Intermittent Fasting and Keto Diet: Can You Combine Them?

In 30 seconds

Fasting and keto raise ketones by different routes. No source here tests them together, so the combination is unproven. In one 3-month trial in adults with obesity, a ketogenic diet raised fasting ketones more than time-restricted eating or alternate-day fasting, but the diets were tested separately. If you have diabetes or take glucose-lowering medicines, do not combine them without your clinician.

• Fasting raises ketones as liver glycogen runs down; a ketogenic diet raises them by keeping carbohydrate very low.
• In a 3-month trial in 160 adults with obesity, a calorie-restricted ketogenic diet raised fasting blood ketones more than time-restricted eating or alternate-day fasting.
• No source reviewed here tests fasting and a ketogenic diet combined, so the combination's benefits and safety are not established.
• If you have diabetes or use insulin or other glucose-lowering medicines, do not combine fasting and a ketogenic diet without your clinician.

How fasting and keto raise ketones

People sometimes combine intermittent fasting with a ketogenic diet, expecting faster ketosis. The two work differently: fasting raises ketones as the body uses up its stored glucose, while a ketogenic diet keeps carbohydrate so low that ketones rise even with regular meals. The fasting side is explained in fasting and ketosis.

During fasting, ketones rise as liver glycogen runs down; reviews note that the timing depends on glycogen stores at the start of the fast and on activity.Source 11Source 22

What a head-to-head trial found

One randomized trial compared a ketogenic diet with fasting schedules such as alternate-day fasting. It tested each approach on its own rather than together, and every group ate about 600 kcal a day less than it needed.

In a 3-month randomized trial in 160 adults with obesity, weekly fasting blood ketones were highest with a calorie-restricted ketogenic diet (5% of calories from carbohydrate); they did not differ between early or late time-restricted eating and modified alternate-day fasting, and were lowest with a Mediterranean diet.Source 33

Blood markers did not separate the diets over this short trial.

At 3 months, fasting glucose, insulin resistance, cholesterol, triglycerides and blood pressure did not differ between any of the diets and the Mediterranean diet, and no serious adverse events were reported; the results do not apply to people with diabetes.Source 33

What is not known

No source reviewed here tests intermittent fasting and a ketogenic diet together. The only direct comparison lasted 3 months, restricted calories in every group and does not apply to people with diabetes, so it cannot show whether the combination works better, speeds up ketosis or is safe in the long term.Source 33

Who should be careful?

Talk to a qualified health professional before fasting if you are pregnant or breastfeeding, under 18, living with or recovering from an eating disorder, have diabetes, take insulin or a medicine affected by meal timing, are an older adult, or are acutely ill. Do not change the dose or timing of any medicine without your clinician. See Blimbi’s safety guide.

Related guides

• Does Intermittent Fasting Reduce Belly Fat? What Reviews Show

  Does intermittent fasting reduce belly fat? No review shows it beats calorie-matched diets for belly fat; trials do show lower body weight.

• Intermittent Fasting vs Calorie Deficit: Is One Better?

  Most intermittent-fasting schedules and a daily calorie deficit give similar weight loss in adults, with small, inconsistent cardiometabolic effects.

• Intermittent Fasting and Ketosis: When It Starts

  Reviews describe fasting ketones rising 8–12 hours in and a shift to fat and ketones 12–36 hours after the last meal: ranges, not rules for ketosis.

• Alternate Day Fasting: How It Works and What Trials Found

  Alternate day fasting alternates fasting and eating days; it lost slightly more weight than other diets only in shorter trials. See who should ask first.

• Circadian Rhythm Fasting: What the Timing Trials Found

  Circadian rhythm fasting: eating earlier may modestly improve fasting insulin, glucose and weight, but trials are short and vary.

Frequently asked questions

Does a ketogenic diet make you enter ketosis faster when fasting?

This is not established. In one trial a ketogenic diet raised fasting blood ketones more than time-restricted eating or alternate-day fasting, but the diets were tested separately. Fasting reviews say the switch toward ketones depends on starting liver glycogen and activity.Source 33Source 22

Does a ketogenic diet improve cholesterol or blood sugar more than fasting?

In the one trial that compared them, none of the diets changed fasting glucose, insulin resistance, cholesterol, triglycerides or blood pressure differently from a Mediterranean diet at 3 months. Longer trials are needed.Source 33

Is it safe to combine intermittent fasting and a ketogenic diet?

Safety of this combination is not established in the sources reviewed here, and it depends on your health and medicines. If you have diabetes or use insulin or other glucose-lowering medicines, plan any change with your clinician, because blood sugar can go too low or too high. Otherwise, talk with your healthcare provider before making significant changes.Source 44Source 55

Sources

1. 1

   Effects of Intermittent Fasting on Health, Aging, and DiseaseRafael de Cabo, Mark P. Mattson. Massachusetts Medical Society, December 26, 2019.DOI: 10.1056/NEJMra1905136 · PMID: 31881139Narrative or mechanistic review

2. 2

   Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of FastingStephen D. Anton et al. Wiley, February 1, 2018.DOI: 10.1002/oby.22065 · PMID: 29086496Narrative or mechanistic review

3. 3

   Effect of a ketogenic diet, time-restricted eating, or alternate-day fasting on weight loss in adults with obesity: a randomized clinical trialJosé Ignacio Martínez-Montoro et al. Springer Science and Business Media LLC, July 1, 2025.DOI: 10.1186/s12916-025-04182-z · PMID: 40598397Human randomized trial

4. 4

   Fasting Safely with DiabetesMartin M. Grajower. National Institute of Diabetes and Digestive and Kidney Diseases, August 26, 2020.Official guidance

5. 5

   Calorie restriction and fasting diets: What do we know?National Institute on Aging. National Institute on Aging, August 14, 2018.Official guidance