GSTM1 and GSTT1 affect the content of vitamin C in the blood. Vitamin C intake should be increased if one of the genotypes is deficient. If both genotypes are deficient, the concentration of vitamin C will be 20% lower than usual, affecting the Antioxidant capacity in the body.
 
Drug or toxins removal is carried out through two-stage metabolism in the liver. The GST enzyme family is responsible for coupling the toxins or drugs reacted in the first stage with hydrophilic chemical functions.
Therefore, the metabolites are water-soluble and then rapidly excreted from the body in sweat or urine. In addition, GST also can resist oxidation and scavenge free radicals. Studies have found that vitamin C levels in the blood will decrease when the GST gene is defective. Thus, it will decrease the body's ability to resist oxidation and may eventually cause carcinogens to damage cells' DNA. Thereby causing cell mutations leading to cancer. There are seven types of GST family, among which GSTT1 and GSTM1 are the two most frequently mutated genes. About 10-50% of people in Asian ethnic groups lack GSTT1 and 20-60% lack GSTM1. Studies on oral cancer have pointed out that people who lack both GSTT1 and GSTM1 have a three times higher risk of developing cancer than those who have both normal genes, and those who lack both genes have a 1.79 times higher risk of developing liver cancer.