Can Someone Have Type 1 and Type 2 Diabetes?

A person with type 1 diabetes can develop the hallmark feature of type 2, insulin resistance, creating a condition researchers call “double diabetes.” It is not an official diagnostic category in most clinical guidelines, but the overlap is real and increasingly common. The concept challenges the neat separation most people assume exists between the two forms of the disease, and it carries practical consequences for treatment and long-term health.

What Double Diabetes Actually Means

Type 1 diabetes is driven by the immune system attacking the insulin-producing cells of the pancreas. Type 2 diabetes centers on the body’s tissues becoming resistant to the insulin the pancreas still produces. For decades, these were treated as two entirely separate diseases. Double diabetes describes what happens when both problems exist in the same person: an individual already living with the autoimmune destruction of type 1 also develops the insulin resistance and metabolic features characteristic of type 2.

The term has been used to describe people with type 1 diabetes who are overweight, have a family history of type 2 diabetes, or show clinical signs of insulin resistance such as high triglycerides, high blood pressure, or excess abdominal fat.¹ Some researchers also use “hybrid diabetes” or “type 1.5 diabetes,” though these labels overlap with other concepts and can create confusion. A 2024 review described double diabetes as “the presence of the insulin resistance characteristic of metabolic syndrome in individuals diagnosed with T1D.”²

An important distinction: double diabetes is not the same as latent autoimmune diabetes in adults, sometimes called LADA. LADA typically involves someone initially diagnosed with type 2 who later turns out to have slow-progressing autoimmune destruction of their insulin-producing cells. Double diabetes goes the other direction. It starts with a clear type 1 diagnosis, and the metabolic syndrome features layer on later. The two conditions share some genetic territory but are not the same thing.

Why This Is Becoming More Common

The rising prevalence of overweight and obesity worldwide has not spared people with type 1 diabetes. Up to 35% of young patients with type 1 diabetes are now estimated to be overweight or obese, a striking number for a condition long associated with a lean body type.³ Intensive insulin therapy itself plays a role. Tight blood sugar control requires more insulin, and higher insulin doses promote weight gain, which in turn increases insulin resistance. The cycle can be frustrating: better blood sugar management leads to weight gain, and weight gain leads to needing even more insulin.

Lifestyle factors that would push anyone toward insulin resistance, including a sedentary routine and a calorie-dense diet, apply equally to people with type 1. The difference is that when a person with type 1 develops insulin resistance, the consequences are layered on top of an already complex metabolic picture. They still make little or no insulin of their own, and now their body also responds poorly to the insulin they inject.

How Doctors Identify It

There is no single blood test that stamps a “double diabetes” label on someone’s chart. Instead, doctors piece together clues from autoantibody testing, insulin production markers, and the clinical picture.

C-peptide, a molecule released alongside insulin, is one of the most useful tools. A joint consensus report from major diabetes organizations recommended checking C-peptide more than three years after diagnosis in adults receiving insulin who test negative for autoantibodies. Very low values, below about 0.20 nmol/L, point toward type 1, while values above 0.60 nmol/L suggest type 2.⁴ Someone with confirmed type 1 who also shows clinical markers of insulin resistance, such as rising insulin requirements, central obesity, abnormal cholesterol, or skin changes like acanthosis nigricans, fits the double diabetes profile regardless of what a single lab value says.

Misdiagnosis is a genuine problem, especially in adults diagnosed later in life. A study from the Diabetes Alliance for Research in England found that 38% of people with type 1 diabetes who were diagnosed after age 30 were initially misdiagnosed as having type 2.⁵ When a heavier adult walks into a clinic with high blood sugar, the assumption often defaults to type 2. If the real diagnosis is type 1, or type 1 with overlapping insulin resistance, delayed insulin treatment can lead to dangerous metabolic crises. This is one of the reasons autoantibody testing and C-peptide measurement matter so much in ambiguous cases.

The Cardiovascular Risk

One of the most serious concerns about double diabetes is what it does to the heart and blood vessels. Type 1 diabetes alone raises cardiovascular risk, primarily through chronic high blood sugar and the vascular damage it causes. Type 2 diabetes raises cardiovascular risk too, largely through insulin resistance, abnormal cholesterol and fat distribution, and chronic low-grade inflammation. When both mechanisms operate at once, the potential for accelerated atherosclerosis goes up.

A review in Nature Reviews Endocrinology described how the combination of chronic high blood sugar and abnormal lipid partitioning in double diabetes can accelerate the atherothrombotic process, meaning the buildup and rupture of arterial plaques.⁶ Many young patients with type 1 diabetes who are also obese present with additional risk factors for large-vessel disease and are recognized to have metabolic syndrome, a cluster that was traditionally only associated with type 2.³

For the person living with double diabetes, this means that managing blood sugar alone is not enough. Cholesterol, blood pressure, and weight all need attention in ways that a traditional type 1 management plan might not emphasize. The clinical approach needs to borrow heavily from the type 2 playbook, even though the underlying autoimmune disease remains.

