Chapter 20 Diabetes mellitus and other disorders of metabolism

Diabetes mellitus

Hyperglycaemia, insulin and insulin action

Introduction

Diabetes mellitus (DM) is a syndrome of chronic hyperglycaemia due to relative insulin deficiency, resistance or both. It affects more than 220 million people worldwide, and it is estimated that it will affect 440 million by the year 2030. Diabetes is usually irreversible and, although patients can lead a reasonably normal lifestyle, its late complications result in reduced life expectancy and major health costs. These include macrovascular disease, leading to an increased prevalence of coronary artery disease, peripheral vascular disease and stroke, and microvascular damage causing diabetic retinopathy and nephropathy. Neuropathy is another major complication.

Insulin structure and secretion

Insulin is the key hormone involved in the storage and controlled release within the body of the chemical energy available from food. It is coded for on chromosome 11 and synthesized in the beta cells of the pancreatic islets (Fig. 20.1). The synthesis, intracellular processing and secretion of insulin by the beta cell is typical of the way that the body produces and manipulates many peptide hormones. Figure 20.2 illustrates the cellular events triggering the release of insulin-containing granules. After secretion, insulin enters the portal circulation and is carried to the liver, its prime target organ. About 50% of secreted insulin is extracted and degraded in the liver; the residue is broken down by the kidneys. C-peptide is only partially extracted by the liver (and hence provides a useful index of the rate of insulin secretion) but is mainly degraded by the kidneys.

Figure 20.1 Part of a beta cell. The ribosomes manufacture pre-proinsulin from insulin mRNA. The hydrophobic ‘pre’ portion of pre-proinsulin allows it to transfer to the Golgi apparatus, and is subsequently enzymatically cleaved off. Proinsulin is parcelled into secretory granules in the Golgi apparatus. These mature and pass towards the cell membrane where they are stored before release. The proinsulin molecule folds back on itself and is stabilized by disulphide bonds. The biochemically inert peptide fragment known as connecting (C) peptide splits off from proinsulin in the secretory process, leaving insulin as a complex of two linked peptide chains. Equimolar quantities of insulin and C-peptide are released into the circulation via the ‘regulated pathway’. A small amount of insulin is secreted by the beta cell directly via the ‘constitutive pathway’, which bypasses the secretory granules.

Figure 20.2 Local forces regulating insulin secretion from beta cells. Glucose enters the beta cell via the GLUT-2 transporter protein, which is closely associated with the glycolytic enzyme glucokinase. Metabolism of glucose within the beta cell generates ATP. ATP closes potassium channels in the cell membrane (a). If a sulfonylurea binds to its receptor, this also closes potassium channels. Closure of potassium channels predisposes to cell membrane depolarization, allowing calcium ions to enter the cell via calcium channels in the cell membrane (b). The rise in intracellular calcium triggers activation of calcium-dependent phospholipid protein kinase which, via intermediary phosphorylation steps, leads to fusion of the insulin-containing granules with the cell membrane and exocytosis of the insulin-rich granule contents. Similar mechanisms produce hormone-granule secretion in many other endocrine cells.

An outline of glucose metabolism

Blood glucose levels are closely regulated in health and rarely stray outside the range of 3.5–8.0 mmol/L (63–144 mg/dL), despite the varying demands of food, fasting and exercise. The principal organ of glucose homeostasis is the liver, which absorbs and stores glucose (as glycogen) in the post-absorptive state and releases it into the circulation between meals to match the rate of glucose utilization by peripheral tissues. The liver also combines 3-carbon molecules derived from breakdown of fat (glycerol), muscle glycogen (lactate) and protein (e.g. alanine) into the 6-carbon glucose molecule by the process of gluconeogenesis.

Glucose production

About 200 g of glucose is produced and utilized each day. More than 90% is derived from liver glycogen and hepatic gluconeogenesis, and the remainder from renal gluconeogenesis.

Glucose utilization

The brain is the major consumer of glucose, and its function depends upon an uninterrupted supply of this substrate. Its requirement is 1 mg/kg bodyweight per minute, or 100 g daily in a 70 kg man. Glucose uptake by the brain is obligatory and is not dependent on insulin, and the glucose used is oxidized to carbon dioxide and water. Other tissues, such as muscle and fat, are facultative glucose consumers. The effect of insulin peaks associated with meals is to lower the threshold for glucose entry into cells; at other times, energy requirements are largely met by fatty-acid oxidation. Glucose taken up by muscle is stored as glycogen or metabolized to lactate or carbon dioxide and water. Fat uses glucose as a source of energy and as a substrate for triglyceride synthesis; lipolysis releases fatty acids from triglyceride together with glycerol, a substrate for hepatic gluconeogenesis.

