December 2018 article published in Canadian Family Physician. However, participants slowly regained the weight, presumably when they reintroduce carbs back into the diet — which simply means that keto is not very sustainable in the long run.

And a recent review concluded that IF is comparable to simply restricting calories all day in terms of weight loss, according to December 2018 research published in the Journal of Translational Medicine. So, if you find that fasting isn't for you, you might reap the same weight-loss results by just cutting calories.

If you're thinking of pairing the two trendy diets, here are the pros and cons to consider:

• You'll likely stick to both more easily. A high-fat diet is really good at getting rid of those hunger pangs and that could make it easier to keep up with IF.
  
  
• You may reach ketosis much faster with IF. It can take several days for your body to convert its fuel source from glucose to ketones, but restricting your intake and thus depleting your glycogen stores may force your body to switch energy sources sooner, per January 2018 research in Nature Reviews Neuroscience.

• It's hard to stick with. Separately, keto and IF can be hard to follow due to the restrictive nature of each. Plus, the ketogenic diet induces higher-than-average immediate weight loss because of the diuretic effect of the diet (aka, it can de-bloat you). However, this can give a false sense of success.
  
  
• You might regain the weight. People following these diets usually notice weight loss; however, when they revert back to normal eating, more often than not, that weight is regained. Combining the two may be a recipe for disaster when you quit the eating regimens.
  
  
• You may feel fatigued. For active individuals and athletes, it's just not a smart choice. "Athletes and active people, in general, are already at risk for low energy availability — and adding two dieting mechanisms would further this risk," Sports dietitian Marisa Michael, RD of Real Nutrition, LLC, tells LIVESTRONG.com.
  
  
• The health risks are unknown. Especially for the keto diet, there's a limited amount of quality long-term studies on the risks associated with following the plan for an extended period of time. The risk for nutrient deficiencies with both IF and keto is known to be high. Plus, "there are many health consequences to low energy availability, such as increased injury, gastrointestinal disturbances, mood disturbances, decreased bone density, amenorrhea and more," Michael points out.

Research has shown that both of these diets work in the short term for weight loss, but combining them isn't recommended if you're trying to lose weight fast and safely. Aside from the potential health risks that may arise, doing both simultaneously is not necessary.

A June 2020 study in The American Journal of Medicine reviewed both keto and IF diets and concluded that while these diets have some favorable effects, they allow foods that are known to increase the risk of heart disease. The researchers stress that if you do try either diet or both in tandem, focus on heart-healthy foods.

There are definitely some folks who should steer clear of both diets together and separately. If you have any medical conditions with your thyroid, gallbladder, pancreas and/or liver, you might want to rethink your decision to follow the keto diet since it may exacerbate the condition, according to the University of Chicago Medicine.

If you have a history of eating disorders, it's best not to try either of these diets. "Keto plus IF sounds like a recipe for disordered eating," Michael says, suggesting that the combo may result in binge eating.

In addition, people with diabetes should not try either of these diets until they have a thorough conversation with their doctor. Diabetes management can be difficult with these diets, so do not start either of these plans until you're cleared to do so.