Malabsorption is a clinical term that encompasses defects occurring during the digestion and absorption of food nutrients by and infections of the gastrointestinal tract. The digestion or absorption of a single nutrient component may be impaired, as in lactose intolerance due to lactase deficiency. When a diffuse disorder, such as celiac disease or Crohn's disease, affects the intestine, the absorption of almost all nutrients is impaired.
Although presenting symptoms, such as diarrhea and weight loss, may be common, the specific causes of malabsorption are usually established based on physiologic evaluations. The treatment often depends on the establishment of a definitive etiology for malabsorption.
To understand the mechanisms of malabsorption, understanding the normal physiologic process of digestion and absorption by the intestinal tract is necessary.
In general, the digestion and absorption of food materials can be divided into 3 major phases: luminal, mucosal, and postabsorptive.  The luminal phase is the phase in which dietary fats, proteins, and carbohydrates are hydrolyzed and solubilized by secreted digestive enzymes and bile. The mucosal phase relies on the integrity of the brush-border membrane of intestinal epithelial cells to transport digested products from the lumen into the cells. In the postabsorptive phase, reassembled lipids and other key nutrients are transported via lymphatics and portal circulation from epithelial cells to other parts of the body.
Perturbation by disease processes in any of these phases frequently results in malabsorption.