The Science of Low Energy Availability and Performance
This article originally appeared on Trail Runner
I will always remember going to my first college track meet when I was 18 years old. I wasn't racing. If I had asked for an "unattached" entry, my 5K PR at the time would have made the meet director laugh in my face. I was just getting into running after quitting football, and seeing those athletes light the track on fire, I was absolutely blown away.
They were so damn fast. The "D" heats of every race were still some of the fastest athletes I had ever seen. Each of the distance events was won in times that felt unthinkable. Looking back, the feeling that sticks with me most is an awestruck ambivalence. "Those are some superheroes!" I thought, followed by a heaping dose of cognitive dissonance. "I'm not sure how I feel about running right now. I am so damn behind."
I realize now that a different feeling characterized my perspective, along with way too many of the people in that stadium: naivety. I had no idea what could be possible long-term, because my brain wasn't equipped to think about where my body could take me in the decades ahead. Many of the athletes were taking shortcuts, knowingly or unknowingly (and probably naive either way), which would derail their careers before they even really started. And based on recent stories, many of the coaches probably turned the other way, feigning ignorance (or worse) as some athletes undercut their futures at the dinner table.
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Training (And Fueling) For The Long Haul
My guess is that only a few percent of the athletes at those meets are still running competitively, with many of them having their athletic development undercut by underfueling. I chugged along, eventually becoming a pro runner, thankful that my 5K PR wasn't fast enough back then to face those pressures in a team setting when I wasn't ready to handle them. Some of the coaches are probably still there, letting cultures of underfueling persist, destabilizing careers for a few meaningless seconds when there are minutes left to pursue.
Yes, this is another article about the perils and complexities of underfueling (see here, here, here, and here for write-ups of past research areas). I write about this topic all the time because the damn myths persist, with younger and older athletes alike thinking they may need to restrict food intake to perform at the top level. The opposite is true. To paraphrase Lauren Fleshman: Grown-ass athletes do grown-ass shit on the track, roads, and trails. And if you want to see your true potential, you better be ready to do some grown-ass training.
Fleshman's incredible book "Good For A Girl" outlines how this process can unfold for young athletes, and that book inspires the question that I want everyone reading this article to ask themselves: "What approach to fueling will let me do the grown-ass shit required to see what's truly possible?" Training for long-term growth is not easy or poetic, as much as it can seem that way in the pages of a magazine or in a carefully edited YouTube video. Grown-ass training can be brutal, feeling like it's ripping bones and viscera to shreds before doing it all again a few days later.
Think about running down a steep 20% grade, every muscle fiber in your quadriceps writhing with each footfall. That's the memory I want athletes to take to the buffet. Because when you're doing grown-ass training, whether it's bombing a downhill or hammering a track workout, the body is going to snap in half if it's held together by a calorie-controlled salad. And those greens might be "healthy," in some ways, but they aren't going to be what powers adaptation that makes the body better the next time.
I'm sorry, that got a bit out of hand for an intro. Every time I talk about this topic, I get so frustrated thinking about why the culture of underfueling persists in some areas of the sport, whether that be college teams or comment sections. In my most charitable moments, I imagine that's because some athletes and coaches assume that this is all just a bit touchy-feely, hiding the truth of what athletes actually do to appeal to the masses. I promise: fueling enough is a universal requirement for long-term adaptation.
Athletes can do lots of things for a high school or college PR. But all grown-ass champions earn it at the dinner table.
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New Study Overview
Let's get to the point of the article! A fantastic new study in the Scandinavian Journal of Medicine & Science in Sports from a research team led by Dr. Anna Melin outlines the impacts of underfueling on sports performance, along with some of the nuances. This study is looking at performance impacts, rather than health impacts (that will eventually undercut performance), so it introduces a layer of complexity that I think is key to understand for an evidence-based fight against cultures of underfueling. I love this study because it highlights the complexities of a topic that is not as simple as throwing cheeseburgers at every problem, which is my preferred approach. If it were that simple, we wouldn't have an epidemic of underfueling affecting the long-term health of endurance athletes.
