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What is type 2 diabetes?

Diabetes refers to a group of chronic metabolic diseases characterized by high levels of glucose in the blood. This occurs when insulin production by the pancreas is defective, or when insulin cannot be used by the body.1,2

In the U.S. alone, more than 30 million people across all ages have diabetes.3 Among the different types of diabetes, type 2 diabetes (T2D) is by far the most common in adults, comprising between 90-95% of all cases.1,2

After the body has broken down food into usable compounds, like glucose, it uses the hormone insulin to help the body’s cells uptake that glucose for energy. In the case of T2D, the body no longer responds to insulin effectively, resulting in ‘insulin resistance’. T2D individuals have varying degrees of insulin resistance and deficiency, though this deficiency is not absolute as in type 1 diabetes. Since T2D progresses slowly, symptoms only appear late in the disease, and can remain undetected for many years. An estimated 7 million adults with T2D in the U.S. are undiagnosed.2,3,4

What are the common symptoms?

Symptoms are a consequence of hyperglycemia (too much glucose in the blood) and include:

  • Blurred vision
  • Fatigue
  • Polydipsia (excessive thirst)
  • Polyuria (excessive urination)
  • Weight loss, sometimes with polyphagia (excessive appetite)

Because T2D symptoms may appear late in the development of the disease, between 24-64% of people with diabetes go undiagnosed and untreated.2

When untreated, T2D can have severe, long term, or life threatening complications, including ketoacidosis or hyperosmolar hyperglycemic state. Diabetes can cause damage to the heart, blood vessels, eyes, kidneys, and nerves. This can lead to heart disease, stroke, loss of vision, and end-stage renal disease.1,2

Hyperosmolar hyperglycemic state is a complication of T2D consisting of severe hyperglycemia, extreme dehydration, hyperosmolality, and altered consciousness.

What are the causes?

T2D develops through a complex interaction between genetic, metabolic, and environmental factors, which ultimately lead to increased levels of blood glucose. For example, T2D is two to six times more prevalent in the non-white population of the U.S., which includes African Americans, Hispanic Americans, and Native Americans.5 At the metabolic level, insulin resistance is considered the major force behind the development of hyperglycemia in T2D.6 Finally, environmental factors such as weight gain, obesity, and decreased physical activity increase the risk of developing diabetes.6

The risk factors associated with the onset of T2D include1,2:

  • Being overweight or obese
  • Ethnicity
  • Family history
  • Poor diet
  • Physical inactivity
  • Sleep duration
  • Smoking
  • Unhealthy lifestyle

When an individual has an elevated level of blood glucose that does not meet the criteria for diabetes, they can be diagnosed with prediabetes, meaning that they have a higher risk of developing T2D. The criteria for a prediabetes diagnosis can include one or more of the following:

  • Impaired fasting glucose (a blood sugar level between 100 and 125 mg/dL). Fasting is defined as no caloric intake for at least 8 h.
  • Impaired glucose tolerance (a blood sugar level of 140 to 199 mg/dL two hours after an oral glucose tolerance test, or OGTT).
  • Glycated hemoglobin (HbA1c between 5.7 to 6.4 percent).1

In turn, the criteria for a diabetes diagnosis can include one or more of the following:

  • Glycated hemoglobin (HbA1c equal to or greater than 6.5 percent).1
  • Elevated fasting plasma glucose (a blood sugar level equal or more than 126 mg/dl).
  • Oral Glucose Tolerance Test (a blood sugar level equal or more than 200 mg/dl, during the 2 hour test).
  • In a patient with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasma glucose ≥200 mg/dl.
The amount of hemoglobin to which glucose is bound within red blood cells. This amount reflects the average blood glucose level over the past 3 months, and is usually elevated in persons with uncontrolled diabetes.

How does this topic relate to my microbiome?

