A human body at rest requires approximately 1,500 to 2,000 calories per day to maintain basic metabolic function. Physical activity, cold, and altitude increase this requirement substantially. At 3,660 meters in cold weather, the survivors’ actual caloric need was likely well above 2,500 calories per day for anyone moving around, and somewhat below that for those forced to remain still by injury.
The daily ration from the aircraft provisions was a fraction of this. Three small squares of chocolate. Perhaps a cracker. A small measure of wine distributed carefully. The total caloric value of a daily ration was well under 500 calories — perhaps significantly under. The survivors were, from the first days, in caloric deficit. They were losing body mass every day.
After the aircraft provisions were exhausted, the available nutrition from the bodies of those who had died was also limited. The survivors were careful and disciplined about consumption — approaching the task as a necessity rather than an abundance — and the amount available did not dramatically exceed what had been available from the aircraft stores. The survivors remained in caloric deficit throughout the 72 days.
The physical deterioration that resulted from this sustained malnutrition was visible and progressive. The survivors lost significant body weight over the 72 days. Roberto Canessa, who had been heavier than Parrado at the time of the crash, weighed 44 kilograms when he was weighed at Los Maitenes after the rescue — a figure that represents a dramatic reduction from his pre-crash weight. Parrado, by his own account, had lost a comparable proportion of his body weight.