Intermittent fasting (IF) and ketogenic diets are all the rage now. They both promote fat burning.
But how well do these two approaches combine?
Ketogenic diets promote fat burning by restricting carbs, which forces you to rely more on dietary fat.
IF restricts all calories for a large part of the day, usually 16 hours or more. Due to your body’s oxidative priority, stored carbs (glycogen) are burned off first. This triggers conditions granting you greater access to burn stored body fat.
Overall fat burning increases in both the proportion as well as the total amount.
Truth be told, ‘fat burning’ is a bit of a marketing term. It’s used in the diet industry to purposely mislead people into thinking that fat burning equals fat loss.
Despite this, it’s also true that fat burning is a very important metabolic state for humans to be in. Modern science is starting to (re)appreciate this fact.
There are different kinds of ketogenic diet, as well as ways to fast intermittently. Before diving into the pros and cons of combining a ketogenic diet and IF, we’ll define both of these briefly.
To IF, simply do not eat for 16 hours during your day (or have caloric drinks). 16 hours of fasting is a loose rule based on subjective expert consensus. Up to a full 24 hours of fasting can still be considered IF.
If you want to IF at a certain time of day like early in the morning, for example, that’s called early time-restricted feeding (eTRF). This means having no more than an 8-hour feeding window. For example, an 8 am breakfast and a late lunch at 4 pm.
eTRF is based on the idea that eating early in the morning, when you’re generally more insulin sensitive, is better than in the evening when you’re less so (1).
Circadian rhythms science tells us that in addition to what and how much we eat when you eat or don’t also matters to your health. Our body operates using a variety of biological clocks affecting many things, like for example our ability to handle food depending on the time of day (2).
Alternate-day fasting (ADF), where you eat a normal amount of calories one day and only 0 to 25% the next, is a popular form of IF. It lends itself to those who notice for themselves that with food ‘total abstinence is easier than perfect moderation’.
Eating about a quarter of your usual calories on some days is also called a fat fast if about 80% come from fat and very few from carbs. It’s used by clinicians who can’t get patients to fully fast right away and so ease them into it this way.
A ketogenic diet is technically any way of eating that produces an agreed-upon amount of blood ketones, like 0.2, 0.5 or >1.0 mmol/L blood beta-hydroxybutyrate.
Typically this is done by eating very few carbs (e.g. 20 g of net or total carbs) and a high proportion of calories from fat (e.g. ~ 80%).
Ketogenic diets cause people to secrete less of the hormone insulin compared to people on standard western diets. By not secreting more insulin from your pancreas a so-called low insulin environment can take hold. This physiological environment also accounts for much of the renowned benefits of fasting (3, 4).
What are the pros and cons of combining this fat burning diet (keto) with a fat burning pattern of eating (intermittent fasting)?
Cons of IF and keto
The cons of IF whilst on a ketogenic diet are minimal and basically come down to a few examples of particular contexts where other dietary patterns are optimal.
Benefits of IF and well-formulated ketogenic diets include filling up quickly (satiation) and staying full for long (satiety).
When might that work against you?
A professional bodybuilder taking full advantage of their regimen will struggle to reach their calorie goal while staying within a restricted feeding window for weeks at a time. It would be much easier for them to eat more often throughout the day and stimulate their appetite with some carbs.
Crossfitters, and in particular swimmers who lose a lot of calories to their liquid environment, may also find it hard to eat enough food for proper recovery within a restricted feeding window.
Skipping a meal when you’re in a social setting simply because you’re not hungry can be a difficult situation to navigate. The social expectation to eat 3 meals a day and snack is pervasive and ingrained.
Social pressure commonly leads people to drop their ketogenic diet or IF pattern. Keto friendly meals and foods are much less available compared to vegetarian options, for instance.
Adherence to ketogenic diets would likely be higher were more options widely available.
Recovering from sarcopenia or cachexia
People with cachexia or sarcopenia must regain precious muscle mass that is hard to come by. Making that a priority means not limiting how often they can eat in a day or the size of their eating window.
In this context, the metabolic benefits of not eating for two-thirds of the day are either overshadowed or even counterproductive.
This doesn’t mean intermittent fasting on a ketogenic diet is bad for maintaining or putting on muscle mass. The ketogenic diet is adequate on both accounts as is intermittent fasting provided enough protein and calories are eaten (5, 6).
