Franziska Spritzler, RD, CDEIntermittent fasting (IF) has become widely popular due to its simplicity and benefits like weight loss, increased energy, and better overall health. (1) However, if you want to achieve and maintain nutritional ketosis, IF alone might not be enough. While intermittent fasting offers many perks, pairing it with a ketogenic diet can significantly boost your chances of reaching and staying in ketosis, even if you indulge in a non-keto meal occasionally.
Why IF Alone Might Not Be Enough
Intermittent fasting involves alternating between eating and fasting periods, which can help lower insulin levels, increase fat burning, and improve your metabolic health. (2) But here’s the catch: Just shortening your eating window doesn’t automatically put you into ketosis.
If you’re consuming too many carbs during your eating periods, your body will continue to rely on glucose, making ketosis difficult to achieve––even if you fast for 18-24 hours.
Getting into ketosis requires a sustained significant reduction in carb intake, which forces your body to use up its liver glycogen stores and switch from burning glucose to burning fat. It can take a few days or up to a week or more to achieve nutritional ketosis, which begins at a blood ketone level of 0.5 mmol/L.
The amount of time and degree of carb restriction needed to enter and maintain nutritional ketosis varies from person to person. Some individuals may be able to get into ketosis via intermittent fasting relatively quickly. But studies suggest that people who are not already eating low carb may need to fast for up 72 hours (three days) to enter nutritional ketosis. (3) And it can take several weeks to months to become fully adapted to a ketogenic diet.
So, if you aren’t keto-adapted and you eat a carb-heavy meal, fast for 24 hours, and then test your ketones, don’t be surprised if you find that you’re not in nutritional ketosis.
The Power Combo: IF and Keto
The high-fat, moderate-protein, very-low-carb ketogenic diet is designed to put your body into ketosis. When you combine keto with intermittent fasting, the effects can be even more powerful. Fasting helps lower insulin and deplete glycogen stores, priming your body for ketosis. (4) Then, by sticking to keto-friendly foods, you supply your body with the nutrition it needs to stay in this metabolic state.
This combination not only makes your body more efficient at burning fat but also helps maintain ketosis during fasting periods. In short, it accelerates your journey to ketosis and makes it easier to stay there.
The Importance of Testing Ketones
To truly know if you’re in ketosis, it’s important to test your blood ketone levels once or twice a day at first and occasionally later on, especially after making dietary changes. This gives you a clear picture of how your body is responding to your diet and fasting routine. Without testing, you might think you’re in ketosis when you’re not, or you may miss the impact of eating non-keto meals. Testing helps you make adjustments to your diet and fasting schedule that can ensure you continue progressing in the right direction.
Why Metabolic Flexibility Matters
Metabolic flexibility is the body’s capacity to adjust to varying metabolic demands during periods of fasting and feeding. Humans evolved to efficiently switch from using fat for energy during periods of food scarcity to prioritizing glucose as a fuel source when food was available, providing a survival advantage. (5)
In the context of keto, metabolic flexibility means your body can easily switch between burning carbohydrates and fats for energy. When you’re keto-adapted and metabolically flexible, you can occasionally have more carbs than usual without any issues.
For example, if you indulge in a higher-carb meal at a social event, your blood glucose will rise and your ketones will decrease temporarily. However, you should be back in light ketosis by late morning the next day, if you don’t eat until then or eat a keto breakfast. This flexibility shows how well your body is adapted to using fat as its primary fuel source.
If you discover that you’re not metabolically flexible, it’s best to stick with keto or low carb while doing IF for now. You may develop metabolic flexibility a little further down the road and be able to eat “off plan” from time to time.
Measuring your ketones after making adjustments to your diet or IF regimen can help you assess your metabolic flexibility and understand your bioindividuality.
REFERENCES
- Intermittent fasting and health outcomes: an umbrella review of systematic reviews and meta-analyses of randomised controlled trials, The Lancet 2024
- The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis, Journal of Clinical Medicine 2019
- Ketones and Human Performance, Journal of Special Operations Medicine 2017
- Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting, Obesity 2017
- Metabolic Flexibility and Its Impact on Health Outcomes, Mayo Clinic Proceedings 2022
