Summary Mannitol is a six-carbon sugar alcohol that is widely distributed in plants. Sugar alcohols are widely used in various food products because of their numerous beneficial health effects. The present study investigated the effects of mannitol consumption on digestion, large gut fermentation and nutrient metabolism in normal and cecectomized male Wistar rats. After 28 d of feeding with three kinds of diet containing 0, 4 or 8% mannitol, mannitol consumption dose-dependently lowered the digestibilities of crude fat and crude protein, the ratio of body fat accumulation to energy absorbed and the hepatic and serum triglyceride levels in normal rats. After 24 d of feeding with three kinds of diet comprising a control diet, a 5% mannitol-containing diet and a 5% fructooligosaccharides (FOS)-containing diet, mannitol lowered the digestibility of fat and the ratio of body fat accumulation to energy consumed and absorbed in cecectomized rats. On the other hand, FOS consumption had no effect on the accumulation of body fat, but lowered the digestibility of fat. FOS consumption greatly improved the accumulation of body ash in cecectomized rats. These results suggest that mannitol has a lowering effect on body fat accumulation, and further indicate that the cecum is not essential for the appearance of effects of mannitol on digestion, absorption and metabolism. Key Words mannitol, digestion, fat accumulation, rat, cecum Excessive accumulation of body fat is considered to be a major risk factor for several chronic disease conditions such as coronary heart disease, cancer, diabetes and hypertension. Fatness is also a specific reason for the persistence of chronic fatty liver, representing one of the major causes of hepatic diseases such as acute steatohepatitis and chronic liver failure ( 1 ). Proper weight management is one of the key components of a healthy lifestyle that avoids the onset of these diseases.Mannitol is a naturally-occurring six-carbon sugar alcohol or polyol that is widely distributed in plants, including algae, onions, grasses, olives and pumpkins. It is used as an osmotic diuretic and cleared by the kidneys, in a similar manner to inulin ( 2 ). Mannitol is not easily digested in the small intestine, but is fermentable in the large intestine ( 3 , 4 ). In a previous study, up to 50% of uniformly labeled 14 C-mannitol orally administered to humans was recovered in urine and stool samples after 48 h and as much as 18% of a dose was recovered as expired CO 2 , whereas little metabolism was observed after intravenous administration ( 5 ). In another human study, the blood glucose level and respiratory quotient remained unaffected within 2 h after administration of mannitol ( 6 ). These findings suggest that part of the ingested mannitol is utilized as an energy source, possibly after being metabolized to organic acids by intestinal microflora and absorbed from the large intestine, although mannitol can be used as a sweetener by people with diabetes.Non-digestible but fermentable oligosaccharides stimulate...