REDs in Runners: What Relative Energy Deficiency Really Looks Like

Episode: S6E22 | Guest: Melissa Lodge, PhD, Fed Collaborative | Aired July 2, 2026 | 1 hr 1 min

Author: Nick Klastava

Podcast Hosts: Nick Klastava & Amanda Katz

Listen: Spotify | Apple Podcasts | YouTube

Relative Energy Deficiency in Sport, or REDs, is one of the most misunderstood risks in endurance running, largely because it doesn’t come with a single visible sign. Coach Amanda and I sat down with REDs researcher Melissa Lodge, PhD, to break down what low energy availability actually does to the body, why it can’t be spotted by looks alone, and how runners, parents, and coaches can catch it earlier.

Key Takeaways

•             REDs is caused by a mismatch between energy intake and exercise energy expenditure, and it affects both male and female athletes, though research shows female athletes are at greater risk.

•             Weight loss is not a reliable sign of REDs. Some underfueled athletes maintain or even gain weight due to the body’s compensatory mechanisms.

•             Losing your period while on hormonal birth control can mask a real warning sign, since a withdrawal bleed is not the same as a true menstrual cycle.

•             Adolescence is a critical, non-recoverable window for building peak bone mass, making early nutrition education especially important for young athletes.

•             Getting enough total calories isn’t sufficient if carbohydrate intake is too low or if eating is concentrated into a narrow window of the day (called within-day energy deficits).

•             REDs is a diagnosis of exclusion, meaning other medical causes need to be ruled out, which makes self-diagnosis unreliable and professional support important.

What REDs in Running Actually Is

REDs describes impaired physiological and psychological functioning caused by low energy availability: essentially, what’s left over for your body’s normal functions once exercise energy expenditure is subtracted from what you’re eating. When that number runs too low for too long, the body starts deprioritizing systems it considers non-essential for immediate survival, which is why REDs shows up as a cascade of seemingly unrelated symptoms rather than one clear signal.

Why Weight and Appearance Are Misleading Indicators

One of the biggest misconceptions Melissa addressed directly: REDs does not have a “look.” Cross-sectional research has found that underfueled athletes can end up at a similar or even higher body weight than adequately fueled athletes, in part because the body activates compensatory mechanisms that hold onto fat stores when it isn’t confident about when its next meal is coming. Meanwhile, fat-free mass, the tissue athletes work hardest to build, is often what gets broken down first to support essential functions. Judging risk by appearance alone misses a large share of athletes actually affected.

Hormonal Birth Control Can Hide a Warning Sign

Melissa was direct about a common blind spot: bleeding while on hormonal birth control is a withdrawal bleed, not a true period, and it cannot be used as a reliable vital sign the way a natural menstrual cycle can. This doesn’t mean birth control is the wrong choice for an individual athlete, but it does mean that losing a natural cycle indicator requires looking at other signs and lab markers more closely, since the usual red flag may not be visible.

Adolescence Is a Window You Can’t Get Back

Bone mass peaks in the early twenties for females and mid-to-late twenties for males, and the adolescent years are when the majority of that peak bone mass is built. Underfueling during this window has consequences that can’t be fully recovered later in life. Melissa also flagged a specific, actionable benchmark: not having a first period by age 15 is a signal worth raising with a physician, since delayed menarche is linked to long-term health and performance consequences.

Getting Enough Calories Isn’t the Whole Picture

Two subtler patterns can produce REDs-like symptoms even when total daily energy looks adequate on paper. The first is under-fueling carbohydrates specifically, even while hitting overall calorie targets. The second, called a within-day energy deficit, happens when eating gets backloaded into a narrow window, going more than three to four hours without food during the day even if the daily total evens out. Both patterns send the body the same signal as genuine underfueling: food isn’t reliably available, so it’s safer to conserve.

How to Support a Teammate or Athlete You’re Worried About

Melissa’s guidance for stepping in: lead with observed behavior and genuine concern rather than assumptions or direct accusations. Noticing a teammate skipping group dinners, talking negatively about their body, or getting injured without a clear cause are entry points for an “I’ve noticed…” conversation. Since diagnosing REDs requires ruling out other medical causes, the goal of a teammate or coach isn’t to diagnose, it’s to open the door to a physician or dietitian who is actually trained in the framework, since many general practitioners still are not.

Guest Bio

Melissa Lodge, PhD, is a REDs researcher and founder of Fed Collaborative, where she translates sports nutrition and exercise science research for athletes, coaches, and support staff. She is working toward becoming a registered dietitian and was previously a competitive runner at the collegiate and post-collegiate level. Find her on Instagram at @fed_collaborative.

Frequently Asked Questions

Can you tell if someone has REDs just by looking at them? No. Weight loss is not a reliable indicator of REDs, since research shows underfueled athletes can maintain or even gain weight due to the body’s compensatory responses. REDs can affect athletes across a wide range of body sizes and appearances, which is part of why it’s frequently missed or misattributed.

Does losing your period always mean something is wrong? Not necessarily on its own, but a missed period in an athlete who isn’t on hormonal birth control is a signal worth investigating, since it can indicate the body is deprioritizing reproductive function due to low energy availability. On hormonal birth control, a withdrawal bleed does not serve as the same reliable indicator, which means other signs and lab markers become more important to monitor.

Why is nutrition education during adolescence so important for young athletes? Adolescence is a critical, largely non-recoverable window for building peak bone mass and going through key physiological development. Underfueling during this period has been linked to long-term health and performance consequences that are difficult or impossible to fully reverse later in life.

Is it possible to have REDs even if you’re eating enough calories overall? Yes. Underfueling carbohydrates specifically, or concentrating your eating into a narrow window of the day (a within-day energy deficit), can produce symptoms similar to overall underfueling, even if your total daily calorie intake looks adequate on paper.