If you're not eating enough.
D000368:Aged; D000369:Aged, 80 and over; D001066:Appetite; D004435:Eating / Q000502:physiology* / Q000523:psychology; D004734:Energy Metabolism; D005260:Female; D006304:Health Status; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity; D009752:Nutritional Status; D015431:Weight Loss