Diabetes mellitus

Diabetes mellitus is a group of metabolic conditions characterized by chronic hyperglycemia, namely a persistent increase in blood glucose concentration caused by impaired insulin secretion, reduced effectiveness of insulin on target tissues or, more commonly, a combination of both mechanisms. This definition should not be interpreted too narrowly, because diabetes is not merely a disease of blood sugar. It is a systemic disorder that alters carbohydrate, lipid and protein metabolism, impairs vascular function, involves the kidneys, retina, peripheral nerves, heart, brain and microcirculation, and over time reshapes the biological and prognostic profile of the entire body.

Its epidemiological importance is enormous. Diabetes is now one of the leading chronic noncommunicable diseases worldwide, with a steadily increasing prevalence associated with population aging, the greater prevalence of overweight and obesity, physical inactivity, environmental and dietary changes, as well as an improved ability to identify the disease at stages that were previously underdiagnosed. Within this context, type 2 diabetes is the most common form, whereas type 1 diabetes, gestational diabetes and specific secondary or genetic forms require separate assessment because their pathogenesis, natural history, initial treatment, risk of metabolic decompensation and follow-up differ.

In everyday language, diabetes is often equated with high blood glucose, but from a clinical standpoint this equivalence is insufficient. Blood glucose is the measurable sign of a deeper disorder of energy homeostasis. Under physiological conditions, glucose introduced through food or produced by the liver is distributed to the tissues under the finely regulated control of insulin, glucagon, incretins, the autonomic nervous system, hepatic function and the balance between nutrient availability and energy demand. When this network loses stability, glucose is no longer managed appropriately, accumulates in the blood and produces progressive toxic effects on cells and blood vessels.

Diabetes must therefore be understood as a disease of metabolic integration. On one side is the endocrine pancreas, which must detect changes in blood glucose and secrete insulin in the correct amount and at the correct time. On the other are the peripheral tissues, particularly the liver, muscle and adipose tissue, which must respond to insulin by regulating the production, use and storage of energy substrates. If the pancreas does not produce enough insulin, or if the tissues do not respond to it effectively, the balance is disrupted. The liver continues to produce glucose when it should not, muscle takes up less glucose, adipose tissue increases the release of free fatty acids, the beta cell undergoes functional and structural stress, and hyperglycemia becomes chronic.

This is why diabetes cannot be viewed as a purely laboratory-based problem. The number shown in a laboratory report represents the final result of a much more complex disorder involving genetics, immunology, hormonal regulation, nutritional status, body fat distribution, chronic low-grade inflammation, mitochondrial function and disease duration. From this perspective, the chapter on the integrated pathophysiology of diabetes is where all these components are brought together into a unified framework.

The main forms of diabetes are not merely different variants of the same disease, but distinct entities that share the hyperglycemic phenotype while arising from different mechanisms. In type 1 diabetes, the fundamental abnormality is the loss of insulin secretion caused, in most cases, by autoimmune destruction of pancreatic beta cells. The result is marked or absolute insulin deficiency, which readily predisposes patients to ketosis and ketoacidosis and makes insulin replacement therapy necessary from the time of onset or, in any case, at an early stage. This form may develop during childhood, adolescence or adulthood and should not be excluded simply because the patient is not young or does not appear lean.

In type 2 diabetes, by contrast, the dominant feature is generally a combination of insulin resistance and progressive beta-cell failure. At first, the body compensates by increasing insulin secretion, but over time this response is no longer sufficient, particularly when visceral adiposity, physical inactivity, genetic predisposition, advanced age, inadequate sleep, hepatic steatosis, metabolic inflammation or other conditions that amplify insulin resistance are also present. Type 2 diabetes is the most common form and the one responsible for most of the global burden of chronic complications, but its apparent epidemiological “ordinariness” should not lead to an underestimation of the biological and clinical heterogeneity of affected patients.

In addition to these two categories, there are gestational diabetes, monogenic forms and conditions secondary to pancreatic disease, cystic fibrosis, endocrinopathies, medications or transplantation. The essential point is that correctly classifying the patient is not a theoretical exercise: it means predicting the risk of acute metabolic decompensation, deciding whether autoantibodies or genetic mutations should be investigated, selecting the most appropriate initial treatment and correctly interpreting the clinical course. A generic diagnosis of “diabetes” is often insufficient; the correct question is always which type of diabetes is present, through which mechanism and at what stage of progression.

Diabetes may present dramatically or remain silent for a long time. The classic onset includes polyuria, polydipsia, unintentional weight loss, fatigue, blurred vision and sometimes increased appetite, but in clinical practice many patients with type 2 diabetes do not come to medical attention because of these symptoms. Instead, the disease may be identified through an incidental finding of hyperglycemia, the presence of recurrent infections, a gradual decline in physical performance or the emergence of signs of organ damage. This insidious course explains why diabetes may remain unrecognized for years.

When insulin deficiency is severe, as may occur in type 1 diabetes or in some rapidly progressive secondary forms, the clinical picture changes radically. Nausea, vomiting, abdominal pain, dehydration, tachypnea, worsening general condition and the risk of ketoacidosis are added to the osmotic symptoms. In frail older adults or individuals with long-standing type 2 diabetes, a hyperosmolar state with profound dehydration and altered consciousness may instead predominate. These presentations serve as a reminder that diabetes is not only a slowly progressing chronic disease, but can also become a genuine metabolic emergency.

