The Body Keeps the Bill
Four systems. Twenty-eight days of data. What extreme endurance actually costs and what the science says about getting it back.
The finish line is where the real experiment begins.
I love this part.
Not the finish line. Not the numbers. This part. The part where the ice is behind you and the body starts sending invoices and you get to sit with the data and figure out what actually happened.
I’m a performance coach. This is the work. Understanding the machinery, reading what it says, knowing what to do next. Most of the time I’m doing that with other people. Right now the subject is me. The lab was the Greenland ice sheet. The experiment ran for twenty-eight days.
572 kilometres. Unsupported team crossing, west to east. 9.6 kilograms of body weight lost. 97,800 activity calories burned against an intake of 4,000 to 6,500 per day. A caloric deficit on hard days of 3,000 to 5,000 calories. A foot injury from Day 14 with perineurium sheath inflammation at 6x normal size on the left foot and 4.5x on the right, compressing directly onto the plantar digital nerve with every step for the final two weeks.
That’s the input side.
What follows is the output side. Four physiological systems, each one measurably disrupted, each one now rebuilding. Understanding this doesn’t make me feel beaten up. It makes me feel grateful. Grateful for a body that did what it did. Grateful that discomfort has a language, and that the language is readable.
Performance is based on understanding. The data matters. Here’s what it says.
The Baseline
Pre-expedition Whoop data: HRV at 41.7ms. Resting heart rate 64.4bpm. Recovery score averaging 59.3%. Sleep score 83.1%. Blood oxygen 94.8%.
Expedition averages: HRV dropped to 35.2ms. Resting heart rate climbed to 72.4bpm. Recovery score averaged 49.3%. Half my baseline capacity, held for a month.
The floor: Day 3, recovery score 1%. HRV 17ms. RHR 79bpm. The body encountering something outside its reference library and recalibrating hard.
Day 14 overnight: blood oxygen at 74.8%. Normal range is 95 to 100%. Below 90% is hypoxic. Below 80% is a medical emergency in most clinical settings. The body was still moving at first light.
Cumulative tracked sleep debt at the end of the crossing: 2,897 minutes. Forty-eight hours. Two full nights owed to the body, compounded across twenty-eight days of receiving 6.5 to 7.5 hours when it was asking for 10 to 11.
Four systems took the weight of this. Here’s what happened to each one.
01 / Myelin — The Brain Burned Its Own Wiring
What it is
Every nerve fibre in your brain is wrapped in a fatty sheath called myelin. It’s the insulation on the cable, what allows neural signals to travel fast and clean. Without it, signals slow, misfire, don’t complete. The cells that produce and maintain myelin are oligodendrocytes. They are energy-hungry and under the right conditions of sustained depletion they will metabolize their own product to keep the axons firing.
What the science shows
In 2025, researchers published a study in Nature Metabolism using multicomponent relaxometry MRI, a technique that measures myelin water fraction as a direct biomarker for myelin content. They scanned marathon runners before and immediately after a 42km race. The result: widespread, measurable reduction in myelin content at the finish line. Across the corpus callosum. Across frontal white matter tracts. In regions governing motor function, sensory processing, and emotional regulation.
When glycogen depletes during extreme endurance, the brain draws on myelin lipids as an emergency energy source. The brain burns its own wiring to stay online.
That’s not interpretation. That’s what the MRI shows.
Myelin levels substantially recovered within two weeks after a marathon and fully normalised after two months. That was after 42km. I did 572 over twenty-eight days in sustained caloric deficit. The same research confirms that undernutrition compounds myelination disruption. The structural changes and the cognitive disturbances track together. The depletion here wasn’t a single event. It was rolling, cumulative, and without adequate fuel to rebuild overnight.
What it explains
The fog that arrives mid-sentence. The thought that disappears before you finish it. The feeling of operating slightly behind yourself. Not tiredness. A structural state. The insulation is down and the signals are paying for it.
I find this fascinating rather than frustrating. The body didn’t break. It made a calculated trade: sacrifice processing speed to keep the whole system moving. When you understand the mechanism, the fog stops being mysterious and starts being data.
What rebuilds it
The raw material for myelin re-synthesis is fat, specifically long-chain fatty acids with omega-3s as the primary substrate. Fatty fish, walnuts, eggs, olive oil. The oligodendrocytes need substrate to rebuild. Moderate movement also stimulates remyelination, well-documented in stroke recovery neuroscience and applicable here. Moderate matters. Hard training during this window depletes further.
Timeline: Meaningful cognitive recovery, weeks six to eight. Full normalization, three to four months.
02 / Autonomic Nervous System — Stuck in the Wrong Gear
What it is
The autonomic nervous system runs two branches. Sympathetic: threat response, resource mobilisation, survival allocation. Parasympathetic: rest, repair, rebuild. They balance under normal conditions. Sympathetic ramps when you need it. Parasympathetic takes over when you don’t.
