The relationship between carb loading and the ketogenic diet is complex, with various approaches and potential benefits and drawbacks. This article explores the cyclical ketogenic diet (CKD), its variations, and its implications for athletes and individuals seeking weight loss or muscle gain.
Introduction to the Cyclical Ketogenic Diet (CKD)
The cyclical ketogenic diet (CKD) involves periods of very low carbohydrate intake, similar to a standard ketogenic diet, alternated with periods of higher carbohydrate consumption (carb loading). This approach gained popularity in the fitness and bodybuilding communities in the 1980s, thanks to authors like Michael Zumpano and Dan Duchaine. The CKD, as described by McDonald, hinges on maintaining ketosis during the carbohydrate restriction days.
Understanding Ketosis and Keto-Adaptation
The ketogenic diet drastically reduces carbohydrate intake (typically to 20-50 grams per day), forcing the body to use fat as its primary fuel source. This process leads to the production of ketones, which the body then uses for energy. After following the ketogenic diet for a few days, the body enters ketosis.
Keto-adaptation refers to the metabolic changes that occur as the body adjusts to using fat for fuel. These adaptations include increased efficiency in oxidizing fat and decreased efficiency in oxidizing carbohydrates. These are just some of the many factors altered by keto-adaptation and carb-load, and it’s not possible to know all those numerous changes and give a full answer.
The Rationale Behind Carb Loading on a Ketogenic Diet
The cyclical reintroduction of carbohydrates with carb-load could be cause for concern to maintain an intact keto-adaptation, for whatever reason that wants to be maintained. Temporary carbohydrate reintroduction during a ketogenic diet inhibits ketosis, “disturbing” the new metabolic balance established over several weeks. However, in the early 24 hours of carbohydrate reintroduction (500 g) the metabolism continues to rely on fat so that carbohydrate is partitioned towards the muscle glycogen resynthesis, while the lipid balance remains exceptionally negative. This would suggest that in the short-term (≤24 hours), keto-adaptation is not significantly compromised.
Read also: Safety of Low-Carb Diets During Lactation
Strategies for Optimizing Carb Loading on a Ketogenic Diet
Several strategies can help minimize the potential disruption of keto-adaptation during carb loading:
Establish Initial Keto-Adaptation: Introduce the carb-load only once the metabolism has well adapted to this new condition.
Reduce Carb-Load Duration: Reduce the duration of the carb-load to allow the liver to anticipate glycogen depletion. The sooner the liver can be depleted, the sooner ketogenesis will be recovered. The liver should normally take around a maximum of 24 hours to deplete.
Prolong Ketogenic Phases: Increase the spacing between carb loads. Technically, the more keto-days are held consecutively, the deeper the state of ketosis may become day after day.
Control Carbohydrate Intake: Have greater control of carbohydrate ingested during the carb-load.
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Accelerate Liver Glycogen Depletion: Restore ketogenesis as soon as possible by accelerating the liver glycogen depletion rate through physical activity beyond that foreseen by the resistance training program. It’s important to choose low-intensity activities so as to depleting glycogen stored in the liver and not that stored in the muscle since the latter is significantly affected in the moderate-intensity range and above.
Time Carb-Load with Exercise: Exercise boosts muscle-specific insulin sensitivity for approximately 24 hours post-workout. That means that if the carb-load falls within this period of time, the muscle will be more efficient in uptaking the ingested glucose than delaying it the next day.
Prioritize Muscle Glycogen Depletion: Since muscle has priority over the liver in post-workout for uptaking exogenous glucose sources, trying to emphasize this priority may further limit liver glycogen re-synthesis. The extent of muscle glycogen depletion depends on the number, the type, and the volume of the workouts, and on how much the last pre-carb-load workout is glycolytic. In general, it is suggested to make the last workout highly depleting by involving all the muscles (total body).
Practical Implementation of the Cyclical Ketogenic Diet
Keto cycling involves following the keto diet for a certain amount of time and then taking a day (or more) off. Keto cycling doesn't have a strict definition. Some people choose to have five or six days on keto followed by a day or two off. Others will do keto for 10 to 12 days followed by three to four days off.