Research shows that the gut microbiome in many T2D patients is imbalanced, or in dysbiosis.7,8 Studies analyzing data from individuals with T2D, prediabetes, or normal glucose levels show differences in the composition and function of the gut microbiota. Compared with healthy subjects, microorganisms like the Roseburia species are less abundant in T2D subjects, while others, such as Akkermansia muciniphila, are more abundant.8,9

Though no unique microorganisms have been linked to the disease, there are theories for how the gut microbiome may contribute to the development of T2D. Possibilities include increased lipopolysaccharide (LPS) levels, modifications to bile acid metabolism, and alterations in short chain fatty acids (SCFA).10 SCFA deficiency is associated with T2D.11

A component of the outer layers from several bacteria.

Which diseases/topics are related to type 2 diabetes?

T2D is a metabolic disorder, which is a disease that involves problems in the body’s metabolic processes.

Other metabolic disorders include:

  • Obesity
  • Prediabetes syndrome
  • Non-alcoholic fatty liver disease

Obesity is also a T2D comorbidity. Aside from obesity, T2D comorbidities include3,12,13:

Comorbidities are one or more diseases co-occurring with the primary disease.
Abnormal levels of lipids in the blood or serum.

How can I take action?

First, consult your healthcare provider. The best way to manage T2D is with an early diagnosis, since the consequences of undiagnosed and untreated diabetes are severe.

A crucial criterion for the diagnosis of T2D is based on the measurement of glucose levels in a blood sample, either taken while the patient is in a fasting state, or 2 hours after ingesting a 75g oral load of glucose. Another method, the measurement of glycated haemoglobin (HbA1c), can be done even if the patient is not in a fasting state.2 The selection of specific tests and treatments depends on how advanced the condition is.

Blood sugar monitoring, insulin, and oral medications may play a role in effective treatment.2 Given that approximately 90% of people with T2D are overweight or obese, adopting a healthy lifestyle – including weight management, healthy diet, and getting regular physical activity – is important in managing diabetes.2,3

More specific treatments may be necessary for correct management of T2D.


1. American Diabetes Association. Diagnosis and classification of diabetes mellitus. (2010). Diabetes Care, 33 (Suppl. 1), S62–S69.

2. World Health Organization (2016). Global Report on Diabetes.

3. Centers for Disease Control and Prevention (2017). National Diabetes Statistics Report.

4. Beagley, J., Guariguata, L., Weil, C., & Motala, A. A. (2014). Global estimates of undiagnosed diabetes in adults. Diabetes Research and Clinical Practice, 103(2), 150–160.

5. Spanakis, E. K., & Golden, S. H. (2013). Race/Ethnic Difference in Diabetes and Diabetic Complications. Current Diabetes Reports, 13(6), 814–823.

6. Stumvoll, M., Goldstein, B. J., & van Haeften, T. W. (2005). Type 2 diabetes: principles of pathogenesis and therapy. The Lancet, 365(9467), 1333–1346.

7. Li, X., Watanabe, K., & Kimura, I. (2017). Gut microbiota dysbiosis drives and implies novel therapeutic strategies for diabetes mellitus and related metabolic diseases. Frontiers in Immunology, 8, 1882.

8. Qin, J., et al. (2012). A metagenome-wide association study of gut microbiota in type 2 diabetes. Nature, 490(7418), 55–60.

9. Karlsson, F. H., et al. (2013). Gut metagenome in European women with normal, impaired and diabetic glucose control. Nature, 498(7452), 99–103.

10. Upadhyaya, S., & Banerjee, G. (2015). Type 2 diabetes and gut microbiome: at the intersection of known and unknown. Gut Microbes, 6(2), 85–92.

11. Zhao, L., et al. (2018). Gut bacteria selectively promoted by dietary fibers alleviate type 2 diabetes. Science, 359(6380), 1151–1156.

12. American Association of Clinical Endocrinologists (2018). Managemente of common comorbidities of diabetes.

13. Luijks, H., Schermer, T., Bor, H., van Weel, C., Lagro-Janssen, T., Biermans, M., & de Grauw, W. (2012). Prevalence and incidence density rates of chronic comorbidity in type 2 diabetes patients: An exploratory cohort study. BMC Medicine, 10(1), 128.