In the lead-up to the The North Face 50 Mile Championships in December, I was looking for any “edge” that could help me race to my potential and to offset some of the advantages that a comparably talented runner had over me, like more ultra racing experience and a training cycle longer than 6 weeks. Hence why I extrapolated a goal time based on my training, why I examined pacing data from past TNF50 races, and why I gave serious consideration to my clothing, footwear, and equipment selections.
Naturally, I also sought to find the optimal race day nutrition plan. Conventional wisdom for endurance sports lasting 2+ hours advocates consuming 240 calories per hour, mostly or exclusively from carbohydrates. This equates to one gram per minute, with each gram containing 4 calories. I saw this figure again and again: Ultrarunning Magazine, Runners World, Outside Online, elite ultra runners, and legends of the sport like Marshall Ulrich and Scott Jurek.
Shortly after I’d placed an order of Clif Shot Bloks and Clif Shot Gels — carbohydrate bombs that I expected to need during long training runs at a rate of, that’s right, 240 calories per hour — I received an email from Melissa Lott, a Minnesota-based kinesiologist who took a Backpacking Fundamentals course with me last year and who read my TNF50 training plan in which I’d cited this conventional wisdom.
Missy’s contention was not with the 240 calories/hour recommendation, but rather with the high-carbohydrate low-fat (HCLF) diet that would mandate this level of caloric consumption to sustain my energy level throughout the race, i.e. to avoid “bonking” or “hitting the wall.” The next day I received from her, Metabolic Efficiency Training, by Bob Seebohar. For an in-depth explanation of metabolic efficiency, read this earlier post. Basically, Seebohar argues that endurance athletes can train their bodies to rely more on fat stores for energy (rather than carbohydrates) through deliberate diet and exercise, reducing the need to ingest food calories during training and races.
Seebohar is not the only person making this case. See Dr. Jeff Volek and Dr. Stephen Phinney, Dr. Tim Noakes, Dr. Philip Maffetone, and personal trainer Ben Greenfield. Several ultra runners adhere or have tried to adhere to their suggested high-fat, low-carb (HFLC) diet. See Timothy Olson, Matthew Laye, and — my favorite — Jonathan Savage.
Generally, I’m skeptical of diets, e.g. Paleo, vegan, low carb, gluten-free, etc. Many sound too good to be true; benefits are anecdotal, individual-dependent, and/or “proven” by biased researchers; and for every diet out there, there is another that advocates the exact opposite. Besides, I’ve never followed a diet and my results have been pretty good over the years.
But I was intrigued by Seebohar’s argument. His scientific explanation seemed rational, and he had extensive lab data to support it. But perhaps more convincing, my experience in training sessions was very consistent with his predicted outcome for fat-adapted athletes:
- I found that I could sustain my energy level on just 100-150 calories per hour, or 40-60 percent of the recommended amount.
- I was having no GI distress and I was carrying astonishingly few calories for 25-35 mile training runs. And,
- I had become much leaner than when my diet had been more carb-heavy.
Conveniently, Seebohar is a sports dietitian for eNRG Performance in Denver. I soon scheduled a metabolic efficiency test with his colleague, Dina Griffin, to learn exactly the state of my fat adaptions.
A metabolic efficiency test requires two pieces of equipment: a treadmill (or bike trainer) and a metabolic cart. The cart 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.
Before the test I purchased a heart rate monitor for my Suunto Ambit 2 watch, which I’d been wanting to do anyway. It’s not necessary for the test but it’s very beneficial since heart rate is a more reliable measure of effort than pace, which is notably impacted by current fitness, cumulative fatigue, vertical gain and loss, altitude, and pack weight.
Before I stepped onto the treadmill, Dina and I first discussed what I hoped to learn. With that understanding, we devised a test that would generated the needed data. We ran two tests:
1. Hiking test. I kept a steady walking pace of 3.o MPH, and every 4 minutes we increased the incline by 5 percent. We started at a 0.0 percent incline and increased to 5.0, 10.0 and finally 15.0. The test was 16 minutes long. I was not wearing a backpack.
