Diabetes Mellitus: Types, Diagnosis and Complications

Diabetes mellitus is a condition in which the body either does not produce enough insulin or cannot respond to insulin normally, causing blood glucose levels to remain abnormally high. Between 11% and 14% of adults worldwide have diabetes, and the condition is a leading cause of cardiovascular disease, kidney failure, blindness and lower-limb amputation when left unmanaged.

How Blood Sugar and Insulin Work

The carbohydrates in food are broken down in the digestive tract into simple sugars, principally glucose. Glucose is absorbed into the bloodstream and serves as the body's primary fuel source.

Insulin — a hormone produced by the pancreas — enables glucose to move from the blood into cells. When blood glucose rises after a meal, the pancreas releases more insulin. As glucose returns to baseline, insulin output falls. In healthy adults, blood glucose is maintained within a narrow range of approximately 70–110 mg/dL (3.9–6.1 mmol/L) throughout the day.

When the pancreas cannot produce sufficient insulin, or when cells stop responding to it normally (insulin resistance), glucose accumulates in the blood rather than reaching the cells that need it. This is diabetes.

Types of Diabetes

Type 1 Diabetes

In type 1 diabetes, the immune system attacks and destroys the insulin-producing beta cells in the pancreas. The result is little or no insulin production. Most cases are diagnosed before age 30, though onset can occur at any age. People with type 1 diabetes require lifelong insulin therapy.

The exact trigger is not fully understood, but a combination of genetic susceptibility and early-life environmental factors — such as viral infections or nutritional exposures — is thought to initiate the autoimmune process.

Type 2 Diabetes

Type 2 diabetes accounts for 90–95% of all diabetes cases. The pancreas continues to produce insulin — sometimes at above-normal levels early in the disease — but the body's cells become resistant to its effects. Over time, the pancreas loses capacity and insulin output declines.

Type 2 diabetes was once considered a condition of middle age, but it is increasingly diagnosed in younger adults and adolescents. Risk rises progressively after age 30 and continues to increase with age.

Certain medications and medical conditions can impair insulin utilisation and contribute to type 2 diabetes, including:

• High-dose corticosteroids (e.g. prednisone, prednisolone)
• Cushing's syndrome
• Some antipsychotic drugs and protease inhibitors

Prediabetes

Prediabetes refers to blood glucose levels that are above the normal range but below the threshold for a diabetes diagnosis. People with prediabetes carry a higher risk of developing type 2 diabetes as well as cardiovascular disease. A 5–10% reduction in body weight through diet and exercise significantly reduces the likelihood of progression.

Diagnostic thresholds for prediabetes:

• Fasting blood glucose: 100–125 mg/dL (5.6–6.9 mmol/L)
• 2-hour glucose after oral glucose tolerance test (OGTT): 140–199 mg/dL (7.8–11.0 mmol/L)
• HbA1c: 5.7%–6.4%

Gestational Diabetes

Gestational diabetes develops during pregnancy because of the insulin resistance caused by pregnancy-related hormonal changes. It generally resolves after delivery, but women who have had gestational diabetes face a markedly higher lifetime risk of developing type 2 diabetes.

Less Common Types

Other forms of diabetes include:

• Monogenic diabetes – caused by single-gene defects affecting insulin secretion or action
• Latent autoimmune diabetes in adults (LADA) – an autoimmune form with slower progression than typical type 1, sometimes initially misdiagnosed as type 2
• Pancreatogenic diabetes (type 3c) – resulting from pancreatitis, haemochromatosis, or surgical removal of the pancreas
• Drug-induced diabetes – associated with glucocorticoids, beta-blockers, calcineurin inhibitors, immune checkpoint inhibitors, and certain antiretrovirals

Symptoms

Diabetes symptoms vary by type and can be subtle, particularly in type 2 diabetes:

• Increased thirst (polydipsia) – elevated blood glucose draws fluid from tissues, causing dehydration
• Frequent urination (polyuria) – the kidneys excrete excess glucose through urine
• Unexplained weight loss – more pronounced in type 1; the body breaks down fat and muscle when glucose cannot enter cells
• Fatigue – cells are deprived of their primary energy source
• Blurred vision – osmotic changes alter the shape of the eye lens
• Slow-healing wounds and recurrent infections – high glucose impairs immune function and circulation
• Tingling or numbness in hands and feet – an early sign of nerve damage

Many people with type 2 diabetes have no symptoms for years. The condition is frequently discovered incidentally during routine blood tests.