Too little for too long
People who cannot go for more than a handful of hours without snacking are unfortunately addicted to food or are most commonly deeply diabetic. Either way, both problems may take considerable time and effort to overcome.
Some can jump straight into intermittent fasting while others can’t without ending up bingeing. Those same people may find more success by first swapping out unhealthy snacks (e.g., potato chips) for healthier ones (e.g. macadamia nuts) before comfortably doing IF.
These strategic questions are important for people to ask themselves and seek answers from health professionals. This is particularly true if you’re on medications as the dosage may need to be adjusted.
False positive blood tests!
You could get a blood test back saying your fasting triglycerides are in the unhealthy elevated range, when in fact they’re not. Why is that?
If you IF while on a ketogenic diet, your body will be relying on lots of fat to power itself. Therefore, lots of fat is transported through your blood, especially right after a high-fat meal.
If you finish a high-fat dinner at 9 pm, make sure you wait until at least until 9 am or 11 am the next day before drawing blood. This will give your triglycerides enough time to return to their baseline, fasting levels. You’ll have avoided what engineer and low-carb expert Dave Feldman coined as the carry-over effect.
In a different context such as a typical higher carb diet, triglycerides will return to fasting levels faster (about 10 hours).
There are many reasons to combine ketogenic diets with IF (or the other way round).
Less insulin, less fuss?
Whole-body insulin sensitivity varies throughout the day, starting out higher in the morning and then lower in the evening (7).
Compared to low-carb or ketogenic diets, higher carb diets are more insulinogenic. Given the comparatively larger metabolic response of higher carb diets, the more insulinogenic your diet, the more careful you’ll want to be about when you eat.
The rule of thumb suggests having your largest and most carb-rich (insulinogenic) meal in the morning, when you’re most insulin sensitive (8). But what if you don’t have any carbs for breakfast and have a steak with fried eggs instead?
This ketogenic (and carnivorous) meal carries a low insulin burden. This is easier for your metabolism to handle. Thus maybe when you eat it (breakfast vs dinner) matters a little less.
IF is well-timed timed calorie restriction that lowers insulin levels, and ketogenic diets minimize the most insulinogenic macronutrient, namely carbohydrate. This double-hit can powerfully normalize insulin signaling, which is central to enhancing lifespan and healthspan (9, 10, 11).
Circadian friendly, ancestral eating patterns
IF and ketogenic diets are an interesting combination that together drive down insulin and establish an ancestral eating pattern. The latter better aligns with our biological clocks (circadian rhythm) (12).
Circadian rhythm entrainment is a fancy term describing the orchestration of your cells processes. They’re cued to your internal (e.g. suprachiasmatic nucleus) and external (e.g. sunlight) clocks. For instance, proper circadian entrainment is crucial to blood sugar regulation or effective muscle growth and repair (13, 14).
People losing fat on a ketogenic diet may experience a keto plateau, where progress slows or even reverses. This can be remedied by implementing IF in addition to the ketogenic diet, thus leveraging both food quality (keto) and timing (IF) for fat-loss.
Does this mean IF should only be done with breakfast and no dinner? No, humans adapt well to a range of meal times. A 6-week study where people skipped breakfast but still ate the same amount of calories showed neutral results in terms of short-term metabolic responses (18).
Fat-loss and body composition
IF and its variations are effective and sustainable fat-loss strategies. Let’s look at them separately first.
1 to 3 months of Alternate-Day Fasting, a type of IF, can decrease body weight by 3% – 7% and fat mass by a respectable 3 – 5 kg. Repeatedly fasting for 24 hours for two months can help you drop 5% of your fat mass. Over 3 to 6 months, it can decrease your body weight by 3% – 9% (19, 20).
Lower carbohydrate diets (including ketogenic ones) are better for fat-loss than higher carb alternatives (21). For every 10% of calories from carbs removed from your diet, you gain a daily 52 kcal advantage in terms of energy balance (22, 23).
52 calories is about 15 pistachios.
