This blog post is an article I wrote for Science Driven Nutrition, a journal that you can check out here: http://sciencedrivennutrition.com
In this article I lay out what we presently know about low carbohydrate diets and how we can manipulate carbohydrate stores to make us faster as athletes over varying exercise durations and intensities.
Carbohydrate Periodization – a tool for cyclists and other endurance athletes?
Since the gain in popularity of low carbohydrate and ketogenic diets and other trends that limit carbohydrate intake, many athletes and performance driven individuals have been manipulating their diet and adopting a to low carbohydrate (CHO) diet. These athletes are in the search of better health, body composition changes, or performance benefits. In a country where diabetes will affect more than half of the population by 2020, CHO are a macronutrient that is more of a concern for Americans than it’s ever been. In the realm of sports nutrition, the role of CHO has been well documented, and the role that glycogen plays in endurance exercise has also been well established. It is now known that glycogen depletion negatively impacts endurance exercise performance, while glycogen content for resistance exercise is less clear. For the purposes of this article, let’s try to focus our discussion on endurance exercise- though that still includes high intensity work in the form of HIT.
Carbohydrate manipulation for sports has long been a tradition in endurance sports, with carbo-loading a commonly known term. Researchers, athletes and coaches have been looking into ‘fat adapting’ athletes for a long time (1985-2005) in order to decrease glycogen utilization, increase fatty acid oxidation and improve performance. We know that a habitually adapted athlete to a high CHO diet will in turn utilize a higher percent of CHO for fuel during exercise [1], when compared to a fat adapted athlete. Thus, there is a theory behind a potential performance enhancement when an athlete adapts a low CHO, high-fat (LCHF) diet. This will then allow a fat adapted athlete to have a nearly unlimited resource of energy derived from fat (triglycerides) stores due to the augmented and sensitized fatty acid enzymes. If we train our bodies to need CHO less, the hope is that we will not limited by their depletion during exercise.
Thus far, very few studies have shown that a LCHF will effectively improve performance. A summary below demonstrates what we know about LCHF diets and some important physiological changes that happen when glycogen is low:
Low-glycogen availability causes a shift in substrate metabolism, both during and after exercise. It also induces an increase in systemic release of amino acids and increases fat oxidation – causing a drop in exercise intensity and a rise in perceived effort – important considerations for athletes. [2]
Expression of genes that stimulate fat catabolism and mitochondrial biogenesis are promoted – causing an improvement in oxidative capacity and a rise in fat oxidation at rest and during exercise by up to 3x. [3]
It takes as few as 5 days to ‘retool’ muscles to increase fat oxidation and reduce CHO oxidation [4]
Adaption or ‘retooling’ occurs by way of changes in concentrations or activity of proteins or metabolites that regulate fatty acid availability (transport, storage, oxidation) and a decrease in dependency on glucose for fuel [5].
Because of this ‘retooling’, the ability to utilize glycogen is severely limited – leading to a reduction in CHO oxidation. Proponents consider this a ‘sparing effect’, while critics may consider it an ‘impairment effect’. This is in part due to the down regulation of PDH, showing a decrease in metabolic flexibility. [12]
It is unknown how long it takes in the face of increased CHO availability to wash out the ‘retooling’ effects of a LCHF diet – though we know they persist in the face of at least 36 hours of aggressive CHO loading and consumption.
There is little in the research to indicate the LCHF diets are beneficial to endurance performance, though studies have shown very mixed results. In general, only submaximal/low intensity exercise was benefited- and even those findings were mixed [6].
Regardless of the lack of immediate benefits – the muscle ‘retooling’ effects of a LCHF diet are very intriguing and have lead many, including myself to consider what the results would be once CHO are reintroduced in the diet of the LCHF athlete. This might mean that we introduce CHO for days leading up to competition day, shorter duration < 3 days, or only during exercise.
However, it would appear that there are no clear benefits of adapting an athlete to a LCHF diet and then flooding the system with available CHO to improve performance. It is very important to mention that with all the studies cited up to this point there are limitations. Varying dietary protocols, exercise prescription, statistical crunching, and training levels of the subjects. Though as the number of interventions and adaptions increase, the likelihood that individual variation in underlying factors such as insulin sensitivity, cellular level adaptions, hormonal and cognitive influencers, and even daily life stresses and activities will undermine the validity of the study’s results.
