Sunday, November 21, 2010

Can Geeking Out on Energy Pathways Improve Your Performance and Sex Life?

This post discusses how our body uses energy during exercise, the primary ways it replaces energy in order to keep moving, and some of the dietary-, health-, and body composition-related implications of exercise at various intensities.
Estimated read time: 9 minutes + videos

If you have a background in exercise physiology, have been to a level one certification, or remember the CFJ article What is Fitness, then this discussion of energy pathways will probably look familiar to you and hopefully provide some further insights and topics for reflection.

If you don’t have any interest in physiology, then this may bring back nightmares of your biology or biochemistry classes.

The graph below shows the three primary energy pathways and ~ how much they contribute during activities of various intensities and duration.

Graph courtesy of What is Fitness

Before we get our elbows dirty, let’s preface with the agreement that we are almost never using exclusively one energy pathway. Rather, at any given time, we are using multiple energy pathways/fuel sources in various amounts dictated by the type, duration, and intensity of activity.

We’ll start with short duration high intensity and work out way to long duration, lower intensity.

The phosphagen (a.k.a. ATP-CP) energy system can produce the greatest power outputs, but it is depleted rapidly.


ATP (Adenosine Tri-Phosphate) is the energy currency of life. It is a molecule containing 3 phosphates, and the bond to each of those phosphates releases energy when it broken (when ATP releases energy, a phosphate is released and it becomes Adenosine Di-Phosphate – now containing 2 phosphates).


CP (Creatine-Phosphate) a.k.a PCr (Phosphacreatine) is another high energy molecule that can rapidly replenish ATP by donating it’s phosphate to ADP.

CP + ADP = ATP + Creatine

There is only enough ATP in the body to fuel a few seconds of activity, or to sprint ~ 15-20 yards. There Is about 3-5 times as much PCr stored in the body, and as such, the ATP-CP system [if running exclusively] could fuel about 10 seconds of activity or sprint just under 100 yards.

Note: the primary logic behind creatine supplementation is in that if we are able to store a greater amount of PCr in the body, then we could maintain maximal efforts for an extra few seconds (or perhaps an extra few pounds or an extra couple reps). Hence, studies have found creatine monohydrate to be useful to performance in short duration activity, resistance training, and power sports, but relatively useless to aerobic/endurance exercise.

Asleep yet? Here’s a video showing the ATP, CP system and how it’s replenished.

Also see:


Is that when we start using carbs and fats for energy? Yes, and, No. We will not use carbohydrates or fats directly to fuel exercise. Rather, we will process macronutrients (carbohydrates, lipids, and to a MUCH LESSER extent, amino acids) in order to make more ATP. We will focus on the carbohydrates and fats for now as they are our primary fuels. The processes in which we can convert carbs and fats into ATP are the anaerobic glycolytic ( a.k.a. lactic acid ) energy pathway and the oxidative (a.k.a. aerobic) energy pathway. Why so many names for the same thing? "Why is the sky blue? Why is the grass green? How does posi-trak on a Plymouth work? Some things we'll never know." – Joe Dirt. (note: all those things are actually well understood. I just don’t feel like it right now)

The Glycolytic energy pathway can refuel ATP pretty fast. Not as fast as PCr system, but still pretty darn fast and it has the added benefit of a larger substrate pool to draw from--blood glucose and glycogen (stored glucose in muscles or liver).  There are a few down sides of this system though. One, the system works faster than our ability to provide oxygen, hence anaerobic (without air), and results in a build up of lactic acid (hence lactic acid pathway). Two, our cardiorespiratory system’s ability to remove CO2 from the tissues (via blood circulation) and replace it with oxygen cannot match the rate at which we are producing CO2 and other byproducts during high intensity exercise--this is among the reasons that we eventually need to slow down or stop. You simply cannot run a marathon at the same rate that you’d run a 200m sprint. And of course, that is why you would be gasping for air after an all out sprint, yet might walk away from a light jog without breaking a sweat. Third, it appears only carbohydrate (more specifically, glucose) can be “burned” anaerobically. An implication of this is that the greater the time we spend fueled in the anaerobic energy system, the greater our need for carbohydrate.

Aside … I’ve mentioned before (here) that we can make carbohydrates from amino acids and the glycerol backbone of triglycerides (fats)—a process called gluconeogenesis. Additionally, we can recycle lactic acid to make glucose via a process called the cori cycle.  If one eats a diet sufficient in protein and energy and VERY carefully moderates their time doing anaerobic/high intensity work, then one could survive and even perform somewhat well on a zero or very low carbohydrate diet.  However, as our time spent in high intensity work increased, the need for dietary carbohydrate becomes very real. Enter sweet potatoes, yams, fruit, coconut water … choose your own adventure.
Very short, high power boosts are fueled primarily by ATP and PCr; as high intensity activity is prolonged,  the anaerobic breakdown of glucose becomes the primary fuel source; the longer the duration of the effort and/or the lower the intensity of the effort, the greater the contribution from aerobic metabolism of carbohydrate and fat.

