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Carb Cycling: Is It All It's Cracked Up To Be?

Carbohydrate Cycling, or ‘Carb Cycling’, has been utilised by bodybuilders and prescribed by their coaches for around two decades, and this diet methodology remains common in bodybuilding circles. Over the years, a number of the most popular bodybuilding diets, including Dan Duchaine’s ‘Ultimate Diet’ and Dr Mauro Di Pasquale’s ‘Anabolic’ and ‘Metabolic’ Diets, have been based upon the concept of carbohydrate cycling. The premise behind such diets is that, by fluctuating the intake of carbohydrates, insulin sensitivity will be increased, which in turn promotes carbohydrate storage as muscle glycogen rather than fat, when carbohydrate are consumed. The conversion of carbohydrates to fat, via de novo lipogenesis, will also be reduced by regularly restricting or preventing carbohydrate intake.

The concept of carbohydrate cycling involves the prescription of ‘High Carb’, ‘Low Carb’ or ‘No Carb’ days, and these days are cycled roughly equally. It is proposed that the use of ‘High carb’ days (2-3g/lb of bodyweight for males) are utilised to stimulate increases in metabolic rate and insulin sensitivity, as well as acting as a psychological reward. It is recommended that ‘High Carb’ days are taken on days in which the most challenging workouts with the highest volume will be undertaken. Although consuming large quantities of carbohydrate on days in which training sessions are of the greatest intensity may aid recovery, this may not be optimal for both growth and recovery as the training stimulus is likely to endure for multiple days. Prescribing a ‘Low carb’ day prior to high volume workouts may also be detrimental to work capacity due to reduced levels of muscle glycogen. Additionally, low muscle glycogen has been found to downregulate the mTOR signalling pathway1,2. This pathway appears to be crucial in increasing muscle protein synthesis following resistance exercise, and therefore low muscle glycogen may significantly reduce protein synthesis post-workout. Further, low muscle glycogen has been found to increase AMPK activity3,4, which has been demonstrated to inhibit the mTOR pathway. This issue of low muscle glycogen would be particularly poignant if training on a ‘High carb’ day was undertaken in the morning when muscle glycogen is not likely to have been significantly replenished.

In a typical carbohydrate cycling diet, the prescribed ‘Low carb’ days (0.5-1.5g/lb of bodyweight for males) and ‘No carb’ days (besides residual carbs) are cycled in between ‘High carb’ days, the order of which can be tailored depending upon the individual’s goals and progress. For example, a diet can begin with a ‘Low carb’, ‘High carb’, ‘No carb’, ‘High carb’ day cycle and can be altered to a ‘Low carb’, ‘No carb’, ‘High carb’, ‘No carb’ cycle when fat loss stalls. It must be noted that it is not recommended to put either ‘No carb’ or ‘High carb’ days back to back, and that it is recommended that vegetables and water are regularly consumed regardless of the carbohydrate intake. Additionally, it is almost impossible to truly consume no carbohydrates, particularly whilst easting vegetables, and therefore ‘No carb’ days may in fact represent ‘very low carb’ days. Although the recommendations vary, it has been suggested that protein and fat intake are generally maintained regardless of carbohydrate intake, although it may perhaps be wise to consider slightly reducing fat intake on ‘High carb days in order to mitigate the increased caloric intake. Additionally, it has been proposed that protein intake should be increased on ‘Low carb’ and ‘No carb’ days (1.25-1.5b/lb of bodyweight), presumably to mitigate the detrimental effects of reduced carbohydrate intake upon hypertrophy and strength. However, whether this increased protein intake, when reduced carbohydrate is consumed, does in fact moderate the negative effects of very low carbohydrate intake remains unclear. It may be the case that this increased protein intake only serves to increase caloric intake. It is important to recognise that, despite the fluctuations in daily macronutrient intake, the weekly macronutrient and caloric intake is maintained, and it is questionable whether these recommended fluctuations provide any significant benefit for body composition. It must also be highlighted that these fluctuations may encourage a ‘binge and purge’ mentality, an unhealthy relationship with food, and eating disorders.

