Bacterial overgrowth can cause a variety of symptoms, many of which are also found in other conditions, making the diagnosis challenging at times.
Many of the symptoms are due to malabsorption of nutrients due to the effects of bacteria which either metabolize nutrients or cause inflammation of the small bowel, impairing absorption.
The symptoms of bacterial overgrowth include nausea, flatus, constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness.
SIBO also causes an increased permeability of the small intestine.
Some patients may lose weight.
Children with bacterial overgrowth may develop malnutrition and have difficulty attaining proper growth.
Steatorrhea, a sticky type of diarrhea where fats are not properly absorbed and spill into the stool, may also occur.

Patients with bacterial overgrowth that is longstanding can develop complications of their illness as a result of malabsorption of nutrients.
Anemia may occur from a variety of mechanisms, as many of the nutrients involved in production of red blood cells are absorbed in the affected small bowel.
 Iron is absorbed in the more proximal parts of the small bowel, the duodenum and jejunum, and patients with malabsorption of iron can develop a microcytic anemia, with small red blood cells.
Vitamin B₁₂ is absorbed in the last part of the small bowel, the ileum, and patients who malabsorb vitamin B₁₂ can develop a megaloblastic anemia with large red blood cells.

In older adults, small bowel bacterial overgrowth is associated with a higher frequency of diarrhea, a lower body mass index, and a significantly lower serum albumin concentration.
Bacterial overgrowth can cause a variety of symptoms, many of which are also found in other conditions, making the diagnosis challenging at times. Many of the symptoms are due to malabsorption of nutrients due to the effects of bacteria which either metabolize nutrients or cause inflammation of the small bowel, impairing absorption. The symptoms of bacterial overgrowth include nausea, flatus, constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness. SIBO also causes an increased permeability of the small intestine. Some patients may lose weight. Children with bacterial overgrowth may develop malnutrition and have difficulty attaining proper growth. Steatorrhea, a sticky type of diarrhea where fats are not properly absorbed and spill into the stool, may also occur.
Patients with bacterial overgrowth that is longstanding can develop complications of their illness as a result of malabsorption of nutrients. Anemia may occur from a variety of mechanisms, as many of the nutrients involved in production of red blood cells are absorbed in the affected small bowel.  Iron is absorbed in the more proximal parts of the small bowel, the duodenum and jejunum, and patients with malabsorption of iron can develop a microcytic anemia, with small red blood cells. Vitamin B₁₂ is absorbed in the last part of the small bowel, the ileum, and patients who malabsorb vitamin B₁₂ can develop a megaloblastic anemia with large red blood cells.
In older adults, small bowel bacterial overgrowth is associated with a higher frequency of diarrhea, a lower body mass index, and a significantly lower serum albumin concentration.
Small intestinal bacterial overgrowth