After the initial adaptation, the body enters a more stable phase of operating on reduced resources. Body fat is metabolized first — converted to ketone bodies that serve as fuel for the brain and other organs in place of the glucose that normally comes from dietary carbohydrates. As fat reserves are depleted, the body begins metabolizing muscle protein as well. This is the phase in which visible wasting occurs. The rate at which this proceeds depends on the amount of body fat at the start of the deprivation and on the level of activity during the period — a resting body burns fat more slowly than an active one.
Alvarenga, as an experienced fisherman, would have had a specific kind of body composition at the time of departure: not heavily muscular, not carrying excess fat, but in the kind of functional working condition that sustained physical labor over many years produces. He was lean but not thin; strong but not large. This body type, subjected to extended deprivation with moderate activity levels, would metabolize at a rate consistent with maintaining function over a very long period — much longer than would be possible for someone starting in a state of obesity (too much body fat to metabolize safely) or extreme leanness (not enough reserves to draw on).
The specific observation that drew attention after his rescue — that he was less visually wasted than expected — is explained in part by the timing of his arrival relative to the trajectory of his physical decline. He had been declining throughout the drift; the rate of decline in the final months had accelerated as reserves were exhausted. Had the drift continued for another two or three months, the visible effects might well have been what observers initially expected to see. He arrived at the point in the trajectory where decline had been significant but was not yet terminal — which is consistent with what the physiological timeline of extended starvation predicts.