The Twin Cycle Hypothesis of Etiology of Type 2 Diabetes
Together with evidence of normalization of insulin secretion after bariatric surgery, insights into the behavior of the liver and pancreas during hypocaloric dieting lead to a hypothesis of the etiology and pathogenesis of type 2 diabetes (Fig. 6): The accumulation of fat in liver and secondarily in the pancreas will lead to self-reinforcing cycles that interact to bring about type 2 diabetes. Fatty liver leads to impaired fasting glucose metabolism and increases export of VLDL triacylglycerol, which increases fat delivery to all tissues, including the islets. The liver and pancreas cycles drive onward after diagnosis with steadily decreasing β-cell function. However, of note, observations of the reversal of type 2 diabetes confirm that if the primary influence of positive calorie balance is removed, then the processes are reversible.
Figure 6.The twin cycle hypothesis of the etiology of type 2 diabetes.
During long-term intake of more calories than are expended each day, any excess carbohydrate must undergo de novo lipogenesis, which particularly promotes fat accumulation in the liver. Because insulin stimulates de novo lipogenesis, individuals with a degree of insulin resistance (determined by family or lifestyle factors) will accumulate liver fat more readily than others because of higher plasma insulin levels. In turn, the increased liver fat will cause relative resistance to insulin suppression of hepatic glucose production. Over many years, a modest increase in fasting plasma glucose level will stimulate increased basal insulin secretion rates to maintain euglycemia. The consequent hyperinsulinemia will further increase the conversion of excess calories to liver fat. A cycle of hyperinsulinemia and blunted suppression of hepatic glucose production becomes established. Fatty liver leads to increased export of VLDL triacylglycerol,85 which will increase fat delivery to all tissues, including the islets. This process is further stimulated by elevated plasma glucose levels.85 Excess fatty acid availability in the pancreatic islet would be expected to impair the acute insulin secretion in response to ingested food, and at a certain level of fatty acid exposure, postprandial hyperglycemia will supervene. The hyperglycemia will further increase insulin secretion rates, with consequent enhancement of hepatic lipogenesis, spinning the liver cycle faster and driving the pancreas cycle. Eventually, the fatty acid and glucose inhibitory effects on the islets reach a trigger level that leads to a relatively sudden onset of clinical diabetes. Figure adapted with permission from Taylor.98
How long will diabetes stay away after weight loss? Long-term normal blood glucose control in previously diabetic individuals after bariatric surgery demonstrates that diabetes does not recur for up to 10 years, unless substantial weight gain occurs. These observations are consistent with the twin cycle hypothesis and the existence of a trigger level for adverse metabolic effects of fat in the pancreas. Hence, for a given individual with type 2 diabetes, reducing the liver and pancreas fat content below his or her personal trigger levels would be expected to result in a release from the fatty acid–mediated dysfunction. Individual tolerance of different degrees of fat exposure vary, and understanding this liposusceptibility will underpin the future understanding of genetically determined risk in any given environment. However, this should not obscure the central point: If a person has type 2 diabetes, there is more fat in the liver and pancreas than he or she can cope with.