Nutritional needs of the endurance athlete are aggressively studied and the days of the pre-marathon pasta dinner have been enhanced by a sophisticated understanding of how nutrients can improve long-duration performance. To help your clients perform at their best, it is important to understand the latest research on optimal macronutrient recommendations, and practical strategies for individualizing and maximizing nutritional needs.
Since the first official use of Gatorade by the Gators football team in 1967 (1), much has been learned about the nutritional needs of the endurance athlete. To better understand nutrient demands, it is important to review the basic principles of energy production and the fuel sources involved.
Through energy metabolism, the body can use the energy-yielding nutrients (carbohydrate, fat, and protein) as fuel. Following digestion and absorption, these three macronutrients can be turned into the high-energy compound adenosine triphosphate (ATP).
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The following all influence which fuel is predominant during activity:
- intensity (anaerobic or aerobic ) of activity
- duration of activity
- conditioning of the athlete
- recovery time
- diet composition
Muscles always use a mixture of fuels, never just one. When carbohydrate, fat, and protein enter metabolic pathways they can make ATP which provides the chemical driving force for contractions. Carbohydrates and protein both have 4 kcal per gram and fat has 9 kcal per gram. During rest, the body derives more than half of its ATP from fatty acids, and most of the rest from glucose, along with a small percentage from amino acids. Endurance athletes train for an hour (or hours) at a time and this intensity and duration of training requires a lot of energy.
Elite athletes undergoing strenuous training can have daily energy expenditures 2-3 times greater than those of untrained individuals. Training can use up as much as 40 percent of an athlete's total daily energy expenditure, and energy demands in competition can also be very high.
Although the traditional culture of endurance athletics has focused on carbohydrate intake, the contribution of protein and fat to energy production is the subject for considerable research and each will be examined here, one nutrient at a time.
Carbohydrates For Endurance Athletes
Glucose, stored in the liver and muscles as glycogen, is vital to physical activity. A review of the literature shows that a relatively high daily carbohydrate (CHO) intake (> 6 g/kg/d) and CHO ingestion (30-60 g/h) during exercise appears to delay the onset of fatigue (2). During activity, the liver breaks down its glycogen and releases glucose into the bloodstream. The muscles use this, and their own private glycogen stores, to fuel activity.
In summary, strenuous exercise of all intensities makes great demands on the body's carbohydrate stores and glycogen depletion will lead to fatigue. Because glycogen stores are limited, and because they provide a critical contribution to both anaerobic and aerobic energy production, one important objective of sports nutrition is to protect glycogen and enhance access to fat for long duration, moderate intensity activity.
FatS for Endurance
In contrast to dietary fat, body fat stores are of tremendous importance during physical activity, as long as the intensity is not too high and there is adequate O2 delivery to use fat as a fuel source. Compared to the finite capacity of glycogen, fat stores can usually supply more than 70,000 kcal for activity (3). Fat is stored mainly in the adipose tissues and some is stored in muscle cells.
Endurance training increases the capacity for fat metabolism in the muscles, so that fat metabolism will cover a greater proportion of the energy production of athletes during exercise than for untrained people. Additionally, if the intensity of activity is low enough to allow aerobic energy pathways to predominate, the athlete will have optimal access to fat as an energy source. This will preserve glycogen and minimize the utilization of protein for fuel.
Protein And Endurance Training
While fat and carbohydrate represent the largest contribution of energy expenditure during exercise, the utilization of protein can also be significant. It is preferable to reserve protein as a building material, for the synthesis of lean skeletal tissues and contribution to other body systems for which protein is essential (i.e., hormones, immune system, transport proteins, etc.). Therefore, one objective of sports nutrition is to minimize protein utilization during activity through consuming enough carbohydrate. This will spare proteins from being broken down to create glucose, a process called gluconeogenisis.
While there is little debate that protein needs are greater for highly active individuals than those less active, this is often explained as a function of total energy intake (4). However, the specific percent contribution of protein to total daily intake for endurance athletes has been in question for some time.
The scientific literature to date provides some sound evidence to support an increase in protein requirements for highly-trained and elite endurance athletes (5). Tarnopolsky found that acute endurance exercise results in the oxidation of several amino acids. The total amount of amino acid oxidation during endurance exercise amounts to 1-6% of the total energy cost of exercise. Based on the available literature, sports nutritionists estimate protein requirements for an endurance athlete to be 1.5 to 1.7 g/kg (6). Additionally, low energy and/or low carbohydrate intake will increase amino acid oxidation and total protein requirements.
An examination of each nutrient in isolation, while interesting, has limitations. For example, an adequate protein intake with inadequate carbohydrate or calories will still result in suboptimal nutrition and performance.
Not just a sum of the parts
Regardless of how athletes divide up their macronutrients, if total energy intake is not adequate, performance will suffer (7). The earlier comments of this article indicated that athletes may have up to 40% greater energy needs than sedentary people. A review study of the nutritional needs of endurance athletes concluded that endurance athletes often have negative energy balance, meaning that expenditure is higher than intake (8). This negative balance can compromise performance and will definitely influence the percent contribution of each macronutrient. Perhaps of even greater consequence than macronutrient distribution is the total energy intake in relation to expenditure. One study on energy balance and ultra endurance athletes concluded that gigathlon competitors expend approximately 10,000 kcal/day in competition, more than feasibly could be taken in during each day of the event (9).