It’s best to eat carbs (again, opt for those complex choices) in the daytime when you're most active (if that's true for you) as opposed nighttime.
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It is advisable to not jump right into keto cycling. “I would definitely not try keto cycling until you’re about two to three months into your keto diet,” she says. “That's because you need to make sure your body is fat adapted so that it can get back into ketosis easily.” In her practice, Devine has noticed that if the body is used to being in ketosis, it’ll snap back more quickly after eating a carb-heavy meal than if a person is new to keto, though there’s not enough research to confirm that this is always the case. She doesn’t encourage keto cycling unless the individual has been following a clean keto diet, which calls for sourcing the healthiest version of keto-approved foods, such as grass-fed meat and whole foods, for at least three months and has a regular and rigorous exercise plan. This should allow them to eat higher carbs without seeing weight gain or intense cravings.
Devine says to follow up a high-carb day with vigorous exercise the next morning. The idea is to force the body to burn the glycogen you took in so it can get back to working on fat burn.
Carb Backloading
Carb backloading involves eating carbs later in the day instead of throughout the day. With that being said, carb backloading is best paired with a workout regimen, particularly strength training. The general rule is to eat most of your carbs during the evening after working out. You can eat these carbs until bedtime (around an hour prior to sleep).
The carb backloading protocol is straightforward: Restrict carbs to as low as 30 grams during the day. Eat more carbs in the evening after your workout session, which involves resistance training or strength training. The best meal plan for carb backloading includes ample amounts of healthy fats and protein, as well as whole-food carbs.
Complex carbohydrates, such as whole grains, quinoa, sweet potatoes, bananas, blueberries, and strawberries, are the best options for carb backloading. Complex carbs have fiber in them and digest more slowly.
Carb backloading is unlikely to cause negative effects in people who are healthy. However, caution should be exercised for those with a medical condition involving insulin resistance, such as diabetes and polycystic ovary syndrome (PCOS).
Potential Benefits and Risks of Keto Cycling
Because keto cycling is so new, no studies have examined the benefits and risks yet. Cycling in and out of ketosis - eating carbs then not eating carbs - might be dangerous. “If you eat a high-fat diet one day (like what is recommended in the keto diet) and then go back to eating carbs the next, I think a person could be in danger of storing much of that consumed fat, which can result in high triglycerides and cholesterol,” she says. She explains the body may not have time to convert fat to energy, so it may stick with using the carbs for energy and storing the fat.
There isn't much research on keto cycling, though one small study was published in September 2020 in Nutrients. The researchers found that keto cycling resulted in an amount of weight loss in young, healthy men similar to that engendered by a nutritious diet that cut 500 calories per day. The weight loss for the keto cyclers, however, was from a combination of decreased body fat, water, and lean body mass, whereas the other dieters primarily lost body fat.
Some anecdotal evidence suggests that people are more likely to stick with keto if they incorporate cycling. You could also end up losing more weight if you stick with the diet for a longer period than you otherwise would have.
Potential Benefits for Athletes
Some people believe elite athletes benefit from keto cycling because their bodies use extra carbohydrates as energy to power through difficult workouts and races. “For an avid athlete, timing carbohydrate intake based on intensity and type of activity can have performance benefits,” Devine says.
After all, research has shown that the strict keto diet impairs exercise performance. One study found keto hurts athletic performance more than a high-carbohydrate diet or one that includes periods of high carbs and low carbs (which is similar to keto cycling).
It seems the body uses high-quality carbs as exercise fuel. “Running on glucose (carbs) during intense strength training workouts can be beneficial,” Devine says. Another study, published in Nutrition Today in January 2018, found that carbohydrates are the only macronutrient that can be broken down quickly enough for the body to use during high-intensity exercise.
Challenges Associated with Keto Cycling
Because it’s best to try keto cycling after your body has already adjusted to ketosis, you likely won’t be able to avoid the challenges of starting keto, such as the keto flu. Symptoms of the keto flu include nausea, fatigue, headaches, constipation, weakness, difficulty sleeping and irritability (14).
