Case Study 1 — Devi's Empty Tank: When the Fuel Runs Out at Mile Eighteen

A clinical and physiological case. Devi Raghunathan is an illustrative composite; the physiology and the clinical picture are not.

Setup

Devi Raghunathan came to the clinic because of the stress fractures. She stayed for a conversation about fuel.

She's twenty-one, runs the 5000 m for a Division I programme, vegetarian since she was twelve for ethical reasons, and she is — by any casual measure — doing everything right. She eats vegetables. She doesn't drink. She sleeps. Her coach describes her as the most disciplined athlete on the roster, which is true and which turns out to be part of the problem.

Her numbers at intake:

But the reason this is a Chapter 6 case study rather than a Chapter 23 one is what she described happening in races.

"I feel completely fine, and then at about eighteen minutes there's nothing there. It's not that it hurts. It's that the tank is empty and I can't make my legs go."

That's not a motivation problem and it isn't a fitness problem. It's a fuel problem, and this chapter explains it exactly.

What's happening at eighteen minutes

Walk it through the pathways.

The rate ceiling

Devi is racing at close to her maximum sustainable intensity. At that intensity, ATP demand is enormous and immediate.

Fat cannot supply it. Beta-oxidation (§6.5) is a multi-step process — lipolysis, transport into the cell, carnitine-dependent transport into the mitochondrion, chopping the chain into acetyl-CoA units, then the citric acid cycle and the electron transport chain. Every step takes time and oxygen. Fat is the high-capacity, low-rate fuel.

So racing intensity is overwhelmingly carbohydrate-fuelled — glycolysis, drawing on muscle glycogen, which is the only fuel system that can supply ATP fast enough.

Devi has, in principle, upward of 100,000 kcal of body fat available. At race pace, essentially none of it can be delivered fast enough to matter.

The tank that wasn't full

Now the second half, which is the part her coach missed.

Muscle glycogen (§6.2) is roughly 300–500 g in a well-fuelled athlete — perhaps 1,200–2,000 kcal. It is replenished from dietary carbohydrate, and replenishment takes hours to a day.

Devi is running 60 miles a week on 2,100 kcal/day. Her energy availability is 31.8 kcal/kg FFM, against an optimal figure around 45. She is not arriving at races with full tanks. She has not arrived at a race with a full tank in perhaps two years.

So at eighteen minutes, muscle glycogen in the working muscles runs low, the rate ceiling drops to what fat can supply, and her legs will not go. Precisely as she described it.

"There's nothing there" is a literally accurate physiological report.

The second problem: what her body has been doing instead

Here's where §6.7 and §6.2 combine into something worse than poor performance.

Her brain needs glucose. Liver glycogen (§6.2) holds about a day's worth and she is running a chronic deficit. So her liver has been running gluconeogenesis — persistently, for months.

Gluconeogenesis needs substrate: lactate, glycerol, and amino acids.

Her protein intake is 1.1 g/kg — 61 g/day — which would be adequate for a sedentary adult and is well below what an athlete in energy deficit needs.

So the amino acids have partly been coming from where §6.2 said they'd have to come from: her own tissue. There is no protein store. Using protein for energy or for glucose synthesis means dismantling something that was doing a job.

Combine that with:

• Low energy availability suppressing the hormonal axis that maintains menstrual function and bone formation
• Ferritin of 11 — iron-deficient without anemia, impairing oxygen delivery and mitochondrial function, which directly limits the aerobic side of §6.4
• Nine months of amenorrhea, meaning low estrogen, meaning impaired bone maintenance

Three stress fractures is not bad luck. It's the predictable output of a system that has been selling furniture to pay the electricity bill for two years.

This is RED-S — Relative Energy Deficiency in Sport — and Chapter 23 covers the full clinical picture.

What she'd been told

This is the part that made me angry, and I'll say so plainly.