Heart rate variability is the readout. High HRV means parasympathetic is running the show, the body safe and rebuilding. Low HRV means sympathetic is dominant, the body on guard and allocating to survival.
What the science shows
For twenty-eight days, sympathetic ran almost without interruption. Cold. Navigation in whiteout. Crevasse terrain. A foot injury escalating daily. Caloric pressure. Weather calls that mattered. The nervous system doesn’t distinguish between physical load and existential uncertainty. It reads the whole thing as one sustained threat and responds accordingly.
Research on an unsupported Antarctic polar traverse, the closest published analogue to what we did, measured HRV at 1, 4, and 15 days post-expedition. At fifteen days, autonomic disruption was still measurable. Parasympathetic function had not returned to baseline. Still reorganizing.
The recovery sequence follows a fixed physiological order: lactate normalises first, then sympathovagal reorganisation begins, then parasympathetic function completes its re-establishment, then overall system complexity recovers. Sequential. You can’t reach step three without completing step two.
What the data shows
Pre-expedition HRV: 41.7ms. Expedition low: 17ms on Day 3. Now, twelve days post-expedition: still regularly dipping into the 20 to 40ms range. RHR still sitting 70 to 80 instead of the low 60s.
Exactly what the research predicts. It’s not a problem. It’s a position on a recovery arc. Understanding the arc is what lets you work with it.
What rebuilds it
Slow extended exhales. Any breathing protocol where the exhale is longer than the inhale directly activates the vagus nerve, the main parasympathetic highway running from the brainstem through the heart, lungs, and gut. Five minutes in the morning, five minutes before sleep. This is mechanism-level neuroscience. Beyond that: sleep, low-strain movement, time. Nothing else meaningfully accelerates this process. The nervous system recovers on its own schedule.
Timeline: Full HRV return to personal baseline, six to ten weeks minimum.
03 / Hormonal System — The Ratio That Tells the Truth
What it is
The testosterone-to-cortisol ratio is one of the most validated markers in applied sports physiology for reading whether an athlete is recovering or deteriorating. Testosterone is anabolic: it builds, repairs, supports red blood cell production, underlies drive. Cortisol is catabolic: it mobilises emergency resources, breaks down muscle, blocks protein synthesis. Under normal training load they exist in productive tension. Under extreme sustained effort with caloric deficit, the balance collapses.
What the science shows
The hypothalamic-pituitary-adrenal axis, the HPA axis, the cascade regulating cortisol, runs hot under sustained load and doesn’t reset the moment effort stops. Simultaneously, the hypothalamic-pituitary-gonadal axis, the HPG axis governing testosterone, suppresses when energy availability falls below approximately 30 kilocalories per kilogram of fat-free mass per day. This threshold is well-established under the RED-S framework in sports medicine. I was running significantly below it for a month.
The compound effect: chronically elevated cortisol blocks anabolic signaling and breaks down muscle. Suppressed testosterone means repair mechanisms are running at reduced capacity. Both axes working against each other, recovery caught between them.
What the data shows
Pre-expedition baseline testosterone: 782 ng/dL total, 141.1 pg/mL free. Strong natural numbers for any age, high for fifty. This morning, twelve days post-expedition, blood was drawn. The results will show how far the crossing pushed those numbers down. The Whoop data, persistent sympathetic dominance and suppressed recovery scores, already tells part of the story. The bloodwork will tell the rest.
This is the experiment running in real time. Not retrospective analysis. Live data, live decisions.
What rebuilds it
Calories in surplus. Carbohydrate specifically, because low carbohydrate states keep cortisol elevated by signaling ongoing energy shortage to the HPA axis. Protein at 1.6 to 2.2 grams per kilogram of bodyweight. Zinc, magnesium, and vitamin D are the three micronutrients most consistently linked to testosterone synthesis. Resistance training in the coming weeks, not yet, but two to three sessions of lifting per week shifts the testosterone-cortisol ratio faster than any other single training intervention once the body is ready for it.
Timeline: Cortisol normalizes relatively quickly once load drops and calories return. Testosterone rebound is slower. Full hormonal stabilization: six to twelve weeks.
04 / Sleep Debt — The Deficit You Can’t Feel
What it is
Sleep debt is the accumulated difference between the sleep your body needs and the sleep it receives. The body tracks this precisely. What makes it particularly important from a performance standpoint is what chronic sleep restriction research has shown consistently: after two to three days of insufficient sleep, subjective sleepiness ratings stabilize. You adapt to the feeling. The fatigue becomes background noise. You stop noticing how impaired you are while the impairment continues to deepen.