It must also be considered that many manifestations of diabetes depend not so much on hyperglycemia itself as on its duration. A patient may receive the diagnosis after already beginning to develop albuminuria, peripheral neuropathy, retinal abnormalities, accelerated atherosclerotic disease or diabetic foot involvement. In other words, diabetes often reveals itself not only through its presenting symptoms, but also through its consequences. For this reason, medical history, physical examination and assessment of overall risk are integral components of the initial clinical evaluation and not merely secondary steps to be performed after laboratory confirmation.

The diagnosis of diabetes is based on established biochemical criteria, but their interpretation always requires clinical context. The main diagnostic tools are fasting plasma glucose, plasma glucose measured two hours after an oral glucose load and glycated hemoglobin, or HbA1c. In the presence of classic symptoms and unequivocal hyperglycemia, the diagnosis may be immediate; in other cases, confirmation is often necessary, particularly when the laboratory result is close to the diagnostic threshold. The crucial point is that the test must not be interpreted mechanically: anemia, hemoglobinopathies, kidney failure, pregnancy, rapid clinical progression, the use of certain medications and discrepancies between values may require caution when interpreting the results.

For this reason, the central aspect of the diagnostic process remains the diagnostic criteria and tests, which are used not only to determine whether a patient has diabetes, but also to distinguish overt diabetes, prediabetes, isolated impaired fasting glucose, impaired glucose tolerance and conditions in which a single test is insufficient. A correct diagnosis must then be followed by the second clinical level, namely classification: age at onset, body weight, family history, rate of progression, possible ketosis, autoimmune profile, signs of insulin resistance, history of pancreatic disease, medication exposure and the presence of other endocrine disorders.

In good clinical practice, the diagnostic process does not end with confirmation of hyperglycemia. From the outset, it is necessary to determine how advanced the disease is, whether complications are already present, what the patient’s cardiovascular and renal risk profile is, whether there are features suggesting an uncommon form and how urgently treatment must be initiated. Diagnosis, classification and prognostic stratification are therefore three components of the same process, and separating them too rigidly reduces the quality of clinical assessment.

The sole objective of diabetes treatment is not to lower blood glucose. Reducing glucose levels is essential, but modern treatment has broader aims: preventing complications, limiting hypoglycemia, preserving quality of life, controlling body weight when necessary, protecting the kidneys and cardiovascular system, and adapting treatment targets to age, frailty and comorbidities. This means that there is no treatment that is “correct” in the abstract, but rather a correct strategy for a specific patient at a specific stage of the disease.

In type 1 diabetes, the cornerstone of treatment is insulin therapy, because the body no longer has sufficient endogenous insulin secretion to maintain metabolic control. In type 2 diabetes, by contrast, management must always begin with the underlying pathophysiology and the patient’s clinical profile. In some cases, intensive lifestyle modification may have a substantial impact, particularly during the early stages, but it is very often necessary to use antidiabetic medications, selecting them according to the predominance of insulin resistance, residual beta-cell function, cardiovascular risk, the presence of chronic kidney disease, the risk of hypoglycemia, body weight, costs, adherence and patient preferences.

A decisive aspect of modern treatment is the transition from a purely glucocentric approach to one also focused on organ protection. Some medication classes not only improve metabolic control, but also reduce cardiovascular events, progression of kidney damage or weight gain. At the same time, nutrition, physical activity, diabetes education, self-monitoring, periodic reassessment of treatment targets and the ability to change strategy as the natural history of the disease evolves remain central. Many forms of diabetes are chronic and may progress; treatment must therefore also be dynamic to remain effective.

The true clinical burden of diabetes derives from its ability to cause damage over time. Chronic hyperglycemia, particularly when associated with hypertension, dyslipidemia, visceral obesity, smoking, metabolic inflammation and kidney disease, accelerates endothelial injury, microcirculatory dysfunction, oxidative stress, protein glycation and widespread cellular dysfunction. This leads to microvascular complications, such as retinopathy, kidney disease and neuropathy, and macrovascular complications, particularly ischemic heart disease, cerebrovascular disease and peripheral arterial disease. These categories are useful for educational purposes, but in clinical practice they tend to overlap and amplify one another.

The chronic complications of diabetes are not a separate chapter from diabetes itself, but possible outcomes of the disease whose risk increases when metabolic control and overall risk-factor management remain inadequate over the long term. Retinopathy reflects the vulnerability of the retinal microcirculation, diabetic kidney disease indicates progressive loss of the glomerular barrier and renal function, neuropathy impairs sensation, motor function and autonomic function, while diabetic foot disease demonstrates particularly clearly how neuropathy, ischemia and infection can combine within a single clinical process with a major prognostic impact.

Alongside chronic consequences, there are also acute complications, which demonstrate how rapidly and dangerously metabolic balance can be lost. However, even when acute decompensation does not occur, diabetes affects prognosis through continuous, cumulative damage. Correct management therefore does not consist of “treating a blood glucose value,” but of managing a long-term disease that requires continuity of care, targeted monitoring, periodic risk reassessment and adaptation to the different stages of life and comorbidity.