Diabetes mellitus: what it is, causes and types


Diabetes mellitus is the name given to a group of metabolic disorders that result in elevated blood glucose levels. Known popularly as high blood sugar, there are several types and several causes of diabetes. All types, however, often have similar complications, such as increased risk of damage to the kidneys, eyes, and blood vessels.

Diabetes is one of the most common diseases in the world, and its incidence has increased over the years, mainly due to poor diet and obesity.

What is glucose?

Glucose, also called dextrose, is a simple carbohydrate molecule (monosaccharide) whose main function is to provide energy for cells to function. Virtually all foods of the carbohydrate class have glucose in their composition.

Most carbohydrates in our diet are composed of three monosaccharides: glucose, fructose, and galactose. To make it easier to understand, think of these three molecules as little bricks. The way these bricks are grouped together gives rise to the different types of carbohydrates we eat, from fruit, to cereals, honey, pasta, bread, vegetables, etc.

Examples: the famous table sugar, called sucrose, is the junction of only two monosaccharides, glucose and fructose. The carbohydrate present in milk, called lactose, is the junction of glucose and galactose.

Our body needs glucose to function, it is our fuel. In fact, from bacteria to human beings, we need glucose to survive. Glucose is the only carbohydrate molecule that can provide us with energy. Both fructose and galactose must first be converted into glucose by the liver in order to be used by the cells.

Blood Glucose Control – Role of Insulin

After a meal, the carbohydrates that have been eaten will go through the process of digestion. Digesting a carbohydrate means breaking it down into several micro pieces until all the “bricks” of glucose, fructose, and galactose are released. In the small intestine, these molecules will be absorbed, reaching the bloodstream.

After a meal, a large amount of glucose, fructose, and galactose reach the bloodstream, raising the blood glucose [glycemia = concentration of glucose in the blood]. Whenever blood glucose rises, the pancreas releases a hormone called insulin, which causes the circulating glucose in the blood to enter the cells of our body.

Insulin also stimulates the storage of glucose in the liver, so that in times of need the body has a source of glucose that does not depend on food. These two actions of insulin promote a rapid drop in blood glucose, causing glucose levels to normalize quickly.

What is diabetes?

Diabetes mellitus is the name given to the group of diseases that result in the body having difficulty controlling blood glucose levels to keep them above normal. We say that diabetes is a group of diseases because there is more than one type of diabetes, with different causes and different mechanisms for the dysregulation of blood glucose levels.

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Usually diabetes is caused by a lack of insulin production or by an inability of the cells to recognize the presence of insulin, that is, there is insulin, but it cannot get the glucose into the cells. There are even cases in which the patient has both problems; not only does he produce too little insulin, but it still malfunctions.

The end result of this reduced or malfunctioning insulin production is the accumulation of glucose in the blood. The patient eats, receives a load of glucose in the blood, but the cells cannot capture it, keeping the blood glucose constantly high.

This high blood glucose, called hyperglycemia, causes two major problems. The first, in the short term, is the lack of glucose in the cells, which need it to function properly. The second, which occurs after years of illness, is damage to the blood vessels.

The excess glucose is toxic to the cells of the vessels, causing the arteries to be progressively damaged, leading to the typical complications of diabetes, such as kidney problems, blindness, cardiovascular disease, neurological damage, gangrene of the limbs, etc.


There are several types of diabetes, but three account for the vast majority of cases, they are:

  • Type 1 diabetes.
  • Type 2 diabetes.
  • Gestational diabetes.

Let us explain them.

Type 1 diabetes

Type 1 diabetes mellitus is an autoimmune disease, that is, it occurs due to the mistaken production of antibodies against our own cells, in this specific case, against the beta cells of the pancreas, responsible for the production of insulin.

We do not know exactly what triggers this mistaken production of auto antibodies, but it is known that there is an important genetic factor. However, genetics alone do not explain everything, since there are identical twin brothers in which only one of them has type 1 diabetes.

It is thought that some environmental factor is necessary for the onset of the disease. Possible culprits could include viral infections, contact with toxic substances, vitamin D deficiency, and even exposure to cow’s milk or gluten in the first months of life. The fact is that in some individuals, the immune system suddenly begins to attack the pancreas, progressively destroying it.

As the beta cells of the pancreas are destroyed, the ability to produce insulin is progressively reduced. When more than 80% of these cells are destroyed, the amount of insulin present is no longer able to control blood glucose, and type 1 diabetes mellitus appears.

Type 1 diabetes accounts for only 10% of diabetes cases and usually occurs in young people, between the ages of 4 and 15, but can affect people as young as 30 or 40.

