Energy Systems and You: A Guide For Athletes -The Toast

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Hellooooo athletes!

Yes, you. You are an athlete. Some of you already have one or more athletic endeavors that you enjoy. Others of you may just have not found (or chosen) your activity yet. Some of you just have a body. So many of us are told that we aren’t athletic, and that we can’t be athletic. Society, friends, authority figures, media – there are lots of people and institutions that try to act as gatekeepers of who gets to be called an athlete. Well, I don’t care. As far as I am concerned, you and I are athletes.

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Members of the Houston Heights Ladies’ Lifting Club. Every single person here is a competitive powerlifter who won their weight class at a local meet. Before this meet, many of them had never competed in an athletic event before in their lives. (note: the beer steins are our trophies!)

OK, now that we have that out of the way, let’s talk about some athlete stuff. No worries, this is not the point at which this article devolves into one of those “Blast your abs with these intense crunch exercises!” listicles. Although I have some strong opinions about which athletic endeavors are the coolest, those are just that: opinions. I am not here to tell anyone else what to do or not do. You are an athlete, and you can choose for yourself! Instead, I want to talk about an athlete’s view on nutrition as it impacts athletic endeavors.

Food Is Fuel

As a person who exists in the world, you are already aware that food provides the fuel your body needs to move, and that different foods contain different kinds of nutrients. The most important nutritional components required to keep your body going are protein, fat, and carbohydrates, known as macronutrients

If you’ve done some reading on nutrition you may also have heard various conflicting narratives about which of these macronutrients are good or bad for you, and these narratives may have shifted over time. For example, fat was once held to be The Worst Thing You Could Put In Your Body, whereas these days carbohydrates are the new Voldemort of nutrition.

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Have some delicious carbs, Harry Potter.

To be honest, I don’t care about what nutrition plan you follow. Everyone has a unique set of needs and goals. What I do want to talk about is how your body uses the food you put in it. As athletes, this is an important subject for all of us. Knowing how our bodies use food means we can understand why each macronutrient is important, and in turn can allow us to decide on nutrition goals that support our athletic endeavors out in the world. So, let’s cover the basics of nutrition very quickly.

The (Very) Basics of Nutrition

Each of the three macronutrients provides energy to fuel our bodies, as well as fulfilling other important roles.

Protein is the basic building block for your muscles and connective tissue. Our bodies use protein to repair damaged muscle and to grow new muscle mass when exposed to the right stimulus (that is: exercise).

Fat is used by our bodies to protect against shock, to keep the cells of our bodies intact, to regulate body temperature, and a ton of other stuff. The simplest way to think of it as an athlete is that, just as protein builds muscle, fat allows us to recover from exertion and other stresses on the body.

Not the recommended way to get your daily allowance of fat.

Carbohydrates are the primary source of energy for our bodies, and are made up of various types of sugar. You’ve heard of dextrose, sucrose, fructose, and so on? Those are all sugars, and are all carbohydrates. As an athlete, a good way to think about carbohydrates is that they are the most efficient energy source for our bodies (as compared to fats and proteins).

(Note: There are a number of other aspects to nutrition not covered by the Big 3. Dietary minerals and vitamins both provide chemicals necessary to the various biochemical reactions in our bodies that make up the process of living. For today’s discussion, though, we’ll focus on the macronutrients.)

Food Is Fuel, But How Is It Used?

OK so now we’ve reviewed the basics of nutrition and we have a general grasp on what the food we put into our bodies is used for. Now let’s talk about how our athletic bodies work; specifically, how we power athletic activity. Think of it like this: if you drive a car, or use a bicycle, you don’t have to be an expert mechanic to get around town. BUT, it can be mighty helpful to know some basic facts (for example, car tires need air when it gets really cold). So, this article isn’t going to try to explain the detailed, super-complex biochemical processes that are involved with us doing our athletic thing. We just want some basic facts that give us an understanding of how we move, and what that means for us as we go about our athletic lives like the bosses we are.

So, let’s start out by talking about energy systems. In addition to using the tears of MRAs to keep us going, our bodies use a chemical called adenosine triphosphate, or ATP, to power muscular movement. Any time we move, we consume ATP. ATP is our energy source, and there are three energy systems our bodies use to make sure they have enough of it to continue being awesome.