Treatment Beyond Insulin

Insulin is non-negotiable in type 1 diabetes. Your pancreas is not making it, so you have to inject it. But when insulin resistance enters the picture, simply giving more insulin creates its own problems: more weight gain, more resistance, and a harder time keeping blood sugar stable. This is where medications traditionally reserved for type 2 come into play.

Metformin

Metformin is the most studied add-on. It works primarily by reducing the liver’s glucose output and improving the body’s sensitivity to insulin. In people with type 1 who also have insulin resistance, metformin has been shown to cut daily insulin requirements. A 2025 randomized trial found that metformin reduced total daily insulin dose by about 0.10 units per kilogram per day compared to placebo, a statistically significant reduction.⁷ Earlier studies showed similar patterns, with lower fasting and post-meal blood sugar levels in the group taking metformin alongside insulin.⁸

The catch is that metformin does not reliably improve long-term blood sugar control as measured by HbA1c in this population. Both the 2025 trial and earlier work found no meaningful difference in HbA1c, time in target glucose range, or glycemic variability between the metformin and placebo groups.⁷ It also comes with a somewhat higher risk of gastrointestinal side effects and a small increase in hypoglycemia risk.⁹ So metformin helps reduce how much insulin you need, and it may help with cholesterol, but it does not necessarily make your blood sugar control any tighter. For someone whose main problem is escalating insulin doses and weight gain, that trade-off can still be worthwhile.

GLP-1 Receptor Agonists

The drugs that have gotten the most public attention lately, GLP-1 receptor agonists like semaglutide and liraglutide, are also being explored in type 1 patients with insulin resistance and obesity. A study of people with type 1 diabetes who started GLP-1 therapy found significant weight loss at 12 months, from a mean of about 98 kg at baseline to 90 kg. HbA1c also improved, dropping from 8.2% to 7.6%, and total daily basal insulin doses decreased.¹⁰ These drugs are not yet approved specifically for type 1 diabetes in most countries, so their use in this context is off-label. But for someone with double diabetes who is struggling with weight and rising insulin needs, the results are promising enough that many endocrinologists are already prescribing them.

SGLT2 Inhibitors and the Ketoacidosis Risk

SGLT2 inhibitors, another class of type 2 drugs, work by causing the kidneys to flush excess glucose into the urine. They have clear benefits in type 2 for blood sugar, weight, and heart and kidney protection. But in type 1 diabetes, they carry a real and dangerous risk: ketoacidosis, including a particularly tricky form called euglycemic ketoacidosis, where blood sugar levels appear normal or only slightly elevated even as dangerous levels of ketones build up in the blood.

A case report illustrated how quickly things can go wrong. A patient whose type 1 diabetes had been misdiagnosed as type 2 was started on an SGLT2 inhibitor and developed severe euglycemic ketoacidosis within two days, with a blood sugar of just 4 mmol/L but critically low bicarbonate and arterial pH.¹¹ A larger comparative study found that SGLT2 inhibitor-associated ketoacidosis had milder ketone levels on average than type 1 ketoacidosis but took significantly longer to resolve, a median of 36 hours compared to 18 hours.¹² The normal-looking blood sugar reading is what makes this dangerous: neither the patient nor the care team may suspect ketoacidosis until the person is seriously ill. For this reason, SGLT2 inhibitors are generally contraindicated in type 1 diabetes, and their use in double diabetes requires extreme caution and careful monitoring if attempted at all.

Shared Genetics Between Type 1 and Type 2

The idea that type 1 and type 2 are completely separate diseases at the genetic level is not quite right. Research into genetic susceptibility has found that some of the same gene regions are associated with risk for both types. This is especially clear in LADA, which shares key genetic signals from both type 1 and type 2, including the major immune system genes involved in type 1 and a gene region called TCF7L2 that is a strong risk factor for type 2.¹³ The overlap is not as simple as LADA being a 50-50 mix of the two genetic profiles; the magnitudes and patterns are more complex. But the finding supports the idea that the boundary between types is blurrier at the molecular level than textbook categories suggest.

For someone with type 1 diabetes who has close relatives with type 2, this genetic overlap may be part of why they are more likely to develop insulin resistance and metabolic syndrome features. It is not just the lifestyle factors, though those matter too. The genetic deck may already be stacked toward both problems coexisting.

