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:
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:
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.