Diabetes mellitus is a group of metabolic diseases characterised by chronic high blood sugar (hyperglycaemia) resulting from defects in insulin production, insulin action, or both. It is one of the most common and fastest-growing chronic diseases in the world, affecting over 537 million adults globally — a number projected to reach 783 million by 2045.
Understanding diabetes — its types, causes, warning signs, and management — is essential health literacy in the modern world.
What is insulin and why does it matter?
Insulin is a hormone produced by beta cells in the pancreas. Its primary role is to regulate blood glucose by allowing cells throughout the body to take up and use glucose for energy. When we eat carbohydrates, blood glucose rises. The pancreas responds by releasing insulin, which acts like a key, unlocking cells to allow glucose entry.
In diabetes, this system breaks down — either because the pancreas cannot produce enough insulin, because cells resist insulin's action, or both. The result is chronically elevated blood glucose, which over time damages blood vessels and nerves throughout the body.
Type 1 diabetes
Type 1 diabetes is an autoimmune condition in which the immune system destroys the beta cells of the pancreas, leading to absolute insulin deficiency. Without any insulin production, blood glucose rises to life-threatening levels within hours to days if untreated.
Type 1 affects approximately 5–10% of all people with diabetes. It can develop at any age but most commonly appears in childhood and young adulthood. There is no known prevention, and it is not caused by diet or lifestyle.
Symptoms of type 1 often develop quickly over days to weeks and can include:
- Extreme thirst and frequent urination
- Unexplained and rapid weight loss
- Extreme fatigue
- Blurred vision
- Ketoacidosis (nausea, vomiting, abdominal pain, fruity-smelling breath) — a medical emergency
Type 1 requires lifelong insulin therapy. Modern management includes multiple daily injections or insulin pump therapy, continuous glucose monitoring, and carbohydrate counting. With good management, people with type 1 can live full, active lives.
Type 2 diabetes
Type 2 diabetes accounts for approximately 90–95% of all diabetes cases. It develops when cells become resistant to insulin's action and the pancreas cannot compensate by producing enough additional insulin. It develops gradually over years, often through a stage called prediabetes where blood sugar is elevated but not yet diabetic.
Risk factors include obesity (particularly abdominal fat), physical inactivity, family history, age over 45, certain ethnicities (South Asian, African-Caribbean, Middle Eastern), and history of gestational diabetes. Unlike type 1, lifestyle factors significantly influence risk — though genetics also plays an important role and many people at high risk never develop type 2.
Symptoms of type 2 are often subtle and may be present for years before diagnosis:
- Increased thirst and frequent urination
- Unexplained fatigue
- Blurred vision
- Slow-healing wounds
- Recurrent infections (urinary tract, skin)
- Tingling or numbness in hands or feet
Because symptoms develop slowly, around half of people with type 2 are undiagnosed. Regular screening (HbA1c blood test) is important for those with risk factors.
Gestational diabetes
Gestational diabetes develops during pregnancy and usually resolves after delivery. It is caused by the hormonal changes of pregnancy creating insulin resistance that the pancreas cannot fully overcome. It affects approximately 2–10% of pregnancies and carries risks for both mother and baby, including large birth weight, preterm birth, and increased risk of type 2 diabetes later in life for both.
Prediabetes: the warning stage
Prediabetes is defined as blood sugar levels above normal but below diabetic thresholds (HbA1c 42–47 mmol/mol or fasting glucose 5.6–6.9 mmol/L). An estimated 720 million people globally have prediabetes. Importantly, type 2 diabetes is not inevitable from prediabetes — intensive lifestyle intervention (weight loss of 5–10%, regular exercise) reduces progression by 50–60%.
Complications of uncontrolled diabetes
Chronically elevated blood glucose damages blood vessels and nerves throughout the body. Long-term complications include:
- Cardiovascular disease — two to four times increased risk of heart attack and stroke
- Diabetic nephropathy — kidney damage, leading cause of kidney failure in developed countries
- Diabetic retinopathy — damage to retinal blood vessels, leading cause of blindness in working-age adults
- Diabetic neuropathy — nerve damage causing pain, numbness, and sensory loss, particularly in feet
- Diabetic foot — poor circulation and neuropathy significantly increase infection risk and can lead to amputation
- Increased infection risk — impaired immune function makes infections more severe
Regular monitoring of blood sugar, blood pressure, cholesterol, and kidney function — together with annual eye and foot checks — is essential for people with diabetes to detect and address complications early.
Managing diabetes
The goals of diabetes management are to keep blood glucose within target ranges to prevent complications while maintaining quality of life. For type 2, this involves a personalised combination of:
- Lifestyle modification — diet quality, physical activity, weight management
- Blood glucose monitoring
- Medications — metformin, SGLT2 inhibitors, GLP-1 agonists, sulfonylureas, insulin
- Management of associated cardiovascular and metabolic risk factors
- Regular screening and monitoring for complications
For type 1, insulin therapy is non-negotiable and cannot be replaced by lifestyle changes. However, diet, exercise, and careful monitoring significantly improve control and reduce complication risk.
When to get tested
Testing is recommended if you have any risk factors for type 2 diabetes, have symptoms suggestive of diabetes, are pregnant, or are over 45 with any additional risk factors. A simple HbA1c blood test or fasting glucose test can identify diabetes, prediabetes, or confirm a normal result.
Editorial note: This article was written by the SymptomSense editorial team in accordance with our editorial policy. It is reviewed against NHS, WHO, and Mayo Clinic guidelines and updated regularly. Last reviewed June 2026. This article is for informational purposes only and does not constitute medical advice.