Diabetes Mellitus Explained: Types, Symptoms, Causes, Diagnosis, and Treatment

Diabetes mellitus is a group of conditions that affect how your body uses glucose, the main source of energy for your cells. This guide explains the different types of diabetes, common symptoms, causes, diagnosis, treatment options, and practical steps to help manage blood sugar and reduce the risk of complications.

Written by: Dr. Albana Greca, MD, MMedSc, Family Physician and Medical Author.

Medically reviewed by: Dr. Ruden Cakoni, MD, Endocrinologist.

Last reviewed: July 2026.

Quick Answer

Diabetes mellitus is a group of conditions in which blood glucose becomes too high because the body does not make enough insulin, cannot use insulin effectively, or both. The main forms are type 1 diabetes, type 2 diabetes, gestational diabetes, and other less common specific types.

Diabetes is diagnosed with laboratory blood tests—not symptoms or one home-meter reading alone. It can be managed through individualized nutrition, physical activity, glucose monitoring, education, and medicines or insulin when needed. Early diagnosis and ongoing care can substantially reduce the risk of complications.

Diabetes can feel confusing at first. You may hear about blood sugar, HbA1c, insulin resistance, carbohydrates, medicines, complications, and many different target numbers. This guide explains the essential information in one place and directs you to more detailed pages when you are ready.

For a step-by-step directory of the website’s blood sugar guides, calculators, medication pages, and patient questions, visit the Diabetes Answers and Tools Learning Center.

Overview of type 1 diabetes, type 2 diabetes, gestational diabetes, and insulin resistance
Diabetes includes several conditions with different causes and treatment needs.

What Is Diabetes Mellitus?

Diabetes mellitus is diagnosed when blood glucose is persistently above established laboratory thresholds. It is not defined by one universal number such as 140 mg/dL, and hyperglycemia does not always mean that a person has chronic diabetes. Illness, stress, dehydration, pregnancy, and medicines such as corticosteroids can temporarily affect glucose.

The term mellitus distinguishes diabetes mellitus from the unrelated condition diabetes insipidus. In everyday conversation, “diabetes” usually means diabetes mellitus.

How Glucose and Insulin Normally Work

Glucose is an important fuel for the brain, muscles, and other tissues. It comes mainly from carbohydrate-containing foods and from glucose released or produced by the liver between meals and overnight.

Insulin is a hormone produced by beta cells in the pancreas. After eating, insulin helps glucose move from the bloodstream into many body cells and limits excessive glucose release by the liver.

how glucose and insulin work

The Main Types of Diabetes

Type What happens Typical treatment considerations
Type 1 diabetes An autoimmune process destroys insulin-producing beta cells. It can begin at any age. Daily insulin is essential. Glucose and ketone monitoring, carbohydrate education, and emergency planning are important.
Type 2 diabetes The body becomes resistant to insulin and may gradually produce too little insulin for its needs. Nutrition, activity, weight management when appropriate, and individualized medicines; some people also need insulin.
Gestational diabetes Diabetes first diagnosed during pregnancy that was not clearly present before pregnancy. Pregnancy-specific glucose targets, nutrition therapy, monitoring, and sometimes insulin or another clinician-selected treatment.
Other specific types May result from a genetic condition, pancreatic disease, endocrine disorder, medication, infection, or another identifiable cause. Treatment depends on the cause and may differ from standard type 1 or type 2 care.

Type 1 Diabetes

Type 1 diabetes is caused by autoimmune destruction of pancreatic beta cells. The body makes little or no insulin, so insulin treatment is necessary for survival. It is not caused by eating sugar, body weight, or personal failure.

Symptoms may develop quickly and include thirst, frequent urination, weight loss, fatigue, nausea, vomiting, abdominal pain, ketones, or deep breathing. Adults can also develop type 1 diabetes, so age alone should not be used to determine diabetes type.

Type 2 Diabetes

Type 2 diabetes usually develops through a combination of insulin resistance and gradual loss of adequate insulin secretion. Genetics, age, body-fat distribution, physical activity, sleep, medicines, pregnancy history, and social or environmental factors may all contribute.

Type 2 diabetes is not simply caused by eating too much sugar. Sugary drinks and excess calories can contribute to weight gain and metabolic risk, but the condition has multiple interacting causes.

Gestational Diabetes

Gestational diabetes occurs during pregnancy and often causes no symptoms. Testing is therefore essential. It usually resolves after delivery, but it increases the parent’s future risk of type 2 diabetes and requires postpartum follow-up.

Learn more from our guide to controlling gestational diabetes safely.

Other Specific Types

Less common forms include monogenic diabetes, cystic-fibrosis-related diabetes, diabetes after pancreatitis or pancreatic surgery, and diabetes related to certain medicines or endocrine disorders. Correct classification matters because treatment may change.

What Is Prediabetes?