One way to imagine these three systems in action is to think about that bane of my own childhood existence, the elementary school PE footrace. At the start of the race, you take off running as hard as you can. After a few seconds, maybe 8-10, you have to slow up a bit. If you have a really sadistic PE teacher and it is a long race, you’ll have to slow up again at the 45 second mark, at which point you’re either trotting or walking. That’s the body’s three energy systems in action. Let’s look at each of them.

The ATP/PCr System (or, Let’s Race! Wait, Why Am I So Tired?)

Remember when I said the power was inside you all along? That’s literally true. ATP is stored in the cells of our bodies, where it is ready for use when needed. However, only a small amount is stored at any one time. This means that our muscles are like high-power, low-capacity batteries:  they can give you a real jolt, but they run out of juice quickly. Generally an athlete has enough ATP stored to power about 5 seconds of movement. This stored ATP is used regardless of what the movement is, or its intensity. That’s because it is right there in the muscle cells, and thus can be used additional chemical conversion. At lower levels of activity, you never actually run out of stored ATP, as your body is able to replenish it as you use it. But if you are exerting yourself in an athletic activity (or any other strenuous effort), you will use up all of the ATP you have stored.
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Whether you are Shelly-Ann Fraser-Pryce winning the 100m sprint at the 2012 Olympics or former NFL player Rosey Grier doing needlepoint, your body uses its stored ATP first.

Once we are out of stored ATP, we have to find some more, which is where the PCr part of the ATP/PCr system comes in. Consider super-high-intensity activities, like a 100m all-out sprint. Since even the fastest 100m sprints take between 9 and 10 seconds, there must logically be another way to quickly make ATP available to our muscles. And there is! There’s another handy material in our cells, creatine phosphate (or PCr), which is used to rapidly manufacture more ATP. This starts happening as soon as we begin using our stored ATP.

Altogether, the ATP/PCr system can give us about 8-15 seconds of maximal effort activity before the cells run out of stored energy. That’s why you can run really hard for a short distance, but then you “run out of gas.” It happens to all of us athletes – even Olympians are bound by this limitation. Well, Olympic athletes are. I guess Athena and friends are a different matter.

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Screw your mortal limitations! I do what I want. Like wear my helmet way up on my head so you can better see my scornful gaze.

The cool thing, though, is that these stores of energy get refilled over time. After a short rest, our bodies have replenished a bit of stored ATP, and can go again. How much rest? It depends on a lot of things. This is one place where being an athlete is handy. The more active we are, the more efficient our bodies become at replenishing these energy stores. For example, after an intense heavy lifting effort (weights, bags of mulch, recalcitrant toddlers, etc.), it can take as little as one minute and as much as five minutes for us to be ready for action again.

Glycolytic Energy (or, Maybe I’ll Just Jog for a Bit)

Obviously, we don’t necessarily just fall down when we use up our stores of ATP (although in my case I do like a good flop-on-the-floor-and-whinge as a finisher to a tough set of squats); we can keep going, albeit slower (and, in my case, with a lot of complaining).

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“Go on without me. I’m done for.”  — me, after heavy squats or deadlifts

Since there’s a limit to how much ATP and creatine we can store directly in our cells, we need to get energy from somewhere else in our bodies in the form of other chemical compounds. The most readily available of these are carbohydrates, which are stored in the muscles and also in the liver. The carbs stored locally in the muscles are ready for conversion to ATP right away; the carbs stored in the liver, on the other hand, have to go on a journey of personal discovery and growth in order to get to the muscles. OK, OK, that’s over-selling it. But they do get converted to glucose (sugar) and sent to the muscles via the bloodstream. Then they get converted to ATP just like the locally-stored carbs.

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Excuse me. Are these carbs locally sourced?

The conversion of carbohydrates to ATP is known as glycolysis, and it begins after about the first 10 seconds of intense activity, and allows our bodies to keep going at a lower intensity level for about another 30 to 40 seconds. That power drop is necessary, as the rate of conversion of ATP via glycolysis is less efficient than simply using what was already there in the muscle cells, and thus makes less ATP available. This drop in power also prevents a gap in energy availability – once your body has used its stored ATP, as your body will only be able to use ATP as fast as it is created.

