The skinniness is a a result of a process called gluconeogenesis. When the body needs more glucose than is being supplied, it breaks down muscle and converts it to glucose. It's happens during long endurance events--ultras--and is not healthy, of course. It's why marathon athletes look old before their time---their facial muscles get sapped for the glucose, too, and in their 40s or so when the collagen begins to fade and sag sets in, they age dramatically.
