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This content is taken from the London School of Hygiene & Tropical Medicine's online course, Diabetic Eye Disease: Building Capacity To Prevent Blindness. Join the course to learn more.

Skip to 0 minutes and 11 seconds Glucose is the primary source of energy that power cells in our bodies. It is a simple sugar, a monosaccharide, that is obtained from the food we eat, specifically carbohydrates. Once the carbohydrate is digested in the intestine, glucose is absorbed into our bloodstream.

Skip to 0 minutes and 33 seconds Blood glucose levels, or blood sugars, constantly fluctuate. High levels are found following a meal, and low levels after strenuous exercise.

Skip to 0 minutes and 47 seconds These fluctuations of blood glucose levels are regulated by the hormone insulin. Beta cells in the pancreas are stimulated to produce and release insulin into the bloodstream by high blood glucose levels. Insulin is a key enabling blood glucose to move into cells and reduce the levels of glucose in the blood. Within the cell glucose provides an essential energy resource for its functioning. Excess blood glucose is stored in the liver as glycogen, a reserve energy source. Glycogen can be released back into the blood by a hormone called glucagon. Optimum levels of blood glucose are maintained by insulin. In healthy people fasting blood sugar levels are between 72 and 100 milligrams per deciliter.

Skip to 1 minute and 36 seconds Diabetes mellitus is a chronic condition caused by changes in the body’s ability to produce or respond to insulin. This results in either high blood glucose levels, hyperglycemia, or low blood glucose levels, hypoglycemia. Diabetes is due to either the pancreas not producing enough insulin, or the cells of the body not responding properly to the insulin produced. This is known as insulin resistance or insensitivity. In both situations, the lack of insulin activity leads to increased blood glucose levels, hyperglycemia.

Skip to 2 minutes and 14 seconds Persistent hyperglycemia causes irreversible structural changes in our body’s small blood vessels. This leads to decreased blood flow and even blockages known as occlusions.

Skip to 2 minutes and 26 seconds Over time decreased and blocked blood flows can eventually lead to complications, such as blindness, kidney failure, foot ulcers, nerve damage, poor wound healing, and increased risk of heart attack and stroke. There are three main types of diabetes mellitus, type 1, and type 2, and gestational diabetes.

Skip to 2 minutes and 50 seconds People with type 1 diabetes may be genetically susceptible. In these individuals an autoimmune response is triggered in the body following an event, for example, a viral infection. Antibodies fighting the viral infection also destroy the body’s own insulin producing beta cells in the pancreas. And this leads to reduced insulin production and progress has to become diabetes. Over time all the beta cells in the pancreas are destroyed, and insulin injections are needed. In type 2 diabetes a person’s cells become insensitive to insulin. Causes of insulin resistance include genetic predisposition, aging, and obesity. Once tissue cells become resistant, insulin is unable to unlock the pathway, allowing blood glucose to enter cells. So blood glucose levels remain high, and this triggers further insulin production.

Skip to 3 minutes and 42 seconds Initially these metabolic changes may be mild and clinically asymptomatic, pre-diabetes. Gestational diabetes occurs when women with no previous history of diabetes develop high blood sugar levels during pregnancy. Gestational diabetes usually resolves when the baby is born. Clinically both type 1 and type 2 diabetes progress at different rates but through similar stages of an early asymptomatic stage with insulin resistance, impaired glucose tolerance, to mild hyperglycemia, pre-diabetes, and to full diabetes, which can lead to complications and requires medication. Understanding the natural history of type 2 diabetes enables primary health workers to recognize and manage the people at risk of developing clinical diabetes.

Skip to 4 minutes and 33 seconds Signs which indicate that someone is at high risk for developing type 2 diabetes include weight gain around the middle abdomen, high cholesterol, and high blood pressure. Gradually as diabetes progresses, typical symptoms experienced include increased thirst, hunger, and urination. Additional symptoms include dry mouth, mood changes, and fatigue.

Skip to 4 minutes and 58 seconds Measuring blood glucose levels can identify those individuals with impaired glucose tolerance who are at risk of developing diabetes. Two tests are used. A fasting blood glucose test is done after 8 to 10 hours of fasting, and is typically done in the morning before breakfast. The two hour blood glucose test is measured two hours after ingesting 75 grams of oral glucose.

Skip to 5 minutes and 24 seconds To confirm a diagnosis of diabetes, the fasting blood glucose levels need to be greater than 126 milligrams per deciliter. In the absence of symptoms, they should be confirmed using the to hour blood glucose level test. Glycated Hemoglobin, HBA1C, reflects the blood glucose level fluctuations over the previous eight to 12 weeks. This is a standardized laboratory test which can be done at any time of day. An HBA1C level over 6.5% is part of the diagnostic criteria for diabetes.

