An estimated 80% of people with celiac disease do not know that they have this ( source ). Chronic intestinal diseases such as ulcerative colitis and Crohn’s disease can also lead to inexplicable weight loss due to a disruption in the digestion of nutrients in the intestines. Do you want an example slimming weekly menu incl. Recipes and shopping list? Click here and I will send them to you for free. # 4: Loss of muscles Muscle breakdown is a normal process with aging. But the loss of muscle mass can also be caused by malnutrition, an eating disorder or an autoimmune disease.