Intermittent Fasting on the Ketogenic Diet
By Dr. Robert Silverman
Fat is your friend, not your foe—a claim that followers of the ketogenic diet have been supporting for almost 100 years. Compared with the Standard American Diet (SAD), the ketogenic diet is not only safe for helping overweight and obese people lose weight, it also supports brain health and improves energy levels.1,2,3 Those who have experienced the benefits of the ketogenic diet, or keto diet, might also be curious about trying intermittent fasting and whether it’s feasible to combine the two. Good news: It’s not only possible, it’s a weight loss strategy I highly recommend.
Here, I explore the two dietary approaches and discuss their individual and combined health benefits. Be sure to consult with your own healthcare practitioner before embarking on any new diet plan.
What is the ketogenic diet?
The keto diet is based on the idea that eating mostly healthy fats, consuming high-quality protein in moderation, and restricting carbohydrates to less than 50 grams per day can cause your body to go into a metabolic state called nutritional ketosis.4,5 During ketosis, your body no longer relies on glucose as a primary energy source. Instead, your liver converts fat into ketones–which are an alternative source of fuel for your brain.
On the keto diet, you’ll get most of your calories from healthy fats found in foods like avocados, grass-fed butter, olives, olive oil, medium-chain triglyceride (MCT) oil, coconut oil, nuts, and seeds. However, keep in mind that some nuts and seeds are better than others. Choose those that are high in fats and lower in carbs; brazil nuts, almonds, walnuts, chia seeds, and flaxseed are all good options. You can also eat all of the nonstarchy, leafy vegetables you want, as well as other low-carb vegetables like broccoli rabe, peppers, bok choy, cauliflower, spinach, asparagus, cucumber, and zucchini. In moderation, eat protein in the form of grass-fed meats, pasture-raised poultry, cage-free eggs, and wild-caught fish. When it comes to fish, choose fatty fish by remembering the acronym SMASH: salmon, mackerel, anchovies, sardines, and herring. Finally, if you want to reach for something sweet, a small amount of 90% dark chocolate is your best option.
On the list of what not to eat? For starters, remember that the keto diet restricts the intake of carbohydrates to achieve a shift from glucose to ketones as a primary fuel source. In order to avoid food high in carbs, limit fruit consumption, as it’s higher in sugar content, and forego fruit juice altogether. You should also avoid grains or starches such as rice or pasta, beans or legumes, root vegetables, and any low-fat or diet products, as they are typically high in added sugars and highly processed.
What is intermittent fasting?
While there are different types of intermittent fasting, such as daily, weekly, and alternate day, the most common involves only eating during a specified window of time each day. That window typically spans between four and seven hours of feeding during the day, but it can be reduced or expanded depending on your dietary needs. Of its many benefits, intermittent fasting works on both sides of the calorie equation.6 It increases the amount of calories you burn by boosting your metabolic rate and reduces the amount of calories you consume by limiting the food you eat. According to a 2014 review, intermittent fasting can cause weight loss of 3-8% over 3-24 weeks.7 A study also showed that intermittent fasting caused less muscle loss than continuous calorie restriction.8
What are the benefits of fasting on the keto diet?
Individually, the keto diet and intermittent fasting can be effective weight loss tools that help people lead healthier lives. However, there are also a number of health benefits that can be optimally achieved when the two are combined:
- Intermittent fasting activates autophagy—a phenomenon where the body eats its own cells and tissues—in a good way.9 Autophagy helps the body remove harmful and toxic compounds, recycle damaged proteins, and increase the production of ketones quicker than on the standard keto diet. Different processes of autophagy occur when either the body is starved or protein and carbohydrates are restricted. Both of these occur when intermittent fasting is combined with the keto diet.6 Combining the two allows you to reap the benefits of autophagy in an efficient way.
- The combination of intermittent fasting and eating a keto diet can help you get into ketosis faster. The keto diet makes intermittent fasting more manageable because your body is already adapted to fasting with ketones. In addition, most people naturally eat less frequently on keto because of the high satiety level, so you’re likely already used to bigger windows without food.
- One of the foremost reasons people turn to intermittent fasting on the keto diet is to lose weight faster.10 Fasting can help you break through weight loss plateaus in a few different ways: Eating a high-fat keto diet and being in ketosis reduces appetite and increases satiety levels.11 It’s much easier to do intermittent fasting when this is the case, versus a diet of carb-filled foods that often increases cravings.
