The Case of Tarrare
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I don't see how he could remain underweight while consuming that volume of matter. Even with a fast metabolism, the sheer physical mass of stones and animals would have to sit in the gut somewhere before passing.
We have to remember these accounts come from 18th century surgeons who viewed medical oddities as a primary source of professional prestige. The reports are likely exaggerated to make the case study more sensational for their peers.
Even if the numbers are inflated, the descriptions of his constant heat and excessive salivation are very consistent with severe hyperthyroidism. That specific biological detail makes the whole thing feel more plausible.
Hypothetically, the military might have used the spy narrative to justify keeping him under observation in a controlled environment. It would be a more efficient way to study a medical anomaly than paying for his food in a civilian hospital.