How did that epidemic even start, knowing what we know about low energy availability and long-term health risks? I think it's because we have problems of non-uniform time horizons and initial conditions–everyone is starting from different places, and all physiology acts on different timelines, so we end up with a hodge-podge of conflicting anecdotes that can really screw with people's minds (and bodies). Meanwhile, studies can have trouble proving what is causation versus what is correlation, which today's study points out elegantly.
Nutrition recommendations are so complex because everyone's physiology is different. Some studies find that just a few days of low energy availability (particularly when accompanied by carbohydrate restriction) can cause negative hormonal and metabolic responses (2022 review), and other studies find that even within-day deficits can lead to these negative outcomes, but those findings do not universally apply to each individual athlete in a quantitative way, and they might not affect performance right away (performance may actually improve in some cases). Some athletes can buffer short-term low energy availability more readily than others, and that can even change for a single athlete over time, as repeated exposure to underfueling can make the body less resilient to its negative effects
Low energy availability can be unintentional and not associated with disordered eating, especially with the high caloric requirements of hard training. But as low energy availability becomes more chronic, it can cause Relative Energy Deficiency in Sport (RED-S), which a 2014 IOC Consensus Statement in the British Journal of Sports Medicine defines as "impaired physiological function including, but not limited to, metabolic rate, menstrual function, bone health, immunity, protein synthesis, cardiovascular health."
That sounds pretty bad, right? If you've never seen an athlete go through RED-S, it can be even worse than you might guess. Even if they're able to keep training, everything can go to crap, from their performance to their bloodwork to their mental health, creating something that feels like the lovechild of a threesome between mono and depression and chronic illness. Often, the impacts can linger for years or even decades. I have seen 60-year old runners describe pangs of regret from the life-altering effects of chronic (sometimes unintentional) underfueling. This isn't just about running.
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Short-Term Impacts
Short-term low energy availability can be anywhere from a few days to a few weeks. Low skeletal muscle glycogen can be associated with an inability to adapt to training interventions, along with increased fatigue. Some studies find that it takes just days to start seeing markers of reduced bone formation, increased inflammation, worsened immune function, and lower iron absorption, while other studies find neutral or even positive performance outcomes (we'll get to that later). A 2017 study in the Scandinavian Journal of Medicine & Science in Sports found that even when controlling for daily energy intake, within-day deficits in women can contribute to impaired menstrual function, lower estrogen levels, reduced metabolic rate, and higher cortisol levels. A 2018 study in the International Journal of Sport Nutrition and Exercise Metabolism found that excessive within-day deficits correlated with male athletes having suppressed resting metabolic rate, higher cortisol, and lower testosterone:cortisol ratios.
Before low energy availability becomes a health crisis, it seems to turn down the thermostat. That makes sense, evolutionarily. Periods of resource limitation were common, and as a species, we don't want to have to hunt down a mastodon and harvest a field of berries every day. But athletes doing grown-ass training are pushing their bodies in ways that do not have evolutionary precedent, and leaving that thermostat set to frugal-dad levels for too long can turn small (usually harmless) perturbations into full-blown health crises.
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Medium-Term Impacts
Medium-term low energy availability can be anywhere from a few weeks to a few months. This is the time horizon that I see athletes fall into most often. A few days here and there, and I might not notice at all, or there might just be some tired days and a bad workout. After the bad workout, the athlete will eat an entire pizza and often feel so much better. I know I've failed a hill workout, only to eat a triple-cheeseburger, waking up the next day ready to rock. Over weeks and months, though, the evolutionary defense mechanisms can start to crumble, and it may become impossible to ignore as the health consequences start to impact performance.
