We have demonstrated previously that a diet containing skimmed milk (SM) fermented by Lactobacillus gasseri SBT2055 (LGSP) reduces adipocyte size in Sprague–Dawley rats. Two experiments were conducted to extend these observations in order to elucidate the mechanism involved. In experiment 1, lean and obese Zucker rats were fed a diet containing SM or LGSP for 4 weeks. The LGSP diet, compared with the SM diet, resulted in lowering of the mesenteric adipose tissue weight (23 %; P < 0·05), adipocyte sizes (28 %; P < 0·001) and serum leptin concentration (36 %; P < 0·05) in lean rats. Obese Zucker rats did not display such dietary effects. Only the number of smaller adipocytes was increased (P < 0·05) by the LGSP diet in the subcutaneous adipose tissue of obese rats. The LGSP diet significantly reduced the serum and hepatic cholesterol in rats. In addition, the LGSP diet led to an increased excretion of faecal fatty acids and total neutral faecal sterols in both rat strains. In experiment 2, Sprague–Dawley rats with permanent cannulation of the thoracic duct were fed either the SM or LGSP diets and their lymph was collected. The LGSP diet lowered the maximum transport rate of TAG and phospholipids. These results indicate that fermented milk regulates adipose tissue growth through inhibition at the stage of dietary fat absorption in lean Zucker rats.