Type 2 diabetes is a chronic metabolic disease characterized by high blood glucose levels, also known as hyperglycemia, resulting from defects in both insulin action and insulin secretion. Insulin is a peptide hormone produced by the beta cells of the pancreas which regulates carbohydrate, fat and protein metabolism throughout the body. Proper insulin function is necessary to transport glucose from the bloodstream into cells for energy metabolism or storage. In type 2 diabetes, the body develops resistance to the normal effects of insulin and the pancreas gradually loses its ability to secrete adequate amounts of insulin to overcome this resistance. Over time, this leads to chronically elevated blood glucose and serious health complications if left untreated. The exact causes of type 2 diabetes are multifactorial and include genetic and environmental risk factors.
The pathophysiology of type 2 diabetes involves both insulin resistance in peripheral tissues like muscle, liver and adipose tissue as well as impaired insulin secretion. Insulin resistance occurs when cells in the aforementioned tissues fail to appropriately respond to normal levels of circulating insulin. At the cellular level, defects in insulin signaling transduction cause impaired glucose uptake and utilization in peripheral tissues despite normal or elevated levels of insulin in the blood. This leads to reduced postprandial glucose disposal and elevated blood glucose levels. Environmental factors such as obesity, lack of exercise and high-fat diets contribute significantly to the development of insulin resistance on a metabolic level through pathways that have been extensively studied. Adipokines secreted by adipose tissue affect insulin signaling pathways, free fatty acid levels compete with glucose metabolism, and chronic mild inflammation is thought to impair insulin sensitivity.
In addition to insulin resistance, pancreatic beta cell dysfunction plays an equally important role in the hyperglycemia seen with type 2 diabetes. In healthy people, beta cells sense elevations in blood glucose or free fatty acids through various nutrient sensors and molecular pathways and secrete just enough insulin to maintain normal glycemic control. In those predisposed to type 2 diabetes, beta cells gradually lose their ability to adequately compensate for peripheral insulin resistance through insulin hypersecretion – a condition termed relative insulin deficiency. Numerous mediators including hyperglycemia, elevated free fatty acids, genetic predisposition, excess cytokine levels, endoplasmic reticulum stress, oxidative stress and glucotoxicity progressively impair beta cell function over many years leading up to the eventual diagnosis of diabetes. Beta cell deterioration manifests as both impaired insulin secretion and reduced beta cell mass as the disease progresses to more advance stages.
The complex interplay of insulin resistance and relative insulin deficiency results in a disequilibrium between insulin demand and supply that drives chronic hyperglycemia. Without sufficient insulin signaling at the cellular level, excess glucose builds up in the bloodstream instead of entering cells where it can be used or stored for energy. Over time, prolonged hyperglycemia causes damage to vital organs, blood vessels and nerves due to its toxic effects when it exceeds the kidneys’ threshold for glucose reabsorption. This leads to severe microvascular and macrovascular complications that greatly impact quality of life and life expectancy if left poorly controlled. Complications include diabetic retinopathy, nephropathy, neuropathy, cardiovascular disease, peripheral arterial disease, cerebrovascular accidents, limb amputations and others. Proper management of blood glucose through a combination of lifestyle modification, pharmacological and medical nutrition therapy aims to prevent or delay these debilitating outcomes.
The underlying pathophysiology of type 2 diabetes involves a progressive decline from normal insulin sensitivity and secretion to insulin resistance and relative insulin deficiency over many years. Genetic and environmental factors promote the simultaneous development of impaired insulin signaling in tissues and decreased responsiveness of pancreatic beta cells – a vicious cycle that ultimately manifests clinically as chronic hyperglycemia. Both peripheral insulin resistance and pancreatic beta cell dysfunction are necessary for the diagnosis of diabetes, as insulinopenia alone is not sufficient to cause hyperglycemia in the absence of concurrent insulin resistance. Understanding the complex multifactorial metabolic derangements behind type 2 diabetes provides critical insights into optimal prevention, screening and management strategies for this serious chronic disease.