Lack of access to calories (limited resources, poverty), lack of interest in eating (decreased appetite for any reason), impaired ability to eat/chew, or problems w/absorption
Hypercatabolic states (e.g. cancer, chronic infections, autoimmune disorders, metabolic disorders)
Poor dietary choices, specific nutritional deficiencies (e.g. iron, b12, vitamin C)
Substance abuse (e.g. cocaine, etoh, crystal meth, etc) can contribute to lack of interest, resources and/or poor choices