Early on, the two lab-coated technicians seemed concerned. Not that I was in a great position to observe them: I was running, albeit slowly, on a treadmill facing a wall. The silicon respirator strapped to my face was connected by a thick plastic tube to a box with a computer monitor on it and this kept me from turning my head more than a few degrees. And I had chosen “Happy Pop” from the internet playlist: some generic-sounding girl-bop was blasting through the small lab speakers on the wall.
Jenn, who had been monitoring my progress on the computer display, muttered something to Terence in an “are-you-seeing-what-I’m-seeing” kind of way and pointed at the screen. In the periphery of my vision, i could tell she was scowling. I strained to see the what was on the monitor but to no avail. Something was wrong and although I’d been running for only 10 or 12 minutes, I didn’t relish the idea of aborting and starting again. Terence disappeared out of my field of view for a moment then came back and said something reassuring. Jenn’s body language relaxed and as her tension vanished, so did mine. She resumed her encouraging remarks and the test carried on.
This was the second time I had taken a Metabolic Efficiency Test (MET), about a year after the first one, so I knew what I was getting myself into. The idea is that you get connected to a Metabolic Cart, an expensive piece of equipment that measures your respiration gases, and either run on a treadmill or ride a stationary bike. The cart measures the amount of carbon dioxide you produce relative to the amount of oxygen you consume and from this ratio, can determine your substrate utilization. In other words, it can tell you how much carbohydrate and fat you are burning for fuel while you exercise.
Generally speaking, the ratio of fat to carbohydrate that our bodies use varies depending on our level of activity: the more intense the activity, the lower the ratio. Lying on the couch might have us burning 100% fat while sprinting uphill might require 100% carb. The MET attempts to capture this ratio on the spectrum of activity from very low to near maximal. The test protocol then, requires you to perform your exercise of choice at gradually increasing workload until you are essentially maxed out.
The concept of Metabolic Efficiency was created by renowned sports dietician Bob Seebohar and is essentially a measure of how the body utilizes its on-board sources of energy. Specifically, though, its focus is on how well the body uses fat: the greater percentage of fat that one can utilize at any given effort level, the more carbohydrate is preserved. This has a marked impact on endurance activities as I’ll outline later, but it also has an overall effect on everyday life: more consistent energy levels and satiety throughout the day and, it is said, better cognitive function and sleep quality. Better fat-burning can also mean improved body composition and weight.
Unless my understanding of Metabolic Efficiency is off (which it may well be), the word “efficiency” here refers to the measure of the body’s ability to mobilize fat for fuel preferentially over carbohydrate. “Efficiency” in endurance exercise can refer to many things, so I think this is an important distinction. As a point of example, it is generally known that sugar is a more “efficient” form of fuel than fat during exercise (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958805/) in that it requires proportionately less oxygen to break down sugar into energy. So one could argue (and some do) that using primarily fat for fuel is less “efficient” than using sugar. Though there are some very vocal proponents of fat adaptation for endurance performance, there really hasn’t been any conclusive studies that have shown that a high-fat diet, low carbohydrate diet actually improves athletic performance – at best, it is shown to not be an impediment.
So why, then, become fat-adapted? There is much written about the benefits of fat adaptation and endurance activities. The most-mentioned advantages relate to the ability to use the massive on-board fuel storage that is contained in your body fat: even a very lean person may carry 50,000+ calories, or enough to theoretically run for days. Of course this is an over-simplification, as there are so many factors contributing to one’s ability to move. Regardless, though, there are few who participate in endurance sports that would argue that an improved ability to burn fat is a bad thing.
In theory, a fat-adapted athlete requires less food intake during endurance efforts than a carbohydrate dependent one. Less eating during exertion means having to carry less food and a reduced chance of gastric distress. Though it is true that pretty much anyone on any type of diet will use fat for fuel during long endurance efforts at sub-maximal efforts, through diet and training it is possible to vastly increase the percentage of fat that one uses at any given effort. You can also train your body to use fat as the primary fuel source at greater and greater intensities.
