Diabetes can be caused by a variety of hormonal and cellular defects, which result in elevated blood glucose levels. A normal fasting glucose level is less than 110 mg/dL (6.1 mmol) according to the World Health Organization (WHO)⁴ and the European Association for the Study of Diabetes (EASD).⁵ According to the American Diabetes Association (ADA), normal fasting blood glucose is less than 100 mg/dL (5.7 mmol).⁶ This level of fasting glucose is maintained in the body by an intricate balance of hormones, which work to maintain glucose levels at a steady state. Normally, insulin and other hormones are released in response to rising blood glucose levels. These powerful hormones activate cellular storage of glucose, amino acids, and triglycerides in target cells, including the liver, muscle, and fat, with the end result of normoglycemia. To keep glucose levels from falling too low, other hormones, such as glucagon, corticosteroids, growth hormone, and epinephrine, increase insulin resistance to maintain adequate circulating glucose. In the presence of diabetes, there is a diminished or absent insulin response and cellular resistance to insulin. These defects, coupled with a deficiency of other glucose lowering hormones, result in higher fasting and postmeal glucose levels.

To diagnose diabetes, either fasting plasma glucose, random glucose, or a post 75 g glucose challenge glucose level can be used. Currently, there is international consensus that a fasting blood glucose level of ≥126 mg/dL (7 mmol), or a random or post meal glucose tolerance level of ≥200 mg/dL (11.1 mmol) in the presence of symptoms of hyperglycemia confers a diagnosis of diabetes (Table 39-1).⁵ Blood glucose levels that are higher than normal but do not reach the criteria for diabetes indicate future risk of diabetes and heart disease. This category of blood glucose is referred to as prediabetes and includes impaired fasting glucose and impaired glucose tolerance. Impaired fasting glucose is defined as fasting blood glucose of 100 to 125 mg/dL (5.6 to 6.9 mmol/L) by the ADA⁶ and 110 to 125 mg/dL (6.0 to 6.9 mmol/L) by the EASD.⁵ There is international consensus that impaired glucose tolerance is defined as blood glucose of 140 to 199 mg/dL (7.8 to 11.1 mmol/L) 2 hours after a 75 g glucose challenge. Uncontrolled, chronically elevated glucose, often termed “glucose toxicity,” can lead to a multiplicity of vascular complications that start long before the diagnosis of diabetes is made. Identifying and treating hyperglycemia in its earliest stages is critical to prevent complications. Unfortunately, as many as 50% of people with diabetes worldwide remain undiagnosed and untreated.¹

The global prevalence of diabetes will double in the next 30 years due to population growth, urbanization, increasing prevalence of obesity, aging, and physical inactivity.⁷ Table 39-2 illustrates the 10 countries with the highest prevalence estimates for diabetes in 2000 and 2030. The countries with the highest rates of diabetes include India, China, and the United States. In India, the crude prevalence rate is 9% in urban areas⁸ and in the United States, 7% of the population is affected by diabetes.⁹ In developing countries, the highest prevalence of diabetes is the middle productive years of 45 to 64 years of age range. In contrast, the majority of people with diabetes in developed countries are greater than 64 years of age.⁷

In the United States and globally, 90% to 95% of people with diabetes have type 2 and the majority of those are overweight.⁹ Over 50% of the U.S. population is overweight and more than one billion people in the world are overweight, of which at least 300 million are obese.² The United States and other developing countries are experiencing an epidemic of type 2 diabetes in youth. This increase in type 2 diabetes in youth strongly correlates with increasing prevalence of childhood obesity.¹⁰

From an ethnic perspective, type 2 diabetes is more common in the United States among Native American, Hispanic/Latino, non-Hispanic Black, African-American, and Pacific Islander populations. 11 Internationally, however, diabetes affects many diverse ethnic groups.⁷ From a global and a local perspective, the human and economic costs of this epidemic are enormous. In the United States, one out of every 10 health care dollars is spent on treating diabetes and its complications. The total annual economic cost of diabetes in 2007 was estimated to be $174 billion. This includes direct costs of treating diabetes-related complications and indirect costs of lost workdays and disability.¹² The global burgeoning increase of diabetes will inevitably increase the rate of premature deaths from CVD as well as increase the prevalence of other diabetes complications.⁷ Worldwide, one in 20 deaths is due to diabetes, which equals 3.2 million deaths annually. Globally, heart disease and stroke account for about 50% to 80% percent of deaths in people with diabetes and this chronic condition is the leading cause of blindness, amputation, and kidney failure.² The pace of growth and the complications of diabetes demand a concerted, global effort to focus on prevention and appropriate treatment of this epidemic. Multiple large-scale trials have proven that changes in diet, physical activity, or pharmacologic treatment can reduce the incidence of diabetes. Other trials document that complications of diabetes can be delayed or prevented through glucose control and risk factor management. The impressive results of these trials (which will be discussed later in this chapter) provide the impetus to focus on global prevention and aggressive treatment of this serious condition.