And while Devine says keto cycling can be helpful for people who crave higher-carb items from time to time, she warns it’s not for everyone.
“Anecdotally, some people who try keto cycling do find it difficult,” she says. “When you reintroduce carbohydrates back into your diet - and this is more about refined carbs, I’m not talking about eating an apple - you don’t feel very good.” She says some people experience a so-called carb hangover - the body feels kind of bloated and inflamed, and you may develop headaches.
It can also be a challenge not to overindulge on those days off the diet. “For somebody who’s been a carb addict or a sugar addict before going on the diet, all of those cravings for those sugars and those carbs come back pretty strong,” Devine says. “It’s kind of like saying, ‘I’m going to stop smoking, but I’m going to have one cigarette a month.’ Good luck. I just think it's a hard dietary approach to follow.”
Potential Downsides
Reversal of Metabolic Adaptations: From the notions discussed here, it appears that the metabolic adaptations induced by the very low carb/ketogenic diet only start to be reversed with the carb-load, and 24 hours would be a too short period of time to impair keto-adaptation. If by measuring the ketone concentrations in the urine (a gross ketone concentration estimation) there is a lower value than expected, it’s recommended to assess the strategies proposed above to speed up the ketogenesis re-activation.
Weight Gain: Many people may eat too many calories on refeeding days, counteracting the weight loss benefits of the standard keto diet. Also, it should be noted that transitioning from a standard to a cyclical keto diet may result in temporary weight gain - primarily due to excess water that is retained when consuming high-carb foods. In fact, your body stores each gram of carbs in muscle with at least 3 grams of water (16).
Considerations Before Starting Keto Cycling
Be careful about starting keto cycling or keto in general if you’re pregnant or nursing. Kieffer recommends that these women consult their primary care provider first. People with type 2 diabetes should also consult their doctor, and those with type 1 diabetes or kidney problems should avoid the diet, as should anyone with conditions related to their pancreas, liver, thyroid, or gallbladder, according to University of Chicago Medicine.
If you’ve cleared the approach with your healthcare team and weight loss is your ultimate goal, know that keto cycling won’t be nearly as effective as traditional keto, Devine points out. “It can halt your weight loss by taking those days off because during that time you’re not going to be burning any fat and you could be putting more on,” she says. “I have seen ‘cheat days’ turn into cheat months and just backfire for the client completely. It creates a very ‘yo-yo’ approach to weight loss which can be very discouraging.” Kieffer adds that the results of keto cycling are never as dramatic as when the body is kept in a constant state of ketosis.
On the other hand, if you take the approach of following the keto diet most days but cycling helps you stay on course, cycling could help you stick to the keto diet longer.
What it boils down to is this: You need to know yourself and your self-control. “If you’re able to have a higher-carb day that includes healthy carbs and be able to get back on track the next day, then it might work for you,” Devine says. “But if you’re somebody who kind of loses control when you get around sweet foods, and one doughnut means the entire case of doughnuts, you’re going to have trouble with it.”
Scientific Evidence and Research
Research on the cyclical ketogenic diet is very limited. Still, it may provide advantages.
One study in 29 elite race-walkers found that the athletes benefited from periodic high-carb intake - though it did not specifically test the cyclical keto diet.
The study revealed that the walkers who received periodic high-carb feedings prior to training sessions experienced significant improvements in performance compared to those following a standard keto diet (13).
The researchers concluded that athletes who periodically consumed a lot of carbs saw an improvement in performance, while those following a strict keto diet did not.
Ketogenic Diet and Exercise Performance
Athletes in endurance sports, lasting one hour or more, are constantly searching for new nutrition strategies to enhance performance. High carbohydrate diets increase muscle and liver glycogen stores, improving endurance performance, yet at the same time, they increase the rate of carbohydrate utilization during exercise. Having this in mind, scientists and athletes have begun experimenting with dietary procedures that would decrease the rate of carbohydrate utilization, while increasing fat metabolism during prolonged physical work. It seems that such an alternative in exercise metabolism can be induced by a high fat, low carbohydrate diet. On the other hand, there are some data indicating that blood ketones are related to fatigue and perceived effort during exercise. Most of these studies have been conducted with untrained and/or obese subjects. Research with competitive athletes in different sport disciplines is scarce, and conflicting results have been presented.
