Medical disclaimer: This article is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always talk to your doctor or diabetes care team before making any changes to your medications, diet, or lifestyle.

At a Glance

• Diabetes mellitus is a chronic condition where blood sugar (glucose) levels are too high because the body doesn’t make enough insulin, doesn’t use it effectively, or both. [1]

• The main types are type 1, type 2, and gestational diabetes (during pregnancy). [1,2]

• Type 2 diabetes is by far the most common, accounting for about 90–95% of all diagnosed cases. [2,3]

• Untreated or poorly controlled diabetes can damage the heart, kidneys, nerves, eyes, and blood vessels. [2,3]

• Evidence-based lifestyle programs can reduce the risk of developing type 2 diabetes by about 58% in people at high risk. [4,5]

• Some people with type 2 diabetes may achieve “remission”—near-normal blood sugars for at least 3 months without glucose-lowering medication—through intensive lifestyle changes and/or medical treatments, under specialist supervision. [6,7]

What Is Diabetes Mellitus?

Diabetes mellitus is a group of conditions in which the body has trouble keeping blood glucose (sugar) within a healthy range. Glucose is your body’s main fuel. Insulin, a hormone made by the pancreas, acts like a key that helps glucose move from your bloodstream into your cells, where it’s used for energy. [1]

In diabetes mellitus, one or more of the following happens:

• Your body doesn’t make any insulin (or makes very little).

• Your body doesn’t respond properly to insulin (insulin resistance). [1,3]

• Over time, the pancreas can’t keep up, and insulin production falls.

The result is hyperglycemia—blood sugar that stays too high over time. Persistent hyperglycemia is what leads to long-term complications. [1,2]

How Your Body Normally Uses Glucose and Insulin

1. You eat food.

2. Your digestive system breaks carbohydrates down into glucose.

3. Glucose enters your bloodstream.

4. Rising blood glucose signals your pancreas to release insulin.

5. Insulin helps move glucose from the bloodstream into your cells for energy or storage. [1]

When this system works well, blood sugar stays in a narrow, healthy range.

In diabetes, there is a breakdown in this system:

• There is too little insulin (especially in type 1 diabetes).

• There is insulin resistance (cells don’t respond well to insulin), common in type 2 diabetes. [1,3]

• Or a combination of both.

Types of Diabetes Mellitus

1. Type 1 Diabetes Mellitus

• What it is: An autoimmune condition where the immune system mistakenly destroys the insulin-producing beta cells in the pancreas. [1]

• Who it affects: Often begins in childhood or adolescence, but can appear at any age.

• Insulin dependence: People with type 1 diabetes require insulin therapy from diagnosis to survive. [1]

2. Type 2 Diabetes Mellitus

• What it is: A metabolic condition where the body becomes insulin resistant and cannot use insulin effectively. Over time, the pancreas may also produce less insulin. [1,3]

• Who it affects: Historically more common in adults, but increasingly seen in younger people due to obesity, physical inactivity, and other factors. [2,3]

• How common: About 90–95% of all diagnosed diabetes cases are type 2. [2,3]

3. Gestational Diabetes

• What it is: Diabetes first diagnosed during pregnancy, when placenta-related hormone changes make the body more insulin resistant. [1]

• After pregnancy: Blood sugar usually returns to normal after delivery, but people who have had gestational diabetes have a much higher risk of developing type 2 diabetes later in life. [1,2]

4. Other Specific Types

Less common forms of diabetes include:

• Monogenic diabetes (for example, MODY — maturity-onset diabetes of the young).

• Secondary diabetes due to conditions like pancreatitis, certain medications (such as long-term steroids), or endocrine disorders. [1]

These require specialist evaluation and often genetic or endocrine work-ups.

How Common Is Diabetes and Why It Matters

Diabetes is very common and rising worldwide. For example, in the United States: [2,3]

• Type 2 diabetes accounts for around 90–95% of diagnosed cases.

• Diabetes is a leading cause of kidney failure, lower-limb amputation, and adult blindness.