The review article found that some studies identified reduced strength and power relative to controls, along with blunted responses to training. However, this is where performance-related research can get really complicated and unsatisfying, from a purely scientific perspective. Because longer-term low energy availability is accompanied by negative health effects, there are ethical limitations that can prevent intentionally putting athletes in these states, so the research on performance often comes from places like weight-loss studies, which could be catching a lot of confounding variables.
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Long-Term Impacts
Long-term low energy availability goes from a few months to a few years. The authors point out that "investigating the effects of such long periods of LEA in a controlled setting is near impossible." As a result, most of our scientific knowledge stems from seeing the health outcomes of RED-S, and tracing it back to behaviors, like seeing the Colorado River at the bottom of the Grand Canyon and saying "This might sound crazy, but hear me out: I think I know what happened."
Most studies find reduced endurance performance, but it's also not so cut-and-dry as to say "eat enough OR ELSE." Some athletes may improve, temporarily, while others crash-and-burn almost instantly. We know the negative indirect effects, outlined by the authors in detail–impaired recovery and adaptation, lower iron and red blood cell levels, overreaching and overtraining, injury and illness, mood and cognitive impairments, and taking the joy out of training (and sometimes out of life).
But we don't know their time courses, their application to individual athletes, and how they evolve over time. Yes, we can see the Grand Canyon now and think: "Well, the Colorado River officially overdid it," much like all the horror stories that are a constant presence in the running world, featuring careers ended and lives damaged. But maybe there was a time when the changes amounted to something like a nice little beach next to a shady embankment, no big deal, no risk of a tourist falling 5000 feet to their death.
And that gets to the most unsatisfyingly thorny part of the review article. The authors put it succinctly: "It is noteworthy that despite an abundance of data demonstrating many indirect negative effects of severe LEA on performance (e.g., increased injury rates, poor training adaptation), there is still very limited research directly assessing the impact of LEA on direct performance outcomes and providing evidence for a causal, rather than correlative, linkage." As the authors outline, there is even some evidence that low energy availability can be associated with neutral or positive effects on performance in some cases.
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Possible Positive Impacts?
Let's stick one of those thorns in our side with another quote from the authors. "Moderate and well-planned LEA may have a positive effect in the function of some tissues (i.e., skeletal muscle, neurons), but if sustained it may reach a point where the normal function of many other tissues and systems (e.g., bone and iron metabolism) may be disrupted and the likelihood of injury increased, therefore potentially negatively affecting performance. This dynamic interplay is complex and as such difficult to predict a final outcome." That's especially relevant for athletes addressing body composition changes, where there can be a blurry line between healthy practices and self-destruction.
I think that uncertainty is why restrictive practices with food can be so damn dangerous and insidious. Some athletes, including many of the college athletes we talked about earlier, may actually have short-term success in lower-energy states. Maybe they even win a championship! So the contagion is released. And then it can spread, spurred on by coaches who look the other way, commenters who don't understand the science, and athletes who seek optimization but may not know the downsides until it's too late.
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Eat Enough, Always
I write about this topic so much because we need to stop this contagion in its tracks. Eating enough is what it takes to do the grown-ass work to achieve grown-ass outcomes, and the downside of playing around at the margins can be a 5000-foot physiological and psychological cliff. "Dieting and restrictive eating is furthermore often associated with an increased risk of developing disordered eating behavior and eating disorders," the authors say. "Therefore, we are of the opinion that the importance of leanness should be de-emphasized within the sports environments, especially in young and developing athletes."
"Hence, when considering … body composition changes, even if for short periods of time, careful planning with realistic goals is needed, and supervision from a sports dietitian is highly recommended."
Eat enough, always. "Enough" allows for nuance, including nuance related to body composition changes, but if you're trying to see where your potential lies, make sure you're working with a health professional who validates (and ideally guides) your goals.
Because there are some grown-ass goals on the horizon. So let's throw down in training and adapt with some grown-ass meals.
David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. With Megan Roche, M.D., he hosts the Some Work, All Play podcast on running (and other things), and they answer training questions in a bonus podcast and newsletter on their Patreon page starting at $5 a month.
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