The hallmark of diabetes is elevated blood glucose levels. This abnormal elevation is due to the decrease or absence of hormones that lower blood glucose and a cellular resistance to insulin. A review of normal fuel metabolism and the hormones responsible for glucose regulation follows.

Plasma glucose levels reflect the rate at which glucose is absorbed from the intestines and then removed from circulation to be stored as fuel. The first phase of fuel metabolism is called the fed state and encompasses the first 4 hours after ingestion of food. During this phase, the pancreas responds to increasing levels of circulating glucose by activating stored insulin called “proinsulin.” Proinsulin is a protein composed of two chains “A” and “B” of amino acids, which are held together by a bridge called a connecting peptide (c-peptide). On pancreatic detection of elevating glucose, the connecting peptide of proinsulin cleaves off. The resulting activated insulin and c-peptide enter the bloodstream. This initial burst of insulin is referred to as first phase insulin response. First phase insulin is responsible for maintaining postmeal glucose within normal limits. Insulin then passes through the portal vein into the bloodstream. Once insulin enters the bloodstream, it has a half-life of a few minutes. However, since c-peptide has a longer half-life, it is can be measured by a blood test to determine how much endogenous insulin a person releases.

During these first 4 hours of the fed state, insulin promotes the storage of glucose as glycogen in the insulin-sensitive peripheral tissues, including hepatic and muscle cells. Insulin also promotes storage of amino acids as protein and excess glucose is stored as fat. The end result is a lowering of circulating blood glucose, lipids, and other energy substrates. Besides promoting the disposal of nutrients into cells, insulin has the critical role of suppressing glucagon. Glucagon, a hormone released by the α-cells of the pancreas, stimulates the release of stored glucose (glycogen) from the liver.

During this fed state, the β-cells of the pancreas also release the recently discovered hormone amylin, a 37-amino acid polypeptide.¹³ Amylin compliments the action of insulin by preventing postmeal blood glucose excursions in several ways. Unlike insulin, it slows down gastric emptying, promotes a sense of satiety, and decreases the release of glucagon. People with type 1 diabetes make no amylin and those with type 2 make less than normal amounts.

Another group of newly discovered intestinal hormones, the incretins, are also released in response to nutrient ingestion. The incretins include glucose-dependent insulin-releasing peptide (GIP) and glucagon-like peptide-1 (GLP-1). GIP is secreted by the K cells of the upper intestine and GLP-1 is released from the L cells of the intestine.¹³ Both are secreted in response to postmeal glucose elevations and stimulate β-cell insulin secretion. In addition to increasing glucose-dependent insulin secretion, incretin hormones inhibit glucagon secretion, slow gastric emptying, and increase feelings of satiety. Incretins help to keep postmeal blood glucose levels within normal ranges. Both GIP and GLP-1 are rapidly degraded by the enzyme dipeptidyl-peptidase-inhibitor-IV. (New therapies that imitate the incretins and inhibit their breakdown will be discussed in the section “Medication for Type 2 Diabetes.”) People with type 1 diabetes make normal amounts of the incretins, while those with type 2 secrete less of this powerful glucose-lowering hormone.¹³

Four to 16 hours after eating, the body enters the phase II or the postabsorptive state. This phase most often occurs during sleep and marks the end of anabolism or energy storage and begins the phase of catabolism or energy production. During this phase, since the body is not exposed to food, it must revert to stored energy for fuel. Glucagon levels rise and insulin levels decrease to a steady state, often termed basal insulin release. The main function of insulin during this phase is not to promote energy storage, but to prevent hyperglycemia. The high levels of circulating glucagon increase the breakdown of glycogen stores in the liver (glycogenolysis) to ensure an adequate supply of glucose to the brain and other glucose-dependent tissues. In addition, fat cells (adipocytes) break down triglycerides and release free fatty acids (FFAs) to be used as energy by the liver and skeletal muscles. The brain will only use glucose for fuel due to its inability to use FFAs as fuel. Many individuals with type 2 diabetes experience morning fasting hyperglycemia due to the dominance of glycogen and the relative lack of insulin during this phase.¹³

In addition to glucagon, there are other catabolic hormones that increase the breakdown of stored fuel supplies and increase circulating glucose. They increase insulin resistance and glycogen breakdown, causing a net increase in blood glucose levels. The hormones released from the kidney, corticosteroids and epinephrine, are activated during flight-or-fight response or hypoglycemia. Other hormones including growth hormone and cortisol increase insulin resistance in early morning, causing many people with type 1 diabetes to experience the “dawn” phenomena, or an elevation in morning glucose.¹⁴

Glucose homeostasis is reliant on a complex interrelationship of hormones that activate anabolic and catabolic processes. When this precise balance is disrupted through the loss or dysfunction of insulin and other hormones, the end result is hyperglycemia.