If organized in priority order, fluid would sit at the top of the list. While not energy-yielding, fluid plays a critical role in optimal performance and safe athletics. The combination of heat stress, dehydration, and exercise imposes perhaps the most-severe physiological challenge for the human body short of disease or serious bleeding (10). Exercise requires the body to attempt to cope simultaneously with competing demands for cardiovascular homeostasis, thermoregulatory control, and maintenance of muscle energetics. When dehydration is superimposed upon this scenario, the results can be catastrophic for both health and performance.
Sweat evaporation provides the primary cooling mechanism for the body, and for this reason athletes are encouraged to drink fluids to ensure continued fluid availability for evaporation and circulatory flow to the tissues. A water loss of even one to two percent of body weight can reduce an individual's capacity to do muscular work (11). In prolonged exercise, sweat losses of 2-3 liters/hour are possible and during a marathon race at high ambient temperatures, runners may lose as much as 8% of body weight, corresponding to about 13% of total body water (12).
The major electrolyte in sweat is sodium with smaller amounts of potassium and magnesium. Loss of substantial amounts of sweat will inevitably reduce the body's reserve of these electrolytes, which can also impair performance. Conversely, excessive drinking can lead to hyponatremia severe enough to cause fatalities. A more reasonable approach is to urge participants not to drink as much as possible but to drink no more than 400-800 mL/hour (13).
To Sum it All Up
In addition to securing the right macronutrient distribution, athletes should be encouraged to make the most nutrient dense choices possible. While a discussion of micronutrients is outside the scope of this article, if athletes are taking in adequate calories and making healthful food choices, they will be better protected against vitamin and mineral deficiencies as well. Timing also is critical and must be individualized to the sport and to each athlete. Science indicates that pre-competition/pre-game meals should be easy to digest and deliver a tolerable blend of carbohydrate, fat, and protein. Nutrients taken during endurance competition should be primarily carbohydrate (sports rehydration beverages, carbohydrate gels and goos and other carbohydrates) to deliver this valuable fuel when glycogen may be running low. Likewise, eating carbohydrates after a training session will enhance glycogen storage and some research indicates that a combination of carbohydrate and protein will further promote glycogen replenishment (14).
There are numerous considerations in designing nutrition protocols for individual athletes. As with other any sport, maximizing the nutritional needs during endurance competition begins in training. Successful implementation of sport nutrition guidance requires that coaches, athletes, and support personnel are made aware of the practical benefits of adequate fluid replacement and nutrient needs, and that appropriate fluid/nutrient-replacement strategies are developed and implemented over time. The competitive advantage will definitely shift in favor of those athletes whose coaches and trainers recognize the fundamental value of fitness, acclimation, hydration, and nutrition for keeping athletes cooled and fueled.
Training can use up as much as 40 percent of an athlete's total daily energy expenditure and energy demands in competition can also be very high
In addition to securing the right macronutrient distribution, athletes should be encouraged to make the most nutrient dense choices possible.
Successful implementation of sport nutrition guidance requires that coaches, athletes, and support personnel are made aware of the practical benefits of adequate fluid replacement and nutrient needs.
- The History of Gatorade, www.gatorade.com. Retrieved on May 11, 2007.
- Lambert EV, Goedecke JH. The role of dietary macronutrients in optimizing endurance performance. Curr Sports Med Rep Aug 2003;2(4):194-201.
- Wilmore, JH, Costill, DL. Physical Energy: Fuel Metabolism, Nutrition Reviews 2001;59:S13-S16.
- Paul GL. Dietary protein requirements of physically active individuals. Sports Med Sep1989;8(3):154-76.
- Tarnapolsky M. Protein requirements for endurance athletes. Nutrition 2004;20:662-668.
- Gaine PC, Pikosky MA, Martin WF, et al. Level of dietary protein impacts whole body protein turnover in trained males at rest. Metabolism 2006;55:501-507.
- Hoffman CJ, Coleman E. An eating plan and update on recommended dietary practices for the endurance athlete. J Am Diet Assoc 1991;91:325-330.
- Nogueira JA, Da Costa, TH. Nutritional status of endurance athletes: what is the available information? Arch Latinoam Nutr Mar 2005;55(1):15-22.
- Knechtle, B et al. Energy metabolism in long-term endurance sports: a case study. Schweiz Rundsch Med Prax Apr 2003;92(18):859-64.
- Murray R. Rehydration strategies--balancing substrate, fluid, and electrolyte provision. Int J Sports Med Jun 1998;19 Suppl 2:S133-5.
- Sawka NM, Montain SJ. Fluid and electrolyte supplementation for exercise heat stress, Amer J Clin Nutr 2000;72:564S-572S.
- Murry R. Fluid and electrolytes in sports nutrition: A guide for the professional working with active people, 3 rd ed. Ed C.A. Rosenbloom. Chicago: ADA; 2000. p 95 - 106.
- Noakes T. Fluid replacement during marathon running. Clin J Sport Med Sep 2003;13(5):309-18.
- Tipton, KD, et al. Timing of amino acid-carbohydrate ingestion alters anabolic response of muscle to resistance exercise. Am J Physiol Endocrinol Metab 2001;281:E197-E206.