Metabolic Efficiency Testing: Explanation and motivations

Example of a runner’s metabolic efficiency at various intensity levels (pace). From “Metabolic Efficiency Training” by Bob Seebohar.

Tomorrow I am having tested my “metabolic efficiency” by Dina Griffin of eNRG Performance in Denver. In this post I will explain the concept and its relevance for backpackers and runners. In a second post, I’ll share my results.

What is Metabolic Efficiency?

Our bodies rely primarily on two sources of energy:

  1. Carbohydrates
  2. Fat

Carbohydrates are most quickly converted into energy. But because the body can only store about 2,000 calories of carbohydrates (in the form of glycogen), this energy source is depleted by 2-3 hours of moderate exercise. In contrast, fats are more slowly converted into energy. But the body has a near infinite store of them: tens of thousands of calories, even on a lean endurance athlete.

Metabolic efficiency is the measure of how well the body utilizes fat as an energy source. Its efficiency can be improved through exercise and nutrition, specifically by:

  • Exercising more at lower intensities, especially early in a training cycle; and,
  • Supporting stable blood sugars by eating more lean proteins, healthy fats, fruits, and vegetables instead of high carbohydrate foods.

If you have heard about the crazy exploits of some endurance athletes on minimal calories — like Kilian Jornet’s 12-hour FKT record on Mt. McKinley fueled by just 300 ml of energy gel (about 250 calories) — this is the explanation: their bodies are extremely efficient at burning body fat, negating the need for food calories during their efforts.

Why is metabolic efficiency important?

It’s in the interest of both backpackers and runners (and any athletes really, especially endurance athletes) to be better fat-burners:

A runner who can rely more on their fat stores for energy will:

  • Reduce their risk of GI distress, because they need not eat so much and not as early in an effort; and,
  • Carry less food weight during training sessions and races that are longer than 2-3 hours.

A backpacker who can rely more on their fat stores for energy will:

  • Pack fewer food calories per day, by instead utilizing their body fat for calories;
  • Need fewer carbs to maintain their energy level, allowing them to carry more food weight in the form of fat, which contains 240 percent more calories per weight than carbohydrates, thereby further reducing their overall food load.

I was turned onto the concept of metabolic efficiency by Melissa Lott, a Minnesota-based kinesiologist who took a Backpacking Fundamentals course with me earlier this year. Missy pointed me to Bob Seebohar, who has an excellent book on the topic, Metabolic Efficiency Training: Teaching your body to burn more fat (Second Edition). I’d highly recommend reading it — it was a game-changer for me. Bob and Gina work together at eNRG Performance in Denver.

How is metabolic efficiency tested?

To test my metabolic efficiency, Dina will use a metabolic cart, which measures the volume of oxygen that I inhale relative to the volume of carbon dioxide that I exhale. Because different amounts of oxygen are used in metabolizing fats and carbohydrates, the cart can determine my ratio of carb- to fat-burning.

My metabolic efficiency will change with intensity. Specifically, I should burn more fat relative to carbohydrates at lower intensities, and more carbohydrates to fat at higher intensities. The point at which I transition from predominantly burning fats to predominantly burning carbohydrates is my Crossover Point. The data makes for a nice chart like this:

Example of a runner's metabolic efficiency at various intensity levels (pace)

Example of a runner’s metabolic efficiency at various intensity levels (pace)

My efficiency can be paired with my pace and my heart rate, which are the two barometers that I use everyday when training. For instance, if we conclude that I burn more fat calories between 8- and 10-minute pace and between 130-145 beats per minute than any other pace or heart rate zone, then I know the effort I need to sustain during training to maximize my fat burning.

By training in this zone, I will help my body make adaptations to improve its fat-burning efficiency. By training more intensely than this zone, my body will instead learn to rely on carbohydrates as a fuel source. By training less intensely than this zone, my body’s fat-burning adaptations will remain less than their potential.

Of course, my optimal fat-burning pace and heart rate zones will change day to day due to vertical gain/loss, footing quality, muscle fatigue, altitude, pack weight, weather and some other factors. But knowing my base base statistics is really valuable.

It might be easier to understand the test by watching the video below.

I’ll write again later this week with my results.

Posted in on November 30, 2014


  1. Val on November 30, 2014 at 8:04 pm

    Coming from a background in Exercise Physiology and Nutrition I am fascinated by the increased in the metabolic testing for endurance athletes. It used to be so expensive that it was really only accessible to elite Olympic athletes.

    One fair warning that we often saw metabolic efficiency changes on a day to day basis in most people based on every factor you can imagine. We encouraged a lot of athletes to eventually learn to perceived the change from one fuel source to another based on their perceived work rather than actual physical measures such as heart rate or pace which can fluctuate much more frequently.

    Good luck with the new knowledge 🙂

  2. Wes on November 30, 2014 at 9:34 pm

    The Fuel4mance test was well worth it for me. I took the test in April 2013 and received objective stats on how few carbs I was using while running at different paces. I haven’t used a gel since that day and I have had zero GI issues during races now that I don’t overload my gut with unneeded fuel. At this point, I have saved hundreds of dollars from cutting back on using carb products in training and races. It would also be nice to go back to Alaska with you and carry a lot less food weight on those brutal hikes!

    • Andrew Skurka on November 30, 2014 at 9:56 pm

      If not gels or other carb-y foods, what are you eating? I’d assume you didn’t finish UTMB without eating some food along the way. Also, did you change your diet, or are you just a natural fat burner?

      Indeed, bushwhacking through alder would be a lot easier with a heavy pack.

