What Is Diabetes Mellitus? Key Facts to Know

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About 38 million Americans have diabetes, and the US Centers for Disease Control and Prevention (CDC) estimates that one in five don’t even know they have it. According to the Harvard T.H. Chan School of Public Health, about nine in ten cases of prediabetes or Type 2 diabetes in the US are preventable—but the disease continues to grow more widespread.

“In the US, it is expected that the prevalence of diabetes will rise from about 9% of people now to about 18% by 2060,” says Richard Siegel, MD, a double-board-certified endocrinologist and the Co-Director of the Diabetes and Lipid Center at Tufts Medical Center in Boston, Massachusetts. 

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What is diabetes mellitus? 

Diabetes mellitus is a disorder where the body’s blood sugar (also known as glucose) level is higher than normal, which is a condition called hyperglycemia, Dr. Siegel explains. This occurs when your pancreas doesn’t make enough (or any) of the hormone insulin—or your body isn’t using insulin properly. 

To explain insulin’s role in the body, experts use a “lock and key” analogy. Glucose is our cells’ go-to energy source, which makes its way into our bloodstream from the foods we eat (mainly carbohydrates.) Dr. Siegel adds that our cells also store this sugar for future use, especially in muscle and fat—but too much sugar can be toxic for cells. Insulin is what regulates this glucose balance. 

In other words: 

• All cells have metaphorical “locks” that keep out excess blood sugar.
• Insulin functions like a “key,” allowing the transfer of glucose into cells.  

But with diabetes, this system fails. The pancreas might not produce enough insulin “keys” to deliver glucose into cells, and/or the cell “locks” resist the insulin keys, preventing glucose from passing through cell walls. Either way, the cells don’t get necessary glucose, and excess sugar builds up in the bloodstream, causing hyperglycemia. 

Types of diabetes mellitus

“There are several subtypes [of diabetes] that can lead to hyperglycemia,” Dr. Siegel says. Broadly speaking, diabetes is categorized into three subtypes: Type 1 diabetes, type 2 diabetes, and gestational diabetes. 

“There have been major strides in the understanding, monitoring, and treatment of diabetes mellitus in the last few decades,” Dr. Siegel explains—including a better understanding of the genetics of different diabetes subtypes. In particular, researchers are moving away from a “one-size-fits-all” approach to diagnosing and treating type 2 diabetes, which makes up 90 to 95% of all adult diabetes cases, according to the CDC. 

Type 1 diabetes

“Type 1 diabetes is an autoimmune disease in which the immune system mistakenly attacks the insulin-producing cells of the pancreas,” explains Jacqueline Yuey Lonier, MD, a double-board-certified endocrinologist at the Naomi Berrie Diabetes Center and assistant professor in the Department of Medicine at Columbia University. “It results from an inherited predisposition to autoimmune disease and immune system activation against pancreatic insulin-producing cells, resulting in insulin deficiency and high blood glucose.”

In short: With type 1 diabetes, the pancreas doesn’t produce insulin “keys,” so glucose builds up in the bloodstream. 

The exact cause of type 1 diabetes is unknown. Genetics play a big role, but someone with a family history doesn’t necessarily inherit the disease—they inherit a genetic susceptibility or risk for developing type 1 diabetes, according to 2022 research published in Frontiers in Endocrinology. Many studies have investigated potential environmental factors that “favor” type 1 diabetes in those who are predisposed, including diet, vitamin D intake, infections, and gut microbiome health. 

Type 2 diabetes

Type 2 diabetes occurs when your pancreas doesn’t make enough insulin (a shortage of “keys”), there’s difficulty using the insulin at the cells (faulty “locks”), or both, according to Dr. Siegel. 

This type of diabetes is characterized by insulin resistance. “Insulin resistance means that insulin is being produced in the endocrine part of the pancreas, but it’s not working well at cells in key organs, including the liver, muscle, fat, and even brain,” he says. With your body not responding to insulin properly, the pancreas tries to make more insulin to deal with increasing levels of glucose stuck in your bloodstream—but even with more insulin “keys,” the cell “locks” won’t budge. 

“Over time, the insulin-producing cells [in the pancreas] can no longer make enough insulin to keep up with the body’s needs, and high blood glucose develops,” Dr. Lonier says. 

Cases of type 2 diabetes have increased by almost 20% in the last decade, according to 2024 research published in Diabetes, Obesity and Metabolism—and projections expect this rate to keep accelerating. One concerning report from the CDC estimates a 700% increase in type 2 diabetes diagnoses among young Americans by 2060.

Research published in 2024 in Diabetes Research: Open Access explains that with this disease burden ahead, researchers are working to further specify subtypes of type 2 diabetes to better understand and treat the disease. These subtypes include:

• Severe insulin-deficient diabetes: There’s insufficient insulin production (too few “keys”). 
• Severe insulin-resistant diabetes: Cells don’t respond to insulin (“locks” stay locked). 
• Mild obesity-related diabetes: Cells sometimes respond to insulin (“locks” don’t always work). 
• Mild age-related diabetes: There’s not enough insulin production, and cells don’t always respond properly to insulin (too few “keys” and stubborn “locks”). 

Gestational diabetes

“Gestational diabetes develops during a pregnancy and [usually] resolves with delivery,” Dr. Siegel explains. It affects about 5 to 9% of pregnant women in the US and usually develops around the 24th week, but if you have diabetes risk factors (including being overweight, having a family history of type 2 diabetes, or having PCOS,) your doctor may want to test you earlier. 