Devi had been to two clinicians about the fractures. Both had focused on the bone. She'd been given calcium and vitamin D and advised on load management. Neither had asked what she ate, and neither had calculated energy availability — which requires three numbers she could have supplied in four minutes.

She had also been told, by a well-meaning teammate, that her fatigue was because she was vegetarian and needed more protein powder.

That advice was one-third right and pointed in the wrong direction. She does need more protein. But her primary problem is not protein composition or vegetarianism — it's total energy, and adding a protein shake to a 2,100 kcal diet in a 60-mile-a-week runner addresses perhaps a tenth of the deficit while confirming to her that the amount she eats is fine.

⚠️ When to see a professional. Amenorrhea in an athlete is not a sign of fitness and is not something to train through. Recurrent stress fractures, absent or irregular periods, unexplained performance decline, or persistent fatigue in a training athlete require assessment by a sports physician and a sports dietitian. Energy availability should be calculated, not assumed, and ferritin should be checked — hemoglobin alone will miss iron deficiency in exactly this population.

The intervention

Not "eat more protein." Not "take iron." Both of those were needed and neither was the lead.

The hardest part was not the plan. It was the framing.

Devi had spent four years in a sport that rewards being light, in which restriction is publicly praised, and she experienced "eat 800 more calories a day" as an instruction to undo the thing she had been most disciplined about. She said, almost exactly: "So everything I've been good at is the problem."

That sentence is why this case connects to Chapter 34, and why anyone working with athletes needs to have read it.

Analysis

1. The physiology diagnosed the problem before the labs did. "Nothing there at eighteen minutes" is a glycogen story, and glycogen stories are energy stories. A clinician who knows §6.2 and §6.5 can form a hypothesis from a sentence.

2. Enormous fat stores are irrelevant at high intensity. Devi carries far more energy than she needs. The constraint is rate, not quantity — the single most useful thing §6.5 gives you.

3. "Eating healthy" and "eating enough" are different problems. Devi's diet quality is genuinely good. Her diet quantity is ending her career. Almost all public nutrition messaging optimizes the first and is silent on the second, which makes athletes systematically vulnerable to advice that is correct for the general population and harmful for them.

4. The absence of a protein store has clinical consequences. §6.2's driest line — "protein is not a storage form" — is what turns a chronic energy deficit into bone loss and tissue breakdown rather than just fatigue.

5. Nobody asked. Two clinicians, three fractures, no dietary history. Energy availability takes three numbers and four minutes.

Discussion Questions

1. Devi described the failure at eighteen minutes precisely and accurately, and no one had connected it to fuel. What does that suggest about how athlete symptoms get triaged?

2. Her teammate's protein-powder advice was "one-third right and pointed in the wrong direction." Why is partially-correct advice sometimes worse than clearly wrong advice?

3. Calculate what Devi's energy availability would be if she increased intake to 2,500 while maintaining training. Is that sufficient? Show the arithmetic.

4. "So everything I've been good at is the problem." How would you respond to that sentence? Write the actual words, and consider what a wrong answer would cost.

5. Devi's ferritin is 11 with a normal hemoglobin. Why does standard screening miss this, and what does §6.4 tell you about why it matters for an endurance athlete specifically?

Your Turn

Calculate energy availability for yourself, or for someone you train or coach:

Energy availability = (daily intake − exercise energy expenditure) ÷ fat-free mass in kg

• ~45 kcal/kg FFM — generally considered optimal
• 30–45 — a grey zone where problems begin to appear
• below 30 — associated with the hormonal, bone, and metabolic consequences above

You'll need three numbers, and you should be honest that all three are estimates: intake (Chapter 4's diary, with its ±20%), exercise expenditure (your wearable, with its considerably worse error), and fat-free mass (estimate from body fat percentage if you have one, or accept a rough figure).

If the number comes out below 30, that is worth a conversation with a professional — not a recalculation, and not a decision to train harder.

And if you're not an athlete, do it anyway. It takes four minutes, and understanding why the denominator is fat-free mass rather than body weight is the part that teaches you something.