Keto and intermittent fasting can be combined for quick weight loss results, but is it safe or healthy? A nutritionist weighs in.

By Carolyn Williams, PhD, RD
June 28, 2018
Credit: ThitareeSarmkasat/Getty Images

Both intermittent fasting (IF) and ketogenic diet are popular eating approaches, but some have suggested taking things a step further by combining the two. So is there any evidence that this works, or, more importantly, is it a healthy approach to weight loss?

What is the IF keto diet?

IF is a short-term fast where one eats as little as 500 daily calories a day, or only eats only during a set, 8-hour time window each day. There are several types of fasting approaches, and the practice has become increasingly popular. It’s also got some research behind it to suggest that rather than decreasing metabolic rate as many assumed, IF appears to maintain and possibly increase metabolic rate to support weight loss, blood glucose management, and cholesterol, as well as decrease inflammatory markers in the body. These effects appear to be triggered by forcing the body to use existing fat stores for energy—stores that are rarely tapped into when one eats or snacks every few hours.

The ketogenic diet takes a similar (but slightly different) approach. Rather than restricting calories or eating windows, the breakdown of nutrients is manipulated so that fat supplies the majority of daily calories (70 to 80%), and very minimal carbohydrates (often less than 20 grams) are consumed. The goal of this eating approach is to put the body into a state of ketosis—a situation where inadequate carbohydrate intake forces the body to break down fat stores whose fatty acids can then be broken down into ketone bodies and used as temporary fuel.

The ketogenic diet already has the body in a state of utilizing fat stores for energy with additional reliance on fat stores, so when it’s combined with fasting, the body is forced to break down even more fat for energy—especially during the last hours of a fasting window. The combined eating approach is suggested by some to be one of the most effective weight loss methods because of its heavy reliance on fat stores to keep the body running. In fact, one Keto IF diet book suggests the combination the two approaches is the “secret to high-powered, long-lasting weight loss.”

What can you eat?

Before talking about specific foods, it’s important to understand how keto diet recommendations affect nutrient intakes. Let’s say one’s estimated calorie needs are 1800, and we’ve decided to follow a ketogenic diet that has a  breakdown of 75% fats, 5% carbs, and 20% protein. This breaks down to approximately 265g fat, 22g carb, and 90g protein.

In order to achieve these numbers, food intake is predominantly fats and oils, about 10 to 13oz protein, and carbs from non-starchy, low-carb vegetables. Dairy foods, nuts and nut butters should be used in minimal amounts, and most all grains, starchy vegetables, and fruits, and added sugars should be avoided to keep carbohydrate intake low. Add in the IF component and this usually means consuming these foods within an 8-hour window each day.

What are the benefits of a keto/IF diet?

Proponents of combining IF with a keto diet for weight loss suggest that the metabolic effects are one of the biggest benefits.

Unlike many weight loss strategies that focus on significantly reducing total calories, the Keto/IF approach doesn’t necessarily require a decrease in calories—only a change in where those calories come from, and when you eat them. Many individuals opt to decrease total daily calories, but only slightly.

The result is that your body burns fat stores due to ketosis and fasting rather than low calorie intake. When one’s goal weight is met, it’s suggested that maintaining weight loss is not as difficult since total calorie level and metabolic rate for not forced to decrease significantly.

Another benefit suggested by Keto/IF books is that fat provides satiety and suppresses appetite. Think about the last time you ate a meal really high in fat; then think about how you really didn’t even think about food, hunger, or appetite for a large remainder of the day. Since the majority of calories come from fats and oils, there’s natural appetite suppression—a particularly nice perk when one is trying to maintain a 16-hour fast.

What are the concerns around a keto/IF diet?

I never like ruling a theory or idea out—particularly when it’s an eating approach that we know very little about (like combining the ketogenic diet and IF.) I could find no published research studies that have examined the effects of the combination in regards to metabolism, health or weight loss. But at the same time, the fact that we don’t know much—in terms of safety, metabolism, effectiveness, and impact on long-term health—is also the problem.

One of my biggest concerns is the lack of essential nutrients that a keto diet provides. When you rule out grains, starchy vegetables, most dairy, and fruits, it’s almost impossible to meet daily intake recommendations for fiber, folate, vitamin C, potassium, calcium, and Vitamin D. Because of this, a keto approach isn’t a healthy long-term approach for most individuals in terms of overall health and disease prevention.

My other concern stems from the suggested benefits. I agree that very low-calorie diets are not practical and usually not maintainable, partly because the body adjusts to a lower calorie level. I can also see how appetite suppression might be a nice perk, not only when trying to lose weight, but also when fasting. But because of the appetite suppression and high level of fat intake, it’s almost impossible to consume all daily calories in an 8-hour window.

In fact, a friend who tried the Keto//IF diet combo a few months ago said that her calorie intake was usually below 1200 calories, simply because she was too full and couldn’t eat any more fat-laden calories before her 8-hour eating window ended.

The Bottom Line

Recent research on both the ketogenic diet and intermittent fasting is promising and intriguing to say the least. We know very little about the short- or long-term effects when the two eating approaches are used together, and combining the two ends up appears to equate to a pretty restrictive intake that is inadequate in calories and nutrients. Right now, individuals are better off trying an eating approach that we know more about.