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I was going to find a relevant image from Dune with a “the spice must flow!” quote, but this picture of Sting in a metal mankini declaring victory was too good to pass up

Sometimes, though, there is a need for more efficient, slower creation of ATP. This brings us to our third energy system:

The Oxidative System (or, Nah, I’ll Just Walk A Bit Instead)

You might be wondering why do fast glycolysis isn’t just called “glycolysis.” It turns out there is another, slower process for converting carbohydrates to ATP, known as slow glycolysis. This process is more efficient than fast glycolysis, but (and I’m sure you saw this coming) takes more time.

As we begin to run out of carbohydrates that can be converted to ATP via fast glycolysis, our bodies begin a complex series of chemical reactions known as the Krebs cycle to convert additional stored carbs as well as fatty acid (in the form of stored body fat) to replenish the body’s stores of ATP. This energy system, known as the Oxidative system, takes much longer and is primarily used to replenish stores of ATP over the long term and to power lower-intensity activity.

The increase in use of the oxidative system comes with yet another drop in the power output of our muscles. We can go for a longer period of time, but at a much lower level of effort. Long runs and walks are good examples of this. Interestingly, although the oxidative system is generally associated with long-term, low-effort activities like running or walking, it is also used extensively by our bodies to replenish energy stores consumed by more intense activities like sprinting or weightlifting, and these activities, along with the energy consumption required for building/re-building muscle fiber, mean that these activities are generally great choices for athletes who want to change their body composition by reducing their relative percentage of body fat.

Working Together

It is tempting to think of these three energy systems as totally independent from one another, and to some extent that is true. You can, for example, hold your breath on a heavy squat and complete the rep pretty much solely using your ATP-PCr and fast glycolytic systems. Indeed, that’s how you do a heavy squat. But in truth all three of these energy systems are all going at once; which one is being used the most is a matter of degree depending on what you are doing, how hard it is, and how long you’ve been doing it.

So, now you know how your athlete self is powered. Let’s return to our discussion of nutrition and talk about what all of this means for you, the athlete who is thinking about their nutrition plan.

Nutrition 2: Electric Boogaloo

The kind of athletic activity you enjoy is going to determine your nutritional focus. Overall, the general recommendation you see has nutrition guidelines of something like 30% protein, 30% fat, and 40% carbohydrates, with variations of +/- 10% depending on who is doing the recommending. There are a lot of varying opinions about how this should be adjusted in order to get leaner. We don’t care about that in this article; we care about understanding how to adjust our food intake for our sports. So let’s just use that general guideline as our starting point and think about what the various sorts of athletic activities mean in terms of nutrition.

Let’s talk fat first: any kind of regular athletic activity really taxes your system, so you’ll need good dietary fats to help you recover. Get that ~30% in your body! You can get very into the details of what kinds of fat you consume, and there is research showing that some fats are more useful than others. As a general guideline, though, the kinds of fats found in naturally-growing foods that humans are adapted to eat (pigs, avocados, etc.) are the kinds you need.   

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Bacon has fat and protein and is basically candy. Perfect.

If you are a strength athlete such as a weightlifter, powerlifter, strongwoman/man, or thrower, you’ll need muscle mass. You’ll move more weight, and move it faster, with more muscle. Strength requires muscle, and to do that you need lots of protein. A common recommendation is 1 to 2 grams of protein per day per pound of lean body weight (i.e., your body weight after you subtract out your body fat). If you don’t want to mess with figuring out your lean body weight, you could just start with ½ to ¾ grams of protein per pound of total body weight and go from there. Eat more protein if you aren’t gaining muscle. Eat less if you are gaining too much relative to your athletic needs.

You’ll also need carbohydrates to power your strength movements. If you do quick, intense strength athletics such as powerlifting and weightlifting, you’ll mostly be using your ATP-PCr energy, and you’ll be replenishing that energy from carbohydrates and fats stored in your body. Thus, you’ll need to replace those carbs. That said, you won’t need that many compared to other kinds of athletes.

If, on the other hand, you enjoy strength-endurance hybrids such as 15-person rugby, American-rules football, strongwoman/man competitions, highland games, or the like, where you exert yourself in extreme bursts over and over again throughout the day, you’ll use your ATP-PCr cycles, but you’ll also heavily leverage your glycolytic and oxidative systems, so you’ll need more carbs than powerlifters or weightlifters.

World’s Strongest Woman 2014 Olga Liashchuk showing some dude what’s up with a medley race.

