Susceptibility to Obesity and Gallbladder Stasis Produced by a Protein- and Fat-enriched Diet in Male Mice Compared with Female Mice

Background: The frequency of Japanese subjects over 20 years old with metabolic syndrome is 45.6% in men but just 16.7% in women. The reason why Japanese male subjects are more susceptible to metabolic syndrome than women is unknown. One possibility is the higher frequency of Japanese male subjects (40-70 years old) who had a drinking habit (67%), while that of female subjects was only 25%. In addition, daily fat intake was markedly increased in Japanese subjects (from 9% to 25%), and cholesterol cholelithiasis is one of the most rapidly increasing digestive diseases during the past 50 years. The object of this study is to examine whether a potential sex-related risk factor exists in the manifestation of metabolic syndrome as well as gallstone formation.

Methods: Gallbladder dysmotility accerelates gallstone formation and gallbladder contraction depends on cholecystokinin (CCK) and its receptor (CCK-1R). We developed CCK-1R gene knockout (-/-) mice. The effects of the fat- and protein- enriched diet OA-2 on body weight, hyperlipidemia, and frequencies of sludge and gallstone formation were examined, and compared between wild-type and CCK-1R(-/-) male and female mice. The OA-2 diet contains slightly higher protein and fat (7.9 % fat and 27.6 % protein) compared with a standard diet (CRF-1) (5.6 % fat and 22.6 % protein), but their total energies are similar. After weaning, CRF-1 was provided until 3 months of age in all animals. Administration of an OA-2 diet was started when age-matched CCK-1R(-/-) and wild-type male and female mice reached maturity, at 3 months of age. Administration of CRF-1 was continued in the rest of the animals. Mice were sacrificed by guillotine at 6 and 12 months of age and the blood was collected to measure plasma levels of triglyceride and cholesterol. The gallbladder was removed and classified as normal (clear gallbladder), clouded (sludge formation), and/or containing gallstone formations.

Results: As long as CRF-1 was provided, the frequency of sludge and/or gallstone formation in CCK-1R(-/-) male mice was 3 of 8 (35%) and 4 of 9 (45%) in females at 12 months of age, whereas no gallstone formation was observed at 6 months of age. On the other hand, male mice fed OA-2 increased their body weight and plasma lipid concentrations, compared with those fed CRF-1 regardless of genotype. Under the OA-2 diet, sludge and gallstone formation was observed at 6 months of age, not only in CCK-1R(-/-) male mice but also in wild-type male mice. In contrast, parameters in female mice did not differ between the two diets.

Conclusion: Male mice were more susceptible to protein- and fat-enriched diet-induced obesity than female mice, and hyper-nutritional status accelerated sludge and gallstone formation in male mice.