Diabetes Mellitus Causes, Symptoms, and Treatment

Authored by , Reviewed by Dr Sarah Jarvis MBE | Last edited | Meets Patient’s editorial guidelines

This article is for Medical Professionals

Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find the Diabetes (Diabetes Mellitus) article more useful, or one of our other health articles.


Treatment of almost all medical conditions has been affected by the COVID-19 pandemic. NICE has issued rapid update guidelines in relation to many of these. This guidance is changing frequently. Please visit https://www.nice.org.uk/covid-19 to see if there is temporary guidance issued by NICE in relation to the management of this condition, which may vary from the information given below.

Diabetes mellitus is a metabolic disorder characterised by persistent hyperglycaemia (random plasma glucose more than 11 mmol/L) with disturbances of carbohydrate, protein, and fat metabolism resulting from defects in insulin secretion (leading to insulin deficiency), insulin action (leading to insulin resistance), or both.

The main types of diabetes mellitus are:

  • Type 1 diabetes mellitus: results from the body's failure to produce sufficient insulin.
  • Type 2 diabetes mellitus: results from resistance to the insulin, often initially with normal or increased levels of circulating insulin.
  • Gestational diabetes: pregnant women who have never had diabetes before but who have high blood glucose levels during pregnancy are said to have gestational diabetes. Gestational diabetes affects about 4% of all pregnant women. It may precede development of type 2 (or rarely type 1) diabetes.

The diagnosis of type 1 diabetes is based on clinical grounds in adults presenting with hyperglycaemia (random plasma glucose more than 11 mmol/L), bearing in mind that adults with type 1 diabetes typically (but not always) present with one or more of the following:

  • Ketosis.
  • Rapid weight loss.
  • Age of onset younger than 50 years.
  • Body mass index (BMI) below 25 kg/m2.
  • Personal and/or family history of autoimmune disease.

Suspect type 1 diabetes in a child or young person presenting with hyperglycaemia (random plasma glucose more than 11 mmol/L) and the characteristic features of:

  • Polyuria.
  • Polydipsia.
  • Weight loss.
  • Excessive tiredness.

Diagnosis of type 2 diabetes:

  • Suspect a diagnosis of type 2 diabetes if persistent hyperglycaemia that may be accompanied by clinical features:
    • Symptoms such as polydipsia, polyuria, blurred vision, unexplained weight loss, recurrent infections, and tiredness. NB: these may be mild or absent.
    • Signs such as acanthosis nigricans (a skin condition causing dark pigmentation of skin folds, typically the axillae, groin, and neck), which suggests insulin resistance.
    • The presence of risk factors (see below).
  • Persistent hyperglycaemia is defined as:
    • HbA1c of 48 mmol/mol (6.5%) or more.
    • Fasting plasma glucose level of 7.0 mmol/L or more.
    • Random plasma glucose of 11.1 mmol/L or more in the presence of symptoms or signs of diabetes.
  • If the person is symptomatic, a single abnormal HbA1c or fasting plasma glucose level can be used, although repeat testing is sensible to confirm the diagnosis.
  • If the person is asymptomatic, do not diagnose diabetes on the basis of a single abnormal HbA1c or plasma glucose result. Arrange repeat testing, preferably with the same test, to confirm the diagnosis. If the repeat test result is normal, arrange to monitor the person for the development of diabetes, the frequency depending on clinical judgement.

Severe hyperglycaemia in people with acute infection, trauma, circulatory or other stress may be transitory and is not diagnostic of diabetes.

Other types of diabetes include:

  • Maturity-onset diabetes of the young (MODY) includes several forms of diabetes with monogenetic defects of beta-cell function (impaired insulin secretion), usually manifesting as mild hyperglycaemia at a young age and usually inherited in an autosomal-dominant manner.[3] .
  • Secondary diabetes: accounts for a small minority of patients with diabetes mellitus. Causes include:

Some patients with type 2 diabetes require insulin, so the old terms of insulin-dependent diabetes mellitus (IDDM) for type 1 diabetes and non-insulin-dependent diabetes mellitus (NIDDM) for type 2 diabetes are inappropriate. Type 2 diabetes is increasingly diagnosed in children and adolescents and so the old term maturity-onset diabetes for type 2 diabetes is also inappropriate.

