This section summarizes current clinical information on diabetes mellitus for the general reader, as well as relevant epidemiologic information not available at the national level in Canada. Unless otherwise noted, the information source for this section is Diabetes in America, 2nd Ed., National Institutes of Health, 1995. Additional information sources include journal articles, and publications of the Canadian Diabetes Association (CDA) and the Centers for Disease Control and Prevention (CDC) in the United States.
Diabetes mellitus is a condition resulting from an inability of the body to sufficiently produce and/or properly use insulin. Insulin, a hormone secreted from beta cells in the pancreas, assists with the conversion of glucose into energy. Without insulin, glucose cannot be sufficiently absorbed from the bloodstream into the cells of the body (primarily in muscle, fat and liver tissue). Chronic high levels of blood glucose due to diabetes are associated with long-term damage, dysfunction and failure of various organs, especially the kidneys, eyes, nerves, heart, and blood vessels.
There are three main types of diabetes: Type 1 diabetes, Type 2 diabetes, and gestational diabetes. Impaired fasting glucose and impaired glucose tolerance are conditions that can lead to diabetes. Most of our information on diabetes in terms of life expectancy, complication rates, and risk factors comes from US data sources.
Type 1 diabetes, previously known as insulin-dependent diabetes, typically occurs in childhood or adolescence and requires multiple daily injections of insulin for survival. The most common form of Type 1 diabetes is caused by auto-immune destruction of beta-cells, resulting in an inability of the pancreas to produce insulin. Type 1 diabetes may account for 5% to 10% of all diagnosed cases of diabetes(51).
According to US data, the reduction in life expectancy seen in people with Type 1 diabetes is dependent on the age at diagnosis. At a minimum, life expectancy is shortened by 15 years. One study using life insurance records showed a reduction in life expectancy of 27 years in those diagnosed before age 15. A study of patients at the Joslin Clinic, a US clinic specializing in diabetes, found only a 16 to 17 year reduction in life expectancy for those diagnosed at ages 10 or 15. However, people who are treated at the Joslin Clinic would tend to be from wealthier families and would likely receive better medical care.
Type 2 diabetes, previously known as non-insulin dependent diabetes, typically begins after age 40. Type 2 diabetes may account for 90% to 95% of all diagnosed cases of diabetes(51). The onset of Type 2 diabetes is a two-stage process: (1) resistance to insulin's action, often exacerbated by obesity, followed by (2) the pancreas failing to increase insulin enough to compensate adequately. This form of diabetes has recently been detected in Aboriginal youth and is known as Paediatric Type 2 diabetes or Type 2Y diabetes ("Y" referring to youth).
Life expectancy is reduced by 5 to 10 years in middle-aged people with Type 2 diabetes. Life expectancy varies with age of onset, the reduction being greater the younger the age at diagnosis.
Gestational diabetes mellitus (GDM) is defined as glucose intolerance first recognized during pregnancy. Usually this form of diabetes is a transient condition that disappears by six weeks postpartum. Gestational diabetes is associated with an excess incidence of fetal macrosomia ("big babies"), pre-eclampsia, and Cesarean section. Women who have had gestational diabetes are at increased risk for later development of Type 2 diabetes. There is some evidence that Type 2 diabetes develops in approximately one third of women who have ever had gestational diabetes(52).
People with blood glucose levels greater than those considered normal but less than the level that is diagnostic of diabetes on standard tests are classified as having impaired fasting glucose (IFG) (if given a fasting glucose test), or impaired glucose tolerance (IGT) (if given an oral glucose tolerance test). People in these groups are considered to be at high risk for diabetes and/or its complications. IFG or IGT prevalence rates are not available at the national level in Canada. In the United States, an estimated 7% of the population have IFG(51). A study conducted in the UK from 1990-1992 found the prevalence of IGT, based on World Health Organization criteria, to be 16.8% among those 40 to 65 years of age(3) Researchers are investigating how to predict which of those people with IFG or IGT will go on to have diabetes and how to prevent such progression.
The classic symptoms of diabetes mellitus are fatigue, polyuria (frequent urination), polydipsia (unusual thirst), and unexplained weight loss. In Type 1 diabetes, the symptoms usually progress quickly and are often dramatic. In Type 2 diabetes, since symptoms are slower to progress, it is possible to have no apparent symptoms and be diagnosed on a non-related medical examination.