While gestational diabetes typically goes away after delivery, it’s important to treat and manage the condition as soon as possible because roughly half of women with gestational diabetes go on to develop type 2 diabetes, according to the CDC. Untreated or poorly controlled gestational diabetes also increases the risk of harm to the baby, including a higher risk of developing obesity or type 2 diabetes as they grow up, according to the American Diabetes Association (ADA). 

Can I prevent type 2 diabetes? 

“As with many medical diseases, type 2 diabetes develops from a combination of genetic abnormalities and environmental influences,” Dr. Siegel explains. Researchers have identified more than 600 regions in the genome associated with type 2 diabetes risk, per a 2024 study published in Nature. In particular, having a family history of type 2 diabetes—a biological parent or sibling—or being of Black, Hispanic, Native American, Asian American, or Pacific Islander descent are known risk factors for type 2 diabetes. 

However, “An important environmental contributor is weight gain, leading to obesity,” Dr. Siegel says. This is because excess fat tissue is a known cause of insulin resistance. 

Physical inactivity, smoking, a diet heavy in sugary and processed foods, high blood pressure, high cholesterol, polycystic ovarian syndrome (PCOS), and age are also considered type 2 diabetes risk factors. “The ADA has a type 2 diabetes risk calculator on their website, which can be a helpful tool for [your] risk assessment,” Dr. Lonier says.

She says that if you have risk factors for type 2 diabetes, you should talk with your doctor about regular screening for diabetes. A blood test called A1C (short for hemoglobin A1C) is the standard test that reflects your average blood sugar levels over the past three months. 

Does diabetes have symptoms? 

“Early diabetes [is] often ‘silent’, without any specific symptoms,” Dr. Siegel says. The same is true for prediabetes, a state where your blood sugar levels are elevated but not high enough to be diagnosed as diabetes. Prediabetes is considered a warning sign for type 2 diabetes—an estimated 70% of people with prediabetes develop type 2 diabetes, according to Frontiers in Clinical Diabetes and Healthcare. But prediabetes has been shown to effectively be reversed through diet and lifestyle changes, according to a 2023 review of studies published in the American Journal of Preventive Medicine.

“If diabetes is not treated, people can develop symptoms related to high blood sugar, including frequent urination, frequent thirst, and fatigue,” Dr. Siegel says. “Uncontrolled diabetes is also associated with weight loss as the body is unable to convert dietary carbohydrates into energy and begins to rely on fat stores to fuel metabolic processes,” Dr. Lonier says. “High blood glucose can also cause blurry vision due to changes in the fluid content of the lens of the eye.” 

Why is diabetes dangerous? 

“Chronic hyperglycemia over the course of many years can damage the small vessels of the eyes and kidneys as well as peripheral nerves—we call these diabetes complications retinopathy, nephropathy, and neuropathy, respectively,” Dr. Lonier says. “Chronically uncontrolled diabetes also increases the risk of atherosclerotic cardiovascular disease leading to heart attack and stroke.” 

Due to these effects throughout the body, diabetes now kills more than 100,000 Americans each year, according to a 2022 report from the BMJ. 

How do you treat diabetes?

Type 2 diabetes treatment

Lifestyle measures are used in diabetes treatment for all people, Dr. Siegel says. However, “Many people with type 2 diabetes can be managed with lifestyle changes alone, often with a goal of weight reduction.” According to 2022 research published in Diabetes Care:

• Weight loss of 10-15% can have a disease-modifying effect and can lead to the remission of diabetes.
• Healthy eating patterns can significantly reduce hemoglobin A1C (the measure of your average blood sugar over the past three months) and help prevent, delay, and treat diabetes-related complications.
• Regular exercise improves blood sugar levels and management and reduces the risk of cardiovascular disease and all-cause mortality. 
• Quality sleep is associated with a lower risk of obesity and improved glucose metabolism. 

But if someone’s blood sugar is still higher than healthy levels, oral or injectable medications (like insulin) can be added to their treatment plan, Dr. Siegel says. And there have been notable advancements in recent years. “Newer medications for type 2 diabetes have been shown to reduce the risk of heart and kidney disease, and in particular [a class of diabetes medications called] GLP-1 receptor agonists have been highly effective in managing type 2 diabetes and obesity and have changed the lives of many people living with these conditions,” Dr. Lonier says. 

“Medications for type 2 diabetes can be reduced or stopped in many people with an intensified lifestyle program, and so may not be lifelong,” Dr. Siegel says. In other cases, metabolic (bariatric) surgery may be advised, depending on the subtype of diabetes and level of blood sugar. 

Type 1 diabetes treatment

“Type 1 diabetes requires lifelong insulin therapy,” Dr. Siegel says—with one exception. If standard treatments aren’t successful, a pancreas transplant may be an option (though transplant recipients must take immunosuppressant medications for the rest of their lives). Other people may be candidates for pancreatic islet cell transplantation, which is a newer, minimally invasive treatment that can temporarily reverse diabetes. 

But most people with type 1 diabetes will be treated by insulin replacement via injection under the skin or via an insulin pump. “Diabetes technology—insulin pumps and continuous glucose monitors—has improved dramatically in the last several years and revolutionized the day-to-day management of people with diabetes, particularly type 1 diabetes,” Dr. Lonier says. There is also a new, recently FDA-approved medication called Teplizumab that may delay the onset of type 1 diabetes and reduce the potential for severe long-term complications, adds Dr. Siegel. 

For more wellness updates, subscribe to The Healthy @Reader’s Digest newsletter and follow The Healthy on Facebook, Instagram, and Twitter. Keep reading:

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