Consumption of lycopene-rich tomatoes improved glucose homeostasis in rats via an increase in leptin levels

The lycopene content of tomatoes is important because of its effects on vital physiological functions such as improvement of glucose tolerance and non-alcoholic fatty liver disease. To investigate the influence of the lycopene content of tomatoes on glucose tolerance and hepatic lipid content, homogenates of lycopene-rich (LR) or lycopene-free negative control (NC) tomato varieties were administrated to normal rats for 4 weeks. At the end of the experiment, an oral glucose tolerance test (OGTT) was performed. Rats were fed once and then dissected. According to the OGTT results, plasma glucose levels in the LR group were 10% and 9% lower at 15 min and 30 min, respectively, than those in the NC group, whereas plasma insulin levels did not differ between the groups at either time point. Upon dissection, plasma leptin levels in the LR group were higher than those in the NC group, while plasma adiponectin levels did not differ between groups. With the exception of retinol palmitate, no carotenoids were detected in the liver by HPLC analysis. Hepatic retinol palmitate levels and hepatic triacyl glyceride levels did not differ between the groups. We concluded that in normal rats, a lycopene-rich tomato variety improved glucose tolerance via an increase in plasma leptin levels that enhanced insulin sensitivity but did not affect carotenoid accumulation or lipid metabolism.