      • Wes on December 1, 2014 at 6:44 am

        At UTMB, I had 2 servings of UCAN, cheese, salami, dark chocolate, chicken broth and MCT oil. At the end, I was falling asleep, so I did drink some Pepsi to get some caffeine. I’ll send you my exact nutrition data from Wasatch 100 – there I took in 3,000 calories over 27 hours with about 1200 calories coming from carbohydrates.
        I know I used to be a carb burner because I used to bonk on long runs if I ran out of gels. After switching to a low carb diet, I no longer bonk (ever). I think diet is by far the most influential variable in becoming more fat adapted. You can do fasted workouts and/or run in specific fat burning zones, but diet is where you can make the biggest impact.

  3. Kenneth Posner on December 1, 2014 at 6:07 am

    Hi Andrew
    I’m a big believer in burning fat, and interested to see how this test comes out for you.

    However, one question for you to consider as you review the results of your metabolic efficiency test: just because you’re burning a certain ratio of fats/carbs at certain paces, why does that mean that’s an ideal training zone?

    For one, the ratios are interesting, but if you multiplied them by the calories burned, you might find that you are burning higher amounts of fat at faster paces.

    Second, by burning up your carb stores at high intensity, you may be stressing the body to amp up fat burning capabilities.

    Third, the body can apparently convert fat into carbs. Therefore, looking at this ratio may not be shedding much light on the underlying processes, which might be more complicated than simply carbs vs. fat.

    Personally, I like to mix it up with long training runs, sometimes all day, with no calories at all. But I still do high-intensity workouts several days a week to improve strength and aerobic capacity.

    Will be following your experiences with interest!

  4. Missy on December 1, 2014 at 3:13 pm

    Well written explanation! I am beyond eager to hear your results, and excited for you to gain even more insight on the topic! I truly hope the results will assist you in your future athletic endeavors!

  5. Alan Dixon on December 1, 2014 at 6:52 pm

    Andrew, there is certainly compelling evidence to train your body to favor burning fats vs. carbs for long races, e.g. ultra runs 50+ miles or ironman triathlons, etc. That is, he who can whisper the loudest wins—he who can go the fastest while still burning fats will do very well. I have no doubt with your all your high mileage training that you will have an extremely high fat burning metabolism and do quite well in your next race.

    I have had my metabolic efficiency tested. I have been on a Paleo diet (very low carbs) for about five years, and do at least one long slow run, and one very long slow bike each week as part of my Triathlon training. It has definitely helped with my Triathlon performance even at Olympic distances (I’m pretty consistent getting on podium). But, some of us just aren’t genetically disposed to burn fat as well as others even with diet and training (that would be me). Can’t get blood out of a turnip. So even with a low carb diet and scientifically coached training to increase my metabolic efficiency, I do not burn enough fat to be competitive and ironman distance. Such is life.

    But I am wondering about the utility of fat based training for activities that are pretty much non-aerobic like backpacking. Seems like even without any fat based training, most people are inherently efficient fat burning at a walking pace. I use a high fat, moderate carb diet for backpacking [in terms of food weight and nutrition it is not practical to have very low carb diet] and have been very successful to put in back-to-back, long, hard days.

    But regardless of training, a low carb diet with lots of fruits and veggies, lean healthy meats, and good fats makes sense for pretty much everybody.

  6. Dave on December 2, 2014 at 1:08 am

    Not an advocate of any kind of diet or lifestyle as all camps have valid scientific principles behind their theories, but this came to mind while reading about carbohydrate versus fats.

    Hope some of these will explain when fat is preferred, and when carbohydrate is required.

  7. Stephen on December 2, 2014 at 6:46 pm

    One thing to keep in mind is that the brain really prefers glucose as an energy source. Glucose comes only from glycogen, external carbohydrates or conversion of protein to glucose. Fat can not be converted to glucose, but it is a very efficient energy source in the presence of sufficient oxygen. If carbohydrate intake is too low, muscle will break down to meet the basic needs for glucose. BTW, this minimal amount of carbs is probably on the order of 100-150gm/d or 400-600 calories.

    • Dave on December 4, 2014 at 4:59 pm

      That’s odd.

      There have been people who have been on 20 g of carb per day for years and don’t suffer from muscle-loss. But again, one of the primary sources for glucose is protein, so they could be converting protein into glucose; whereas with fat, it is a bit more difficult to undergo gluconeogenesis.

      That being said, a lot of professional lean, active athletes try to target 100 to 150 g per day in order to maintain weight without losing or gaining anything.

  8. John Shannon on December 19, 2014 at 8:12 pm

    The results were never shared. This test is performed in a fasting state (from what I have seen on the web) and results are being extrapolated to fuel burning in a fed state. That is deceptive and probably not accurate no matter what mumbo jumbo they are “feeding” you.

    • Andrew Skurka on December 20, 2014 at 12:09 pm

      Working on the post with my results. Been dealing with some end-of-year stuff recently, plus the race.

      Perhaps Dina could chime in on why the test is done in a fasting state — it’s a valid question.

    • Dina on December 21, 2014 at 8:14 pm

      The protocol we used for Andrew did require him to be in a fasted state so that we could get an accurate assessment of his metabolic status. This allows for a clear “picture” of his substrate oxidation rates without the influence of food.

      eNRG Performance does offer a second type of Metabolic Efficiency assessment whereby the athlete consumes a usual pre-race snack or meal 2-3 hours prior. The test results show the influence of the consumed food upon substrate oxidation during exercise. Both tests have different protocols and different outcome measures.

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