What the science shows
Research published in the journal Sleep found subjects restricted to six hours per night for fourteen days showed cognitive performance equivalent to two full nights of total sleep deprivation. They didn’t feel that bad. Their output said otherwise. The implication: self-assessment of cognitive function under chronic sleep debt is unreliable, because the system doing the evaluating is the one that’s compromised. Recovery takes approximately four days of adequate sleep per hour of accumulated debt. For substantial chronic debt, the timeline is weeks to months. Vigilant attention and mood regulation take the longest to come back.
What the data shows
Sleep need during the crossing: 10 to 11 hours most nights. Actual sleep received: 6.5 to 7.5 hours. Two to four hours short, almost every night, for four weeks. Cumulative tracked sleep debt at the end: 2,897 minutes. Forty-eight hours.
Post-expedition sleep is still fragmented. Nine wake events in a single night in the first week back, consistency at 65%. The body is working to clear the debt. It’s not a straight line.
What I keep coming back to: the debt accumulated because the body was doing something more important than sleeping. It was moving. Every hour of sleep traded was in service of forward progress. That’s not a system failure. That’s the system making exactly the right call under the conditions it was given.
What rebuilds it
No alarm clock where possible. Let the body determine wake time. This is the highest-leverage intervention available and it costs nothing. Twenty-minute naps where the pull is strong, short enough to stay out of deep sleep cycles and wake refreshed. Cold dark room, no screens before sleep, consistent wake time as the circadian anchor. Every night, without exception.
Timeline: Weeks to months for full cognitive restoration from chronic debt of this magnitude. Feeling recovered comes well before being recovered.
The Connected System
This is the part most people miss.
Four systems. But not four problems. One loop.
Poor sleep keeps cortisol elevated. Elevated cortisol locks the autonomic nervous system in sympathetic dominance. Sympathetic dominance fragments sleep architecture, which means the debt deepens instead of clearing. Disrupted hormones starve the cellular repair processes that myelin resynthesis depends on. Reduced myelin efficiency degrades the brain’s own regulatory signalling, which makes autonomic balance harder to maintain, which keeps the stress response running hot.
Round and round. Each system making the others worse. Each one waiting for the others to move first.
This is why you can’t fix one thing and expect the rest to follow. This is why “just sleep more” doesn’t work when cortisol is elevated. Why “just eat more” doesn’t shift the hormonal picture when the nervous system is still in threat mode. Why the fog doesn’t lift just because the body is no longer on the ice.
The systems fell together. They come back together.
That has a timeline. Meaningful improvement across all four by weeks six to eight. Full recovery, HRV at personal baseline, hormones stabilized, cognitive function restored, sleep debt cleared, three to four months from the finish line.
Not a guess. Not optimism. The expected, documented, physiologically predictable cost of what we did. The body runs the numbers precisely. It always has.
None of this is complaint. I’d do it again tomorrow.
What the ice gives you, what this recovery is giving me, is specificity. When you go somewhere genuinely hard, genuinely outside the ordinary range of experience, something shifts in how you see everything else. The ordinary becomes more vivid. The things that matter become obvious. The things that don’t fall away without effort.
Gratitude gets easier when you’ve been somewhere that strips everything back. Sleep, food, warmth, movement: these stop being background and become foreground. The body becomes something you’re in conversation with rather than something you’re managing.
That’s not a side effect of the crossing. That’s part of the point of it.
I’ve spent years working in extreme environments, leading expeditions, coaching performance, watching what happens to people when they go to the edges of what they thought they were capable of. The consistent finding: hard things don’t just test you. They clarify you. They make everything more intentional, more specific, more real. The data from this crossing is the most intimate version of that clarification I’ve ever had.
Performance is based on understanding. Real understanding, not theory, not borrowed frameworks, but data from the edge of what’s actually possible, changes how you think about everything. Every hour of discomfort on that ice is teaching material now. Lived. Tested. Mine.
The body keeps the bill.
I’m happy to pay it.
This morning, twelve days post-expedition, I had blood drawn. Full hormonal panel, stress markers, the complete picture. Results pending.
That’s the point. This isn’t retrospective. It’s a live experiment. The Whoop data tracks daily. The bloodwork tracks at intervals. The protocol adjusts as the numbers come in. I’ll report back: what the results showed, what changed, what the body did next.
Four systems. Real data. Ongoing.
If you’re going through something similar, a hard event, a long training block, a body asking for more than you’ve been giving it, the science applies whether you were on an ice sheet or not. Understanding what’s happening is the first step to working with it rather than against it.
More to come.
G.
All data sourced from Whoop 5.0 and Garmin Fenix 8 tracked across the crossing.
Full expedition dataset and daily breakdowns at gerryvanderwalt.com/greenland.
You haven't even had time to recuperate from this expedition and the pain in your feet and the other things is put you through and you're already going into another expedition. Do you think this was a good idea or do you think you should have waited.