Type 2 diabetes often presents on a background genetic predisposition and is characterised by insulin resistance and relative insulin deficiency. Insulin resistance is aggravated by ageing, physical inactivity, and overweight (body mass index [BMI] 25-29.9 kg/m²) or obesity (BMI >30 kg/m²). Among obese patients, weight loss often reduces the degree of insulin resistance and may delay diabetes onset or ameliorate diabetes severity and thereby reduce risk of long-term complications. Insulin resistance affects primarily the liver, muscle, and adipocytes, and it is characterised by complex derangements in cellular receptors, intracellular glucose kinase function, and other intracellular metabolic processes. The complexity and variety of these intracellular derangements suggest that what is now classified as type 2 diabetes may be in fact a larger group of conditions that await future definition.
The precise mechanism by which the diabetic metabolic state leads to microvascular and macrovascular complications is only partly understood but probably involves both uncontrolled blood pressure (BP) and uncontrolled glucose, increasing the risk of microvascular complications such as retinopathy and nephropathy. Mechanisms may involve defects in aldose reductase and other metabolic pathways, damage to tissues from accumulation of glycated end products, and other mechanisms. With respect to macrovascular complications, high BP and glucose raise risk, but so do lipid abnormalities and tobacco use. One unifying theory postulates the existence of a metabolic syndrome that includes diabetes mellitus, hypertension, dyslipidaemias, and obesity, and predisposes to coronary heart disease, stroke, and peripheral artery disease. However, this theory is not universally accepted as more clinically useful than assessing individual cardiovascular risk factors.
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