Let's start off with an analogy: Every car on the road has to be fueled in some fashion; most cars run on regular gasoline while there are also diesel engines as well as electric cars.  It is common knowledge to the driver as to which gasoline is the most efficient type for the vehicle they drive. 

As humans, and more specifically exercising humans, it should be a goal to embody the most efficient form of nutrition for which type of "vehicle" we choose to be.  What that means is that as people trying to become as physically fit as possible, our nutritional intake should match the activities we are doing.

Taking a field trip to exercise physiology class: there are three energy systems in which all life processes take place.  First is the ATP-CP system, second is glycolysis and third is aerobic phosphorylation.
ATP-CP- This is the shortest living energy system, it runs anywhere between 8 and 12 seconds.  The ATP-CP system is used for quick movements that are NOT long in duration

Glycolysis- this energy system, while slightly running during the ATP-CP system, really takes over once the ATP-CP system is exhausted and has no more energy to give off.  This system lasts generally between the 12 second mark and stops around 2 minutes into the exercise bout.  Here, we still have yet to start utilizing oxygen to create energy.  As athletes or exercisers, an awesome goal to have is to be able to stay in the "glycolytic pathway" as long as we can before the switch to the third system.

Aerobic Phosphorylation- have you ever been on a run and eventually your breathing rate is very deep and slow? That's how you know when you're using this third system! Here, we're using oxygen to our advantage to assist us in breaking down our fat storage for the energy being used.  While there is also some carbohydrate breakdown, especially when exercise comes to a high intensity, that is an indicator that the individual is about to reach the point of fatigue.

Now, SPORTS!

Looking across all different sports, there are diet plans or eating philosophies that seem to be better suited for required energy output that athletes have.  For our purposes, let's look at the time under tension that the athlete undergoes to determine what macronutrient is their main source of fuel as well as how much of that specific macronutrient the athlete needs!

ATP-CP- what sports usually have athletes undergo short bouts of explosive movements?

• Baseball/Softball
• Football
• Swimming (select events)
• Diving
• Golf
• Tennis
• Track and Field
• Olympic Lifting
• Gymnastics

What macronutrient is being broken down here? These sports don't involve hard breathing so its not fat, we never want to break protein down for energy, so that leaves carbohydrates! Glucose that resides in the muscle is broken down immediately for energy as well as some from the blood.

HOLD ON…WAIT! If you're a 50m sprinter reading this article, please don't take this as an excuse to eat a box of pasta a day, that is the last thing I want you to get out of this.  Please don't be scared, but some numbers are about to start to get thrown around.  The body has right around 5.5 Liters of blood.  The highest possible "safe" blood glucose is 125 mg/dl of blood.  Doing the math, that only really leaves less than 7 grams of sugar running through the blood at any time, while a plate full of pasta is around 40g of carbohydrates (and let's be honest, many people take in 2 or 3 servings with dinner).  So for my ATP-CP athletes out there, carbs are important, but don't go overboard!

Extra nerdy side note: Low carb diets have also been gaining speed in the science world and are generally thought to accompany decreased quality of performance for anaerobic athletes (before 12 weeks usually), but another study suggests that even on a low carb high fat (LCHF) for 7 days decreased body mass while keeping anaerobic capacity.  This information could be especially useful for athletes that compete in events that have a maximum weight requirement such as wrestling or power lifting.

See first link for further information

Glycolysis- what sports usually have time under tension that lasts around 2 minutes?

• Soccer
• Football (select positions)
• Basketball
• Field Hockey
• Lacrosse
• Cycling
• Swimming (select events)

aNAerobic, meaning they don't utilize oxygen, the body breaks down two things: intramuscular glycogen (sugar in the muscles) in sugar drawn from the blood.  Now again, just like the ATP-CP athletes, I'm not giving a free go-ahead to eat as many carbohydrates as you'd like, although you can be a little more liberal as far as carbohydrates are concerned, especially if you're exercising for a lengthy period of time.
Aerobic Phosphorylation- what sports does an athlete generally get to a steady state of oxygen consumption?

• Cross Country
• Swimming (distance events)
• Outdoor track (distance events)
• Field Hockey (select positions)
• Lacrosse (select positions)

Now I must warn you, this is where the science is in two distinct camps: A carbohydrate rich fuel plan and a low carb (high fat) eating style.  First, the carb-rich plan.  Because fat takes more oxygen to burn than carbohydrates, many researchers are lead to believe that even at the "steady state" of oxygen consumption carbohydrate is still the preferred fuel source of the body and should be used as fuel still during longer exercise bouts.  Some researchers even go as far as saying that if an individual goes on a low carbohydrate fuel plan their performance will suffer.

See second link for more information

Now, the low carb camp.  I feel as though its important to make note of the man who first said to "eat a plate of pasta the night before a race" was Tim Noakes.  The Tim Noakes who ran multiple marathons, was a leading nutritional researcher and the same Tim Noakes who contracted type 2 diabetes mellitus while previously thought of as "healthy."  Noakes and other scientists suggest that if an individual undergoes a LCHF plan their oxygen utilization as well as fat burning efficiency will increase without performance suffering.   Towards fatigue, the body switches back from burning primarily fat to carbohydrates, this is called "the crossover point," scientists in the low carbohydrate camp believe that if an athlete undergoes a LCHF fuel plan, this point will take longer to reach, and therefore delay fatigue.

See third link at the bottom to get more information

Last side note:  Once the food and macronutrient percentages have been picked out, it is up to the athlete to be honest with how much energy they are putting out.  For example, it is not the best idea for an athlete who is injured watching their teammates lift to be eating the same amount of food that their peers who are exercising are eating.  Whether you're on a standard carb or high fat fueling plan, good ole' thermodynamic rules still apply.  One must still be aware as to how many calories are coming into the body as well as coming out.

As athletes (or regular individual for that matter) there are resources available at your disposal.  As a Towson University athlete, Christine Turpin (RD, CSCS) is always available for nutritional consults. If you have any thoughts or questions as to what your main energy system is, or what your overall nutritional intake should be, she is the perfect person to go to.
 
Sources:
New study: high intensity exercise on a low carb diet. (n.d.). Retrieved February 12, 2018, from http://caloriesproper.com/new-study-high-intensity-exercise-on-a-low-carb-diet/
Fat Burning – Why It's Overrated for the Competitive Endurance Athlete |. (2016, December 3). Retrieved February 12, 2018, from http://www.eatsleep.fit/endurance-sports/fat-burning-why-its-overrated-for-the-competitive-endurance-athlete/
Chang, C., Borer, K., & Lin, P. (2017). Low-Carbohydrate-High-Fat Diet: Can it Help Exercise Performance?. Journal Of Human Kinetics, 56(1), 81-92.