Intermittent Fasting – 80/20 Endurance

Among the benefits of traveling internationally is that it gives you a different perspective on your own country. For example, in 2015 I spent two weeks in Kenya conducting research for my book The Endurance Diet, and it was there that I came to fully appreciate how screwed up America’s relationship with food is.

As part of my research, I ate nothing but traditional Kenyan food at every meal, and I returned to the States two pounds later than I’d been when I left. The most fattening characteristic of any diet is how processed its constituent foods are, and the Kenyan diet is minimally processed. But that’s not the only reason I lost weight during my East African sojourn. The other reason is that there are no food commercials on Kenyan television. None. Not that I actually watched a lot of television while I was there, but the larger point is that food is not constantly shoved in people’s faces in Kenya as it is here. And it has an effect. By the end of my stay, I found myself thinking about food a lot less than I did back home.

I thought of this experience recently when, against my better judgment, I waded into an online debate about fasted workouts. My goal in doing so was to help the other parties recognize fasted workouts as the simple, straightforward practice they are, but this proved impossible, and I think it’s because America has a screwed-up relationship with food.

I was up against two separate factions in this debate, both of which viewed fasted workouts as extreme and radical, each in a different way. One faction regarded the practice of completing a workout on an empty stomach as a kind of torture—a gratuitous sufferfest ending in degrees of exhaustion never approached in regular workouts. This is absurd. A fit and well-nourished athlete who chooses to delay breakfast until after a workout is only minimally compromised by having their metabolic fuel tanks less than fully topped up. The science is rather abundant in this area. For example, in a 2012 study by Australian researchers, trained cyclists completed 60-minute time trials in fasted and non-fasted conditions, averaging 282 watts after a good breakfast and 273 watts on empty stomachs. That’s a 2.8 percent difference in performance.

In prolonged exercise bouts, the impact of skipping breakfast is greater but still far milder than some athletes seem to believe. A 1999 study by Tim Noakes and colleagues found that moderately trained subjects lasted an average of 136 minutes in a time-to-exhaustion cycling test at 70 percent of VO₂max in the fed state compared to 109 minutes in the fasted state. That’s an 18 percent difference. In practical terms, this means you should feel no more fatigued at the end of a 14-mile fasted training run than you would be at the end of a 16.5-mile post-breakfast run. If you factor in the additional effect of withholding calories during the run, which fasted workouts require, the difference becomes somewhat greater, but even then it’s nowhere close to extreme.

Why, then, do some athletes regard fasted workouts as extreme? I think it’s because we are overfed as a society. In America, unlike Kenya, not only are our television screens filled with (junk) food advertisements, but these advertisements are carefully designed to brainwash us into being disposed to overeat, attaching positive associations to words like “crave” and “stuffed” and negative associations to “hunger” (and I’m talking about normal appetite, not chronic malnourishment). Consider Snickers’ “You’re not you when you’re hungry” ad campaign, which encourages viewers to keep a 2-ounce calorie bomb in their pocket wherever they go so they can cram it down their gullet the moment they feel the slightest hint of want in their tummies. We all like to think we’re immune to such mind-meddling, and we’re all wrong, and the fact that a certain faction of athletes finds it unimaginable to occasionally delay breakfast until after they’ve completed a workout proves it.

Another thing that Kenya has a lot less of than America does is disordered eating. Not all pockets of American society have high rates of eating disorders, but endurance sport is one pocket that does. Research has shown that people who struggle with disordered eating tend to have a history of trying popular diets (e.g., keto) and dietary practices (e.g., intermittent fasting). This is not to say that such diets and practices cause eating disorders; rather, it’s evidence that individuals who are predisposed to disordered eating tend to be attracted by such things.

Fasted workouts fit this mold. They are the type of practice that certain athletes are drawn to for the wrong reasons. Although I don’t know of any real cases, I can easily imagine that some athletes have overused or misused fasted workouts and subsequently developed eating disorders. Because this risk exists, a certain faction within the endurance sports community believes that fasted workouts should not be promoted. But to me this is like banning automobiles because some people drive while impaired. Depriving all athletes of the opportunity to benefit from this practice is unfair to the majority of athletes who are at low risk of developing an eating disorder and it is also not a legitimate solution to the problem of disordered eating within the athletic community.