Prediabetes means glucose is above the usual healthy range but below the diagnostic range for diabetes. It is a warning of increased risk rather than a guarantee that type 2 diabetes will develop.

Prediabetes also provides an opportunity for prevention. Structured nutrition, physical activity, weight reduction when appropriate, sleep improvement, and sometimes medication can reduce progression risk.

Insulin resistance often contributes to prediabetes and type 2 diabetes. Read more about what insulin resistance means and how it is assessed.

Common Diabetes Symptoms

Possible symptoms include:

  • Frequent urination, including during the night
  • Increased thirst or dry mouth
  • Increased hunger
  • Unexplained weight loss
  • Fatigue or weakness
  • Blurred vision
  • Slow-healing cuts or repeated infections
  • Tingling, burning, or numbness in the feet or hands

Type 2 diabetes and gestational diabetes may cause few or no symptoms. Symptoms are also nonspecific and can occur with many other conditions. Testing—not guesswork—is required.

Our detailed guide explains the symptoms of diabetes and when to arrange testing.

What Causes Diabetes?

There is no single cause that applies to every type:

  • Type 1: An autoimmune process in a genetically susceptible person, probably influenced by environmental triggers that are still being studied.
  • Type 2: A combination of genetic susceptibility, insulin resistance, impaired insulin secretion, aging, body-fat distribution, inactivity, sleep problems, and other metabolic or environmental factors.
  • Gestational diabetes: Pregnancy-related hormonal and metabolic changes that increase insulin resistance in a susceptible person.
  • Other types: Specific genetic changes, pancreatic damage, medicines, hormonal disorders, or other diseases.

Explore these mechanisms in more detail on our causes of diabetes overview.

Where Does Blood Glucose Come From?

Blood glucose comes from two main sources:

  1. Food: Carbohydrates are digested into glucose and other simple sugars. The liver converts some of these substances into forms the body can use or store.
  2. The liver: Between meals and overnight, the liver releases stored glucose and makes new glucose to maintain the body’s energy supply.

Protein and fat can also influence glucose indirectly, especially over several hours. This is why fasting glucose can be high even when a person has not eaten and why food is only one part of diabetes management.

How Is Diabetes Diagnosed?

Diabetes is diagnosed using laboratory blood tests. Common criteria for nonpregnant individuals include:

Test Prediabetes Diabetes
Fasting plasma glucose 100–125 mg/dL 126 mg/dL or higher
HbA1c 5.7–6.4% 6.5% or higher
2-hour glucose during a 75-g OGTT 140–199 mg/dL 200 mg/dL or higher
Random plasma glucose No single prediabetes threshold 200 mg/dL or higher with classic symptoms or hyperglycemic crisis

Unless hyperglycemia is unmistakable or a crisis is present, an abnormal result usually needs confirmation. Pregnancy uses different testing pathways and thresholds.

A home glucose meter or continuous glucose monitor can identify concerning patterns, but it cannot independently establish a diabetes diagnosis. See our complete guide to blood tests for diabetes, including fasting glucose, HbA1c, and the OGTT.

How Diabetes Is Managed

Diabetes care should be individualized. The plan may include:

Glucose Monitoring

Finger-stick testing or a continuous glucose monitor can help identify fasting, after-meal, overnight, exercise, illness, and medication-related patterns. The schedule depends on diabetes type, medicines, insulin use, pregnancy, and risk of low glucose.

Balanced Nutrition

A diabetes-friendly eating pattern is not a “sugar-free” diet. Most people benefit from appropriate carbohydrate portions, vegetables, fiber-rich foods, protein, healthy fats, whole fruit rather than juice, and fewer sugar-sweetened drinks and heavily refined foods.

Learn how meals affect glucose in our guide to foods, drinks, and blood sugar.

Physical Activity

Regular activity can improve insulin sensitivity, cardiovascular fitness, mobility, and well-being. The type and amount should reflect age, fitness, heart health, eye disease, neuropathy, medications, and hypoglycemia risk.

Medicines and Insulin

People with type 1 diabetes require insulin. Type 2 diabetes treatment may include metformin, GLP-1 receptor agonists, SGLT2 inhibitors, DPP-4 inhibitors, sulfonylureas, insulin, or other therapies. The choice may consider glucose, heart and kidney disease, weight goals, hypoglycemia risk, cost, and patient preferences.

Read our overview of diabetes medicines, benefits, and side effects.

Weight, Sleep, and Emotional Health

Weight management may improve type 2 diabetes for some people, but advice should avoid blame and preserve nutrition and muscle. Sleep apnea, poor sleep, depression, chronic stress, and diabetes distress also deserve assessment and support.

Routine Preventive Care

Diabetes management also includes blood-pressure and cholesterol care, kidney testing, dilated eye examinations, foot checks, dental care, vaccinations, smoking cessation, and regular review of medicines and treatment goals.

key areas of diabetes care

Can Diabetes Be Cured or Reversed?

