What Causes Type 2 Diabetes? Genetics, Risk Factors, and What You Can Change

Type 2 diabetes does not have a single cause. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says it usually results from a combination of risk factors, and that researchers still do not fully know what triggers the underlying problems [3][4]. What is clear is that some of those factors, like genes and age, are fixed, while others can be changed.

What Goes Wrong in the Body

The CDC compares insulin to a key that lets blood sugar into the body's cells [2]. In type 2 diabetes, cells in the muscles, fat and liver stop responding well to that key, which is called insulin resistance [2][3]. The pancreas tries to make up for it by producing more insulin, but over time it cannot keep up, and blood sugar rises [2][11]. The NIDDK describes type 2 as the result of both problems together: not enough insulin and trouble using it [3].

For a closer look at this process, see Insulin Resistance: What It Is and Why It Matters.

Is Type 2 Diabetes Genetic?

Partly. The American Diabetes Association (ADA) explains that people inherit a predisposition to diabetes, and then something in their environment triggers it. In its words, "genes alone are not enough" [10].

The ADA says type 2 has a stronger link to family history than type 1, and that twin studies show genetics play a very strong role [10]. Even so, when one identical twin has type 2 diabetes, the ADA puts the other twin's risk at "three in four at most" [10]. It is an upper limit from twin research, not the chance of a parent passing type 2 to a child.

Family history is on both US and UK risk lists. The CDC and the NHS both name having a parent, brother or sister with type 2 diabetes as a risk factor [1][6], and the NIDDK lists a family history of diabetes [4].

Genes or Shared Habits?

Families share more than DNA. The ADA notes that obesity tends to run in families and that relatives often have similar eating and exercise habits. When diabetes runs in a family, it can be hard to tell whether lifestyle or genes are responsible, and the ADA says it is "most likely" both [10].

Single-Gene Diabetes Is Different

Some rare forms of diabetes are caused by a change in a single gene, which parents can pass to their children. The NIDDK groups these as monogenic diabetes, separate from typical type 2 [3]. For how the main types differ, see Type 1 vs Type 2 Diabetes: Understanding the Key Differences.

Risk Factors You Cannot Change

Age

Risk rises with age, but health agencies use different ages in their guidance:

• CDC (US): lists being 45 or older as a risk factor [1].
• NIDDK (US): lists being 35 or older [3].
• NHS (UK): says you are more at risk if you are white and over 40, or over 25 and from a South Asian, Chinese, Black African or Black Caribbean background [6].
• NICE (England, 2012, updated 2017): encourages risk assessment for adults 40 and over, and from age 25 for people of South Asian, Chinese, African-Caribbean, Black African and other higher-risk minority ethnic backgrounds [8].

These are the ages each agency uses in its risk lists or risk checks, not a biological cut-off. The NIDDK notes that type 2 can develop at any age, even in childhood [4], and NICE states that age is no barrier to being at high risk [8].

Ethnic Background

Risk lists in the US and UK name different groups because they describe different populations:

• In the US, the CDC lists African American, Hispanic or Latino, American Indian and Alaska Native people, and notes that some Pacific Islander and Asian American people also have higher risk [1]. The NIDDK lists a similar set of groups [4].
• In the UK, the NHS and NICE list people of South Asian, Chinese, Black African and Black Caribbean backgrounds [6][8].

Ethnicity also affects how body weight is judged. The NIDDK says Asian Americans are considered overweight from a body mass index (BMI) of 23, compared with 25 for most adults and 26 for Pacific Islanders [4]. NICE likewise considers a blood test for people aged 25 and over of South Asian or Chinese descent whose BMI is above 23 [8].

Risk Factors You Can Influence

Body Weight and Waist Size

Overweight and obesity appear on risk lists from the CDC, NIDDK, NHS, NICE and the World Health Organization (WHO) [1][4][6][8][9]. Where weight is carried matters too. The NIDDK lists a large waist as a risk factor even for people with a normal BMI, and gives US markers of more than 40 inches for men and more than 35 inches for women who are not pregnant [4]. These are risk markers, not personal targets.

Physical Activity

The CDC lists being physically active less than 3 times a week [1]. The NIDDK includes not being active, for example because of a job that involves long periods of sitting or difficulty moving around [4]. WHO names not getting enough exercise as a contributing factor [9].