Double Diabetes in Children and Adolescents

The pediatric picture is especially concerning. Childhood obesity rates have risen sharply in recent decades, and this trend has hit the type 1 diabetes population as well. As noted earlier, estimates suggest up to 35% of young people with type 1 diabetes are overweight or obese. Many of these children present with features that were previously only seen in adults with type 2: abnormal cholesterol, elevated blood pressure, non-alcoholic fatty liver disease, and in adolescent girls, polycystic ovary syndrome.³

This creates a clinical challenge that didn’t exist a generation ago. Pediatric endocrinologists managing type 1 now have to think about metabolic syndrome screening and prevention strategies that used to belong exclusively to the type 2 world. It also raises questions about long-term outcomes. A child who develops both autoimmune-driven insulin deficiency and obesity-driven insulin resistance early in life faces decades of compounded cardiovascular and metabolic risk. Getting the weight trajectory and insulin resistance under control during childhood and adolescence could matter enormously for what happens in their thirties and forties.

The Emotional and Psychological Weight

Managing one type of diabetes is already demanding. The constant glucose monitoring, carbohydrate counting, injection or pump management, and fear of hypoglycemia create what researchers call diabetes distress, a specific form of emotional burden distinct from clinical depression. Prevalence estimates for significant diabetes distress run above 20% in both type 1 and type 2 populations.¹⁴

For someone with double diabetes, the burden can feel compounded in ways that don’t have a neat clinical label. You are managing insulin injections and counting carbohydrates like someone with type 1, while also trying to lose weight, watch cholesterol, and fight insulin resistance like someone with type 2. The dietary advice can feel contradictory: eating enough to prevent hypoglycemia while simultaneously restricting calories to reduce insulin resistance. And there is an identity confusion that comes with not fitting cleanly into either diabetes community. Online support groups, educational materials, and even clinical care pathways are almost always organized around a single type. If your body doesn’t cooperate with the categories, you can feel like you are falling through the cracks of a system designed for cleaner diagnoses.

Healthcare providers who recognize double diabetes as a real and distinct clinical pattern can make a meaningful difference simply by naming it. Knowing that your experience has been described in the medical literature, that other people deal with the same overlapping problems, can reduce the isolation. It also opens the door to a more tailored treatment conversation, one that draws from both the type 1 and type 2 toolkits rather than forcing a choice between them.

Why It Is Not Yet an Official Diagnosis

Despite a growing body of research, double diabetes does not appear as a standalone category in the major classification systems used by endocrinologists worldwide. The American Diabetes Association and the World Health Organization both recognize type 1, type 2, gestational diabetes, and a handful of specific genetic forms, but no hybrid category. Part of the resistance is practical: the line between “type 1 with insulin resistance” and “double diabetes” is inherently blurry. How much insulin resistance is enough to qualify? What combination of metabolic markers crosses the threshold?

Some researchers have argued that formalizing double diabetes as its own diagnosis would force clinical guidelines to address the unique management needs of this group, particularly the cardiovascular risk screening and the use of adjunct medications like metformin and GLP-1 agonists.² Others worry that adding another category to an already complex classification system could create more confusion than clarity, especially since the underlying continuum of autoimmunity and insulin resistance does not break neatly at any one point.

For the person living with overlapping features, the classification debate matters less than the clinical response. Whether your chart says “type 1 diabetes with metabolic syndrome” or “double diabetes,” what counts is whether your doctor is addressing both the autoimmune insulin deficiency and the insulin resistance simultaneously, and tailoring your treatment plan accordingly. The label is useful mainly as a shorthand that reminds everyone involved that this is not a standard type 1 case and should not be managed as one.

1
PubMed. Double diabetes: A distinct high-risk group?
2
PubMed Central. Double diabetes-when type 1 diabetes meets type 2 diabetes: definition, pathogenesis and recognition
3
PubMed Central. Multi-Faceted Influence of Obesity on Type 1 Diabetes in Children – From Disease Pathogenesis to Complications
4
PubMed Central. C‐peptide determination in the diagnosis of type of diabetes and its management: A clinical perspective
5
PubMed Central. Mistaken Identity: Missed Diagnosis of Type 1 Diabetes in an Older Adult – Section: Discussion
6
PubMed. Cardiovascular risk in double diabetes mellitus–when two worlds collide
7
Nature Communications. Effect of metformin on insulin resistance in adults with type 1 diabetes: a 26-week randomized double-blind clinical trial
8
PubMed Central. The effects of metformin in type 1 diabetes mellitus
9
PubMed Central. A Review of Insulin Resistance in Type 1 Diabetes: Is There a Place for Adjunctive Metformin?
10
Obesity Medicine. GLP-1 agonists in Type 1 diabetes – Indications and use
11
Journal of Diabetes and Endocrine Practice. Euglycemic Diabetic Ketoacidosis Triggered by SGLT2 Inhibitor Use in Undiagnosed Type 1 Diabetes
12
JAMA Network Open. SGLT2 Inhibitor–Associated Ketoacidosis vs Type 1 Diabetes–Associated Ketoacidosis
13
PubMed. Overlap of genetic susceptibility to type 1 diabetes, type 2 diabetes, and latent autoimmune diabetes in adults
14
The Lancet Diabetes & Endocrinology. Diabetes distress