When exercise intensity is low and/or exercise duration is long, the oxidative/aerobic pathway becomes  our primary source of ATP.  We can use both glucose and fatty acids aerobically--fats provide 70-90% of the energy during rest and normal activities.  As we increase intensity, carbohydrate contributes increasingly more to our efforts.  We have a much greater fuel supply to draw from aerobically (a hypothetical athletic male with full muscle and liver glycogen stores and an athletic level of ‘leaness’ would have enough glycogen to travel 15-20 miles and enough energy stored in bodyfat to travel ~800 miles).  During aerobic metabolism, we are working at a rate equal to or less than our rate of oxygen availability and we are better able to keep on top of H+ and CO2 production.  Hence, we have far greater endurance while working in the oxidative pathways.  The downside?  Our power output takes a hit in exchange for the increase in time to exhaustion.
Data from Williams, MH. Nutrition for Health, Fitness, & Sport. 9th edition. New York, NY. McGraw-Hill. 2010. Based on running at an energy cost of 100 calories on average. 

So if I burn primarily fat during rest and low intensity activity, should I quit “working out” and walk or play X-Box in order to lose weight?

If you engage exclusively in low-intensity "exercise" and follow a diet lower or moderate in carbohydrate with a reasonable calorie level, then you will burn mostly fat for energy throughout the day, and will likely have some success in developing leaness.  However, your “fitness” will degrade to match the demands of your work output.

Conversely, while high intensity work uses a lower PROPORTION of fat, the total energy “burnt” is higher.  Additionally, high intensity activities increase our metabolic rate in the hours (or days) that follow the training session.  So while we “recover”, we’ll burn a few extra calories per hour of couch time.  Finally, high intensity work has greater potential to improve our “fitness”—you get faster, stronger, and generally more useful by progressively working harder in various activities.  High intensity workouts do fantastic things for strength, conditioning, and readiness when they are strategically chosen and not taken to exceedingly long durations with high frequency.


The New York Times has published a few attention-getting articles over the years suggesting that exercise is not effective for weight loss: The Scientist and the Stairmaster, Phys Ed: Why Doesn’t Exercise Lead to Weight Loss, and Weighing the Evidence on Exercise.  In short, when the average person begins to "exercise", they begin a mileage-, time-, or heart rate-based routine on one of the various hamster wheel contraptions in the air conditioned cardio room at the gym.  Prolonging time spent in the moderate-to-high intensity realm increasingly depletes glycogen, resulting in increased hunger--especially for carbohydrate.  If you choose to "replenish" with sports drinks, bagels, smoothies and GUs ... enter the insulin roller coaster ... and glycogen won't be all that you are "storing".  Without consciously controlling the diet, most often exercisers will compensate for their increased energy expenditure by eating more and will not lose weight or may even gain weight.  Excessive moderate/high intensity exercise can also lead to increased cortisol (blood-glucose-raising, catabolic stress hormone) and various other hormonal variations which can impair body composition efforts (or worse).  You’ll commonly see group exercise instructors who are carrying around a few extra pounds (usually located in the mid-section) despite hours of exercise each day.

A good approach for general wellness may simply include a smart diet, and an exercise regimen perhaps akin to the recommendations of Mark Sisson or Art Devany.  Accumulating a good amount of movement at very low intensity (primary fat metabolism) with occasional very short, very high intensity workouts to improve speed, strength, and boost resting metabolic rate.  Which very strangely mirrors anthropological assumptions of our ancestral movement patterns.  Lots of walking (forage, gather, migrate, play) and occasional all out efforts (hunt down prey, escape a predator).  Don't forgot sleep.  To push fitness to higher or elite levels, a relatively greater intensity and/or frequency will be required; and with that will come a greater need for nutrition focused on the fuel demands of your type, duration, and frequency of exercise.  Use your smart.


Fine. That’s probably plenty for today. Let's summarize with this archaic, yet useful video reviewing some of what we touched on today:

There’s several more things I want to take a look at with this thread, so keep an eye out for updates coming soon.

A few things to think about in until then ...

How can we best prioritize different fuel systems?

Is there a benefit to prioritizing or avoiding fuel systems? (To performance? Body Composition? Health?)

Will “pacing” strategies during workouts (or specific work:rest intervals) emphasize different fuel systems or recovery rates?

How should I eat before, during, or after my workouts based on the energy pathways and types of fuels that are used?

Can we balance an exercise program to keeping insulin, cortisol, and other hormones in check, obtain a desirable body composition, maintain functional fitness into our later years, and avoid health and disease?  How can we push work output to greater levels without negative health consequences?

Finally, one of the Crossfit definitions of fitness is: "he who is fittest is he who is best blended in all 3 energy pathways".  e.g. High work capacity over broad time and modal domains ... What does that mean to you and how would we use our knowledge of energy systems to best accomplish this blended fitness?


  1. Great post! What I'm also interested in though is this: a) How does the brain differ from the rest of the body in using energy, leading to b) What are the implications for increased performance? I was in the hospital recently, and my nurse said that during a conversation that only the brain can store and use carbohydrates without insulin present. Kinda blew me away. I'll look into this and see what I can dig up. Reason I'm interested is, I have a recreational interest in intermittent fasting and how it promotes gluconeogenesis. =) Happy Thanksgiving!

  2. Hey Tom,
    Thanks for the post.
    With the gluconeogenesis and IF. A lot would depend on how saturated your glycogen stores are during the fast. And also whether or not you exercise or have some other stressors during the fast and how intense.
    Hit me up again (here or on FB) with some more specifics of what you're looking for and hopefully I can clarify.


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