Smoking

The NIDDK lists smoking, including tobacco products and e-cigarettes, and exposure to secondhand smoke as risk factors [5]. WHO includes avoiding tobacco among the ways to prevent or delay type 2 diabetes [9].

Pregnancy History

Two pregnancy-related factors raise risk later in life, according to the CDC and NIDDK: having had gestational diabetes, and giving birth to a baby weighing 9 pounds or more [2][4].

Gestational diabetes usually goes away after birth, but the CDC and NHS both say a history of it makes type 2 more likely later [1][7]. NICE lists it among conditions that raise risk [8], and the NIDDK notes a professional may recommend regular testing [3].

The CDC, NHS and WHO differ in how firmly they describe the effect on the child:

• The CDC says the baby is more likely to have obesity as a child or teen and to develop type 2 later in life [1].
• The NHS says "some research has suggested" these babies may be more likely to develop diabetes or become obese later [7].
• WHO says the children are "possibly" at increased risk [9].

More on pregnancy is in Gestational Diabetes: Risks, Management, and What to Expect.

Prediabetes

Prediabetes means blood sugar is higher than normal but not high enough to be type 2 diabetes [5]. The CDC lists it as a risk factor in its own right, sharing almost all the same risk factors as type 2 [1][2]. People with prediabetes usually have no symptoms, the NIDDK notes, and it estimates about 97.6 million US adults had it in 2021 [5].

Prediabetes is not a guarantee of diabetes. The NIDDK says people with prediabetes have a high chance of developing type 2 within 5 to 10 years, but not everyone does [5], and WHO says progression is "not inevitable" [9]. The CDC says prediabetes can be reversed to prevent or delay type 2 [1]. Signs, tests and steps to take are covered in Prediabetes: Early Warning Signs and How to Reverse It.

Other Health Conditions and Medicines

Several conditions are linked to higher risk:

• CDC (US): non-alcoholic fatty liver disease [1]. See Fatty Liver and Diabetes: A Hidden Link.
• NIDDK (US): polycystic ovary syndrome (PCOS), Cushing's syndrome, acromegaly, sleep apnea and viral illnesses such as COVID-19, which it links to insulin resistance and prediabetes [5].
• NHS (UK): conditions such as high blood pressure [6].
• NICE (England): heart and circulation disease, high blood pressure, stroke, PCOS and mental health problems. NICE also names people with learning disabilities among groups to consider for a risk check [8].

The NIDDK also notes that some medicines taken over a long time can raise risk [5]. If you take a long-term medicine, ask your doctor whether it affects your risk rather than stopping or changing it yourself [4].

Children and Teens

WHO says type 2 diabetes was until recently seen only in adults and now occurs increasingly often in children [9], and the CDC says more children, teens and young adults are developing it than in the past [2]. The NHS notes it is still less common in children [6].

Children share the adult risk factors, and the NIDDK adds two of their own: being born with a low birth weight, and a mother who had gestational diabetes while pregnant with them [4]. The NHS lists ethnic background, weight, activity and family history as factors that can raise a child's risk [6]. The ADA says type 2 runs in families partly because children learn habits from their parents, and that it can be delayed or prevented in young people [10].

The NHS advises seeing a GP if you think you or your child may have type 2 diabetes, or are at higher risk [6].

What You Can and Cannot Change

The NIDDK says reaching a healthy weight, being physically active and not smoking may help prevent or delay type 2 diabetes [3][4], and WHO adds a healthy diet [9]. In England, NICE recommends a two-step approach for people at high risk: a risk assessment first, then a blood test where needed, with lifestyle-change programmes for those at highest risk [8]. The NHS offers a free NHS Health Check to people aged 40 to 74 without an existing condition [6].

Conclusion

If several of these factors apply to you, that is a reason to ask about a risk check, not a reason to assume diabetes is coming. Bring your family history and any pregnancy history to the appointment, along with a list of the medicines you take. For the wider picture of diabetes, including symptoms and treatment, see Diabetes: Causes, Symptoms, Diagnosis, Complications, and Modern Treatment (Complete Guide).

Medical Disclaimer

This article is for general information and does not replace advice from a qualified health professional. Talk to your doctor about your own risk.