Carbohydrate Intake for Endurance Training: Redefining Traditional Views

Burke, L.M., et al. 2011. Carbohydrates for training and competition. Journal of Sports Sciences, 29 (Suppl. 1), S17–S27.

Nutrition is emerging as an essential preparation component for everybody from novice exercisers to Olympic competitors. The most common nutritional tactic for recreational athletes is to focus on carbohydrate intake. After all, the availability of carbs within the body plays a central role in performance, especially for endurance athletes.

Carb consumption—type, timing and amount—is a decisive factor in how the body uses and replenishes glycogen, the stored form of glucose in muscles and the liver. This column will explore key concepts in using carbohydrates to prepare for competition.

Introducing “Carbohydrate Availability”

Estimating the carbohydrate needs of a recreational athlete is complicated, because each person has a different training schedule, volume of training and type of competition (triathlon, half-marathon, marathon, Ironman®, etc.). Furthermore, the exerciser’s food preferences and carbohydrate tolerance have to be considered.

Burke et al. (2011) conclude that when designing nutrition recommendations for an athlete, the trainer needs to ensure that the body has enough “carbohydrate availability” to meet the demands of exercise. Simply consuming a “high-carbohydrate” diet (measured by the proportion of carbohydrate calories in the exerciser’s total energy intake) does not necessarily mean that a recreational athlete is consistently meeting his carbohydrate needs. Burke and colleagues suggest that the focus should be on how much carbohydrate in the body is “available” for the specific type of exercise the athlete will perform—as well as how much postexercise carbohydrate, stored as glycogen, will be needed during recovery. Table 1 provides guidelines for ensuring that carbohydrate fueling and refueling are able to meet exercise demands.

Acute Manipulation of Carbohydrate Status

To promote sufficient carbohydrate availability during competition, many recreational athletes have traditionally chosen a carbohydrate-loading strategy. Common “carb-loading” practices include a tapering of carbohydrate restriction for 3–4 days, followed by increased carbohydrate intake for 2–3 days prior to competition. It now appears that the initial 3–4 days of depletion is not required for increased glycogen storage and carbohydrate availability (Burke et al. 2011). In fact, high glycogen storage concentrations have been achieved after as little as 24 hours of rest with high carbohydrate intake (Burke et al. 2011).

It is also important for exercise professionals to inform their clients that carbohydrate loading does not always translate into an increase in athletic performance. Success in competition depends on an athlete’s physiology, metabolism, psychology and nutrition, so changing just one factor would not necessarily be decisive.

Carbohydrate Types: Using the Glycemic Index

Rather than focusing on carb loading, some recreational athletes choose specific carbohydrate types and carefully time their consumption of these. For example, many recreational athletes choose carbohydrates based on the glycemic index, consuming low-GI foods/drinks prior to exercise and high-GI foods/drinks during and after exercise. The purported main benefit of this tactic is to achieve better maintenance of plasma glucose concentrations during and after exercise. However, the evidence that eating low-GI foods prior to exercise improves athletic performance (Burke et al. 2011) is inconsistent. In addition, Burke et al. note that carbohydrate intake during exercise may offset the effect of low-GI carbohydrate intake after exercise.

Carbohydrates During Exercise

Consuming carbohydrates during exercise competitions lasting longer than 1 hour has become a common way to boost performance. Indeed, consistent carbohydrate intake during sustained exercise maintains blood glucose while sparing glycogen and muscle catabolism (breakdown). But there is no one-size-fits-all approach. During events lasting longer than 60 minutes, carbohydrate intake consisting of 30–60 grams per hour, accompanied by adequate hydration, is recommended. Consuming more than 60 g per hour during an event may lead to gastrointestinal upset.

Many of the sports drinks, energy gels and bars designed to meet competitors’ needs during exercise include multiple types of carbohydrates—such as sucrose, fructose, glucose and maltodextrin. Consuming such a product during exercise may lead to optimal carbohydrate fuel utilization (Burke et al. 2011).

A New Way for Endurance Athletes: Train Low; Compete High

The muscle adaptations that take place during exercise include increased mitochondrial (ATP synthesis factory) size and number; heightened enzyme activity to break down foodstuffs; and improved structure of some muscular contractile components, which enhances contractility. Interestingly, a decrease in cellular energy triggers a muscular response that increases mitochondrial size and number, which eventually translates into better aerobic performance (Burke et al. 2011).

Further, Burke and colleagues observed that subjects experienced greater increases in mitochondria and enzymes controlling fatty-acid and carbohydrate catabolism when exercising with lower glycogen stores or under fasting conditions. These observations have led to the development of a new concept called “Train low; compete high.” Competitive athletes who follow this new paradigm train with lower carbohydrate availability and then switch to higher carbohydrate availability prior to competition.

However, it is a misconception that athletes must adhere to a low-carbohydrate diet in order for “Train low; compete high” to work. Rather, athletes are encouraged to simply train under depleted circumstances and then meet carbohydrate needs throughout the remainder of the day. More research is needed in this area to eventually provide healthy guidelines and recommendations for serious athletes.

Summary: Complexities of Carbohydrate Consumption

The complexity of nutrient type and timing for exercise performance is vast, and it depends on an enormous number of variables. Much remains unknown about ideal training strategies. Competitive recreational athletes need to be reminded that their needs may be different from the needs of others because of body size, age, gender, level of competition and type of athletic event (see Figure 1 for general dietary recommendations).

Indeed, since there are such great differences among various sports and competitions, it is constructive for recreational athletes to experiment with carbohydrate intake strategies and to develop preferences based on the foods they like and can tolerate. These athletes are also encouraged to consider working with registered dietitians and/or certified sports dietitians to design dietary plans that meet both performance goals and nutrient needs throughout the day.