Because diabetes can quietly damage organs for years before symptoms appear, early diagnosis and proactive management are essential. [2]

What Causes Diabetes Mellitus?

The causes vary by type, but usually involve a mix of genetics, immune function, environment, and lifestyle. [1,3]

Causes of Type 1 Diabetes

• Autoimmune attack on the beta cells of the pancreas.

• Likely triggered by a combination of genetic susceptibility and environmental factors (such as viral infections). [1]

People with a family member with type 1 diabetes have a higher risk, but many people who develop type 1 have no known family history.

Causes of Type 2 Diabetes

Type 2 diabetes develops gradually, often over many years. Key drivers include: [1,3]

• Insulin resistance (cells become less responsive to insulin).

• Excess body weight, especially around the abdomen.

• Physical inactivity.

• Unhealthy dietary patterns (high in refined carbohydrates, sugary drinks, and ultra-processed foods).

• Family history of type 2 diabetes.

• History of gestational diabetes.

• Age, certain ethnic backgrounds, and other metabolic conditions (like fatty liver disease).

Over time, the pancreas may also produce less insulin.

Causes of Gestational Diabetes

• Hormones produced during pregnancy increase insulin resistance.

• If the pancreas can’t produce enough extra insulin to overcome that resistance, blood sugar rises and gestational diabetes develops. [1]

Understanding Insulin Resistance

Think of insulin as a key and your cells as locks on a door.

• In a healthy system, the key works smoothly.

• With insulin resistance, the locks are stiff or jammed. The body still produces insulin (sometimes a lot), but the cells don’t respond well. [3]

What happens next:

1. The pancreas tries to produce more insulin to overcome resistance.

2. Over time, the beta cells become overworked and may fail to keep up.

3. Blood sugar levels rise, leading to prediabetes and eventually type 2 diabetes if nothing changes. [3]

Insulin resistance can be present for years before diabetes is diagnosed, which is why early screening in at-risk individuals is so important. [3]

Is Diabetes Hereditary?

Genetics and Type 2 Diabetes

Family history matters—but it isn’t destiny. Some estimates suggest that if one parent has type 2 diabetes, a child may have roughly a 40% lifetime risk of developing it, and if both parents have type 2 diabetes, the risk may be closer to 70%. [8]

These numbers vary by study, ethnicity, lifestyle, and other risk factors.

The good news: Even with a strong genetic predisposition, intensive lifestyle interventions (nutrition, activity, weight management) have been shown to reduce the risk of type 2 diabetes by about 58% in people with prediabetes. [4,5]

Genetics and Type 1 Diabetes

• Type 1 diabetes also has a genetic component, but the risk patterns are different.

• Specific genes increase susceptibility, but environmental triggers (such as infections) appear to play a major role. [1]

How Is Diabetes Diagnosed?

Most guidelines, including the American Diabetes Association (ADA) Standards of Care, use the following blood tests and cut-offs (usually confirmed on a second test unless the person has clear symptoms): [1,9]

1. A1C (Glycated Hemoglobin)

Reflects average blood sugar over approximately 3 months.

• Normal: < 5.7%

• Prediabetes: 5.7–6.4%

• Diabetes: ≥ 6.5%

2. Fasting Plasma Glucose (FPG)

Measured after at least 8 hours without calories.

• Normal: < 100 mg/dL

• Prediabetes: 100–125 mg/dL

• Diabetes: ≥ 126 mg/dL

3. 2-Hour Oral Glucose Tolerance Test (OGTT)

Measured 2 hours after drinking a 75 g glucose solution.

• Normal: < 140 mg/dL at 2 hours

• Prediabetes: 140–199 mg/dL

• Diabetes: ≥ 200 mg/dL

4. Random Plasma Glucose

Diabetes can be diagnosed if random plasma glucose is ≥ 200 mg/dL with classic symptoms (such as increased thirst, frequent urination, unexplained weight loss, or blurred vision). [1,9]

Your clinician will choose tests based on your risk factors, symptoms, and overall health.