Diabetes (Diabetes Mellitus)

How diabetes works

After you eat, carbohydrate is broken down into glucose, which enters the blood. The pancreas releases insulin, a hormone that lets cells take up glucose for energy and tells the liver to stop releasing stored glucose. In diabetes:

• Type 1: the immune system destroys the insulin-making beta cells, so there is little or no insulin.
• Type 2: the body's cells respond poorly to insulin (insulin resistance), and the pancreas gradually cannot make enough to compensate.

Glucose builds up in the blood. Over years, high glucose damages blood vessels and nerves throughout the body.

Types of diabetes

Prediabetes means glucose is higher than normal but below the diabetes range. It affects about 1 in 3 US adults, most of whom do not know it.

Early signs and symptoms of diabetes

• excessive thirst and dry mouth
• frequent urination, including at night
• increased hunger
• tiredness and weakness
• blurred vision
• unexplained weight loss (more typical of type 1)
• slow-healing cuts and frequent infections such as thrush, UTIs and skin infections
• tingling or numbness in the hands or feet
• dark, velvety skin patches in the neck or armpits (acanthosis nigricans), a sign of insulin resistance

Type 1 symptoms usually develop over days to weeks and can progress to diabetic ketoacidosis. Type 2 develops slowly over years, and many people have no symptoms, which is why screening matters.

Risk factors

Type 2 diabetes

• overweight or obesity, especially around the waist
• age 35 or older
• family history of type 2 diabetes
• physical inactivity
• Black, Hispanic/Latino, American Indian, Alaska Native, Asian American or Pacific Islander heritage
• history of gestational diabetes or a baby over 9 lb (4 kg)
• polycystic ovary syndrome
• high blood pressure, low HDL cholesterol or high triglycerides
• sleep apnea, and medicines such as steroids and some antipsychotics

Type 1 diabetes

• family history of type 1 diabetes
• other autoimmune conditions such as thyroid disease or celiac disease
• certain genes; no lifestyle cause is known

Diagnosis

Without clear symptoms, an abnormal result is confirmed with a repeat test. Autoantibody tests and C-peptide help distinguish type 1 from type 2 when it is unclear.

Who should be screened

The American Diabetes Association recommends screening all adults from age 35, and earlier for adults who are overweight and have another risk factor. The USPSTF recommends screening adults aged 35 to 70 with overweight or obesity. Pregnant women are screened, typically at 24 to 28 weeks.

Treatment

Lifestyle foundation

• Eating pattern: emphasize vegetables, whole grains, legumes, fruit, nuts, fish and lean protein; limit refined carbohydrates, sugary drinks and processed foods. Several patterns work, including Mediterranean and lower-carbohydrate approaches. A registered dietitian or diabetes educator helps.
• Physical activity: at least 150 minutes a week of moderate activity, plus strength training twice a week, and breaking up long periods of sitting.
• Weight loss: losing 5 to 10% of body weight improves glucose, blood pressure and cholesterol. Larger losses can put type 2 diabetes into remission in some people.
• Stop smoking, limit alcohol, and get enough sleep.

Medicines for type 2 diabetes

• Metformin: usually the first medicine; inexpensive and weight-neutral.
• GLP-1 receptor agonists (such as semaglutide, dulaglutide, liraglutide) and dual GIP/GLP-1 agonists (tirzepatide): lower glucose, promote weight loss; some reduce heart attack and stroke risk.
• SGLT2 inhibitors (such as empagliflozin, dapagliflozin): lower glucose through the urine and protect the heart and kidneys.
• DPP-4 inhibitors, sulfonylureas, pioglitazone: other options; sulfonylureas can cause low blood sugar.
• Insulin when other medicines are not enough.

Doctors choose medicines based on heart and kidney disease, weight, low blood sugar risk, cost and preferences.

Type 1 diabetes

Insulin is essential for life, given by multiple daily injections or an insulin pump. Continuous glucose monitors (CGMs) track glucose every few minutes, and automated insulin delivery systems ("hybrid closed loop") adjust insulin automatically. Teplizumab can delay the onset of type 1 diabetes in some high-risk people.