So what’s the point of discussing this? A recent study by Marquet LA, et al. called Enhanced Endurance Performance by Periodization of CHO Intake: "Sleep Low" Strategy peaked my interest in this topic once again and lends valid consideration that CHO/glycogen manipulation in accordance with training may work under certain training and timing conditions. This study is summarized below:
·21 triathletes were split into two groups for 3 weeks- a ‘sleep low’ group and a control group.
·Both groups consumed 6g/kg/day CHO, but with different timing to manipulate availability during training sessions.
·The ‘sleep low’ group performed interval training sessions (HIT) in the evening with high-CHO availability, and then were restricted of CHO overnight (‘sleeping low’), and then performed ‘train-low’ sessions with low endogenous and exogenous CHO availability.
·The control group undertook the same training protocol, but without CHO restriction and with high CHO availability for all training sessions.
·The sleep low treatment significantly improved TTE at 150% of aerobic peak power and 10km running performance. Fat mass was also decreased to a larger extent in the sleep low treatment while muscle mass was not. [20]
This study is important because it used a novel approach to CHO manipulation and showed significant performance benefits. It sheds some light on the possible protocols to ‘retool’ muscles for fat adaption without compromising high intensity performance, while working them into a program that utilizes both exercise intensities. It shines light on the gray area of CHO manipulation. In this study, over-all CHO intake is not limited, only the timing is changed. Athletes were also completing all high-intensity work with high CHO availability. However, this study utilizes both exercise intensities on the same day – not necessarily a common practice amongst most athletes or cyclists – though more common among triathletes, track cyclists, and professional athletes. It would be interesting to see this study undertaken on a more macro-scale – restricting CHO during training periods of prolonged submaximal exercise, and ensuring high CHO availability during the ‘peak’ or times when mostly high intensity exercise is performed.
Consider periodization in the role of program creation for athlete’s in terms of strength or endurance training. All coaches will tell you that it is important in order to elicit training adaptions. Why shouldn’t nutrition periodization beyond caloric totals and timing of macronutrients be implemented in accordance with training under the supervision of a sports nutritionist? The research doesn’t indicate a clear path towards how this will happen, and more research certainly needs to be done in the area – but the idea is an interesting one.
Why should a cyclist who’s clocking in 25+ hours of training in a week at very low intensities be consuming roughly the same diet (besides caloric content) of when they are training at other intensities, durations, and frequencies later in the season? No study has been done where cyclists consume a LCHF diet over their ‘base’ period where all exercise is done at sub-maximal intensities, and then switched to a high-CHO diet once they commence higher-intensity training. Based off studies done and an understanding of the underlying mechanisms, this could be of benefit to athletes. In a more acute sense, in regards to the previously mentioned study, some cyclists (or any other athlete who partakes in both aerobic and high intensity exercise) could benefit from manipulating CHO intake between sessions.
Too often in the realm of nutrition are things stated in black and white. We as humans like to compartmentalize overly-complex processes and make them easier to understand and adhere to- and we often like to think about diets in black and white terms. It is important that research, as well as the field, attempts to bridge this understanding in the body’s adaptions to low CHO availability, and how to implement it without raising the fatigue levels of athletes to a compromising degree. Training ‘low’ has many implications for fatigue and therefore requires careful treading. It is also abundantly clear that a lack of CHO is detrimental to high intensity performance. As a sports nutritionist, I think it’s important that we continue to advance our field and not settle for blanket dietary recommendations regardless of interpersonal, lifestyle and environmental differences for athletes.
With the advancement of personalized testing for nutritional, enzymatic, and hormonal inadequacies – this change is already taking place. I believe it is possible, with further research and field work that we can use these tests in the future to make evidence-based nutrition programs utilizing micro and macro periodization of CHO intake for athletes in order to improve their performance, and ultimately hit their peak when they need to – rather than searching for a yearlong dietary strategy that will improve performance all year long.
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