As an endurance coach and writer, I’m focused on promoting fasted workouts responsibly. Although not inherently risky, fasted workouts are an advanced method that only makes sense for athletes who are already doing the basic things that yield bigger improvements in fitness, which include training at high volume, maintaining high diet quality, obeying the 80/20 rule of intensity balance, and eating enough throughout the day every day. Fasted workouts are meant to help athletes who’ve already realized 99 percent of their genetic potential through these and other fundamentals squeeze out that last 1 percent. For everyone else—including all youth athletes—they can wait, and for those who have any kind of history of disordered eating or any reason to believe they might be at risk for it, they can wait forever.

By way of closing, I would just like to mention that in Kenya, for cultural rather than scientific reasons, most runners do their first and hardest run of the day before breakfast every single day. Someone needs to tell them how radical and extreme this practice is so they can stop doing it and finally get good at running!

Have you ever tried intermittent fasting, or considered trying it, as a way to improve your endurance performance? Then you’ll be interested in a new study that just came out of UC Davis. Led by nutritionist Ashley Tovar, it aimed to determine the effects of a 16/8 “time-restrictive feeding” (i.e., intermittent fasting) program on body composition and performance in runners.

Twenty-seven male runners between the ages of 21 and 36 participated in the experiment. Each subject completed four weeks of eating within an eight-hour window each day (16/8) as well as four weeks of eating on a normal 12/12 schedule, the order of these two schedules being randomized. The runners were instructed to eat the same types and amounts of food as normal on either schedule, so that only the timing differed.

Before and after each four-week period, the subjects underwent body composition testing and ran a 10K time trial. It was found that body fat decreased slightly, from 16.8 to 15.8 percent, on the time-restricted feeding program, while no change occurred on the normal eating schedule. Improvement in 10K times was about equal on both diets.

It’s easy to draw the wrong conclusion from these findings. In order to draw the right conclusion, we need to understand that it’s not easy to lose body fat and yet fail to improve running performance. That’s because shedding body fat boosts running economy. The fact that time-restrictive feeding failed to improve 10K performance more than normal eating despite triggering fat loss indicates that something about it counteracted the boost in running economy the runners got from getting leaner. In other words, intermittent fasting seems to have made these runners less fit at the same time it made them leaner.

The authors of the study speculated that reduced carbohydrate availability may have been the main factor neutralizing the expected performance-enhancing effect of fat loss on the time-restrictive feeding program, writing, “Extreme depletions of carbohydrate availability have been shown to limit catecholamine responses, suppressing the effect of epinephrine in inducing glycogenolysis and the formation of lactate. The hypothesis that this effect was demonstrated in this investigation may be further supported in that suppression of catecholamines as a result of a persistent lower carbohydrate availability may reduce oxidizable fuels and lead to decreases in VCO₂, as observed in this experiment.”

The authors went on to suggest that the self-sabotaging underfueling effect of intermittent fasting is likely to have even greater negative consequences on fitness and performance at race distances both longer and shorter than the 10K distance used in this study, explaining, “The indication that a [time-restrictive feeding] diet may lower lactate at higher intensities (90% VO_2peak) suggests that performance during longer duration events requires a greater total contribution of carbohydrate as a fuel. Therefore, a ≥21.1 km race, and shorter duration events requiring a higher reliance on glycolytic type IIa muscle fibers, such as a 5 km race, may be more affected by the 16/8 diet.”

If all this talk of glycogenolysis and catecholamines is a bit over your head, here’s an analogy: Practicing time-restrictive feeding is like racing with a carbon-plated super shoe on one foot and a hiking boot on the other. In this analogy, the super shoe is the equivalent of the fat-loss effect of intermittent fasting and the hiking boot stands for its glycogen-depleting effect. One makes you faster, while the other makes you slower, and the net result is that you’re neither faster nor slower.

If TRF were the only way to lose body fat, certain athletes might still want to consider it as a way to improve, and least in races of moderate duration. But there are in fact other ways for athletes to lose body fat that do not leave the body underfueled and thus actually improve endurance performance. Improving overall diet quality and practicing intuitive eating are two such methods, which, unlike time-restrictive feeding, are practiced widely by elite athletes to promote a lean body composition without robbing the muscles of precious fuel.

I look forward to the time when the intermittent fasting fad has run its course in endurance sports. In the meantime, as an individual athlete, you can use it to your advantage. While others jump on the time-restrictive feeding bandwagon for reasons they believe are rational but are in fact psychosocial, and are fooled into thinking the diet has benefited them because they’ve lost a bit of body fat when in fact this would-be benefit has been neutralized by a form of semi-starvation, you can do the rational thing and simply emulate the less gimmicky eating habits shared almost universally by the world’s best endurance athletes and get both leaner and faster.

