Antidiabetic effects.

Abstract

Abstract Diabetes mellitus (DM) is a disease in which a person has chronically high blood sugar levels. There are various types of DM, but approximately 90% of the cases in Japan are type 2 DM caused by lifestyle factors stemming from eating and exercise habits. Long-term hyperglycemia can cause capillary disorders and lead to DM-related complications such as retinopathy, kidney diseases, and neuropathy. A number of studies have indicated that the ingestion of green tea or tea catechins is effective in preventing a rise in blood sugar levels. Several mechanisms of action are involved in this effect including: (i) inhibition of α-amylase activity in the digestive juice, which is involved in producing sugar from starch, resulting in a reduction in glucose production and uptake in the digestive tract; (ii) promotion of the glucose intake into skeletal muscle and adipose tissue; (iii) enhancement of sensitivity of insulin, a hormone that lowers blood glucose levels, and protection of pancreatic β cells; and (iv) suppression of hepatic gluconeogenesis (e.g. glucose production from non-carbohydrates) to prevent a rise in postprandial blood glucose levels. Recent cellular and animal studies revealed molecular mechanisms underlying gluconeogenesis suppression by green tea catechins in which epigallocatechin gallate, a main constituent of green tea catechins, inhibits gene and protein expressions of transcriptional factors involved in the gluconeogenesis. In human studies, amelioration of insulin resistance by green tea and catechins is observed. Several epidemiological studies have suggested that the habitual drinking of green tea reduces the morbidity risk of DM. Although further detailed analyses are required to evaluate the beneficial effects on humans, drinking of green tea appears to prevent and improve DM through the multiple activities of its constituents. Because DM increases the risk of colon and liver carcinogenesis in addition to obesity and arteriosclerosis, habitual drinking of green tea would be a promising strategy for the primary prevention of not only DM but also these related disorders.