Lyle McDonald outlined a detailed strategy for a variation of a carbohydrate cycling diet, known as The Cyclical Ketogenic Diet (CKD), in his book (‘The Ketogenic Diet’, 1998), which includes daily macronutrient guidelines and workout programmes. However, in the introduction to the CKD chapter, it is highlighted that this diet strategy is not recommended for beginners, due to the full muscle glycogen depletion, and thus the large amount of training, required, and that this strategy is not optimal for mass gains. This is likely to be due to the low insulin and anabolic hormone levels, and the depleted muscle glycogen, associated with low caloric and carbohydrate intakes. It is unlikely that significant muscle mass can be gained during the 24-48 hour carbohydrate loading utilised in a CKD, and therefore carbohydrate based diets with a caloric intake greater than that of maintenance levels will be more beneficial for gaining muscle and strength. Although alternative CKD strategies aimed at mass gaining are also proposed, these are also likely to be less effective in increasing lean body mass compared to diets involving the daily caloric consumption over that of maintenance levels.

McDonald’s strategy for training whilst utilising a CKD, involves two full body workouts early in the week to reduce muscle glycogen and increase fat utilisation during the week. Then, later in the week, usually a Friday, another full body workout is undertaken to fully deplete muscle glycogen and stimulate glycogen resynthesis prior to a weekend carbohydrate load. However, it must be recognised that muscle glycogen cannot be accurately depleted without precise measurement methods and the required workload to fully deplete muscle glycogen will vary between individuals. The ketogenic nature of such a diet has led to anecdotal reports of a lack of energy and ‘fuzziness’ during the initial adaptations which may persist for 2-3 weeks in a standard ketogenic or even longer during a CKD.

McDonald also outlined that the degree of carbohydrate loading required to fully stimulate glycogen resynthesis will depend upon the degree of depletion. The type and timing of carbohydrate consumption may also affect level of glycogen resynthesis, however it should be noted that the amount of carbohydrate consumed is a more important consideration. If sufficient carbohydrates are consumed, glycogen supercompensation can occur, in which greater than normal levels of stored glycogen can be achieved, although a maximal level of glycogen resynthesis in a given time period exists. It is also proposed that, after carbohydrate depletion, the consumption of carbohydrates above caloric maintenance levels is more likely to be stored as muscle glycogen, and that minimal fat gain as a result of the conversion of carbohydrate to fat (de novo lipogenesis) is likely to occur over the first 24 hours of a carbohydrate load. However, it is likely that, during the second 24 hours of the carbohydrate load, greater fat gain may occur due to the previous replenishment of muscle glycogen and that a significant amount of water is likely to be drawn into the muscle cells as muscle glycogen is restored. It must be acknowledged that carbohydrate loading should not be seen as an excuse to ‘binge’ and that caloric intake significantly over that of maintenance levels will lead to small amounts of fat gain, regardless of previous muscle glycogen depletion, and may stunt weight loss progress over time.

Although little published literature examining cyclical ketogenic diets exists, a number of previous studies have investigated the effects of low calorie diets and refeeding upon body composition and exercise performance. Early studies in obese individuals noted that prolonged fasting or a very low carbohydrate diets (500kcal/day) significantly reduced resting energy expenditure, nitrogen balance and leucine balance5,6. This would suggest that the metabolic rate, net protein balance, and protein synthesis of fasting individuals was reduced, which would not be considered to be favourable for individuals looking to improve body composition and for which the maintenance of muscle mass should be considered imperative. Further, it has been determined that a very low calorie diet induced significant reductions in serum Thyroid hormone (T3), however very low calorie diets in which carbohydrates were also low induced even greater reductions in serum Thyroid hormone7. These findings have significant implications for those aiming to improve body composition, as Thyroid hormone is known to influence carbohydrate, fat and protein metabolism, as well as protein synthesis. However, a week of refeeding has been found to reverse this effect.

Refeeding after calorie restriction has been shown to restore resting energy expenditure and therefore restore metabolic rate5,6. More recently, it has been determined that a two week refeed after energy restriction restored insulin sensitivity to normal levels8. Additionally, it was noted that carbohydrate refeeding did not increase net nitrogen balance whereas protein refeeding did induce a significant increase. Experiments in rats, in which they were fed hypoenergetic diets, identified that these low calorie diets induced lower muscle glycogen and increased fatigue compared to controls9. Therefore, it is likely that, although significant energy restriction is likely to be effective for fat loss, very low calorie diets are likely be detrimental to exercise performance. The rate of energy restriction and fat loss must also be considered, and it has been determined that, although faster reductions in calorie consumption may result in more rapid fat loss, slower reductions in calorie consumption have been found to allow for increased fat free mass, likely attributed to muscle gain, which was found not to be the case for more rapid reduction in caloric intake10.