Since type 1 diabetes is a disease that usually appears in the first years of life, it usually causes complications at a young age. A patient who is only 25 years old can have diabetes for more than 20 years, thus suffering the consequences of the disease while still young, especially if the diabetes control has not been well done during all these years.

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Since type 1 diabetes is caused by a lack of insulin, its treatment basically consists of the regular administration of insulin to control blood glucose levels.

Type 2 Diabetes

Type 2 diabetes mellitus is a disease that also has some degree of decreased insulin production, but the main problem is a resistance of the body to the insulin produced, causing the cells to be unable to capture the circulating glucose in the blood.

Type 2 diabetes occurs in adults, usually obese, sedentary, and with a family history of diabetes. Being overweight is the main risk factor for type 2 diabetes. The association between obesity and type 2 diabetes is so strong that many patients can even stop being diabetic if they manage to lose weight.

The way the body stores fat is also relevant. People with fat accumulation predominantly in the abdominal region have a higher risk of developing diabetes.

Type 2 diabetes is often accompanied by other conditions, including high blood pressure and high cholesterol. This constellation of clinical conditions (hyperglycemia, obesity, hypertension, and high cholesterol) is referred to as the metabolic syndrome, and is a major risk factor for cardiovascular disease.

Besides obesity and a sedentary lifestyle, there are other risk factors for type 2 diabetes:

  • Age over 45 years.
  • Family history of diabetes.
  • Hypertension.
  • Prior history of gestational diabetes
  • Fasting blood glucose higher than 100 mg/dL (pre-diabetes)
  • Polycystic ovary
  • High cholesterol
  • Prolonged use of medications such as corticoids, tacrolimus, cyclosporine, or nicotinic acid
  • Smoking.

A diet high in saturated fats and carbohydrates and low in vegetables and fruits.
Type 2 diabetes can initially be treated with oral medications. These are usually drugs that stimulate the pancreas to produce insulin or increase the sensitivity of the cells to the insulin present.

Over time, the hyperglycemia itself causes damage to the beta cells of the pancreas, causing a progressive reduction in insulin production. For this reason, it is common for patients with type 2 diabetes, after many years of illness, to need insulin to control their blood glucose levels.

Gestational Diabetes

Gestational diabetes is a type of diabetes that appears during pregnancy and usually disappears after delivery. This type of diabetes occurs due to a resistance to the action of insulin.

During pregnancy the placenta produces a number of hormones, some of which inhibit the action of circulating insulin, causing the mother’s blood glucose to rise. It is thought that part of this effect is to ensure a good amount of glucose for the developing fetus. It is good to remember that the pregnant woman needs glucose for herself and for the fetus. If this anti-insulin action did not exist, there would be more risk of hypoglycemia during periods of fasting, such as during the night’s sleep.

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In most women this insulin resistance does not cause major problems, since the pancreas is able to control blood glucose by increasing its production of insulin. Pregnant women produce on average 50% more insulin than non-pregnant women.

The problem arises in pregnant women who already have some prior degree of insulin resistance or whose pancreas is unable to increase its insulin production beyond baseline. The main risk factors for gestational diabetes are being overweight, late pregnancy, and pre-diabetes (I will explain further in the pre-diabetes topic).

Gestational diabetes usually appears only after the 20th week of gestation, when the anti-insulin hormones start to be produced in large amounts.

Gestational diabetes is associated with several problems for the fetus, including premature birth, respiratory problems, hypoglycemia after birth, outsized babies, and an increased risk of type 2 diabetes for both mother and child.


Pre-diabetes is a condition in which the body cannot keep blood glucose at normal levels, but it is not yet high enough to be diagnosed as diabetes.

In people with normal insulin function, fasting blood glucose (at least 8 hours of fasting) is always below 100 mg/dl. A fasting glucose persistently above 126 mg/dl is required for a diagnosis of diabetes. Therefore, everyone with a fasting glucose between 100 and 125 mg/dl is considered pre-diabetic.

Usually, what occurs in patients with altered fasting glucose is a lack of response of the body to the insulin produced. The pancreas may function well, but the cells do not respond as they should to the insulin present in the blood, causing the passage of glucose to the tissues to be impaired.

The main cause of this insulin resistance is excess weight and fat accumulation in the abdominal region. Fat cells have more difficulty in using insulin than muscle cells. In addition, excess fat produces several chemical mediators that decrease the effect of insulin in the body. As we can see, the risk factors and mechanisms of prediabetes are similar to those of type 2 diabetes.

Patients with prediabetes have a high risk of progressing to type 2 diabetes in the short to medium term. In fact, for every 100 patients diagnosed with prediabetes, 11 develop diabetes within just one year. Within 10 years, more than 50% of patients will have progressed to diabetes.