These symptoms are also compatible with diabetic ketoacidosis (DKA), a condition resulting from absolute insulin deficiency and excess contra-insulin hormones, which can lead to coma and death. DKA is much more common among those with Type 1 diabetes. Before the discovery of insulin, people with Type 1 diabetes usually died of DKA(34).
The Canadian Diabetes Association (CDA) released new criteria for the diagnosis of diabetes in 1998 in its revision of the Clinical Practice Guidelines for the Management of Diabetes in Canada(14). The new diagnostic criteria are based upon a number of recent studies demonstrating that the previous diagnostic criteria using the fasting plasma glucose test (FPG) lacked sensitivity (proportion of diabetes cases accurately detected) compared with the sensitivity of the oral glucose tolerance test (OGTT). However, the OGTT has problems of less specificity (proportion of non-diabetes cases accurately identified) compared with the FPG, as well as a lack of test-retest reliability. Also, because the OGTT is a more difficult and expensive test to perform, it has not received wide clinical acceptance.
The new FPG diagnostic criteria increase the sensitivity of that test by lowering the cutpoint for a diagnosis of diabetes, from 7.8 to 7.0 mmol/L. The use of the new FPG criteria for a diagnosis of diabetes should increase the number of people with diagnosed diabetes as well as decrease the proportion of undiagnosed cases.
The primary goal of therapy for diabetes is the maintenance of the patient's health in terms of his or her quality of life and overall sense of well-being, as well as the avoidance of acute and chronic complications. Since virtually every aspect of daily life may be affected by treatment, the person with diabetes is the key member of the diabetes health care team. For most people with diabetes, improving metabolic control (such that blood glucose is near normal levels at all times) will achieve the primary goal of preventing long-term complications. Depending upon the type of diabetes and the therapy required to achieve near normal levels of blood glucose, this objective may be more or less difficult to achieve without causing acute side effects. The metabolic goals of treatment/management must therefore be individualized. Education of health care providers and of the people affected with diabetes and their families is a cornerstone of effective treatment and self-management of this condition.
Lack of insulin production by the pancreas makes Type 1 diabetes particularly difficult to control. Treatment requires a strict regimen that typically includes a carefully calculated diet, planned physical activity, home blood glucose testing up to several times a day, and multiple daily insulin injections(51).
Treatment typically includes diet control, exercise, home blood glucose testing, and in most cases, oral medication and/or insulin. Approximately 40% of people with Type 2 diabetes require insulin injections(51).
Treatment for women with gestational diabetes includes dietary counseling, regular and moderate exercise, glucose monitoring, and insulin therapy if diet alone does not achieve target blood glucose. The value of treatment for gestational diabetes remains controversial.
Race and Ethnic Background. Race and ethnic background are accepted risk factors for Type 1 diabetes. Rates are higher among non-Hispanic whites than blacks or Hispanics in the United States.
Age. Incidence increases with age through childhood and adolescence but decreases during adulthood.
Geographic Variability. Comparisons among countries facilitated by the World Health Organization (WHO) have found the highest rates of Type 1 diabetes in Scandinavia, intermediate rates in Canada and the United States, and low rates in Japan and Tanzania.
Seasonal Variability. Most studies show some seasonal variation, with incidence rates lowest in the warm summer months.
Genetic Susceptibility/Family History. Genetic susceptibility is a factor in Type 1 diabetes but is not as strong a factor as in Type 2 diabetes.
Possible Links to Diet and Exposure to Viruses. There is some evidence of a protective effect of breast-feeding against Type 1 diabetes.
Note. Type 1 diabetes affects males and females equally.
Genetic Susceptibility/Family History. Increased risk of diabetes with a positive family history of diabetes is probably due to genetic and environmental factors. Evidence for genetic factors comes from twin studies where the probability of finding diabetes in both twins is twice as high among identical twins as fraternal twins.
Obesity. Obesity is a well-recognized risk factor for diabetes. Diabetes has been positively associated with each of the following: (a) the level of obesity, (b) the duration of obesity, and (c) body fat distribution (i.e., central deposition of fat).
Physical Inactivity. Physical inactivity is considered a risk factor on the basis of observational and ecologic studies.
Ethnic Background. Ethnic background has been accepted as an independent risk factor based on studies of high prevalence populations (Aboriginal, Black, Hispanic), which have found increased risk for diabetes among these populations compared with the white population after controlling for all other known risk factors.
