LESTER HOLT, reporting: NFL football players are known for their grit, tenacity, and hard knocks. But one of their toughest physical challenges comes at training camp, with its twice-daily practices, known as “two-a-days.” ORLANDO PACE (Former NFL Tackle): Two-a-days, man, it is rough, you know, because it’s 100-degree temperature, you’re hitting twice-a-day in practice. HOLT: Orlando Pace, a seven-time Pro Bowl offensive tackle for the Saint Louis Rams, says two-a-days push players to the limit because they happen in the sweltering months of July and August. PACE: It’s really important for guys to stay hydrated, try to get the right nutrition in them, those type of things. HOLT: NFL players, like all humans, need six-types of essential nutrients to stay healthy during training camp. The most crucial is water, which helps players cool their body temperature by allowing the formation of sweat. Dr. DOUGLAS CASA (Korey Stringer Institute, University of Connecticut): A sweat drop will form on your skin and if the humidity level is not too high, that sweat droplet will likely evaporate. And when that sweat droplet evaporates, it actually is removing heat from the body. HOLT: But sweating also causes dehydration, which can lead to muscle cramps, heat exhaustion, and in extreme cases, heat stroke. Dr. CASA: During hard practices in the heat, almost all athletes are between 102 and 104 degrees Fahrenheit. So you could imagine that if heat stroke is maybe like 105 and higher, then you don’t have a real big wiggle room to deal with. HOLT: At Pittsburgh Steelers training camp, the intensity of workouts causes some players to sweat 10 or more pounds of weight per practice. More than water is lost–so are minerals, such as sodium, potassium and electrolytes. To keep players healthy, trainers make water and sports drinks available at all times, even during live drills. HINES WARD (Wide Receiver, Pittsburgh Steelers): You’re constantly drinking even though you’re not thirsty. I get to the point where I’m sick of water, but it’s a necessity. HOLT: Players also weigh themselves before and after practice to gauge how much fluid they need to re-hydrate. RYAN GROVE (Assistant Athletic Trainer, Pittsburgh Steelers): Our goal is in between practices, if you lose 5 pounds, you need to replace those 5 pounds of fluid loss before the next practice. HOLT: Beyond replacing fluids and minerals, players need to replenish calories. The average adult male needs 2,200 to 3,000 calories a day. Players can burn that many in a single practice, and so they need to eat more. GROVE: It’s not uncommon that these athletes are probably consuming anywhere between 5 and 10 thousand calories a day. HOLT: While calories are often associated with food, they are also a unit of energy. A physics calorie measures the amount of heat it takes to raise the temperature of one gram of water by one degree Celsius. A food calorie, or kilocalorie, equals 1,000 physics calories and is a measure of how much energy is available in foods. Dr. NANCY RODRIGUEZ (University of Connecticut): A food calorie is the equivalent of gas to a car. It’s fuel for your body. So calories provide heat or energy for you to do physical work or to think or to breathe. SCOTT PAXSON (Nose Tackle, Pittsburgh Steelers): I like to eat everything they put in front of me. HOLT: At Steelers camp, players get their calories by eating three full meals and a late night snack every day. JOHN NORWIG (Head Athletic Trainer, Pittsburgh Steelers): You can eat almost anything you want as long as you do it modestly and not in any extreme nature, where you’re getting the food groups that are necessary for you to perform at a higher level. HOLT: These food groups supply the other essential nutrients: especially proteins, carbohydrates, and fats. Proteins, found in fish and steak, are made of compounds called amino acids that help build and repair muscle. Carbohydrates, found in pasta and fruits, are starches and sugars the body uses for energy. And fats, found in such foods as dairy products, help supply energy and maintain cell membranes. ANTWAAN RANDLE EL (Wide Receiver, Pittsburgh Steelers): You don’t want too many fats, of course, but you still want to have some, but not too many. Carbohydrates and proteins are essential to every meal that you have. PACE: Really, as a pro athlete, you always try to, year round, you try to really try to pay attention to what you’re eating, because what you put in your body, you kind of get out on the field. I will say always eat right, stay hydrated, and I think you’ll be healthy throughout the year. HOLT: An NFL year, that starts in the heat of summer, and hopefully lasts long into winter.