There’s no evidence that P. T. Barnum actually said, “There’s a sucker born every minute.” But if he didn’t say it, he should have, because it’s true—there is a sucker born every minute.

I encounter suckers almost every day in the domain of endurance sports nutrition. Athletes come to me asking why they’ve lost not only weight but also fitness and performance on a ketogenic diet, what I think of intermittent fasting, which antioxidant supplement is best–suckers, all of them!

It’s tough to know whom to blame. On the one hand, I want to blame the suckers. After all, there was a time when I knew nothing about endurance nutrition, yet I never fell for any of the gimmicks being peddled to athletes in those days (we’re talking late 1990s). My instinct was always to take my cues from elite athletes and mainstream science, and both of these sources consistently led me to focus on maintaining a balanced, inclusive diet based on natural, unprocessed foods and to practice a few fine-tuning measures such as eating within an hour after completing big workouts.

Twenty years later, not much has changed. As I write these words, the whole world is talking about Eliud Kipchoge’s jaw-dropping marathon world record of 2:01:39. I can assure you that Kipchoge does not follow a ketogenic diet or practice intermittent fasting or take an antioxidant supplement.

Whenever I make this point, some clown counters that Eliud Kipchoge and his ilk are so genetically different from the rest of us and/or train so much more than the rest of us do that we can’t possibly use them as dietary role models. This is nonsense. The relatively few genes that distinguish elite talents from the masses have absolutely nothing to do with how food is digested and metabolized. And as for training, there is very solid evidence that athletes with average talent get the best results when they emulate elite training practices except at a different scale, so why shouldn’t the same be true of diet?

Anyway, a part of me wants to say that athletes should know better than to adopt diets and nutritional practices that are followed by cult-like athletic subcultures rather than world champions and supported by stories of biological plausibility rather than real science. On the other hand, I recognize that in our society athletes and nonathletes alike are systematically trained to reach for dietary gimmicks and magic bullets. So a lot of responsibility falls on the shoulders of those who try to make suckers out of us.

As one whose job is to help athletes perform better through better nutrition, I find it frustrating to know that if I wrote a book with a catchy shtick that either capitalized on or anticipated the next big fad, a book that made huge promises but was filled with bad information, hence sure to yield poor results for most people, I would make a lot more money than I would if I wrote a book that offered athletes solid, proven guidance on how best to eat for health, fitness, and performance. Most people don’t want the truth about diet—they want a miracle.

This is why I have mixed emotions concerning Marni Sumbal’s new book, Essential Sports Nutrition. On the one hand, I think it’s a good book—a credible, comprehensive primer on eating for fitness and performance. On the other hand, for this very reason, I fear that the endurance athlete market will, on the whole, pass it over in favor of The Keto Alternate-Day Starvation Breakthrough!

I knew that I was going to like Sumbal’s offering by Page 2, where I encountered the following paragraph:

Yet many athletes are misled to believe that there’s only one “right” way to eat. I often hear from my athletes that dairy is bad or that sugar is off-limits during competition season. Right now, the current sports nutrition trend is to restrict carbohydrate intake. I tell athletes that being mindful of what you eat is important, but adhering to only one set of sports nutrition principles is short-sighted. Applying a restrictive approach to sports nutrition often ignores long-term health and performance consequences—especially if the diet is seen as a “quick fix” to boost performance or change body composition. In this book, I take a more all-inclusive approach. I’ll give you practical nutrition strategies to help you enhance sports performance, fitness, and long-lasting health.

An on the very next page, this:

Eating should never cause anxiety, worry, or frustration.

Can I get an amen?

The book is divided into four sections. In the first, Sumbal provides a basic (for many, remedial) education on human nutrition. Part Two focuses on matters of nutrition timing, such as eating for post-exercise recovery. The next section comprises seven chapters aimed at special populations within the broader athlete community, such as children and those pursuing weight loss. Finally, Part Four presents recipes for pre-exercise, during exercise, post-exercise, and non-exercise days.

If you want to avoid being the next endurance athlete suckered into adopting inferior nutritional practices, or if you’re tired of being suckered, read Essential Sports Nutrition. Shtick sells, but if you want to get faster, you need to know what’s true and do what works.

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