2. Running test. I started at a pace of 5.5 MPH (a 10:54 minutes/mile pace), and every 4 minutes my pace increased by 0.5 MPH, with the last interval at 9.5 MPH (a 6:19 minutes/mile pace). We kept the incline at 1.0 percent. The test was 36 minutes long.
To reset the measurements between tests, I recovered by walking at 3.5 MPH for 4 minutes at a 1.0 percent grade.
Before sharing the test results, there is some important context I wish to share:
- The test was done in a fasting state. It started at 8 AM, and I had not ingested any calories since dinner the night before. Also, no morning coffee.
- I was as fit as I’d been in years, after backpacking all summer and running intensely all fall.
- Even before learning about “metabolic efficiency,” my diet and exercise was consistent with metabolically efficient athletes. Amanda and I generally avoid processed goods and refined grains; all summer I had been running in the morning (on an empty stomach) to avoid the heat; and for 75+ days last year I was out backpacking, an activity that is precisely in the fat-burning cardio zone.
Hiking Test Results
At a normal hiking speed and on common gradients, I overwhelmingly burn fat stores for energy. Even while keeping a 3 MPH pace on a 15.0 percent grade — equivalent to climbing just over 4,000 vertical feet in an hour — three-quarters of my caloric expenditure was from fat. At that effort, I could hike for almost 12 hours before depleting the 2,000 calories of glycogen (carbs) stored in my muscles and liver, at which point I would finally need to start ingesting carbs in order to avoid bonking.
I don’t have a clear explanation for why the fat/carb differential continued to narrow after 16 minutes even though the last interval (3.5 MPH at 1% incline) was less taxing than the two before it, as evidenced by my heart rate and perceived effort rating. Perhaps it was an emotional response to the breathing tube, or perhaps my body had not properly warmed up yet.
Dina sent my test results in an 18-page PDF full of tables, charts, and graphs. I won’t share them all, but I thought this one was worth sharing too:
Notice the dramatic increases in my caloric burn (“oxidation”) as the incline increases. At a 15 percent grade, I burn 3x as many calories versus level terrain, almost 2x as many calories versus a 5% grade, and 30% more calories versus a 10% grade. This explains why I notice an uptick in my appetite on days with above-average vertical gain and loss — I’m working much harder to carry myself and my backpack up those climbs.
Running Test Results
Finally at 6:44 minute/mile pace and at a heart rate of 158 bpm, my body reach its “crossover point” — it began relying more on carbohydrates than body fat for energy. At slower paces, it relied predominantly on fat.
For context, my average heart rate during TNF50 was 142 bpm. At that effort, two-thirds of my energy is from fat. This explains why I did not bonk despite only consuming 100-150 calories per hour during the race and avoiding food entirely for the first 90 minutes. According to the data, I could actually have run for over 6 hours before depleting my glycogen stores and needing additional carbohydrates from food. At 142 bpm, I burn 318 carbohydrate calories per hour.
To further improve my metabolic efficiency — essentially, to push the crossover point to the right, perhaps even off the chart — I can work on my diet and/or my training. The chart below shows my optimal training zones to become more fat-adapted. My normal training pace is a bit faster — I’m usually mid-140’s bpm and 7:00-7:30 min/mile pace.
Implications of test results on hiking and running
The test results give me a baseline, inform my training and diet decisions, and/or validate my anecdotal experiences and past decisions.
On future backpacking trips, I plan to start carrying less food weight overall by carrying more high-fat foods. Fat is 2.4x as calorically dense as carbohydrates (240 cal/oz versus 100 cal/oz) and a high-fat diet will weigh less than a high-carb diet with the same number of calories. Importantly, my body does not need many carbs to sustain my energy while hiking since it relies primarily on fat stores at normal hiking paces and gradients. A backpacker who is less fat-adapted may have less success with this approach. Also, there is a limit to high-fat: I draw the line when I feel like I’m eating couscous olive oil soup, and I never will snack on sticks of butter.
For ultra running, the test results simply give me confidence. Confidence that I can legitimately ingest fewer calories during training sessions and races than conventional wisdom suggests I need. This reduces my risk of GI distress, which can can cost many hours and places, and sometimes even a finishing buckle. It also allows me to carry less food weight on training sessions and/or self-supported races.