Early research into ketogenesis discovered that, in fasted rats, liver ketogenesis, the production of ketone bodies from fatty acids which can be utilised for fuel, was increased and the formation of triglycerides was reduced11. Additionally, it was found that carbohydrate refeeding reduced ketone body formation and increased liver glycogen and plasma glucose. These findings indicate that, as is commonly known, when carbohydrate intake is limited, alternative substrates (gluconeogenesis), including fatty acids, are metabolised and that the re-introduction of carbohydrates will reduce this. More recently, a number of studies in humans have been published comparing low carbohydrate, high fat diets (Ketogenic) and high carbohydrate, low fat diets. Two studies in the early 2000’s determined that, similarly to the early rat studies, after adaptation to a high fat diet, fat oxidation increased and carbohydrate oxidation reduced during exercise12,13. These findings have been supported by the discovery of a reduced Pyruvate Dehydrogenase (PDH) activity, an enzyme involved in glycolysis, and an increased Hormone-sensitive Lipase (HSL) activity, an enzyme responsible for the conversion of stored triglycerides to free fatty acids13,14. Carey and colleagues12 identified that although power output was higher in high fat diet subjects, there was no significant difference in 4 hour time trial cycling performance between high fat and high carbohydrate subjects. Similarly to the early rat studies, it was determined that carbohydrate refeeding for a single day returned muscle glycogen to normal levels after high fat diet adaptation13. Interestingly, as a result of a study in which protein balance was compared between individuals consuming a low carbohydrate diet and a high carbohydrate diet, it was found that net protein balance was lower at rest, as well as before and after exercise, in subjects consuming a low carbohydrate diet15. Further, it was demonstrated that this reduced protein balance in these individuals was mainly due to increased protein breakdown, but also lower protein synthesis. The increased leucine oxidation during exercise in the low carbohydrate group is likely to have contributed to this increased protein degradation.

Helms and colleagues16 have also recently noted that low carbohydrate diets are likely to result in reduced performance and an increased risk of lean body mass loss, indicating that low carbohydrate diets for fat loss may lead to loss of muscle mass. The authors did acknowledge that intramuscular triglyceride can be used as fuel for heavy resistance training, which may act as a viable fuel source for individuals who have already adapted to high fat, low carbohydrate diets. However, it was also highlighted in this publication that the ketogenic diets often utilised in the scientific literature result in both reduced caloric intake as well as increased protein intake. Therefore, it is unclear from the literature whether the reported results of ketogenic diets are associated with low carbohydrate and high fat intake or with reduced caloric intake and increased protein intake. In the view of Helms et al16, the increased protein intake in low carbohydrate diets are likely to be key to their success. It must also be recognised that significant inter-individual variation in carbohydrate and fat utilisation as a percentage of energy expenditure exists, with these percentage varying up to 4 fold between individuals depending upon muscle fibre composition, diet, age, training, glycogen levels and genetics. This may significantly influence macronutrient requirements between individuals and thus further study is required.

Although the level and duration of energy restriction may be limited during carbohydrate cycling, compared to complete fasting, it should be acknowledged that detrimental complications as a result of refeeding after fasting have been identified. Fasting has been shown to lead to reduced intracellular minerals, for example Sodium and Potassium, with serum mineral levels remaining normal17. By refeeding with carbohydrates, insulin secretion is increased and glucose absorption is increased using the Sodium Potassium pump to shuttle glucose into the cells as well as minerals that were present in the blood. This causes a rapid decrease in serum mineral levels which can lead to neurologic, pulmonary, cardiac, neuromuscular, and haematologic complications and can be fatal if untreated. Although such extreme cases of ‘Refeeding Syndrome’ are extremely unlikely whilst utilising a carbohydrate cycling diet, the dangers of significant fasting and refeeding must be recognised.

To conclude, carbohydrate cycling may be effective for fat loss and may be a useful method for individuals who struggle to follow prescribed macronutrient recommendations. A diet that an individual can follow and maintain is likely to be more effective in the long term than ‘the perfect diet’ that an individual cannot maintain. However, it is likely that a diet in which an individual follows prescribed carbohydrate, protein and fat macronutrient values whilst, when necessary, slowly reducing caloric intake and increasing energy expenditure is likely to be just as effective, if not more effective, for fat loss and may allow for greater retention of fat-free mass. Additionally, in contrast to a carbohydrate cycling diet, a prescribed macronutrient based dieting protocol does not encourage a ‘binge and purge’ mentality, rather it encourages moderation. It is also apparent that carbohydrate cycling is not an effective diet for muscle and strength gains, or for beginner trainees.



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