Estimated read time: 6.13 minutes (about as long as it takes to calculate your macros, second-guess them, and then just eat a banana 🤔)

Hey Performance Nerds! Jonah here. 🤓

Your bones started weakening weeks before the stress fracture showed up on the MRI. Your hormones shifted days before you felt slower.

And your blood work looked "normal" the entire time.

That's chronic underfueling. Not a crash. A quiet, coordinated downshift.

Here's what you'll learn today:

(Augie has never counted a calorie in his life. He eats whatever hits the floor and still outruns me to the mailbox. Some of us aren't that lucky.)

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Takes 10 seconds. Your answer shapes what I build first. Now, let's talk about what happens when you don't eat enough.

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💡 See this week’s full Stryd training tip at the end of this newsletter.Stryd

Energy availability (EA) is simple math. Calories eaten minus exercise calories, divided by your lean body mass.

When that number drops too low, your body starts triaging. It protects the essentials.

Reproduction, bone repair, thyroid output, immune function? Deprioritized.

Marathon runners are especially vulnerable. High training volume burns through calories fast, and accidental underfueling pushes you into the danger zone without trying.

Three practical EA ranges (IOC 2023, Loucks):

30 isn't a magic threshold. The IOC 2023 consensus treats LEA as a continuum.

Individual responses vary widely. Some runners hit trouble sooner than others.

The point isn't precision. It's direction:

Hormones shift before you feel slower. By the time performance drops, the damage has been accumulating for weeks.

The evidence for these endocrine patterns is strong (IOC 2023, Ihle & Loucks 2004). The exact timeline in real-world runners is less certain, since most data comes from lab studies.

But the direction is consistent. Your body downgrades quietly, then loudly.

Running loads your bones with every step. Your body repairs that microdamage through constant remodeling.

But remodeling requires energy and nutrients.

Under LEA, bone building slows while bone breakdown speeds up (Logue et al. 2020). The gap between damage and repair widens with every mile.

Collagen and bone repair peaks about 24 to 72 hours after loading. If you're under-resourced when that window opens, microdamage stacks faster than your body can fix it.

Warden, Edwards & Willy (2021) frame it clearly: stress fractures happen when load exceeds the bone's current capacity. LEA lowers that capacity.

You can modify your biomechanics and add bone-loading exercises. But if energy status is insufficient, those tools have limited benefit.

(You can't outrun a deficit.)

The 2025 Delphi consensus now recognizes LEA as a central modifiable risk factor for bone stress injuries.

The part most runners miss:

Haines et al. (2023) studied male runners with lower energy availability. They had worse bone density, worse bone structure, and lower estimated bone strength.

Even with testosterone in "normal" ranges (though lower estrogen was still a red flag).

"Normal" blood work doesn't mean safe bones. Lower estrogen, even within normal limits, was still linked to poorer bone outcomes.

This isn't just a female athlete issue. Male runners can't assume they're immune.

Three more systems take a hit. Each one chips away at your ability to train and recover.

Metabolic adaptation:

Your metabolic rate drops several percent beyond what weight loss alone explains (Trexler et al. 2014). Larger deficits and longer durations produce bigger drops.

For runners who aren't losing dramatic weight, the signs are subtler. Lower leptin, lower T3 (thyroid hormone), higher cortisol (stress hormone).

Your body gets "efficient" at conserving energy:

Muscle building:

Energy deficits of roughly 500-1000 kcal/day below maintenance reduce resting muscle growth, even with high protein. Areta et al. (2014) showed that resistance training plus adequate protein can rescue it acutely back to energy-balance levels.

But chronic deficit still chips away at lean mass.

Your endurance adaptations (mitochondria, oxidative capacity) may look fine initially.

Structural capacity quietly erodes underneath.

You keep hitting your paces. Your bones and muscles are telling a different story.

Immune function:

The IOC 2023 consensus highlights that LEA athletes get sick more often. Heavy training plus reduced energy weakens your front-line defense against respiratory infections.

The runner who keeps getting sick during marathon blocks may have an energy problem, not an immune problem. (More fuel in, more defense out.)

You can manage body composition. Just not chronically, not deeply, and not during your hardest training.

The principle is energy matching (Stellingwerff, Burke). More fuel on hard and long days. Strategic deficit (if needed) only on easy days.

Match nutrition to training load the same way you periodize running.

Think of it like this: your Tuesday tempo and Saturday long run get full fuel. Your Monday recovery jog can absorb a small deficit.

Your off-season base phase can handle a planned 4-6 week body comp block. Your 12-week marathon build? Feed it.

The evidence for harm from chronic LEA is strong. The exact parameters for "safe deficit" duration are consensus-based rather than proven by long-term trials.

But the physiology is clear. Short, moderate deficits during lighter training carry far less risk than chronic restriction during hard blocks.

Bottom line: Your body doesn't announce it's underfueled. It quietly downgrades everything. Feed the machine.

Welcome to the prove you’re a nerd section. Each week, I ask a question about a common running science myth.

Answer correctly, and you’ll be entered into a weekly raffle to win a package of Jonah’s favorite supplements.

Heat can be a weapon in a marathon build. It can also quietly wreck your key sessions if you layer it in carelessly.

Most of you played this one exactly right.

The correct answer?
A. After runs (easy or hard), 15–30 minutes, start 2x/week and build gradually, avoiding race week 🔥 ✅

Heat exposure works by expanding plasma volume and improving thermoregulation. But that adaptation happens with repeated, manageable doses. Post-run sessions let you stack the stimulus without compromising the quality of the workout itself.

The goal is adaptation, not exhaustion.

Here’s how the votes shook out:
🟩 A. After runs, 15–30 minutes, start 2x/week and build gradually, avoiding race week 🔥 – 230 ✅
⬜️ B. Only after hard workouts to maximize adaptation from stacked stress 🥵 – 28
⬜️ C. The day before long runs or workouts to boost plasma volume ahead of time ♨️ – 31
⬜️ D. One long 45–60 minute session weekly to create a bigger adaptation signal 🚿 – 24

A few of you leaned toward “stack more stress” or “make it longer.”

But heat adaptation isn’t about crushing you. Longer or pre-workout sessions increase fatigue and dehydration risk, which can blunt the very workouts driving your marathon fitness.

Heat is a supplement to training, not the main course.

Bottom line?

Use heat after runs, dose it progressively, and protect your key workouts at all costs. Adaptation is earned through consistency, not suffering. 🔥

I had a big training week recently and my paces still looked fine. But something felt off. My legs felt flat, like running on dead springs. Less bounce, more effort for the same splits.

I checked my Stryd data and there it was. My Leg Spring Stiffness had dropped steadily across the week. Fatigue was showing up in the data before it showed up in my pace.

Think of your legs as springs. When you're fresh, your tendons handle most of the elastic work cheaply. When fatigue accumulates, your muscles pick up the slack at a higher energy cost. Same pace, more effort. Stryd's LSS captures that shift.

Why it matters:

After a hard week, check your LSS trend before adding more volume.

👉 Try it on your next run