Several studies with competitive athletes have indicated that low carbohydrate ketogenic diets do not compromise aerobic endurance and explosive strength performance, while decreasing body weight and fat mass. Most studies with endurance athletes have indicated that prolonged ketosis results in an adaptation, after which free fatty acids become the major metabolic fuel, and carbohydrate utilization is markedly reduced during moderate, but exhausting exercise. Having this in mind, new concepts of improving endurance performance have been created, with the hypothesis that a low carbohydrate high fat diet will increase the rate of free fatty acid (FFA) metabolism during exercise, while muscle glycogen will be preserved for later stages of an event, especially for the more intense parts. The major drawback in fat loading is the fact that per unit of time, more ATP can be generated from carbohydrate than from fat oxidation. When blood-borne FFA are oxidized, the maximum rate of ATP resynthesis is about 0.40 moL/min, while an aerobic or anaerobic breakdown of glycogen can generate from 1.0 to 2.0 mol of ATP/min. During high intensity exercise, the rate of ATP breakdown is too high to be matched by the rate of ATP synthesis from FFA. This phenomenon limits the use of fat loading in sport disciplines that require high intensity efforts from the athletes. High intensity exercise also suppresses lipolysis, thereby reducing the availability of fatty acids to the muscles.
A major metabolic adaptation to endurance training is an increased capacity for fat oxidation. Cross country cycling is a predominantly endurance sport event in which training sessions last from 1 to 4 h. The intensity of effort in this sport discipline varies from low to maximal; thus, both metabolic pathways improve significantly with training. The contribution of fat to the total energy expenditure increases after endurance training at both relative and absolute exercise intensity. Most importantly, the trained muscles of athletes have a greater mitochondrial and capillary density, which enables them to oxidize more fat compared to the untrained muscles of sedentary subjects. This sparing of glycogen effect allows endurance athletes to exercise longer before experiencing glycogen depletion and associated fatigue. Endurance trained individuals deliver more blood and oxygen to the working muscles due to a higher cardiac output and an increased arteriovenous oxygen difference. These athletes also produce less lactate at the same load due to a higher lactate threshold. Both of these adaptive changes facilitate fat oxidation. Theoretically, since endurance athletes can metabolize fat more efficiently, low carbohydrate, high fat diets should be preferred to carbohydrate loading as a nutritional strategy for increased performance. Numerous studies examining the benefits of these dietary procedures with athletes of different sport disciplines and different sports level have given conflicting results.
A ketogenic diet is high in fat and low in carbohydrate and protein content. Although the “ketogenic diet” is deficient in one nutrient (carbohydrates), it provides an alternative fuel source for the brain and skeletal muscles, which includes ketones. These ketone bodies include β-hydroxybutyrate and acetoacetate. Despite some therapeutic benefits, ketogenic diets create several physiological consequences of which the most significant for physical exercise includes ketosis. Other side effects of ketogenic diets for sport performance include dehydration, hypoglycemia and increased risk of kidney stones.
Study on Off-Road Cyclists
The main objective of this research was to determine the effects of a long-term, low carbohydrate, ketogenic diet, rich in polyunsaturated fatty acids, on aerobic performance in off-road cyclists. Additionally, the effects of this diet on body mass and body composition were evaluated, as well as those that occurred in the lipid and lipoprotein profiles, due to the dietary intervention. The effects of the high fat diet on resting and exercise concentrations of chosen hormones and metabolites were also determined. The main hypothesis stated that a long-term, low carbohydrate, ketogenic diet, applied in off-road cyclists, would decrease body mass and body fat content, while increasing FFA metabolism during continuous exercise with moderate intensity, with a concomitant decrease in insulin levels and glucose uptake. It was also hypothesized that the ketogenic diet would allow the subjects to maintain the level of aerobic power and capacity as evaluated by the value of maximal oxygen uptake (VO2max) and lactate threshold (LT).