- Combining the keto diet with intermittent fasting can help decrease oxidative stress and inflammation in the body.12,13 Inflammation is part of the body’s complex biological immune response to illness, infections, injury, and other forms of stress.14 Up to a point, inflammation is normal and even desirable. But long-term, excessive inflammation can lead to joint pain, digestive issues, and other health problems.
- The combined efforts of intermittent fasting and the keto diet lead to enhanced cognitive performance and neuroplasticity. Cognitively, the brain operates better on ketones versus glucose from sugar or carbs.15,16,17 Ketones are an efficient, slow-burning fuel source with preserved uptake by the brain–lending itself to heightened levels of concentration and longer periods of focus.18 Ketones also increase the brain-derived neurotrophic factor (BDNF), which works to support the brain’s existing neurons while encouraging new neuron and synapse growth.19,20
- Finally, using both these strategies together can help stabilize blood sugar levels as compared to intermittent fasting alone.4 Alternating between glucose and ketones for energy can cause spikes in blood sugar, leading to brain fog, mood swings, low energy, and other side effects. Someone eating the SAD might experience this with intermittent fasting; however, during keto-adaptation, liver and muscle glycogen deposits are maintained. With the absence of glucose to burn, you won’t experience the peaks and valleys of varying blood sugar levels.
While intermittent fasting is not a necessary part of following a keto diet, I definitely recommend the approach if you want to double down on benefits, achieve previously unattainable results, and meet new health goals. Alone, each has its weight loss and healthy lifestyle benefits. Together, they can help you achieve your optimum health from the inside out.
- Gibas MK et al. Induced and controlled dietary ketosis as a regulator of obesity and metabolic syndrome pathologies. Diabetes Metab Syndrome. 2017;11(1):S385-S390.
- Stafstrom CE et al. The ketogenic diet as a treatment paradigm for diverse neurological disorders. Frontiers in Pharmacology 2012; 3:59.
- Hoyer S. Abnormalities of glucose metabolism in Alzheimer’s disease. Annals of the New York Academy of Science. 1991;640:53-58.
- Volek JS et al. Comparison of energy-restricted very low-carbohydrate and low-fat diets on weight loss and body composition in overweight men and women. Nutr Met. 2004;1:13.
- Abbasi J. Interest in the ketogenic diet grows for weight loss and type 2 diabetes. JAMA. 2018;319(3):215-217.
- Klempel MC. Dietary and physical activity adaptations to alternate day modified fasting: implications for optimal weight loss. Nutr J. 2010;9:35.
- Barnosky A. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. ScienceDirect. 2014;164(4):302-311.
- Varady K. Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obes Rev. 2011;12(7):e593-601.
- McCarty M et al. Ketosis may promote brain macroautophagy by activating Sirt1 and hypoxia-inducible factor-1. Med Hypotheses. 2015;85(5):631-639.
- Johnstone A. Fasting for weight loss: an effective strategy or latest dieting trend? Int J Obes. 2015;39(5):727-733.
- Van Wymelbeke V et al. Substrate oxidation and control of food intake in men after a fat-substitute meal compared with meals supplemented with an isoenergetic load of carbohydrate, long-chain triacylglycerols, or medium-chain triacylglycerols. Am J Clin Nutr. 2001;74(5):620-630.
- Mattson M et al. Beneficial effects of intermittent fasting and caloric restriction on the cardiovascular and cerebrovascular systems. Nutr J. 2005;16.
- Johnson J. Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma. Free Radic Biol Med. 2007 1;42(5):665-674.
- Faris M. Intermittent fasting during Ramadan attenuates proinflammatory cytokines and immune cells in healthy subjects. Nutr Res. 2012;32(12):947-955.
- Zhao W et al. Caprylic triglyceride as a novel therapeutic approach to effectively improve the performance and attenuate the symptoms due to the motor neuron loss in ALS disease. PLoS One. 2012:7(11):e49191.
- Kim DY et al. Inflammation-mediated memory dysfunction and effects of a ketogenic diet in a murine model of multiple sclerosis. PLoS One. 2012:7(5):e35476.
- Henderson ST et al. Study of the ketogenic agent AC-1202 in mild to moderate Alzheimer’s disease: a randomized, double-blind, placebo-controlled, multicenter trial. Nutrition & Metabolism. 2009;6:31.
- Volek JS et al. Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci. 2015;15(1):13-20.
- Lee J. Dietary restriction increases the number of newly generated neural cells, and induces BDNF expression, in the dentate gyrus of rats. J Mol Neurosci. 2000;15(2):99-108.
- Volek JS et al. The art and science of low carbohydrate living. Beyond Obesity;2011.
Dr. Silverman is a paid consultant and guest writer for Metagenics.