A Lancet paper proposed a new classification model for adult-onset diabetes.[5] Researchers from Sweden and Finland say the model can identify individuals with increased risk of complications at diagnosis:

  • Cluster 1: SAID (severe autoimmune diabetes) - type 1 diabetes and latent autoimmune diabetes in adults (LADA).
  • Cluster 2: SIDD (severe insulin-deficient diabetes) - high HbA1C, impaired insulin secretion and moderate insulin resistance.
  • Cluster 3: SIRD (severe insulin-resistant diabetes) - obesity and severe insulin resistance.
  • Cluster 4: MOD (mild obesity-related diabetes) - obesity without insulin resistance.
  • Cluster 5: MARD (mild age-related diabetes) - similar to cluster 4 but comprising older adults and only modest metabolic disarrangement.

Individuals in cluster 3 (most resistant to insulin) have a significantly higher risk of diabetic kidney disease than individuals in clusters 4 and 5 who had been prescribed similar diabetes treatment. Cluster 2 (insulin deficient) have the highest risk of retinopathy.

The development of type 1 diabetes mellitus is based on a combination of a genetic predisposition and an autoimmune process that results in gradual destruction of the beta cells of the pancreas, leading to absolute insulin deficiency. There is usually a pre-diabetic phase where autoimmunity has already developed but with no clinically apparent insulin dependency. Insulin autoantibodies can be detected in genetically predisposed individuals as early as 6-12 months of age.[6]

Possible triggers for the process may include viruses, dietary factors, environmental toxins, and emotional or physical stress. Early cessation of breast-feeding has also been linked to increased risk of developing type 1 diabetes, but the association is unproven and controversial.[7]

  • Approximately 8% of those with diabetes have type 1 diabetes - usually juvenile-onset, but it may occur at any age. It may be associated with other autoimmune diseases. It is characterised by insulin deficiency.
  • A sibling of a person with type 1 diabetes has 6-7% risk of developing it themself. This risk increases to 30-70% in identical twins. The risk for a child is 1-9% if a parent has type 1 diabetes.
  • Associated with HLA DR3 and DR4 and islet cell antibodies around the time of diagnosis.
  • Patients always need insulin treatment and are prone to ketoacidosis.
  • The most at-risk population for type 1 diabetes is Caucasian of northern European ancestry. Incidence is high in Scandinavian people.
  • Approximately 90% of those with diabetes; they are usually older at presentation (most often >30 years of age) but it is increasingly diagnosed in children and adolescents.
  • Type 2 diabetes is associated with excess body weight and physical inactivity.
  • All racial groups are affected but there is increased prevalence in people of South Asian, African, African-Caribbean, Polynesian, Middle-Eastern and American-Indian ancestry.
  • It is caused by impaired insulin secretion and insulin resistance and has a gradual onset.
  • Those with type 2 diabetes may eventually need insulin treatment.

The global age-standardised incidence rate of type 2 diabetes significantly increased from 228 per 100,000 people in 1990 to 279 per 100,000 people in 2017.

Risk factors for type 2 diabetes[2]

  • Obesity, especially central (truncal) obesity.
  • Lack of physical activity.
  • Ethnicity: people of South Asian, African, African-Caribbean, Polynesian, Middle-Eastern and American-Indian descent are at greater risk of type 2 diabetes, compared with the white population.
  • History of gestational diabetes.
  • Impaired glucose tolerance.
  • Impaired fasting glucose.
  • Drug therapy - eg, combined use of a thiazide diuretic with a beta-blocker.
  • Low-fibre, high-glycaemic index diet.
  • Metabolic syndrome.
  • Polycystic ovary syndrome.
  • Family history (2.4-fold increased risk for type 2 diabetes).
  • Adults who had low birth weight for gestational age.
  • Patients with all types of diabetes may present with polyuria, polydipsia, lethargy, boils, pruritus vulvae or with frequent, recurrent or prolonged infections.
  • Patients with type 1 diabetes may also present with weight loss, dehydration, ketonuria and hyperventilation. Presentation of type 1 diabetes tends to be acute with a short duration of symptoms.
  • Presentation in patients with type 2 diabetes tends to be subacute with a longer duration of symptoms.
  • Patients with diabetes may present with acute or chronic complications, as outlined in the sections on 'Complications', below.

The World Health Organization (WHO) now recommends that glycated haemoglobin (HbA1c) can be used as a diagnostic test for diabetes. An HbA1c of 48 mmol/mol (6.5%) is recommended as the cut-off point for diagnosing diabetes. A value less than 48 mmol/mol does not exclude diabetes diagnosed using glucose tests.[8] See also the separate Glycated Haemoglobin (HbA1c) article.

