Either you do not eat well or there is increased metabolism as in hyperthyroid states, HIV,
cancer etc. Poor absorption as in irritable bowel
syndromes and malabsorption
syndromes too is possible. Malabsorption syndromes are seen in intestinal tuberculosis, HIV, tropical sprue, parasites in stool and Whipple’s Disease. Diabetes too can be the cause.
The best thing would be to consult a general practitioner or PCP and try and assess what is the cause of your problem.