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Diabetes in Humans
type 1, 2 and 3

Leo Rogier Verberne

5. How to differentiate between diabetes type 1, 2 and 3

1. An 11-year-old child must leave the classroom several times a day to go to the loo; the child drinks and eats unusual quantities but nevertheless remains thin. These symptoms developed in the course of a few weeks and point to juvenile (type 1) diabetes.
2. A corpulent 50-year-old man is constantly thirsty and drinks a lot. He has difficuly concentrating on his work and he feels tired all day. These problems have existed for many months with increasing intensity. Which leads one to suspect adult-onset (type 2) diabetes.
3. In an Aruban family 8 out of 10 children got insulin dependent diabetes at an early age. This seems to be MODY (type 3 diabetes).

However, the pictures of manifest diabetes type 1, 2 and 3 can be confusing. For example: a 16 year old, school-going girl must often leave lessons to have a pee. She has difficulty concentrating, which tires her after only a short while. Her shape is massive, which she tried to change several times by following various diets. But each time she failed after a while and fell back into excessive eating. And so her waist size grew instead of slenderizing. According to her there is no diabetes in the family. How is she to be diagnosed?

The clinical pictures of manifest diabetes type 1, 2 and 3 can be confusing. For example: a 15 year old, school going girl must often leave lessons to have a pee. She has difficulty concentrating, which tires her after only a short while. With a BMI of 27, her shape is massive, which she tried to change several times by following various stringent diets. But each time she failed after a while and fell back into excessive eating. And so her size of waist grew instead of slenderizing. Her only younger brother has no diabetic problems and she has no knowledge of any diabetes in the family medical history. How is she to be diagnosed?

Juvenile (type 1) diabetes generally develops at an age between 2 and 20 years. Adult-onset (type 2) diabetes usually does not manifest itself until middle age and MODY (type 3 diabetes) comes out between 10 and 25 years (table 1). But in some cases, juvenile diabetes does not cause problems until middle age; adult-onset diabetes can sometimes be diagnosed in youngsters (3) and sometimes MODY can be manifest only after the age of 40 (1).

Table 1. Age at which diabetes generally becomes manifest



Age (years)



2 - 20



> 40



20 - 40

Ms J., 57 years old, suffers from thirst and fatigue to an increasing extent. She is no longer able to carry out her work activities as an accountant. The numbers start to dance before her eyes after only a few hours; she cannot concentrate and as the afternoon progresses, she feels dead tired. These symptoms developed gradually over the course of a few weeks. She has difficulty sleeping because of her thirst and because she has to urinate often. During the past week, she has had to drink at least a litre every night. Upon inquiry by the general practitioner, it was found that she has always been in good health. According to her, there is no history of diabetes in her family.
She does not appear to be ill, but she does seem severely tired. Her body weight is 63 kg at a height of 1.69 m. Her blood pressure is 125/70 mmHg. What strikes the eye are the vitiligo segments on the back of her hand and lower arm. The peripheral vessels pulsate adequately and there are no heart murmurs. Lab tests confirm the probability of the diagnosis: the fasting plasma glucose level of the blood is 22 mmol/l. In view of her age, she is diagnosed with type 2 diabetes and treated with 1 mg of glimepiride per day. After a week, a fasting plasma glucose level of 9 mmol/l is measured. And the symptoms worsen again after a few weeks, despite increasing the dosage of glimepiride. When the patient became extremely nauseous and could no longer keep any food or fluids down, she was found to be in keto-acidosis upon being taken to the emergency ward.

The diagnosis of type 2 diabetes was made too readily based on age alone and was most likely incorrect. Type 1 diabetes frequently comes about at a juvenile age indeed, but it can be manifest only at old age. If the classic characteristics of type 2 diabetes (family medical history, overweight, high blood pressure and an altered lipids profile in the blood) are lacking, then the diagnosis of type 1 diabetes should be considered. This certainly applies if there are symptoms that point to auto-immunity, as was the case with this patient (vitiligo). In case of doubt, it is wise to treat with insulin in expectation of diagnostics (antibodies that target GAD antigen) (3).

