In the first chapter of Slaughterhouse-Five, the narrator promises Mary O'Hare that he will write a novel about World War II that will not attract the attention of manly men like John Wayne or Frank Sinatra. One way in which Vonnegut certainly succeeds in making war seem utterly unappealing (besides, you know, the death and pain and misery) is by emphasizing the hunger and illness of the soldiers fighting it. Paul Lazzaro's stomach is shrunken with hunger, Edgar Derby weeps at the taste of syrup, and all the American POWs spend their first night in the British compound with explosive diarrhea. The book really foregrounds the unattractive, absurd realities of male bodies under stress. The only soldiers with big muscles and washboard abs are the English officers, who have been prisoners for the whole war, and who barely fight.
Kurt Vonnegut depicts the bodies of the American POWs as weak and poorly fed to demonstrate that this is a war being fought by fools and children rather than heroic manly men.
The contrast between British and American POWs shows that maintaining a fighting man's body in captivity is one thing, but staying healthy, warm, and strong on the front lines is quite another.