Studies have identified at least 150 DNA variations that are associated with the risk of developing type 2 diabetes. Most of these changes are common and are present both in people with diabetes and in those without. Each person has some variations that increase risk and others that reduce risk. It is the combination of these changes that helps determine a person's likelihood of developing the disease.
Within the hepatocyte, fatty acids can only be derived from de novo lipogenesis, uptake of nonesterified fatty acid and LDL, or lipolysis of intracellular triacylglycerol. The fatty acid pool may be oxidized for energy or may be combined with glycerol to form mono-, di-, and then triacylglycerols. It is possible that a lower ability to oxidize fat within the hepatocyte could be one of several susceptibility factors for the accumulation of liver fat (45). Excess diacylglycerol has a profound effect on activating protein kinase C epsilon type (PKCε), which inhibits the signaling pathway from the insulin receptor to insulin receptor substrate 1 (IRS-1), the first postreceptor step in intracellular insulin action (46). Thus, under circumstances of chronic energy excess, a raised level of intracellular diacylglycerol specifically prevents normal insulin action, and hepatic glucose production fails to be controlled (Fig. 4). High-fat feeding of rodents brings about raised levels of diacylglycerol, PKCε activation, and insulin resistance. However, if fatty acids are preferentially oxidized rather than esterified to diacylglycerol, then PKCε activation is prevented, and hepatic insulin sensitivity is maintained. The molecular specificity of this mechanism has been confirmed by use of antisense oligonucleotide to PKCε, which prevents hepatic insulin resistance despite raised diacylglycerol levels during high-fat feeding (47). In obese humans, intrahepatic diacylglycerol concentration has been shown to correlate with hepatic insulin sensitivity (48,49). Additionally, the presence of excess fatty acids promotes ceramide synthesis by esterification with sphingosine. Ceramides cause sequestration of Akt2 and activation of gluconeogenic enzymes (Fig. 4), although no relationship with in vivo insulin resistance could be demonstrated in humans (49). However, the described intracellular regulatory roles of diacylglycerol and ceramide are consistent with the in vivo observations of hepatic steatosis and control of hepatic glucose production (20,21).
The classic symptoms of diabetes are polyuria (frequent urination), polydipsia (increased thirst), polyphagia (increased hunger), and weight loss. Other symptoms that are commonly present at diagnosis include a history of blurred vision, itchiness, peripheral neuropathy, recurrent vaginal infections, and fatigue. Many people, however, have no symptoms during the first few years and are diagnosed on routine testing. A small number of people with type 2 diabetes mellitus can develop a hyperosmolar hyperglycemic state (a condition of very high blood sugar associated with a decreased level of consciousness and low blood pressure).
Studies in type 1 patients have shown that in intensively treated patients, diabetic eye disease decreased by 76%, kidney disease decreased by 54%, and nerve disease decreased by 60%. More recently the EDIC trial has shown that type 1 diabetes is also associated with increased heart disease, similar to type 2 diabetes. However, the price for aggressive blood sugar control is a two to three fold increase in the incidence of abnormally low blood sugar levels (caused by the diabetes medications). For this reason, tight control of diabetes to achieve glucose levels between 70 to120 mg/dl is not recommended for children under 13 years of age, patients with severe recurrent hypoglycemia, patients unaware of their hypoglycemia, and patients with far advanced diabetes complications. To achieve optimal glucose control without an undue risk of abnormally lowering blood sugar levels, patients with type 1 diabetes must monitor their blood glucose at least four times a day and administer insulin at least three times per day. In patients with type 2 diabetes, aggressive blood sugar control has similar beneficial effects on the eyes, kidneys, nerves and blood vessels.