Overall, the bigger you are, the more food you’ll have to eat to maintain your size. For example, professional strongman Hafþór Bjornsson (aka The Mountain That Rides on Game of Thrones) eats over 10,000 calories per day (7 chickens in one sitting, according to Peter Dinklage). But he’s 6’ 9” tall and weighs almost 400 pounds. The rest of us normal humans can eat regular human amounts of food. Still, the more muscle mass you carry, the more protein you need to maintain it.

On the other hand, if you are an endurance athlete (marathoners, ultramarathoners, people who hike the Appalachian Trail, swimming the English Channel, etc.), you have to focus more on storing energy for the long haul and less on packing on muscle. Indeed, you will probably find that carrying a ton of extra muscle mass around is less helpful for you.

So as an endurance athlete, you’ll need just enough protein to maintain the muscle mass that you need to move your body, but you won’t want to get huge and jacked, as that will work against your athletic goals.

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Hafþór is The Mountain That Rides, not The Mountain That Runs Extremely Long DIstances.

Carbohydrate intake, on the other hand, will be something you prioritize in your nutrition plan. If you are burning carbohydrates all day every day, you need to make sure your carb intake increases accordingly. Just keep in mind that this activity is a much lower level of intensity (otherwise you couldn’t keep it up for a long time), and as a result you won’t be consuming energy at a very high rate; you’ll just be doing it for longer. All in all, you may not need that many more carbohydrates relative to your baseline 40% guideline. It takes a pretty large quantity of endurance exercise to dramatically impact your required carbohydrate intake.

In between these two extremes of strength-focused and endurance-focused sports are… pretty much every other activity. Field sports such as rugby, football (both American and The Rest of The World versions), hockey (field and otherwise), swimming, CrossFit, running/walking in your local 5k or 10k race, and many, many other disciplines all involve a balance of strength and endurance, and this means balancing your nutrition intake as well. If your sport leans towards moving for longer periods, you’ll likely want less muscle mass, and thus relatively less protein. If your move in short, intense bursts, you’ll want more. This can happen even within a single sport based on positions played (e.g., rugby 7s vs. 15s teams, or football tight ends vs. offensive line), and so on. Adjusting your nutrition plan to match the balance of strength and endurance in your athletic endeavor can take some tweaking, but you can use yourself as your own experimental subject. Mad science for athletes!

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Good news, everybody! You are your own mobile nutrition field laboratory!

Some things to keep in mind as you experiment with your nutrition plan:

1. Sleep. Not getting enough sleep jacks your body up and inhibits recovery. You should aim for between 8 and 10 hours of sleep per night, and if you don’t get that much you should expect to see your performance (and your progress towards your athletic goals) slow down, stall, or even reverse. Get your sleep!

2. Feeling “flat,” slow, or weak may mean you are not getting enough carbs. HOWEVER, it is much more likely that you aren’t getting enough sleep.  I mentioned it above, but I’ll say it again: 8-10 hours, athlete! Get in bed!

3. Feeling sore after doing an activity that you do regularly (vs. one that is new to you, which will certainly make you sore as you use your body in new ways) may mean you haven’t gotten enough protein. It could also mean you need to think about doing some post-activity stretching.

4. Getting enough water and staying hydrated will keep your body operating efficiently, speed recovery, and prevent cramping due to dehydration.

So there you go. You now have an athlete’s working knowledge of how your body stores and uses energy, and the beginnings of an understanding of how you can plan your nutrition to fit your goals. Go forth and do fun athletic things!



Zatsiorsky, Vladimir and Kraemer, William. Science and Practice of Strength Training, second edition. ISBN-13: 978-0736056281

Baechle, Thomas R. and Earle, Rodger W. Essentials of Strength Training and Conditioning, 3rd ed. ISBN-13: 978-0736058032

Kilgore, Lon; Hartman, David, and Lascek, Justin. Fit: An Unconventional Guide to Using Conventional Methods for Creating Fitness for the Real World. ISBN-13: 978-0615497068

On the web:

Stumptuous – possibly the best site on the web for beginners to weightlifting, with a ton of great resources for all athletes. Run by Krista Scott-Dixon.

Energy Systems in Sport and Exercise – a good source for more information about our bodies’ energy systems – a site that helps you understand the macronutrient breakdown of various foods

Dave Nix is a competitive masters-level (i.e., old) powerlifter and volunteer coach. He lifts and coaches out of his garage gym.

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