Note. High blood pressure and dyslipidemia (disturbances in lipoprotein patterns, linked to coronary heart disease) are associated with diabetes, although it is not clear at this time whether these factors contribute to a person's risk of getting diabetes, or are caused by diabetes itself. Gender is not an independent risk factor for Type 2 diabetes in the general population. Smoking has been associated with diabetes in a few studies but it is unclear if smoking is a causal factor in the onset of diabetes. It is clear that smoking is a causal factor for cardiovascular disease, one of the major complications of diabetes.
US studies have reported an increased risk of gestational diabetes among African Americans, Hispanic/Latino Americans, American Indians, and people with a family history of diabetes. Obesity is also associated with higher risk.
The main long-term complications of diabetes are as follows: (i) microvascular disease, or small blood vessel damage, including retinopathy (eye disease) and nephropathy (kidney disease); (ii) macrovascular disease, or large blood vessel damage, including ischemic heart disease, stroke, and peripheral vascular disease; (iii) neuropathy (nervous system disease); and (iv) foot problems. Hypertension (high blood pressure) is a risk factor for most complications of diabetes. There is strong evidence that early diagnosis and tight control of blood sugar levels and blood pressure can prevent or delay these complications.
The risk of heart disease is substantially increased for people with diabetes. According to 1996 national survey data (NPHS 1996), Canadians have 2 to 6 times (depending on age) the likelihood of having heart diseases or stroke if they have diabetes as those without diabetes. Heart disease is uncommon in those under 30 with Type 1 diabetes. Adults with diabetes are more likely than those without to have risk factors for heart disease, especially high blood pressure, low levels of HDL cholesterol, and high levels of triglycerides. However, some of the increased risk of heart disease associated with diabetes appears to be independent of these factors.
After 7 to 15 years, 25 to 40% of all patients with Type 1 diabetes develop microalbuminuria. Of these, over 90% progress to proteinuria over time. Kidney function declines at variable rates; it appears to be a slower decline in Type 2 diabetes. After 10 years of persistent proteinuria, the incidence of chronic kidney failure is 11% in those with Type 2 diabetes and 50% in those with Type 1 diabetes.
Three complications of diabetes can lead to blindness: retinopathy, cataracts, and glaucoma. Of people who have had diabetes for at least 15 years, 97% of insulin-taking patients and 80% of those not taking insulin have retinopathy; the most severe manifestation, proliferative diabetic retinopathy, occurs in 40% of those taking insulin and 5% of those not taking insulin. Of people who have had insulin-dependent diabetes for 30 or more years, 12% are blind. Diabetic retinopathy is the leading cause of new cases of legal blindness among the working age population.
Roughly 60% of people with diabetes have some degree of diabetic neuropathy, and in half of these it develops within nine years of diagnosis. Most have a mixed motor/sensory deficit resulting in decreased sensation, increased sensitivity, pain, weakness, and muscle wasting. Frequently, abnormal functioning of the autonomic (self-functioning) nervous system leads to disorders of the heart and circulation, and gastrointestinal and genitourinary tracts.
The risk of lower extremity (limb) amputations following diabetes diagnosis is 6% at 20 years and 11% at 30 years. Lower limb amputations are typically the result of a foot infection that does not heal and eventually becomes gangrenous. The initial wound is often the result of a lack of protective sensory function in the foot due to neuropathy. The inability of the wound to heal properly is due to decreased blood and nutrient flow to the lower limbs, the result of peripheral vascular disease in most cases.
An estimated 40% of people with diabetes in Canada have high blood pressure. High blood pressure is one of the main risk factors for cardiovascular disease.
Periodontal disease (a type of disease that can lead to tooth loss) occurs with greater frequency and severity among people with diabetes. Periodontal disease has been reported to occur among 30% of people aged 19 years or older with Type 1 diabetes(51).
The rate of major congenital malformations in babies born to women with pre-existing diabetes varies from 0% to 5% among women who receive preconception care to 10% among women who do not. Between 3% and 5% of pregnancies among women with diabetes result in the death of the newborn; the rate among women who do not have diabetes is 1.5%(51).
Diabetes can directly cause acute life-threatening events, such as diabetic ketoacidosis and hyperosmolar nonketotic coma, as a result of biochemical imbalance in uncontrolled diabetes. People with diabetes are more susceptible to many other illnesses. For example, they are more likely to die of pneumonia or influenza than people who do not have diabetes(51).