Previously labeled “juvenile diabetes” or “insulin-dependent diabetes,” type 1 diabetes affects approximately 10% of all people with diabetes.⁶ The unique feature of type 1 is its’ progressive autoimmunity resulting in complete destruction of the pancreatic β-cells. Although it can occur at any age, most new cases are expressed during childhood and puberty, when insulin-resistant pubertal hormones are at their peak. To express type 1 diabetes, a genetic propensity and an environmental trigger are necessary.¹³ Research has not identified any one causative agent that triggers the autoimmune attack against the pancreas, but several agents are suspected.¹⁵ Viral triggers such as enteroviruses, coxsackie virus B, congenital rubella, cytomegalovirus, and mumps are suspected culprits. However, these agents are only theorized to initiate the autoimmunity of type 1 diabetes, and research on causation is ongoing. From a prevention perspective, it appears that children who are breastfed are less likely to develop type 1 diabetes.¹³

When 90% of the pancreas is destroyed, there is no longer enough insulin available to maintain euglycemia and the symptoms of hyperglycemia are expressed. The rate of destruction of the β-cell mass with type 1 diabetes is rapid in youth and more gradual in the older age group.¹³ Although the destruction of the β-cells is progressive, the onset of type 1 diabetes is usually abrupt. With only 10% of the pancreas working, there is no longer adequate insulin to maintain euglycemia. Without sufficient insulin to utilize glucose for energy, the body starts breaking down fat stores for fuel. The pace of this fat breakdown and resulting ketone bodies overwhelms the liver and, in a short time, it can no longer clear ketones at a fast enough pace. High levels of circulating ketones result in ketosis and acidosis—also called diabetes ketoacidosis. At this point, the body cells are starved for glucose and the person usually feels ill enough to seek medical help. Depending on the duration and severity of ketosis, the person with a new case of type 1 diabetes appears malnourished due to inability to store fuel, dehydrated due to osmotic diuresis, and may have abdominal pain and nausea from the ketone bodies. In an effort to blow off excess acids, the person may have rapid respirations and a their breath may smell fruity. Treatment includes fluids, insulin, electrolyte replacement, and patient and family education. Clinical presentation is usually enough to make a diagnosis of type 1 diabetes. If unsure, a diagnosis can be confirmed by antibody blood tests. Some tests used to confirm autoimmune β-cell destruction include antibodies to glutamic acid decarboxylase, islet cell autoantibodies, and insulin autoantibodies.¹⁵ Patients with type 1 diabetes will require insulin replacement for the rest of their lives. There is ongoing investigation to evaluate if type 1 diabetes can be prevented or delayed in individuals at high risk of developing type 1 diabetes. To date however, large, randomized clinical trials have failed to demonstrate treatment effect.¹⁶,¹⁷ These studies have improved the understanding of the immunopathogenesis and will hopefully lead to future strategies and treatments to prevent type 1 diabetes.

Unlike type 1 diabetes, type 2 diabetes (formerly referred to as adult onset or non-insulin-dependent diabetes) is not an autoimmune condition. Of all people with diabetes, 90% to 95% have type 2. Most people with type 2 diabetes are overweight and develop hyperglycemia as a result of insulin resistance and insulin deficiency. Besides being overweight, some of the risk factors for developing type 2 diabetes include physical inactivity, first-degree relative with diabetes, women who delivered a baby bigger than 9 lb (4.2 kg), or who had gestational diabetes. Other risk factors include hypertension, impaired glucose tolerance, elevated triglycerides, and other conditions associated with insulin resistance.¹¹ In addition to these risk factors, the social milieu into which a person is born can also increase or decrease the likelihood of the expression of type 2 diabetes. Social research reveals that people of
lower socioeconomic status are more likely to express diabetes.¹⁸ This may be due to a variety of factors including lack of access to safe places to exercise, limited knowledge of healthy eating, and increased prevalence of obesity. Being overweight and obese, especially central abdominal obesity, across all populations increases in the risk of diabetes. New research has discovered that abdominal adipose tissue acts as an endocrine organ, secreting chemical mediators that increase insulin resistance and inflammation.¹⁹