Intestinal cancer with subsequent loss of function of part of the small intestine. There is less surface area for nutrient absorption, hence the weight loss. The lack of nutrients also accounts for the inability to make the hemoglobin and red cells, for the anemia, and for the lack of oxygen-carrying capacity, fatigue.
Gallstones. The hardened bile in the gallbladder affects lipid metabolism, causing weight loss and fatigue. The anemia is due to a lack of bile pigments available to produce red blood cells.
Colitis. The inflammation of the colon makes it impossible for nutrients to be properly absorbed, hence the weight loss and fatigue. The inflammation also disrupts the absorption of various vitamins required to make hemoglobin and red blood cells and reduces oxygen-carrying capacity.
Peptic ulcer. The bleeding ulcer causes loss of blood, resulting in anemia and fatigue. The weight loss is caused by the inability of the stomach to absorb nutrients.