Beyond the ‘metabolic advantage’ of lower carbohydrate diets, there seems to be an appetite advantage too (24). Doctors, patients and the clinical literature consistently report more satiation (the desire to stop eating) and satiety (not wanting to start eating again) on ketogenic diets (25). Ketogenic diets can even blunt the powerful rebound hunger that occurs when drastically and chronically restricting calories (26).
Furthermore, applying calorie restriction intermittently, like with IF, may preserve muscle mass better than chronic caloric restriction does (27).
Health benefits independent from fat-loss
The biggest criticism of IF (and even ketogenic diets) is that they’re merely calorie restriction by another name. And since calorie restriction is what’s really responsible for observed health benefits, they’re not all that exciting (28, 29).
However, pre-diabetics doing eTRF on a diet of 50% carbs, 35% fat and 15% protein do better than those eating on a normal schedule – even when matched for weight and caloric intake (30). The eTRF group lowered their blood pressure, oxidative stress and satiety, while also increasing their insulin sensitivity and β-cell responsiveness.
The extent and frequency of calorie restriction matter too, not just the total caloric load. A near 40% decrease in the satiety hormone PYY occurs after a 2 – 3 day fast (31). This fact may explain why going through daily or periodic fasting may be easier than chronically restricting calories.
Indeed, simply consuming all your calories in 1 meal instead of 3 will most likely lead to improvements in body composition – even when fat-loss isn’t the goal! After six months eating one meal a day, subjects lost 3.5 kg of weight of which 60% (2.1 kg) was from fat. The 3-meal a day group lost no weight (32).
When you eat is shown to matter in animal studies too. Mice put on a fast-mimicking diet (FMD) with the same amount of calories used to maintain their body weight, lost weight as well as visceral fat mass but not lean mass (33).
Inflammation, longevity, healthspan
Food timing is not only important for body composition but also for healthspan, longevity and markers of inflammation.
Research in mice suggests they can live 13.6 % longer on a ketogenic diet as well as see significant improvement in healthspan metrics (34). Given how much better humans are adapted to nutritional ketosis than mice, this is quite promising.
It can’t be said enough, food timing, not only food quality matters. Indeed, an ADF schedule can improve markers of inflammation and oxidative stress even when using nutritionally poor meal-replacement shakes (35). It seems that the better formulated the diet used with IF, the more impressive the improvements in biomarkers are (36).
Studies in mice have even shown how fasting can promote stem cells to switch to ‘regenerative’ mode (37). These results are encouraging, especially since they’re consistent with improvements in human inflammatory profiles: improvement in 8-isoprostane, nitrotyrosine, protein carbonyls, 4-HNE adducts, TNFɑ and BDNF (38).
More free time
The obvious but easily forgotten fact about eating a ketogenic diet alongside IF is the time it saves people. Those who IF usually eat two rather than three meals a day, saving themselves considerable amounts of time.
Poor time management is a major stressor nowadays, so any strategy that frees up time has a considerable selling point going for it. IF provides that free time.
Ketogenic diets, since they can normalize appetite, tend to provide more stable energy levels (e.g. no hypoglycemia) and less intrusive food thoughts. Improvements in both these factors constitute potentially massive improvements in quality of life.
Sustainability is the missing quality in many dietary interventions. Combining intermittent fasting with ketogenic diets can solve the two major pain points that people report for why they couldn’t stick to their diet or fat-loss goal.
The first is that people can’t reach satiation and satiety, leaving them ravenous and prone to making bad food choices. The fat burning aspects of a well-formulated high-fat diet and going without food for a while address this exceptionally well.
The other pain point is people are short on time, simply too busy to shop, cook and clean up as necessary. Intermittent fasting can free up a considerable amount of time, thus providing additional mental space for making better food choices.
A well-formulated ketogenic diet eaten within a restricted feeding window is a powerful tool that can both improve your health and lighten your schedule.
Lastly, for those who are curious to know their precise ketone levels, here is a guide to ketone measuring strips.
Written By Raphael Sirtoli
This article is written by Raphael Sirtoli who has an MSc in Molecular Biology. He’s currently doing a PhD in Health Sciences where he conducts neuroscience research on mental health. He is the co-founder of Nutrita, a company helping people follow a low-carb ketogenic diet to reach their health and performance goals. Nutrita created the first-ever ‘Keto score’, the most accurate Insulin index to date, and an advanced ‘Nutrient density score’.