For me, one of the greatest benefits of fat adaptation is reflected in both my running and daily life – getting off the sugar train. Removal of sugar from my diet has evened out my energy level and temperament during the day: I know longer experience spikes and crashes, cravings or “hangriness.” Likewise, on long runs, my fluctuations in energy are very subtle – unlike the days when I relied on gels to keep me going – and bonking is never a factor. Some have used the analogy of a fireplace to describe the difference between using carbohydrate and fat as fuel. Carbohydrate is like kindling, where on the extreme you have sugar, which is more like paper or lighter fluid; fat is more like seasoned hard wood. Anyone who has tried to heat their house with a wood fire knows that if you only have paper and kindling, you have a fire that lights quickly, but you have to continually feed or ti will just go out. A few nice, seasoned logs might start slower, but can give you hours of heat.
From my previous MET, I had already seen the results of some of my diet and training modifications: I was already a “butter-burner” (You can see those results here. So I was interested to see if there was any further change, one more year in. Today, my Metabolic Efficiency Test ended when my heart rate hit 188. I went through a cool-down period and then got off the treadmill and attempted to stop sweating.
During the post-mortem, Terence revealed to me that the reason for his concern during the early parts of the test was that they thought the Met Cart might not be functioning properly – my carbohydrate utilization was not budging off of 0% through the first few stages. He had then referenced my previous test and realized that it was similar.
As he went through the test results with me, I was admittedly dumb-struck:
As you can see, my fat utilization was still at almost 80% at a heart rate of 175 bpm or 93% of my max heart rate and I did not hit the metabolic crossover point until 185 bpm or 98% of my max heart rate. Further, my Fat Max point, or the point at which I was burning the most overall fat (1.28 grams / minute), was at 175 bpm – again, well into training “Zone 5”. I believe this puts to rest the notion that it is not possible to fuel high intensity efforts on fat.
To put these heart rates into perspective, I use 135-145 as my MAF training zone and hard track intervals will possibly get me to 170 which would conceivably put me at about 85% fat utilization Though these results are quite remarkable and a real testament to what the body is capable of, given the right conditions, the real question is “So what?”
Like so many things that we try to explain with science, we are often left with more questions than answers. This is one of the limitations of scientific experiments – in order to be useful, they have to be extremely specific and because they are so specific, their scope of “truth” is very narrow. Ultimately a Metabolic Efficiency Test is only really good at telling you what your body will do during… a Metabolic Efficiency Test. We don’t really have enough information to predict what this means to performance in endurance activities and even if we did, there are so many confounding factors that we still might not be able to say anything conclusively. My opinion is that tests like these are highly individual, and are really just tools to help you to make good decisions. Everything else that you bring to it is, quite literally, up to you.
All this said, here is what I do know:
n=1 may not be “science” but it has real merit:
You can’t rely 100% on someone else’s scientific study. After reading a study critically and trying to identify biases, confounders and errors, you can possibly walk away with some tidbits of information that you can apply to your life. Everyone is so vastly different in so many ways that I believe there is literally no time not to experiment on yourself if you want to find out what works for you.
The bonk-proofing is real:
The difference between before and after my shift to LCHF from a more “typical” food pyramid way of eating has been significant. I have bonked a few times in my life so I know what it’s like and a well-managed LCHF diet has the potential to create the conditions where it NEVER happens.
Some combination of diet and training can turn you into a good, if not remarkable fat-burner:
In the months leading up to this MET, I ate a consistent LCHF diet of generally 100 grams or less of net carbs per day. I wasn’t militant about it at all, I didn’t count macronutrients or measure ketones and I trained almost exclusively at MAF heart rate. That said, I was consistent and never fell off the wagon.
There is a growing number of advocates and “n=1”s for the benefits of LCHF and fat adaptation. Here are just a couple, who happen to be extremely successful ultra-runners:
Zach Bitter: http://zachbitter.com/blog/
Jeff Browning: https://www.gobroncobilly.com/blog/
Also check out: