Effect of Dietary Carboxymethyllysine on glucose and insulin sensitivity, cecal fatty acids and the plasma metabolome in mice fed the Total Western Diet

Abstract

Carboxymethyllysine (CML) is an oxidized product of lysine and a reducing sugar that is formed in heated foods. Rodent studies indicate that dietary CML is proinflammatory via interaction with the advanced glycation endproduct receptor (RAGE), and that dietary CML promotes insulin resistance. Moreover, previous in vitro studies have shown that heat damaged proteins affect the composition and metabolic activity of the microbiome, which may have systemic effects. Therefore, we hypothesized that increasing levels of CML on a model Western Diet background would decrease insulin sensitivity and possibly modify the metabolic activity of the microbiome. To test this hypothesis, we formulated versions of the Total Western Diet with increasing levels of CML. This was achieved by heating the sugar (glucose and fructose) along with the protein (casein). Subsequently, the protein/sugar mix was added to the diet at three levels. The result is that three diets were produced with <2, 8 and 16 ug CML per gram of diet. Mice (n=10/gp) were fed diets for 8 weeks. An oral glucose tolerance test (OGTT) was performed at week 6 and an insulin tolerance test at week 7. After 8 weeks mice were sacrificed and tissues were collected and frozen in liquid nitrogen. There were no differences in food intake or weight gain. At week 6 mice fed the high CML diet had increased fasting glucose (p<0.05), and yet a lower area under the curve (AUC) in the OGTT (p<0.05). There was also a trend for mice fed the highest CML diet to be the most insulin sensitive (p=0.052). The only difference in plasma metabolites was an increase in proprionic acid in mice fed the high CML diet vs. the low one. Subsequent analysis of the short chain fatty acids in the cecal contents indicate a dose‐dependent increase in proprionic acid as a function of dietary CML. Collectively, these data suggest that glycated protein may increase the production of proprionic acid in the gut as well as systemically, the significance of which is unclear.