The Connections Between Overtraining and Underfueling

According to a new study, overtraining and underfueling share pathways, symptoms, and diagnostic complexities. Those connections have important implications for long-term adaptation and health.

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I think the most important workout I prescribe as a coach happens the day after many races. The workout is given in all-caps, shouted with urgency. 


Or perhaps BURGERS. If we’re feeling spicy, BURRITOS. An athlete can make substitutions within the spirit of the plan, but if they eat a salad, it better be topped with enough bacon and croutons to qualify as a structurally-challenged BLT. 

There are exceptions for athletes that have been told by a doctor or nutritionist to follow different guidelines. The main idea is that the post-race recovery window is a good reminder that eating enough food is the key to long-term adaptation in the context of a healthy life. So I might not always be writing PIZZA, but PIZZA is always implied. Pizza isn’t just a meal, it’s a lifestyle.

The rationale: eating enough is key for the healthy function of the nervous, endocrine, metabolic, and musculoskeletal systems, all of which are needed at full strength to encourage adaptation. Low energy availability–even within a single day during heavy training–is like pulling out an assortment of Jenga blocks from our physiology at random and then facing an earthquake. At first, things might not collapse with a few blocks on the ground. But eventually, when the quake comes (if not sooner), everything will come crashing down.

RELATED: Are You Overtraining?

New Study Overview

A November 2021 review article in Sports Medicine by a wonderful group of researchers led by Trent Stellingwerff found some fascinating links between overtraining and underfueling, underscoring the importance of always eating enough food. Let’s start with some definitions.

Overtraining syndrome is a series of maladaptations to chronic stress loads, with the article using the specific definition of ​​“an accumulation of training and/or non-training stress resulting in long-term decrement in performance capacity.” Short-term performance decreases from days to weeks are called functional overreaching (usually planned, possibly leading to an athlete progressing above baseline) or non-functional overreaching (unplanned and unproductive). The 2021 article uses the term “training-overload/OTS,” reviewing studies that find a sustained and significant decline in performance outcomes.

In 2014, the British Journal of Sports Medicine published the IOC Consensus Statement on 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).

The 2021 article is so important because it answers a question that gets to the heart of athletic performance. May some cases of overtraining be partially attributable to underfueling? 

RELATED:5 Signs You’re Not Eating Enough

And it raised some questions in my mind as a coach. 

How many athletes have undershot their ultimate potential because they (and/or their healthcare providers/coaches) didn’t see the link between fueling and adaptation? How many athletic careers have come crashing down, seemingly without explanation, all due to misinformed understandings of physiology? 

These questions are especially relevant with recent revelations that the University of Oregon adjusted training based on body composition readings in repeated DEXA scans (we talk about the background on the Some Work, All Play podcast here). If overtraining and underfueling are interconnected, then increasing training while failing to appropriately increase fueling could send athletes into unnecessary, almost universal physiological black holes.


Study Principles

A primary difficulty in study design is that both overtraining and RED-S involve diagnosis by exclusion, where other potential causes are ruled out first. On top of that, there is no one presentation of either condition. For both, “performance decline is coupled with a broad and individually variable palette of psychological/emotional, physiological, immunological,and neuroendocrine decrements.” You could imagine someone who rarely exercises and eats plenty going to the doctor with a similar set of symptoms as an athlete with overtraining or RED-S. If it looks like a duck, and it quacks like a duck, it may be a duck (subject to disclaimers). But it could also be a rabbit, depending on how you are looking at it and what you are looking for.

For overtraining, there are some helpful protocols to add certainty to diagnoses. A 2010 study in the British Journal of Sports Medicine used a two-bout exercise protocol, involving two max performance tests four hours apart, finding attenuated changes in cortisol and certain hormones for overtrained athletes. Generally, performance decreases over time can provide diagnostic clues for overtraining. For RED-S, risk-assessment tools and energy availability calculators can evaluate energy availability, and bone health scans and menstrual cycle status are diagnostic factors, but it’s still difficult to directly prove that clinical outcomes result from underfueling in all cases. 

That uncertainty of individual diagnosis is why the review article is so helpful. It examined 57 studies for overtraining and 88 studies for low energy availability to draw conclusions that apply across a broad population of athletes. Interestingly, 78% of the nearly 10,000 participants in the RED-S studies were female, while only 19% of the 1000+ participants in the overtraining studies were female, which could influence how observations/diagnoses have been made over time (i.e. perhaps male athletes have been underdiagnosed with RED-S). There are four conclusions I want to highlight.

RELATED: The Importance Of Eating Enough Food

One: Overtraining and low energy availability share almost all symptoms

There are 13 performance and health outcome groupings for the two conditions, and overtraining and RED-S share all but one (bone health, which only applies to RED-S). They share impacts to: endurance; strength; injury/illness; adaptation; glycogen/protein synthesis; coordination; sex hormones; other hormones; biochemistry; immune function; metabolic/cardiovascular function; and mental health/related variables. Both ailments have at least some hypothalamic-pituitary origin, affecting almost every variable that goes into health and performance.