In type 1 diabetics, the β-cells have been shut down due to an auto-immune reaction. That process of inflammation causes antibodies that target GAD (Glutamic Acid Decarboxylase), an enzyme that helps in the production of insulin (4). So the presence of antibodies that target GAD demonstrates the auto-immune reaction and is therefore evidence of type 1 diabetes. In the case of adult-onset diabetes or MODY, an auto-immune reaction against the β-cells does not occur and so the anti-GAD test is then negative. But anno 2017, the anti-GAD test is carried out only in some certified laboratories (in the Netherlands) and not in the regular labs for general practitioners.

The blood of type 1 diabetics no longer contains naturally produced insulin because the β-cells in these patients have been shut down (4). In the case of adult-onset diabetes and MODY on the other hand, some natural insulin is still present in the blood. That can be demonstrated, even when external insulin has been administered to a diabetic. In the natural production of insulin by the β-cells, pro-insulin is split up into equal amounts of insulin and C-peptide (Connecting-peptide) (4). No C-peptide is formed in the production of synthetic insulin. Therefore the presence of this substance in the blood points to the production of natural insulin (2). So a type 1 diabetic who is injected with synthetic insulin, has insulin in his blood indeed, but no C-peptide. Alternately, the presence of C-peptide can be demonstrated in those suffering from adult-onset diabetes or MODY. But C-peptide determinations need special medical grounds in The Netherlands.

Due to ignorance, MODY in children and youngsters is sometimes mistaken for type 1 diabetes (1). And if discovered at a mature age, it can be mistaken for adult-onset diabetes. As a consequence, people with MODY are not always properly treated. The anti-GAD test is negative, because there is no auto-immune reaction in the pancreas, and the C-peptide test is positive in MODY patients, because there is some residual insulin production, just like in adult-onset diabetics. So to distinguish between type 2 and type 3 diabetes these blood tests are not helpful. But the gene-variants in the DNA are different indeed. So DNA-research can be conclusive between adult-onset diabetes and MODY. The same DNA-research is needed to distinguish between the different types of MODY (1-10). However, in the Netherlands, the gene-variant concerned in humans with MODY is confirmed by DNA-analysis in only 5% of all 20.000 MODY patients (1). Yet that is important for the choice of treatment, which is different for each gene-variant. But, until now, it is only carried out in specialized laboratories and DNA-analysis does not make up part of the routine in making diabetes diagnoses. Because neither the anti-GAD or C-peptide tests are part of routine diabetes diagnosing, the distinction between diabetes type 1, 2 and 3 mainly depends on the age of the patients.

Back to the 16 year old school-girl in the opening of this chapter. Suppose that a complete laboratory analysis is made revealing the following (normal values between brackets):
FPG: 8.0 mmol/l (< 6.1)
2HPG: 15.0 mmol/l (< 7.8)
HbA1c: 65 mmol/mol (< 43)
Anti-GAD65 <3 U/ml (< 35)
C-peptide 0.40 nmol/l (0.26-0.62)
In the DNA research for MODY(1-10), none of these gene-variants appeared to be present. So the diagnosis is: type 2 diabetes, in spite of her juvenile age.

1. The age alone of a diabetic does not offer enough to go on in differentiating between diabetes type 1, 2 and 3 and will sometimes lead to faulty treatment.
2. Juvenile (type 1) diabetics have antibodies in their blood, targeting the enzyme GAD, as a result of the auto-immune reaction in the pancreas; because they do not produce natural insulin they have no C-peptide in their blood.
3. Adult-onset (type 2) diabetics have no antibodies in their blood against GAD because there is no auto-immune reaction against β-cells in the pancreas; they do produce some natural insulin and as a consequence they have C-peptide in their blood.
4. In humans with MODY (type 3 diabetes), the anti-GAD-test is negative because there is no auto-immune reaction in the pancreas; they do have some natural insulin and so there is also C-peptide in their blood.
5. DNA-research is required to distinguish between different types of MODY (1-10); moreover, DNA-research distinguishes between MODY and adult-onset diabetes.

1. Erfocentrum (2017). MODY
2. MedicineNet.com (2016). Definition of C-peptide
3. Tack CJ en Stehouwer CDA. Diabetes mellitus In: Interne geneeskunde. eds. Stehouwer, Koopmans en van der Meer. 14e druk (2010); ISBN 978-90-313-7360-4; p 842-843
4. Wikipedia.en (2016) Insulin
5. Wikipedia.en (2016) Glutamate decarboxylase

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