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As a running coach, you get to see a lot of blood work. And sometimes it can just feel … off.
Many of the knobs seem to be turned in the wrong direction. Stress hormone cortisol might be high, vitamin D low, hemoglobin nowhere near where it could be, other numbers moving up or down seemingly at random. It’s often accompanied by fatigue and lack of adaptation over time. What’s going on?
The answer to that question can vary significantly. It might be stress, underlying health concerns or just natural variability. But one possible answer keeps coming up over and over. And it turned me into an evangelist, preaching the same phrase constantly:
Eat enough, always. Eat too much, sometimes. Eat too little, never.
After those wonky blood tests, a doctor or a blood-testing company like Inside Tracker might point out to the athlete the perils of low energy availability. I think that the physiological stakes of not eating enough are becoming more well-known, but it’s still so easy for many athletes to play if off. Runners don’t need to eat that much, right? Absolutely wrong. Eating enough is one of the most important parts of an athletic life.
What happens next can be a celebratory turning point. An athlete experiencing fatigue or injury or a wonky blood test will increase their food intake. Over time, fatigue and soreness go down. Adaptation goes up. The blood numbers stabilize. Health improves. And performance skyrockets.
What the heck is going on?
I have written about that question many times, highlighting the importance of always eating enough food, avoiding intentional fasted running (particularly for female athletes) and preventing excessive within-day deficits. But, today, I really want to focus on what is happening to the endocrine and metabolic systems when we don’t consume enough calories in the context of an athletic life.
A fantastic 2019 review article in the Current Opinion in Endocrine and Metabolic Research journal described how failing to eat enough food can cause a cascade of negative physiological effects. I think it’s important to use a running magazine platform to dive deeper on those physiological effects because it can be so easy to play them off in our communities.
A commenter might joke: “Yeah, right, like eating too little is a problem!” Maybe someone follows up: “My stomach insists I’m eating enough. ;)” Or most disheartening for me, an athlete might note in their training log how they didn’t really fuel after their long run—they were busy and didn’t want to get in the way of other plans. That last one is real, from just this week. And it broke my heart. It’s so key that we all know the severe consequences that can lay on the other side of underfueling.
Before getting to the physiological details of why eating enough is a high-stakes game, a quick reminder. It can be really, really hard to talk about eating, eating disorders, body image and all of that other stuff that goes into being a human that struggles with existence. I’m not going to get into the details of disordered eating in this article, but those struggles can be strikingly common in the running community. If you are having trouble, know you are not alone. Reach out to a nutritionist, therapist, friend (like me), anyone to start—just get that conversation started. We are in this together, and we got this.
Low Energy Availability
In 2014, the British Journal of Sports Medicine published the IOC Consensus Statement on the Female Athlete Triad and Relative Energy Deficiency in Sport (RED-S), defined as “impaired physiological function including, but not limited to, metabolic rate, menstrual function, bone health, immunity, protein synthesis, cardiovascular health caused by relative energy deficiency.” Low energy availability is when total energy intake doesn’t leave enough energy availability after considering energy expenditure during exercise (undereating or overexercising or both).
In other words, with great athletic power comes great eating responsibility. I’m trying to get Spiderman an endorsement contract with Chipotle.
“OK,” an athlete might say, “Big deal. I don’t need to worry about this—I mean, look at that pro athlete on the cover of the running magazine! I look nothing like that.” And so the fuse is lit.
As the 2019 review says, athletes of all levels face particular problems with low energy availability due to intense training programs, lack of nutrition education and, perhaps most heartbreakingly, “pressures associated with ‘ideal’ body type for performance.”
So they eat a bit too little for a bit too long. BOOM! The physiological bomb goes off.
The stakes are high for the endocrine and metabolic systems.
The 2019 review sets up a helpful framework to understand what’s happening as the gears of the physiological clock start to grind to a halt.
On the hypothalamic-pituitary-gonadal axis, energy is diverted away from reproductive functions. The body is trying to survive, and one of the first things to go is healthy sexual function. Gonadotropin-releasing hormone secretion is disrupted, along with potential variation in luteinizing hormone and follicle-stimulating hormone. For women, estrogen can go haywire, and there may be testosterone changes too, sometimes accompanied with amenorrhea. For men, testosterone can plummet in some cases. In one 2018 study on cyclists in BMJ Sport & Exercise Medicine, testosterone dropped to the very low end of the reference range in competitive athletes with lower energy availability. All of that may affect sex drive and sexual function, too.
The hypothalamic-pituitary-adrenal axis is next, related to stress. Stress-hormone cortisol goes up when the body is facing fueling-related imbalance, like it would evolutionarily when there is an imbalance between the optimal number of lions chasing you (0) and the actual number of lions chasing you (1+). A 2017 study in Medicine & Science in Sports & Exercise found that when controlling for other variables, elite athletes with menstrual cycle disturbances had higher cortisol and worse neuromuscular performance. Perhaps most notably, even controlling for overall caloric intake, men (2018 study) and women (2017 study) with higher amounts of within-day deficits had higher cortisol and worse health outcomes. Chronically elevated cortisol can mess with hormones, adaptation, and just about every other physiological function you can think of. The stress may even accelerate the athletic aging process.