Two: Overtraining and RED-S are related to “under-recovery” 

Every training session introduces stress, leading to some sort of rest and recovery requirement. Overtraining and RED-S are both causes and responses to chronic stresses that are not being adequately processed by the body. Notably, overtraining and RED-S both lead to performance decreases. For athletes dealing with these ailments, increasing training volume and/or intensity will likely make them perform worse, which can be especially difficult in an athletic landscape that sometimes focuses on weekly miles and hard workouts. But even short of long-term performance reduction, stagnation and general fatigue may be related to the same under-recovery processes.

RELATED: Forgive Yourself On Bad Days

Three: Some overtraining-related diagnoses may be due to misdiagnosed under-recovery from underfueling

The next step the article took made me gasp with excitement. I know that sounds dramatic, but that’s how cool it is, and that’s also a clue to how few dates I got in high school.

The authors looked at a subset of 21 studies that implemented training overloads, many of which resulted in overtraining-related outcomes. Next, they calculated energy availability in those studies. In other words, the 2021 article studied the past studies to make new findings.

14 of the 21 studies found an energy availability decrease between the training overload and control groups. For example, a 1988 study in Medicine & Science in Sports & Exercise examined 12 male collegiate swimmers during a 10-day intensive training block. Retrospectively, that study classified swimmers as overtrained or non-overtrained based on subjective feelings of fatigue. Despite similar training and energy expenditure increases in the two groups, the overtrained group ate about 1000 kcal less per day (4682 vs. 3631). Across the studies, differences as low as 15% in daily energy intake could explain some of the overtraining symptoms.

As the study authors concluded, “Even small within-day energy deficits (only 300–400 kcal) have been associated with clinically meaningful RED-S symptom outcomes in cross-sectional study designs. Although a one-off small 300 kcal deficit (either within day, or over an entire day) is most certainly not clinically impactful, when multiplied over months, these small mismatches in energy can become significant (e.g. energy deficit of 300 kcal/day over 1 year = 100,000+  kcal deficit).”

4 of the 21 studies demonstrated an independent effect of low carbohydrate intake. Even without low energy availability, there is some evidence that carbohydrate restriction could lead to RED-S symptoms. At a Halloween party, an athlete with amenorrhea was talking to my co-coach Dr. Megan Roche, and the part of Megan’s advice that jumped out to me (after she heard about the athlete’s background and nutrition) was very simple: “CARBS!”

However, not every study supports the overarching hypothesis that underfueling is an element in overtraining. 3 of 21 studies showed overtraining-related symptoms without evidence of lower energy availability. Interestingly, one was the 2020 study in the Journal Of Applied Physiology on muscle fiber typology and overtraining that was the subject of this Trail Runner article.

The authors summarize the findings: “Taken together, 86% of the training-overload/OTS studies reporting dietary outcomes (18 out of 21) showed reduced EA (n  = 14 studies) and/or CHO availability (n  = 4) between treatment groups or between pre and post concurrent with symptoms consistent with both OTS and RED-S.” 

Eating enough won’t prevent every case of training-overload/OTS, but it may prevent many of them.


Four: Awareness and monitoring is key

To conclude, the authors highlight that increased training load “tends to initially result in improvements in performance, potentially through a combination of increased training adaptations and/or initial changes in body composition outcomes.” That’s adaptation to training, and it’s not a bad thing. But what happens next can be very, very bad.

In some athletes and coaches, the authors say, it can create a “feed forward” misconception that “even more training and/or more decreases in extreme body composition metrics might further drive positive performance outcomes.” That misconception creates “the perfect storm” for overtraining and RED-S.

A few weeks ago, we saw this problem magnified with disturbing clarity at the University of Oregon. Their body-composition monitoring program was a scientific failure because it interpolated from outliers winning Olympic medals while extrapolating from the beginning phases of an athletic life of the college runners in the program. It was more than a problem of trying to fit a square peg into a round hole. It was also a problem of trying to push every athlete into a hole to begin with, whether it was backed by individual physiology or not.

The 2021 article emphasizes the importance of fueling the work you are doing. Under-recovery via overtraining or RED-S is unequivocally bad for health. The next step is elevating a culture that supports long-term recovery and adaptation, which requires plenty of fuel. 

As a coach, I know that if an athlete is eating enough, I can prescribe more training and harder training. Those athletes can pursue big dreams without fear, and they can imagine adventures for decades and not just a few months. 

Eating enough isn’t just about avoiding a big wall in the middle of the road in athletic development. It’s about removing that wall entirely so that we can put the pedal to the freaking metal in training, chasing some scary dreams.


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 wrote a book called The Happy Runner.

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