The Hidden Cost of Underfueling: What RED-S Does to Your Hormones

Most athletes think eating too little just means they'll have less energy for workouts. In reality, your body starts shutting down systems it considers non-essential — including hormones, recovery, bone repair, and metabolism itself. And it does this quietly, without warning, often while you're still training hard and eating what you consider a healthy diet.

In the previous article, we covered what RED-S is — the chronic gap between energy in and energy needed, and the pattern of symptoms that gap produces. This article answers the question that makes the urgency of this condition genuinely clear: why does underfueling affect so many systems in the body simultaneously?

Why isn't it just a matter of having less energy for workouts? Why does a relatively modest caloric gap ripple out to affect hormones, recovery, bone health, immune function, and performance adaptation all at the same time?

The answer comes down to how your body prioritizes resources when it perceives scarcity. Understanding that logic changes how you think about every symptom.

Prefer to watch? The full video is below — or keep reading for the expanded breakdown.

Your Body Runs a Triage System

When energy availability drops below what's needed to fund all physiological functions, your body doesn't distribute the shortage evenly. It follows a survival hierarchy — shaped by millions of years of evolution — about what gets funded and what gets cut.

At the top are the functions your body needs to keep you alive right now: cardiovascular function, brain activity, core temperature regulation. These get energy regardless of what's available. Non-negotiable.

Below that are systems the body considers important but not immediately survival-critical: immune function, reproductive hormone production, bone remodeling, recovery from exercise, metabolic rate itself. These get rationed when the budget is tight.

And at the bottom — in terms of survival priority — is performance adaptation. Getting fitter. Building muscle. Improving endurance capacity. These are long-term investments, and when resources are scarce, they're the first things cut.

This is why RED-S affects so many systems at once. It's not that underfueling happens to cause a collection of unrelated problems. Your body is executing a single, coordinated response to perceived energy scarcity — and the systems that get rationed first are exactly the ones athletic performance and recovery depend on most.

Cortisol: The First Domino

When energy availability drops, the body interprets that as physiological stress. The HPA axis responds by elevating cortisol — and in the context of RED-S, that elevation serves a specific metabolic purpose. Cortisol drives gluconeogenesis: the process by which the body manufactures glucose from non-carbohydrate sources — amino acids from muscle tissue, glycerol from fat. Your body breaks down its own tissue to keep blood sugar stable when food isn't providing enough.

Short-term, that's appropriate emergency physiology. Long-term — which is what RED-S produces because the deficit is chronic, not temporary — persistently elevated cortisol becomes the primary driver of downstream hormonal suppression throughout the entire system.

Because cortisol doesn't just raise blood glucose. It suppresses the hypothalamic signaling that drives reproductive hormone production. It competes for the same adrenal precursor hormones as testosterone and DHEA. It impairs peripheral thyroid hormone conversion. And it directly inhibits the tissue repair processes that make training adaptation possible.

Cortisol is the first domino. Every other hormonal disruption in RED-S runs through it to some degree.

Testosterone: The Adaptive Signal That Goes Quiet

As cortisol rises, testosterone falls through two simultaneous mechanisms covered in detail in the hormone suppression article in this series.

The hypothalamus detects the energy deficit and reduces GnRH output. Less GnRH means less LH from the pituitary. Less LH means less testosterone production — from the testes in men, from the ovaries and adrenal glands in women. And simultaneously, elevated cortisol competes for the shared precursor hormones that testosterone synthesis depends on.

The performance consequences are direct. Testosterone is the primary anabolic signal — the hormone that drives the protein synthesis cascade converting training stress into adaptation. When it drops, the adaptive response weakens. Training produces more stress and less adaptation. Performance declines not because the athlete is doing less work, but because the hormonal environment needed to capture the benefit of that work is no longer present.

This is the specific reason why training harder in response to declining performance makes RED-S worse. You're adding more stimulus to a system that has already lost the hormonal capacity to respond to it.

Estrogen: Consequences Beyond the Obvious

Estrogen decline in RED-S gets most attention in female athletes because menstrual disruption is its most visible marker. But the consequences go significantly further — and affect male athletes too, through the estradiol that's converted from testosterone.

Estrogen supports muscle repair through its effects on satellite cell activity — the muscle stem cells responsible for repairing exercise-induced damage. When estrogen declines, the recovery burden from identical training increases. This is one mechanism behind the performance cliff many female athletes experience in RED-S: training hasn't changed, but the hormonal support for recovering from it has been quietly withdrawn.

Estrogen also modulates serotonin, dopamine, and norepinephrine signaling — which affects mood stability, motivation, and cognitive function. The mood instability and cognitive fog frequently reported in RED-S have a specific neurochemical basis here, not simply in fatigue.

In men, estradiol converted from testosterone supports bone health, cardiovascular function, and mood regulation. When testosterone declines in male athletes with RED-S, estradiol declines with it. These effects apply regardless of sex — largely invisibly in men because estrogen is rarely measured on a standard male panel.