  • Diabetes is usually diagnosed by an HbA1c of 48 mmol/mol (6.5%) or more. If the use of HbA1c is inappropriate (eg, people with end-stage chronic kidney disease), type 2 diabetes is diagnosed by a fasting plasma glucose level of 7.0 mmol/L or greater.
  • In an asymptomatic person, the diagnosis of diabetes should never be based on a single abnormal HbA1c or fasting plasma glucose level; at least one additional abnormal HbA1c or plasma glucose level is essential. If the second test results are normal, it is prudent to arrange regular review of the person.
  • In a symptomatic person (thirst, increased urination, recurrent infections, weight loss, drowsiness and coma), diabetes can be diagnosed with more confidence on the basis of a single abnormal HbA1c or fasting plasma glucose level (although a second test may be prudent).
  • Severe hyperglycaemia in people with an acute infection, trauma, circulatory or other stress may be transitory and should not be regarded as diagnostic of diabetes.

The management plan for a person with diabetes includes:[2]

  • Diabetes education: structured education and self-management (at diagnosis and regularly reviewed and reinforced) to promote awareness.
  • Diet and lifestyle: healthy diet, weight loss if the person is overweight, smoking cessation, regular physical exercise.
  • Maximising glucose control while minimising adverse effects of treatment, such as hypoglycaemia.
  • Reduction of other risk factors for complications of diabetes, including the early detection and management of hypertension, drug treatment to modify lipid levels and consideration of antiplatelet therapy with aspirin.
  • Monitoring and early intervention for complications of diabetes, including cardiovascular disease, feet problems, eye problems, kidney problems and neuropathy.

A global assessment of an individual's cardiovascular risk is essential. See the separate Cardiovascular Risk Assessment article.

See the separate articles:

Type 1 diabetes[1]

  • Without insulin replacement, people with type 1 diabetes would die within days or weeks.
  • With insulin replacement, people with type 1 diabetes can participate normally in the usual activities of daily life but are at risk of complications.
  • The risk of complications is greatly reduced by keeping circulating glucose levels to as near normal as possible in order to reduce tissue damage.
  • Disability from complications that are not avoided can often be prevented by early detection and active management.

Type 2 diabetes[2]

  • Insulin deficiency in type 2 diabetes progresses with time and usually worsens over a period of years.
  • Type 2 diabetes diagnosed in young people is characterised by rapid onset and progression, with more common and progressive comorbidities (such as fatty liver disease and sleep apnoea), and more rapid development of microvascular complications and cardiovascular risk than adult-onset type 2 diabetes, leading to increased morbidity and mortality rates.
  • Young-onset type 2 diabetes is associated with greater mortality rates, more complications, and increased cardiovascular disease risk factors than for people with type 1 diabetes of similar duration.

Type 1: despite a great deal of ongoing research, there are currently no interventions before diagnosis that have shown any benefit.

There is interest as to whether vaccination can be applied in autoimmune and inflammatory conditions. Vaccination may play a role further in the prevention of type 1 diabetes .

Type 2: see the separate Prevention of Type 2 Diabetes article.

Further reading and references

  1. Diabetes - Type 1; NICE CKS, November 2020 (UK access only)

  2. Diabetes - type 2; NICE CKS, February 2022 (UK access only)

  3. Maturity-onset Diabetes of The Young; Online Mendelian Inheritance in Man (OMIM)

  4. Wolfram Syndrome 1, WFS1; Online Mendelian Inheritance in Man (OMIM)

  5. Ahlqvist E, Storm P, Karajamaki A, et al; Novel subgroups of adult-onset diabetes and their association with outcomes: a data-driven cluster analysis of six variables. Lancet Diabetes Endocrinol. 2018 Mar 1. pii: S2213-8587(18)30051-2. doi: 10.1016/S2213-8587(18)30051-2.

  6. Dunger DB, Todd JA; Prevention of type 1 diabetes: what next? Lancet. 2008 Nov 15372(9651):1710-1. Epub 2008 Sep 22.

  7. Knip M, Virtanen SM, Akerblom HK; Infant feeding and the risk of type 1 diabetes. Am J Clin Nutr. 2010 May91(5):1506S-1513S. doi: 10.3945/ajcn.2010.28701C. Epub 2010 Mar 24.

  8. Use of Glycated Haemoglobin (HbA1c) in the Diagnosis of Diabetes Mellitus; World Health Organization, 2011

  9. Primavera M, Giannini C, Chiarelli F; Prediction and Prevention of Type 1 Diabetes. Front Endocrinol (Lausanne). 2020 Jun 211:248. doi: 10.3389/fendo.2020.00248. eCollection 2020.

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