Diagnosis

Fasting Plasma Glucose (FPG)

Blood is drawn after an overnight fast of at least 8 hours.

Random Blood Glucose

A random (non-fasting) blood glucose of 200 mg/dL (11.1 mmol/L) or above, in the presence of symptoms, is sufficient for a diabetes diagnosis.

HbA1c (Glycated Haemoglobin)

HbA1c reflects the average blood glucose level over the preceding 2–3 months. Unlike fasting glucose tests, it does not require preparation and can be performed at any time of day. It must be measured in a certified laboratory.

A diagnosis of diabetes generally requires two separate abnormal results, unless a single result is accompanied by clear symptoms.

Oral Glucose Tolerance Test (OGTT)

The OGTT involves a fasting blood draw, followed by ingestion of a standard glucose solution, with blood drawn again 2 hours later. It is used primarily to screen for gestational diabetes and in older adults with equivocal fasting glucose results.

Complications of Uncontrolled Diabetes

Sustained elevated blood glucose causes progressive damage to blood vessels and nerves throughout the body. Complications are broadly categorised as acute or long-term.

Acute Complications

• Hypoglycaemia – dangerously low blood glucose, usually from excess insulin or medication
• Diabetic ketoacidosis (DKA) – occurs primarily in type 1 diabetes when the body breaks down fat for energy, producing acidic ketone bodies
• Hyperosmolar hyperglycaemic state (HHS) – a serious emergency in type 2 diabetes, characterised by very high blood glucose without significant ketosis

Long-Term Complications

High blood glucose also suppresses immune function, making people with diabetes significantly more susceptible to bacterial and fungal infections.

Treatment Principles

Type 1 Diabetes

Treatment requires lifelong insulin replacement. Regimens combine rapid-acting insulin at mealtimes with a long-acting basal insulin to mimic normal pancreatic function. Continuous glucose monitors (CGMs) and insulin pumps allow more precise glucose control and are increasingly standard of care.

Type 2 Diabetes

Management follows a stepwise approach:

1. Lifestyle intervention — diet modification, regular physical activity, and weight loss form the foundation of treatment. A 5–10% reduction in body weight significantly improves insulin sensitivity.
2. Oral medications — metformin is typically the first-line agent. Additional drugs are added as needed, including DPP-4 inhibitors, GLP-1 receptor agonists (e.g. semaglutide), and SGLT-2 inhibitors (e.g. empagliflozin), the latter two of which offer additional cardiovascular and kidney benefits.
3. Insulin therapy — introduced when blood glucose cannot be adequately controlled with lifestyle changes and oral medications.

Considerations for Older Adults

Strict blood glucose targets that are appropriate for younger patients carry a higher risk of hypoglycaemia in frail or elderly individuals. Falls, confusion, and loss of consciousness from hypoglycaemia can be particularly dangerous in this group. Treatment goals are therefore individualised based on overall health status, life expectancy and other medical conditions.

Diabetes Screening in China

China's major tertiary hospitals — including nationally recognised institutions in Beijing, Shanghai, Guangzhou and Xi'an — operate specialist endocrinology and metabolic disease departments capable of delivering:

• A complete diabetes assessment (fasting glucose, HbA1c, OGTT, insulin function, lipid panel, renal and hepatic function) within a single day
• Multidisciplinary evaluation involving endocrinology, cardiology, nephrology and ophthalmology where indicated
• Transparent pricing — the hospital's published package price is confirmed before you travel, with no percentage commission and no hospital markup

If you have a family history of diabetes, are experiencing symptoms, or are seeking a thorough health assessment before they become a problem, contact a ChinaMedicalTour care navigator to arrange an appointment with full English-language coordination.

Medical content based on MSD Manuals, authored by Erika F. Brutsaert MD (New York Medical College), reviewed by Glenn D. Braunstein MD (Cedars-Sinai Medical Center). Last reviewed December 2025.