On the hypothalamic-pituitary-thyroid axis, low energy availability leads to major disturbances too. The exact lines of causation vary, but as energy intake goes down, the thyroid attempts to compensate in ways that can throw off health, including athletic performance. The 2019 study also notes that low energy availability can alter growth hormone and insulin-growth-factor-1, which like the thyroid can have massive and unpredictable downstream impacts on overall health.
Put it all together, and the body can fundamentally alter baseline functions over time.
For example, the studies on within-day deficits found that failing to eat enough on shorter time scales led to suppressed resting metabolic rate. Other studies find reduced appetite and associated regulating hormones. That’s why appearance is a bad proxy for whether someone is struggling with low energy availability—when the fuel tank is low, the fire may burn more weakly. As the fire burns more weakly, body composition may actually worsen, and adaptation rates may plummet. Those adaptation rates obviously matter for an athlete aiming to chase their potential. But what’s extra scary is to consider what that means for overall health.
On the road or trails, an athlete with low energy availability may excel or they may not—genetic and background variability make it so that energy availability is still one of many variables that are inputs in performance. But even for the athletes whose anomalous genetics can temporarily weather the endocrine storm, there’s a chance that long-term health is being compromised with every under-fueled day. And based on the research, it’s likely that those athletes are going to have to pay the piper sooner rather than later. That’s why it’s so important to not look at snapshots that highlight what an athlete is doing right now, but moving pictures that show what happens over years of performance and health (both mental and physical).
Because this is not just about results.
The science unequivocally states that long-term results require eating enough food, but race-day outcomes are dust in the wind, even gold medals. This goes far beyond the finish line, to what happens to our bodies and brains.
Many studies find reduced bone health in men and women as energy availability goes down due to the connection between bone mineral density and the endocrine system. Cardiovascular health can be impaired as the heart has to adapt to low energy states. A 2020 review in Sports Medicine—Open showed that the immune system can suffer, all-cause injury risk can go up, and almost every physiological variable we can measure in a controlled study faces some risk. Depression and anxiety prevalence may rise too.
The exact timelines and individual responses are uncertain, though. And that’s a big reason why low energy availability can be so difficult to address.
Avoiding Low Energy Availability
For runners, time with low energy availability is like removing a block in Jenga. The problem is that each of us have a different Jenga tower, different sized blocks, different hand steadiness. Keep doing it, and the tower will fall eventually. The question is whether that’s tomorrow or 10 years from now. That uncertainty in quantification and individual response can create an environment where the extent of the problem is easier to downplay.
Perhaps a champion has low energy availability. Their background and genetics may create a context where they can remove blocks and keep standing, at least for now. But what I want to draw attention to is what people don’t always see—all of the scattered blocks of all of the athletes who weren’t outliers.
A 2020 review in the International Journal of Sport Nutrition and Exercise Metabolism highlighted the uncertainty of measuring, preventing and solving low energy availability. With endocrine impacts, for example, the low energy availability thresholds can depend on age, activity level, genetics, or pre-existing hormonal context. Energy availability equations can rely on self-reporting and can fail to account for all relevant variables. Macronutrient distribution matters, as do micronutrient profiles and overall life stresses. It can seem like a chalkboard in a math classroom, with variables spinning in every direction.
That’s why preventing and solving low energy availability is not about numbers as much as it’s about overall approach to training and life. To start, the review says, “Interventions … should focus more broadly on increasing [energy intake] and/or reducing [energy expenditure].” Eat more, with a focus on calorie-containing beverages and fun, fulfilling foods. Carbohydrates are especially important given their role in healthy glycogen stores and endocrine health. Breakfast is key after a natural overnight fast, as is preventing intentional fasted training. Given that within-day deficits cause many of the same health issues even when controlling for daily intake, avoid restrictive periods within a day as well. Working with a nutritionist or other health professional can be immensely helpful.
Consider adding rest days, which can act as a long-term insurance policy for energy availability. Rest is not always about whether the brain or the muscles need it. In fact, I’d argue that most motivated athletes could train every day. Instead, rest is about adaptation time for the endocrine and nervous systems, aiming for cellular-level recovery that can be tough for any athlete to self-assess.
And perhaps most of all, avoid overtraining. Here’s a really interesting question that has been posed in the literature: is it possible to have overtraining syndrome without underfueling? While the answer is uncertain, it underscores how all of this stuff is connected. Training can be an uplifting thing; overtraining is a destructive thing; and the line between the two might be indecipherable in a training plan. Eating can build us up; undereating can tear us down; and it might be impossible to tell where you stand with a simple equation. It’s definitely impossible to tell by looking in the mirror.
Given that ever-present uncertainty, focus on actions in training and fueling that are centered in self-love. For many athletes, the loving thing that embodies true investment in themselves is training to feel good, rather than using mounting fatigue as a proxy for training effectiveness. It’s eating an extra slice rather than ordering the salad. It’s looking in the mirror and loving what that body can do, rather than focusing on what it can’t.
At the cellular level, there are trillions of chemical reactions going on every second to make us tick. We can’t control those reactions, and most of them can’t even be measured. But add them together, and their ongoing healthy function requires plenty of fuel. Let’s give it to them.
From Trail Runner