Insulin resistance (IR) is defined as the inability of a known quantity of exogenous or endogenous insulin to increase glucose uptake and utilization. Several mechanisms have been proposed as possible causes underlying the development of IR and the IR syndrome. IR occurs as part of a cluster of cardiovascular-metabolic abnormalities commonly referred to as “The Metabolic Syndrome.” This may lead to the development of type 2 diabetes, accelerated atherosclerosis, hypertension, dysmenorrhea, hirsutism, and polycystic ovarian syndrome, depending on the genetic background of the individual developing IR. The aim of this study was to assess, in 123 female and 35 male (mean age, 42 y ± 10.3; range 19-75 y) volunteers) whether IR could be partly related to a dietary sugar intolerance and whether there could be a correlation between the ALCAT intolerance test and a mutation of the TCFTL2 gene (it promotes the trascription of the proglucagone and plays a key role in the development of the Langherans islands). Results evidenced that subjects with an intolerance to sugar, also showed a statistically significant complete or incomplete alteration of the TCFTL2 genetic test. Based upon these findings, our study demonstrated that there is a clinical correlation between the ALCAT food intolerance test and the IR. The positivity to the ALCAT test of one of the sugars tested (fructose, sugar cane, and sugar beet) indicates, in the majority of the subjects, the presence of a mutation of the gene TCF7L2 and could contribute to the prevention and treatment of the IR.