Thyroid: The Metabolic Brake

What happens to thyroid function in RED-S is not a failure of the thyroid itself. It's a deliberate downregulation of metabolic rate in response to perceived energy scarcity.

The thyroid produces T4, which is converted to the active T3 form in peripheral tissues — primarily the liver and muscle. T3 is what actually drives metabolic rate, thermogenesis, and energy expenditure. When energy availability is chronically low, the body suppresses T4-to-T3 conversion as a conservation strategy. Less energy coming in — reduce the rate at which energy gets spent.

The consequence for athletes: reduced metabolic rate, persistent fatigue, cold intolerance, cognitive slowing, and body composition that resists everything despite consistent training and careful eating. And because TSH — the standard thyroid screening marker — reflects pituitary signaling about T4 production, not the conversion step, it stays normal. The athlete has significantly impaired active thyroid hormone availability and a completely normal TSH. Standard testing misses it entirely.

There's an important additional layer here: the metabolic recalibration that RED-S creates through thyroid downregulation is one of the primary reasons recovery from this condition takes months rather than weeks. Simply eating more doesn't immediately reverse an adaptation the body has been building and reinforcing over time.

DHEA: The Overlooked Piece

DHEA is produced by the adrenal glands and serves as a precursor to both testosterone and estrogen, with its own independent anabolic effects on tissue repair and immune regulation.

In RED-S, DHEA declines alongside testosterone as the adrenal pathway gets increasingly directed toward cortisol production. The adrenal system has limited output capacity, and when chronic energy deficit keeps demanding more cortisol, DHEA production is the thing that gives way.

For masters athletes, this is compounded by the fact that DHEA naturally declines with age more rapidly than most other hormones — levels peak in the mid-20s and drop significantly through the 40s and 50s. The baseline reserve is already lower, so the additional suppression from energy deficiency tips the balance faster and more significantly than it would in a younger athlete.

Low DHEA compounds impaired tissue repair, reduced immune competence, and a further reduction in the anabolic hormonal environment that training adaptation depends on. It's rarely the first thing anyone tests — but it's one of the most consistently abnormal markers in athletes with RED-S and one of the most useful for understanding how deeply the adrenal system has been compromised.

Where It Shows Up Clinically: Recovery, Bone, and Immune Function

These three systems are where the hormonal disruptions above become visible as symptoms — and where the triage logic becomes most apparent.

Recovery slows because the energy and substrates required for protein synthesis, collagen remodeling, and glycogen replenishment are being rationed. The gap between training-induced damage and repair capacity widens progressively. Soreness lingers. Minor injuries don't resolve. Sessions accumulate fatigue instead of fitness. Fatigue that doesn't respond to rest isn't a training problem — it's a repair budget problem.

Bone density quietly declines because bone remodeling — the constant process of osteoclasts breaking down and osteoblasts building up — depends on both energy and the hormonal signals that have now been withdrawn. Estrogen suppresses bone resorption. Testosterone and DHEA support bone formation. When all three drop simultaneously, the balance shifts toward net bone loss. Stress fractures disproportionate to training load are often the first clinical finding that prompts the right investigation. In masters athletes, where age-related bone loss is already occurring, RED-S accelerates this more quickly than most people expect.

Immune function is rationed in parallel. Secretory IgA — the antibody lining the respiratory tract and gut that acts as a first line of defense against pathogens — declines when energy is insufficient. Susceptibility to upper respiratory infections increases. Illnesses take longer to resolve. The immune system's capacity to efficiently resolve inflammatory responses is reduced, contributing to the persistent soreness and low-grade inflammation that characterize chronic RED-S.

The Complete Picture

When you look at all of this together, the pattern is coherent — not coincidental.

RED-S isn't a collection of unrelated problems. It's a single coordinated physiological response to chronic energy insufficiency: a systematic downregulation of every function the body considers a long-term investment when short-term survival resources are tight.

Cortisol rises. Testosterone drops. Estrogen declines. Thyroid conversion is suppressed. DHEA falls. Recovery slows. Bone density erodes. Immune function is rationed. Each of these individually can look like an isolated finding with its own explanation. Together they're a coherent picture — and that picture doesn't resolve while the underlying energy gap remains open.

Here's the clinical problem: most athletes who develop RED-S have bloodwork that gets reported as normal. Standard panels don't look for these specific markers. The markers that are included often haven't shifted enough — or aren't being interpreted through the right lens — to be flagged as abnormal, even when all of this is well underway.

So how do you actually identify RED-S when standard testing misses it? That's exactly what the next article in this series covers.

Dr. Jason Barker is a naturopathic doctor with over 20 years of clinical experience working with endurance athletes. He is a two-time Ironman finisher and the founder of Natural Athlete Clinic. For individualized evaluation of energy availability and its downstream consequences, visit naturalathleteclinic.com.