Intermittent Fasting on Keto: What to Know Before You Combine the Diets for Weight Loss

The ketogenic diet, also known as keto, is a low-carb, high-fat diet designed to put your body into a metabolic state called “ketosis,” where it burns fat for fuel instead of carbohydrates. The diet has been used since the 1920s to treat medication-resistant epilepsy.

Intermittent fasting (IF) is another dietary approach to weight loss where you alternate between periods of fasting and eating within a specific time window.

Since both diets can promote weight loss, you may be considering combining the two ways of eating.

Currently, research on this combination diet is limited. Additionally, while both diets are often labeled as “lifestyle choices,” keto and IF significantly alter the body and should be introduced under doctor supervision, no matter your health status.

Why Combining Intermittent Fasting With Keto Has Become Popular for Weight Loss

A keto diet or IF can help with weight loss in the short term, but both diets are very restrictive and are certainly not suited for everyone.

Some choose to pair the two approaches to amplify their respective mechanisms. The keto diet increases levels of ketones in the body, and ketones also increase during times of fasting. This may lead to more weight loss.

It can take as little as two to four days to enter ketosis after you start the diet, though it can take over a week for some people.

A study published in 2024 found that both diets significantly reduced body fat in participants, and a combination of both diets had the most significant impact.

People who are trying to lower their blood glucose levels may also be interested in this dual dietary approach, but should speak with a doctor before starting one or both diets for this purpose.

Importantly, anyone interested in combining these two diets should speak with a medical professional before getting started. While research is promising in the field of chronic condition treatment, most studies cite risks involved with specific groups and conditions.

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Who Shouldn’t Try an Intermittent Fasting Keto Approach?

Because of the significant impact both diets have on the body, experts recommend that every person consult with a medical professional before starting keto or IF.

Limited research makes it difficult to identify specific groups who should avoid the IF-keto diet, but you can use the exclusion criteria of current research as your guide; while researchers are currently conducting studies on the benefits of keto and IF for conditions like diabetes, many studies for one or both diets still exclude certain groups of people — this is often to narrow the focus of the study, because these groups are likely to experience different health impacts from the general population.

Using this exclusion criteria, it’s recommended that you consult with your doctor before going on keto-IF if you:

• Have diabetes and are on medications (especially insulin or sulfonylureas)

• Have a liver or pancreatic condition

• Are pregnant or breastfeeding

• Are under 18

• Have a history of disordered eating

• Are underweight

• Have gallbladder disease or gout

• Have an uncontrolled chronic condition

• Have kidney disease or kidney stones

• Are undergoing cancer treatment

The Right Way to Introduce Intermittent Fasting While on Keto

First, there is no substantial body of research suggesting that it’s best to introduce intermittent fasting after you begin keto, but when you practice the keto diet short-term, you may experience symptoms of “keto flu” as your body adjusts to burning an alternative energy source. Symptoms of keto flu include nausea, fatigue, headaches, insomnia, and vomiting. In a fasted state, these symptoms may be more difficult to manage. Common “starter” methods for intermittent fasting are the 12/12 method, where you eat and fast in 12 hour windows, and the 16/8 method, where you reduce your eating window to 8 hours.

The Possible Health Benefits of a Keto Intermittent Fasting Diet

There is little research on the health effects of doing keto and IF together, but ketone levels may increase when the plans are combined. This may accelerate weight loss. Still, everyone responds differently, so it won't be true across the board.

Both diets may reduce insulin levels and improve insulin sensitivity, potentially helping to regulate blood sugar levels, providing therapeutic benefits for people with conditions like type 2 diabetes.

But the research continuously stresses that people with diabetes should not go on these diets without medical supervision.

Ketone bodies, which increase on both diets, may also contribute to health benefits — a rise in ketone bodies is associated with decreased inflammation, and inflammation is the driving force behind many chronic conditions.

Are There Any Known Health Risks of Doing Keto and Intermittent Fasting Together?

There is little research exploring the keto-IF combination diet. But both diets have been linked to health issues, and practicing them together is not an antidote to risk. Additionally, there’s overlap in at-risk groups for both diets.

Studies have shown that the keto diet may cause blood sugar fluctuations and, over the long term, an increased risk of heart disease.

Additionally, as bulleted above, keto is not recommended for people who are pregnant, have an eating disorder, or have a history of disordered eating.

And while fasting has been practiced for centuries and has a substantially larger body of research supporting its benefits, you may still experience adverse side effects, especially if you have type 2 diabetes and are on certain medications (due to the risk of hypoglycaemia) or if you have heart disease. Preliminary research presented in 2024 at the American Heart Association’s Lifestyle and Cardiometabolic Health Scientific sessions found a 91 percent higher risk of death from cardiovascular disease in people who limited their eating to a window of 8 hours per day, a most common “starter” window for intermittent fasters, compared with those who ate over 12 to 16 hours.

Additionally, fasting is not recommended for people who are pregnant or have a history of disordered eating.

Therefore, folks who have diabetes, heart disease, are pregnant, or have a history of disordered eating should consult with a doctor before getting started with either or both diets.

The Takeaway

• The keto diet is a restrictive high-fat, low-carb diet, and intermittent fasting restricts the number of hours you will be eating.

• There is a lack of research on combining the two, so it’s unclear exactly what impact the combination diet could have on your body, especially if you have certain conditions.

• Before you decide to follow keto and IF together, be sure to consult your healthcare team. Your provider can help you determine whether this combined diet plan is a good fit for you.

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