Targets

For many adults, the targets are:

• A1C below 7% (individualized: lower for some, higher for older adults or those at risk of hypoglycemia)
• glucose 80 to 130 mg/dL (4.4 to 7.2 mmol/L) before meals
• with a CGM, time in range (70 to 180 mg/dL) above 70%
• blood pressure below 130/80 mmHg
• statin therapy for most adults aged 40 to 75

Low blood sugar (hypoglycemia)

People on insulin or sulfonylureas can have hypoglycemia (glucose below 70 mg/dL or 3.9 mmol/L). Symptoms include shakiness, sweating, hunger, a racing heart, irritability and confusion. Treat with the 15-15 rule: take 15 grams of fast-acting carbohydrate (4 oz juice, glucose tablets), recheck in 15 minutes, and repeat if still low.

Complications

Short-term

• Diabetic ketoacidosis (DKA): mostly type 1, a dangerous buildup of ketones; also possible with SGLT2 inhibitors
• Hyperosmolar hyperglycemic state: very high glucose and dehydration, mostly in older adults with type 2
• Severe hypoglycemia

Long-term

• Heart disease and stroke: the leading cause of death in people with diabetes
• Diabetic retinopathy: damage to the retina; a leading cause of blindness in working-age adults
• Diabetic kidney disease: the most common cause of kidney failure
• Diabetic neuropathy: numbness, burning and pain in the feet; also affects digestion, bladder and sexual function
• Foot ulcers and amputation
• Erectile dysfunction, gum disease, hearing loss, depression and higher dementia risk

Preventing complications

• yearly dilated eye exam or retinal photos
• yearly urine albumin and blood kidney tests
• foot checks at every visit, and daily self-checks
• blood pressure and cholesterol control
• vaccines: flu, pneumococcal, hepatitis B, COVID-19 and others as recommended

Living with diabetes

Diabetes self-management education programs teach glucose monitoring, medicine use, nutrition, sick-day rules and how to handle exercise and travel. Diabetes distress and depression are common; tell your care team if you feel overwhelmed. With good management, people with diabetes can live long, active lives.

Prevention

Type 2 diabetes can often be prevented or delayed. In the Diabetes Prevention Program, people with prediabetes who lost about 7% of body weight and exercised 150 minutes a week cut their risk of developing diabetes by 58%. Metformin can also help some high-risk people. There is currently no way to prevent type 1 diabetes, though screening relatives for autoantibodies can identify early stages.

Frequently asked questions

What are the first signs of diabetes?

Common early signs are increased thirst, frequent urination, tiredness, blurred vision and, especially in type 1, weight loss. Many people with type 2 diabetes have no symptoms for years. A simple A1C or fasting glucose blood test can check.

What is the difference between type 1 and type 2 diabetes?

Type 1 is an autoimmune condition in which the pancreas stops making insulin, so insulin treatment is needed from diagnosis. Type 2 is caused by insulin resistance and gradually declining insulin production, linked to genes, weight and activity, and is often treated first with lifestyle changes and tablets.

What A1C level means diabetes?

An A1C of 6.5% or higher on two tests (or with other confirming results) means diabetes. Between 5.7% and 6.4% is prediabetes, and below 5.7% is normal. A1C reflects average blood sugar over the past 2 to 3 months.

Can type 2 diabetes be reversed?

Type 2 diabetes can go into remission, meaning A1C stays below 6.5% without medicines, often after significant weight loss through diet, medicines or bariatric surgery. It is not a cure, and glucose can rise again, so regular monitoring continues.

What foods should people with diabetes avoid?

Limit sugary drinks, sweets, white bread, refined snacks and highly processed foods, which raise blood sugar quickly. You do not need to avoid all carbohydrates: focus on fiber-rich whole grains, legumes, vegetables and fruit, with portion control and protein at meals.

Is diabetes hereditary?

Genes play a role in both types. Type 2 has a strong family link: having a parent or sibling with it raises your risk considerably, though lifestyle also matters. Type 1 also runs in families, but most people with type 1 have no close relative with it.