The causes of diabetes are complex and only partly understood. This disease is generally considered multifactorial, involving several predisposing conditions and risk factors. In many cases genetics, habits and environment may all contribute to a person’s diabetes.

To complicate matters, there can be contrary risk factors for the various forms of the disease. For example, autoimmune diabetes (type 1 and latent autoimmune diabetes of adulthood, LADA) is more common in white people, but metabolic diabetes (type 2 and gestational diabetes) is more common in people of other races and ethnicities. Type 1 is usually diagnosed in children, but advancing age is a risk factor for type 2 and gestational diabetes.

Insulin resistance, prediabetes and metabolic syndrome are strong risk factors for type 2 diabetes. Other diabetic risk factors and causes include:

• Genetics and family history. Certain genes are known to cause maturity-onset diabetes of the young (MODY) and Wolfram syndrome. Genes also contribute to other forms of diabetes, including types 1 and 2
  
  Family medical history is also influential to varying degrees. For example, a person whose parents both have type 1 diabetes has a 10 to 25 percent chance of developing that disease, according to the American Diabetes Association, and someone whose parents both have type 2 diabetes has a 50 percent chance of developing that disease.
  
  
• Weight and body type. Overweight and obesity are leading factors in type 2 diabetes and gestational diabetes. Excess fat, especially around the abdomen (central obesity), promotes insulin resistance and metabolic syndrome.
  
  Most people with autoimmune diabetes (type 1 and LADA) are of normal weight, and excess weight has not traditionally been considered to be related to these conditions. However, recent research indicates that obesity may hasten the development of type 1 diabetes and that the increasing rate of type 1 diabetes may be at least partly due to the rise of childhood obesity. Furthermore, patients with autoimmune diabetes who gain weight are susceptible to insulin resistance and double diabetes.
  
  
• Sex. Though men make up less than 49 percent of the U.S. adult population, they account for 53 percent of the adult cases of diabetes, according to the National Institutes of Health (NIH). The prevalence of diabetes in American men and women was similar until 1999, when a growing disparity began, according to an analysis of statistics published by the U.S. Centers for Disease Control and Prevention (CDC). Little or no research has been conducted to explain this trend. One factor may be the documented increase in recent years of low testosterone levels (male hypogonadism), which scientists have linked to insulin resistance.
  
  
• Level of physical activity. Lack of regular exercise is blamed for much of the twin global epidemics of obesity and diabetes.
  
  
• Diet. The effect of diet in the development of diabetes is controversial. Some studies have linked heavy consumption of soft drinks and other simple carbohydrates to risk of metabolic diabetes, and foods low in the glycemic index, such as whole grains, to reduced risk. Yet the ADA states that eating foods containing sugar does not cause the disease. The culprit, rather, is the weight gain due to sedentary habits and excess intake of calories, according to the ADA.
  
  Another dispute centers around whether being fed cow’s milk early in life might be linked to type 1 diabetes. Some researchers have noted a connection, but others have not. Further scientific research is likely on this topic.
  
  
• Other diseases. Medical conditions including high blood pressure, hyperlipidemia (unhealthy levels of cholesterol), polycystic ovarian syndrome, asthma and sleep apnea have been linked to type 2 diabetes. Celiac disease (gluten intolerance) and other autoimmune diseases have been linked to type 1. The many conditions that may cause secondary diabetes include pancreatitis, hemochromatosis, endocrine disorders including hyperthyroidism, Cushing’s disease and acromegaly, and genetic conditions including cystic fibrosis, Down syndrome and some forms of muscular dystrophy.
  
  
• Hormones. These chemical messengers can contribute to diabetes in various ways. For example, stress hormones such as cortisol have been linked to fluctuating glucose levels in type 2 diabetes, and stress hormones in women during pregnancy have been linked to risk of type 1 diabetes in the child. The release of growth and sex hormones during adolescence may make some teens more susceptible to diabetes. A wide range of hormonal treatments including anabolic steroids, growth hormone, estrogens, injected contraceptives, androgen deprivation therapy for prostate cancer and corticosteroids have been linked to secondary diabetes.
  
  
• Medical treatments. In addition to hormonal therapies, medications including diuretics, beta blockers (another class of antihypertensives), immunosuppressives, antiretrovirals (AIDS/HIV drugs) antipsychotics, lithium, and some antidepressants, anticonvulsants and chemotherapy drugs have been linked to an increased risk of secondary diabetes. Pancreatectomy and radiation therapy may also result in secondary diabetes. Drugs including pentamidine (used to treat pneumonia) and L–asparaginase (used to treat leukemia) have been linked to type 1 diabetes.
  
  
• Other chemicals. In addition to these pharmaceuticals, some studies have linked PCBs, other pollutants and certain pesticides including the defoliant Agent Orange and dioxin (its active ingredient) to insulin resistance and type 2 diabetes. Common consumer plastics and plastics ingredients including phthalates and bisphenol A have also been linked to insulin resistance in some cases. Exposure to agricultural pesticides during pregnancy has been tentatively linked to gestational diabetes. A rat poison called pyriminal has been linked to type 1 diabetes.
  
  
• Other environmental factors. Some researchers theorize that free radicals may contribute to the development of type 1 and possibly other forms of diabetes. Free radicals are formed as a result of chemical reactions in the body. Smoke, air pollution and even genetics contribute to the formation of free radicals. When these radicals build up, they can destroy cells, including those involved in the production of insulin.
  
  Cold weather is another possible environmental factor in type 1 diabetes. This disease occurs more commonly in cold climates and develops more frequently in the winter than the summer.
  
  
• Viruses. Some people are diagnosed with type 1 diabetes after a viral infection. Viruses thought to be related to type 1 diabetes include mumps, rubella and coxsackie virus (related to the virus family that causes polio and hepatitis).
  
  
• Smoking. Cigarette smoking is a risk factor for type 2 diabetes and possibly other forms of diabetes.
  
  
• Alcohol. Excessive use of alcohol is a risk factor for diabetes. For example, it can cause pancreatitis. However, some research has found that light drinking may decrease the risk of becoming diabetic.

Most of these risk factors can be described as either uncontrollable, such as genetics and age, or controllable, such as exercise and diet. Some, such as obesity, may involve genetics and lifestyle choices. People cannot alter their uncontrollable risk factors, but they can lower their risk of developing diabetes by reducing controllable risk factors through improved health habits.