There is currently no general cure for type 1 diabetes or type 2 diabetes. Some people with type 2 diabetes can achieve remission, often after substantial and sustained weight loss or metabolic surgery. Remission means glucose remains below the diabetes range without glucose-lowering medication for a defined period; it does not guarantee that diabetes will never return.

Herbs, detoxes, vitamins, chromium, cinnamon, bitter melon, and other supplements should not be presented as proven cures. Some products can interact with medicines or cause side effects. Do not replace insulin or prescribed treatment with a supplement.

Possible Complications

Over time, persistent hyperglycemia can damage blood vessels and nerves. Diabetes may increase the risk of:

  • Heart disease and stroke
  • Chronic kidney disease
  • Retinopathy and vision loss
  • Peripheral and autonomic neuropathy
  • Foot ulcers, infections, and amputation
  • Dental and gum disease
  • Sexual and bladder problems
  • Severe hypoglycemia from some treatments
  • Diabetic ketoacidosis or hyperglycemic hyperosmolar state

Complications are not inevitable. Glucose management, blood-pressure and cholesterol care, smoking cessation, appropriate medicines, and regular screening can substantially reduce risk.

Learn how screening and early treatment help in our overview of diabetes complications and prevention.

Can Type 2 Diabetes Be Prevented?

Type 1 diabetes cannot currently be prevented through diet or lifestyle. Type 2 diabetes can often be delayed or prevented in people at high risk through structured lifestyle intervention.

Helpful strategies may include:

  • Regular physical activity
  • Reducing prolonged sitting
  • A balanced, fiber-rich eating pattern
  • Weight loss when medically appropriate
  • Adequate sleep and treatment of sleep apnea
  • Smoking cessation
  • Managing blood pressure and cholesterol
  • Medication such as metformin for selected high-risk individuals

Prevention advice should be realistic and individualized. Genetics, medication effects, disability, food access, stress, work schedules, and social conditions also affect risk.

When to Arrange Diabetes Testing

Ask about testing when you have symptoms, a previous high result, gestational-diabetes history, polycystic ovary syndrome, cardiovascular disease, overweight or obesity, a close family history, high blood pressure, abnormal cholesterol, or other significant risk factors.

Adults should also receive age-appropriate screening even without symptoms, according to current clinical guidance. Children and adolescents with overweight or obesity and additional risk factors may need screening as well.

When High or Low Blood Sugar Is an Emergency

Seek Urgent Medical Help

Get urgent or emergency care for vomiting, abdominal pain, deep or difficult breathing, fruity-smelling breath, confusion, severe dehydration, fainting, seizures, inability to swallow safely, or moderate or large ketones.

Severe low blood sugar, loss of consciousness, stroke symptoms, or chest pain also requires emergency assistance. Do not give food or drink to an unconscious person.

Very high and very low readings require action based on symptoms, diabetes type, medicines, ketones, and the person’s written plan. Review our guide to determine whether blood sugar may be too high or too low.

Doctor’s Note

Diabetes is not one disease with one treatment. Correct classification, accurate testing, and an individualized plan matter. Ask not only, “What is my glucose?” but also, “What type of diabetes do I have, what are my heart and kidney risks, am I having low glucose, and is my plan practical and safe?”

Most Asked Questions

Is diabetes caused by eating sugar?

No. Eating large amounts of sugary food or drinks can contribute to excess calorie intake and type 2 diabetes risk in some people, but type 1, type 2, gestational, and other forms have different and more complex causes.

Does blood sugar above 140 mg/dL mean diabetes?

Not automatically. Interpretation depends on whether the test was fasting, after a glucose drink, after a meal, or random. Diagnosis uses established laboratory criteria and often needs confirmation.

Can someone have diabetes without symptoms?

Yes. Type 2 and gestational diabetes may cause no obvious symptoms. Screening is important when risk factors are present.

Can type 1 diabetes begin in adulthood?

Yes. Type 1 diabetes can begin at any age. Adults with rapid symptoms, weight loss, ketones, or unexpected insulin deficiency need prompt assessment.

Can people with diabetes eat carbohydrates?

Yes. Carbohydrate type, amount, timing, medicines, and the complete meal affect glucose. Most people need an individualized plan rather than total carbohydrate avoidance.

Can walking lower blood sugar?

Physical activity can improve insulin sensitivity and may lower glucose, but it must be used safely. Do not exercise through severe hyperglycemia, ketones, dehydration, chest pain, or serious illness.

Can supplements control diabetes?

No supplement is a replacement for proven diabetes care. Evidence for many products is limited, and some can interact with medicines or cause liver, kidney, bleeding, or hypoglycemia problems.

Educational safety note: This article provides general diabetes education and does not replace personal medical advice, diagnosis, or treatment. Do not start, stop, or change insulin, diabetes medicines, supplements, carbohydrate targets, weight-loss plans, or exercise without speaking with your healthcare professional.

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