Skip to 5 minutes and 58 seconds The criteria for diagnosing gestational diabetes is different. Gestational diabetes should be diagnosed if the woman has either a fasting plasma glucose level of 95 milligrams per deciliter or above, or a two hour plasma glucose level of 120 milligrams per deciliter or above.

Skip to 6 minutes and 18 seconds In conclusion, blood glucose levels in our bodies are kept in balance by insulin. Changes in the production of insulin, or increasing insulin resistance, leads to increased blood glucose levels, hyperglycemia. In all people with diabetes, persistent hyperglycemia leads to microvascular changes and irreversible organ damage. People with diabetes must manage their blood glucose levels to prevent or limit progression of diabetic complications.

Introducing diabetes: The story of sugar and insulin in the body

A brief history of diabetes mellitus

Diabetes has a long history in the medical literature. In ancient Egypt, about 1500 years before the common era (BCE), diabetes was described as a condition linked with weight loss and excessive urination. During the same period, Indian physicians noted that the urine from these patients attracted ants and classified it as ‘madhumeha’ or honey urine. Until the 11th century, diagnosis was often made by ‘water tasters’, who drank the urine of someone suspected of having diabetes and confirmed if it tasted sweet.

The word diabetes comes from ancient Greece and means to pass through, a reference to the frequent urination. Mellitus means honeyed in Latin, a reference to the sweet urine. The modern medical term ‘Diabetes Mellitus’ captures both the symptoms and the signs of the condition.

1700 to 1800: Understanding the condition

Around 1776, Dobson observed that in some people, diabetes was fatal in less than five weeks and, for others, it remained as a chronic condition. This is the first time a distinction was made between type 1 and type 2 diabetes. However, it took another 80 years, before Claude Bernard made the link between diabetes and glycogen metabolism for the first time. This was followed in 1869 by the discovery of two types of cells in the pancreas (the islets of Langerhans) by a German medical student.

Early 1900s: Linking diabetes with insulin

The link between the pancreas and diabetes was made through Dr Banting’s experiments. He demonstrates that administering secretions from the pancreas of healthy dogs to dogs whose pancreases had been removed, could treat high sugar levels. This discovery led to an award of a Nobel prize in 1923.

Jean de Meyer and Sir Edward Albert Sharpey-Schafer both independently proposed the name “insulin”. It is believed, the name is in reference to the tiny cell islands in the pancreas – the islets of Langerhans. Insulin is latin for “insula”, meaning island.

20th century: Developments in the management of diabetes

In the early 1900s, treatments for diabetes included: the oat-cure, the milk diet, the rice cure, potato therapy, opium and overfeeding to compensate for the loss of fluids and weight. There was also a popular and widely treatment of a restricted diet. Dr Fredrick Allen, tried out more extreme starvation diets till there was no sugar in the urine were also used but often with fatal outcomes for patients with type 1 diabetes.

  • By 1923, Eli Lilly a pharmaceutical company, undertook the first mass production of insulin
  • By 1955, first oral drug, Carbutamide, was developed to help lower blood glucose levels
  • By the 1970s the first synthetic human insulin was being produced using recombinant DNA techniques. Prior to this, insulin manufacturers had to stockpile pancreatic tissue from animals
  • 1980s: Humulin, the first biosynthetic Human insulin, was approved for distribution. It was identical to the structure of human insulin and with the advantage of being less likely to lead to allergic reactions than animal insulin
  • 1985: The first insulin pen delivery system, the NovoPen, allowed patients to inject themselves.

The Diabetes Control and Complications Trial (DCCT) report of 1993 clearly highlighted the need for self-management through good nutrition, activity and monitoring of blood sugar levels to delay the onset and progression of long-term complications in the eyes, kidneys and nerves in people with type 1 diabetes.

Progress to date and the future

By the late 20th century, huge advances in diabetes care had taken place, from the development of insulin pumps to the introduction of genetically engineered insulin. In fact more than 300 insulin analogues had been identified, including 70 from animals, 80 which are chemically modified and 150 biosynthetic insulins.

Looking ahead, there are ongoing advances to achieve the ideal diabetes management. This includes the work of Dr Damiano to introduce a bionic pancreas (iLet) which can deliver both insulin and glucagon every 5 minutes.


Watch the video to learn how diabetes develops as a disease and the functional changes that occur in people with diabetes. We also introduce you to the basic disease classification - Type 1, Type 2 and gestational diabetes – and diagnoses.

As you watch, consider how important control of blood sugars is for managing diabetes. How feasible is self-management of blood sugar levels for people with diabetes in your local setting?

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This video is from the free online course:

Diabetic Eye Disease: Building Capacity To Prevent Blindness

London School of Hygiene & Tropical Medicine