What gasoline is to cars, "ATP" is to humans. "ATP" (adenosine triphosphate) is an energy molecule that powers human muscles (and all organs). As you've probably recognized, ATP contains the chemical element "phosphorus" which makes a match light up or fireworks explode.
ATP molecules are created in plants during photosynthesis and serve as energy carriers. When you eat carbohydrates from plants (or proteins and fats from animals which in turn feed on plants), your body converts (or metabolizes) these nutrients into carbon dioxide, water ... and ATP (with the help of sufficient oxygen supplied by the lungs). As muscles contract and organs do their work, ATP energy is then converted from chemical form into mechanical form. (Here again the "laws of thermodynamics").
Human muscles can only store a little ATP at a time. So it needs to be ingested constantly, particularly during workouts. That's why athletes, in order to keep going, need to re-fuel from time to time with carbs or fats (depending on what sports they are engaged in).
More intensive physical activities (like sprinting, lifting heavy weights, or wrestling) require more carbohydrates for fuel. Carbohydrates are the most effective energy storage systems, because they can release energy quickly, with no need of oxygen from the lungs (this process is called "anaerobic"). Less intensive endurance sports, such as hiking, swimming, or biking will burn fat, which is only possible with the help of oxygen and a lot of heavy breathing (known as the "aerobic" process).
How much energy is contained in carbohydrates and fats? Carbs carry about 4 kcal of energy per gram while fats carry up to 9 kcal per gram. That's why nutritionists recommend that athletes get up to 65% of their daily energy from carbohydrate-rich foods like breads, cereals, rice, pasta, potatoes, beans or lentils. In fact, the fastest runner during the 2008 Olympic Games, Usain Bolt from Jamaica, is said to have obtained his strength from local sweet potatoes called yams. If you've been following the swimming competitions you may have heard Michael Phelps talk about his incredible daily calorie intake of 12,000 kcal to prepare his body for the relentless competition schedule. High value oils made from rapeseed plants (canola) or olives provide the right kind of fats. Lean cuts of meats, eggs or dairy products are great protein providers.
During intense training periods of an hour or more, athletes may burn up to 30% of their daily energy needs. Adequate nutrition is necessary to restore energy balance. Depending on the type of sports, athletes must be very careful about balancing body weight and physical activity in order to reach their highest performance. Many aspiring gymnasts for example, often lose too much weight in order to belong to a certain weight class. Conversely, weight lifters often bulk up too much to lift heavier bars. Unbalanced body weight and physical activity may lead to severe health problems.
On average, an adult spends the following amount of energy (measured in kilo calories) during an hour of each of the following activities:
1. Sitting down: 120 kcal
2. light work standing up, like cleaning: 250 kcal
3. Lifting weights: 390 kcal
4. Swimming breast stroke for just over a mile: 440 kcal
5. Tennis: 470 kcal
6. Soccer: 560 kcal
7. Walking: 590 kcal
8. Skiing, cross country for about 4 miles: 600 kcal
9. Mountain climbing: 610 kcal
10. Running for 10 km or biking for 24 km: 680 kcal