The Right Way to Use Carbs for Optimal Training and Racing

Using carbohydrate intake and timing to break through limits of endurance performance.

By Jennifer Kurtz, PhD, CISSN, CSCS, EP-C

Jennifer Kurtz is an exercise physiologist with expertise in nutrition, energetics and metabolism, and sport and exercise. Her research covers all aspects of cycling performance, ranging from power output to oxygen consumption, heart rate, perceived exertion, NO metabolite production, GI distress, and more. Kurtz favors interdisciplinary approaches to human health-related questions, incorporating physiological approaches and nutritional methods to understand the whole-human response to exercise and nutrition, with a focus on elevating awareness of research on female athletes. In addition to all that, she still has time to compete as a CAT 1 mountain bike racer.

Participation in endurance events has exponentially increased nationally and globally in the past few decades, with over 1.3 million participants in 2021 and 4.2 million participants at its peak in 2015 and 3.5 million individuals worldwide (Shilton, 2019). Surprisingly, ultra-marathon races are becoming more popular, and there has also been a shift from standard running races to races such as mud runs, spartan races, and color runs (Vitale & Getzin, 2019).

As endurance and ultra-endurance event participation has grown and changed, the latest nutrition strategies have also evolved. Although there have been significant advances in understanding the nutritional requirements, still many gaps still exist in the literature. The field of “Sports Nutrition” is constantly evolving and sometimes contradictory, so, the purpose of this blog is to summarize some of the latest and newest evidence regarding macronutrients, micronutrients, hydration, and supplements for endurance athletes.

Carbohydrates (CHOs) before, during, and after endurance events are critical. Most athletes know this; however, research shows that CHO needs are largely unmet by high-level athletes (Burke, 2001; Kerksick et al., 2017). Numerous reviews and investigations continue to highlight the importance of delivering optimal CHOs and replenishing lost liver and muscle CHOs stores, especially for athletes involved in moderate-to-high-volume training (Kerksick et al., 2017).

Daily Carbohydrate Recommendations

Athletes involved in 1-3 hours per day of intense exercise, 5–6 times per week typically need to consume a diet consisting of 5-8 g/kg/day, and athletes participating in ≥4 h/day of intense training,1–2 daily workouts for 5-6 days per week may need to consume 6-10 g/day to maintain muscle and liver glycogen levels (Kerksick et al., 2017; Vitale & Getzin, 2019). It is recommended to consume CHOs in the form of whole grains, vegetables, and fruit while limiting foods that can cause gastrointestinal (GI) distress (digestive and stomach abnormalities) and empty too quickly from the stomach, such as refined sugar and starches. 

Pre-Competition Carbohydrates

To top off glycogen stores and replenish muscle and liver glycogen prior to an event <90 minutes, research recommends at least 6 g/kg and up to 7-12 g/kg in the 24 hours before an event (Vitale & Getzin, 2019).

For events lasting >90 minutes, glycogen “carb” loading in the preceding 36-48 hours may help improve performance (Vitale & Getzin, 2019). Recent studies show that a high-intensity activity that depletes CHO stores followed by a 1-day high (10-12 g/kg/day) intake of CHO achieves “CHO supercompensation which can be maintained for 3 days (Vitale & Getzin, 2019). However, it is important to consider GI distress prior to competition to see what foods work or do not work well with your stomach. In the final 1-4 hours leading up to an event, a single dose of 1-4 g/kg of CHO is suggested for a “top off” of glycogen stores.

"When you nail your fueling it's pretty remarkable to see that you can actually get stronger as the race unfolds."

-Hannah Otto
 Kokopelli Trail FKT (Fastest Known Time)
 137 Miles | 11:53:30

While considering CHO needs for endurance athletes, the American College of Sports Medicine (ACSM) and the International Society of Sports Nutrition (ISSN) recommend a CHO intake of 30-60 g/hour during exercise lasting less than 2.5 hours and 60-70 g (or up to 90 g, if tolerable) if for exercise lasting longer than 2.5 hours (Jeukendrup, 2013; Kerksick et al., 2017). However, elite athletes are often exceeding the CHO recommendations consuming up to 150 g/hour. Ultimately, the consumption of CHO during competition depends on GI distress and tolerability. 

Endurance athletes undergoing prolonged bouts (>2 hours) of endurance training can optimally break down CHO at a rate of 1.0-1.2 g per minute, which is equivalent to 60 g per hour (Kerksick et al., 2017; Vitale & Getzin, 2019). This is a relatively wide range and is independent of the type of activity, the duration of the activity, GI distress, and the level of the athlete. For CHO taken as a hydration mix, several reviews advocate a concentration of 0.7 g/kg/h in a 6–8% solution (i.e., 6–8 g per 100 ml [3.4 fl oz] of fluid) (Kerksick et al., 2017). 

It is now well established that different types of CHOs can be broken down at different rates that result in greater CHO uptake and may help improve GI tolerance. Combinations of glucose and sucrose or maltodextrin and fructose are reported to promote greater rates of CHO utilization when compared to single-source CHO (Kerksick et al., 2017). These studies generally indicate a ratio of 1–1.2 for maltodextrin to 0.8–1.0 fructose to support the highest rates of CHO breakdown and utilization during exercise.

Fat, Keto, and the “Train Low” Strategy

The preferred macronutrient source for athletes during competition is CHOs due to the above-mentioned benefits, but recently some endurance athletes have become interested in ketogenic adaptation to enhance fat utilization. This involves athletes manipulating their CHO intake levels by using a “train low” strategy, consuming fewer CHOs and more fat.

The “train low” state of low CHO availability (<5-10% of total energy intake) can upregulate lipid oxidation enzymes at the expense of damping CHO breakdown (glycolysis). However, it remains uncertain how the macronutrient intake shift from CHO to fat affects endurance training and performance. Training in a low CHO state may potentially stimulate fat oxidative enzyme pathways, but there is an expense of losing the athlete's “high gear” (maximal glucose oxidation), which is often needed during race situations (Vitale & Getzin, 2019).

The effects of the ketogenic diet on athletes during endurance exercise are controversial and not recommended for higher performance output. Endurance athletes are also encouraged to limit pre-race fat intake during a CHO loading phase or if there are GI comfort concerns. Rather than a “train low” ketogenic approach, athletes are encouraged to follow a normal dietary fat intake of 20-25% of their total calories.

Carbohydrate Mouth Rinsing

CHO mouth rinsing is another tactic athletes often employ while racing, but to better effect. CHO rinsing stimulates taste receptor cells and the central nervous system to improve performance without actually consuming CHOs (Vitale & Getzin, 2019).

Rinsing every 5-10 minutes (with at least 5-10 s of oral contact with a 6.4–10% CHO solution may improve performance by ~2–3% in high intensity (>70% VO2max) exercises bouts of up to 1 h (Vitale & Getzin, 2019). Oral CHO rinsing may be of value if the event is 1 h or less. However, for exercise lasting ~2 h or more, formal CHO ingestion is imperative for performance.

Post-Competition Carbohydrates

Carb needs don’t end once exercise does. CHO replenishment combined with protein is recommended to replace lost muscle and liver glycogen and assist with muscle protein synthesis and repair processes, particularly if there is a limited recovery window. The need for optimal CHO intake is essential for maximal endurance performance – for more, see this blog on glycogen repletion.