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Are you periodizing your carbs?

Nutrition for endurance performance relies heavily upon achieving full glycogen stores for the start of an event, as well as having metabolic flexibility to maximize usage of both carbohydrates and fats as fuel sources during the event. 

During exercise, intensity and duration are the main determinants of fuel source with carbohydrates being the preferred source as intensity increases. Effective endurance training can be done to improve the body’s ability to utilize fat to spare carbohydrates and combining training with adequate carbohydrate intake helps to improve performance capacity, especially at higher intensities.

Many athletes have experimented with low-carb-high-fat (LCHF) protocols to increase fat oxidation rates during exercise, but potential consequences of this include decreased performance capacity, in part explained by reduced glycogen stores and/or reduced capacity for carbohydrate oxidation. In an attempt to attain metabolic flexibility without the consequences of LCHF, athletes often turn to carbohydrate periodization, or simply completing lower intensity sessions with low carbohydrate availability or consuming higher carbohydrate diets on training days and LCHF on non-training days. While this seems like a reasonable strategy, it might not yield the results you are looking for, and at the very least, there is a lot of nuance to be considered.

Post-exercise glycogen restoration occurs in two phases and the second phase can last for multiple days, provided enough carbohydrates are ingested. Limiting carb intake on days between hard training sessions could compromise recovery of glycogen stores and subsequent performance in future sessions. In this scenario, reduced training capacity or exercise economy would likely be attributed to reduced glucose oxidation and lower carbohydrate utilization. 

Carbohydrate is still an important energy source during endurance rides. Even though you might be riding in or near your "fatmax" zone, carbohydrate is still being utilized for fuel, and this adds up the longer the ride goes. If you’re doing these sessions with low carbohydrate availability, you could be setting yourself up for compromised performance and possibly other health consequences associated with low energy availability or low carbohydrate availability. 

Intermittent LCHF or low carbohydrate availability could also result in poor mood/well-being, increased feelings of fatigue, low energy, GI issues, and poor concentration or attention. On recovery days, hunger is often high but if you are trying to eat LCHF on an easier day, satiety while maintaining energy balance may also be a challenge since high fat foods will be more energy dense compared to lower fat, higher carb foods. 

Female athletes also need to consider changing metabolic demands based on hormone fluctuations. Carbohydrate oxidation is relatively higher in the follicular phase compared to luteal. And, when estrogen is high, glycogen storage capacity is higher; but, when estrogen is low, females may need to consume more carbohydrates and/or more precisely address nutrient timing to effectively carb load.

So, what’s the best answer for managing your carbohydrate intake?

The answer really depends on you as an individual athlete, your training, goals, and personal preferences. Taking into consideration recent research, there doesn’t appear to be any greater improvement in maximal lactate steady state (MLSS) power, time to exhaustion (TTE), substrate utilization, body weight or body composition with periodized low carb training compared to traditional higher carb diet, as demonstrated in a recent 5 week protocol using trained male cyclists. In this study, only the lower intensity sessions (13 in 5 weeks) were completed under low carbohydrate availability conditions. This might suggest that you wouldn’t want to bother with carbohydrate periodization, at least in the manner of restricting carbs for easier rides.

If you are an athlete who would prefer to eat a higher carbohydrate-based diet around workouts each day, the nutritional quality of carbs do matter. Complex, high fiber carbohydrates are important for general metabolic and cardiovascular health and should still make up a large portion of a cyclist’s diet. A recent study also highlighted that a low glycemic index diet seems to influence substrate oxidation  without compromising performance at higher intensities, compared to LCHF and high glycemic index diet. 

Ultimately it’s important to eat adequate carbohydrates to support your training and performance, time the carbohydrate intake so it’s available in your system in and around training, and eat plenty of complex carbs (high fiber, nutrient-rich, lower GI carbs). For most regularly training athletes, there doesn’t appear to be a benefit to doing longer sessions fasted or with low carbohydrate availability, even when the intensity is lower. It makes sense to scale hourly carbohydrate intake relative to the duration and intensity of the session, and based on your overall training load/goals. 

The takeaway? Combining a diet that has nutritious carbohydrates with an optimal training program and you might have a winning formula for metabolic flexibility and performance. Following general evidence-based recommendations for daily carbohydrate intake of 3-10 grams per kilogram of bodyweight and during-exercise fueling of 30-90 grams of carbohydrate per hour, for sessions longer than 75-90 minutes is a great starting point. From there, individualization is key.

PMID: 29444266, 38600291, 38276556


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