You might do a fast walk around the block each morning, run marathons, lift weights, or perhaps you do a regular gym or jogging session to lose weight. In any case, there are certain eating and drinking behaviors that can maximize your ability to train or compete at your best. This article, and the others in this series, gives you the theoretical background and the practical application.
It’s true enough that top-tier athletes will get most benefit from fine-tuning dietary habits, but even more novice exercisers can benefit from eating the right sustenance at the right time and in the right quantities. It could make all the difference to completing a training session strongly, or being able to get up in the morning with enough energy to go out and do the exercise that you know you have to do.
This article discusses how the body uses energy for particular human activities, and how you can take advantage of those energy systems to optimize your eating and drinking in relation to your training and, or competition.
The Energy Systems of the Body
When we exercise, depending on how hard and long, and whether we've just had a meal, a mix of fuel is used, but often one dominates, depending on the nature of the activity. Food is fuel, and understanding the principles of refueling and types of fuels required for particular activities is important in maximizing exercise performance. There is inevitably some chemistry in this, but the basics are not too complicated.
ATP and PCr System (Alactic)Adenosine triphosphate (ATP) is the primary molecule of energy for human movement. At rest we have a small store of ATP, perhaps for a few seconds worth of intense performance. This is replenished by another important molecule, phosphocreatine or creatine phosphate (PCr). Energy from this system is supplied very quickly but lasts less than 10 seconds before it needs to be replenished with rest. This energy source is not "nutrition critical" under normal circumstances. This is sometimes called the "alactic" system. Doing a single squat or deadlift utilizes the ATP/PCr system.
As you exercise longer than 10 to 20 seconds, additional energy systems become available for more sustainable energy production as exercise duration increases, all of which ultimately produce ATP as the ultimate chemical source of energy.
Lactic Acid System (Lactic or Anaerobic)
Beyond the energy supply of the PCr system, and with continued high-intensity activity, the lactic acid system takes over and produces a supply of ATP, limited by the rate of oxygen availability, for up to about 90 seconds depending on intensity and lactate tolerance. This system will dominate in a 400-meter runner or 100-meter swimmer for example, although most activities longer than about 10 seconds use a mix of energy systems.
Glucose stored in muscle as glycogen is the main source of energy for this system. Performance can be limited by inadequate storage of muscle glucose, or inhibition of the rate at which it is made available to the muscle cells. These two systems, alactic and lactic, supply "fast" energy.
Oxygen supply maintains the aerobic system more or less perpetually as long as a fuel source is available, energy again being ultimately provided as ATP. The aerobic system uses blood glucose, muscle glycogen, blood and muscle fats (triglycerides) and even ketones (breakdown of fats) -- as well as proteins in certain circumstances. Needless to say, this is the dominant energy system of longer exercise duration and day-to-day living. Understand, though, that a mix of fuel sources and energy systems, (aerobic and anaerobic), will occur even in a marathon or long bike race.
Carbohydrate Is the Most Important Fuel
Carbohydrate is the most important fuel for athletes. Even though fat provides a theoretical unlimited supply of energy for most activities, and this is valuable in longer-duration sports, it cannot provide "fast" energy like glucose (and PCr), and this is limiting for most sports. Glucose or "blood sugar," which is also stored in liver and muscle, is the result of the digestion of carbohydrates including starches and sugars from foods like breads, grains, cereals, pasta, vegetables, sugars and fruits.
Low-carbohydrate diets are not suitable for athletes – even endurance athletes that rely substantially on the fat-fueled aerobic system. Failure to maximize glucose and glycogen storage, and its rapid accessibility, will degrade performance and every second or fraction counts in competition.
Examples of the main energy systems in various running events
- 100 meters - ATP/PC (alactic system)
- 200 meters - ATP/PC and anaerobic/lactic glucose
- 800 meters - anaerobic/lactic glucose and aerobic glucose
- 10,000 meters - aerobic glucose and fats, some anaerobic/lactic glucose
- Marathon - aerobic glucose and fatty acids, minor anaerobic/lactic
- Most running team sports (like soccer or basketball) over the duration of a game - all energy systems!
Both lactate from anaerobic glucose metabolism, and glycerol from the breakdown of triglycerides (fats) into fatty acids and glycerol, are recycled for energy use. Ketones are the breakdown product of fats when the body senses glucose is so low that an alternative energy source is required. Ketones are not useful substitutes for glucose as athletic fuels because they cannot be supplied with sufficient rapidity for high performance to working muscle.
Read other articles in the Sports Nutrition series:
- Eating and Drinking for Energy in Exercise and Sports
- Carbohydrates for Energy in Sports and Exercise
- How Much Protein for Athletes and Exercisers
- Ideal Body Fat Percentage for Athletes and Active People
- Burke and Deakin, Clinical Sports Nutrition, 3rd Edition, McGraw-Hill Australia Pty Ltd, 2006
- Med Sci Sports Exerc. 2009 Mar;41(3):709-31. American College of Sports Medicine position stand. Nutrition and athletic performance. American Dietetic Association; Dietitians of Canada; American College of Sports Medicine, Rodriguez NR, Di Marco NM, Langley S.