Hormonal regulation

Insulin is a major regulator of intermediary metabolism, although its actions are modified in many respects by other hormones. Its actions in the fasting and postprandial states differ (Fig. 20.3). In the fasting state, its main action is to regulate glucose release by the liver, and in the postprandial state, it additionally promotes glucose uptake by fat and muscle. The effect of counter-regulatory hormones (glucagon, epinephrine (adrenaline), cortisol and growth hormone) is to cause greater production of glucose from the liver and less utilization of glucose in fat and muscle for a given level of insulin.

Figure 20.3 Fasting and postprandial effects of insulin. In the fasting state, insulin concentrations are low and it acts mainly as a hepatic hormone, modulating glucose production (via glycogenolysis and gluconeogenesis) from the liver. Hepatic glucose production rises as insulin levels fall. In the postprandial state insulin concentrations are high and it then suppresses glucose production from the liver and promotes the entry of glucose into peripheral tissues (increased glucose utilization).

Glucose transport

Cell membranes are not inherently permeable to glucose. A family of specialized glucose-transporter (GLUT) proteins carry glucose through the membrane into cells.

GLUT-1 – enables basal non-insulin-stimulated glucose uptake into many cells (see Fig. 6.29).

GLUT-2 – transports glucose into the beta cell, a prerequisite for glucose sensing, and is also present in the renal tubules and hepatocytes.

GLUT-3 – enables non-insulin-mediated glucose uptake into brain neurones and placenta.

GLUT-4 – enables much of the peripheral action of insulin. It is the channel through which glucose is taken up into muscle and adipose tissue cells following stimulation of the insulin receptor (Fig. 20.4).

Figure 20.4 Insulin signalling in peripheral cells (e.g. muscle and adipose tissue). The insulin receptor consists of α- and β-subunits linked by disulphide bridges (top right of figure). The β-subunits straddle the cell membrane. The transporter protein GLUT-4 (bottom left of figure) is stored in intracellular vesicles. The binding of insulin to its receptor initiates many intracellular actions including translocation of these vesicles to the cell membrane, carrying GLUT-4 with them; this allows glucose transport into the cell.

The insulin receptor

This is a glycoprotein (400 kDa), coded for on the short arm of chromosome 19, which straddles the cell membrane of many cells (Fig. 20.4). It consists of a dimer with two α-subunits, which include the binding sites for insulin, and two β-subunits, which traverse the cell membrane. When insulin binds to the α-subunits it induces a conformational change in the β-subunits, resulting in activation of tyrosine kinase and initiation of a cascade response involving a host of other intracellular substrates. One consequence of this is migration of the GLUT-4 glucose transporter to the cell surface and increased transport of glucose into the cell. The insulin-receptor complex is then internalized by the cell, insulin is degraded, and the receptor is recycled to the cell surface.

Classification of diabetes

Diabetes may be primary (idiopathic) or secondary (Table 20.1). Primary diabetes is classified into:

Type 1 diabetes, which has an immune pathogenesis and is characterized by severe insulin deficiency

Type 2 diabetes, which results from a combination of insulin resistance and less severe insulin deficiency.

Table 20.1 Aetiological classification of diabetes mellitus, based on classification by the American Diabetes Association (ADA)

Note: Patients with any form of diabetes may require insulin treatment at some stage of their disease. Such use of insulin does not, of itself, classify the patient.

(Adapted from ADA. Diagnosis and classification of diabetes mellitus. Diabetes Care 2008; 31(Suppl 1):S55–S60.)

The key clinical features of the two main forms of diabetes are listed in Table 20.2. Type 1 and type 2 diabetes represent two distinct diseases from the epidemiological point of view, but clinical distinction can sometimes be difficult. The two diseases should from a clinical point of view be seen as a spectrum, distinct at the two ends but overlapping to some extent in the middle. Hybrid forms are increasingly recognized, and patients with immune-mediated diabetes (type 1) may, for example, also be overweight and insulin resistant. This is sometimes referred to as ‘double diabetes’. It is more relevant to give the patient the right treatment on clinical grounds than to worry about how to label their diabetes. The classification of primary diabetes continues to evolve. Monogenic forms have been identified (see p. 1007), in some cases with significant therapeutic implications. Although secondary diabetes accounts for barely 1–2% of all new cases at presentation, it should not be missed because the cause can sometimes be treated. All forms of diabetes derive from inadequate insulin secretion relative to the needs of the body, and progressive insulin secretory failure is characteristic of both common forms of diabetes. Thus, some patients with immune-mediated diabetes type 1 may not at first require insulin, whereas many with type 2 diabetes will eventually do so.

Table 20.2 The spectrum of diabetes: a comparison of type 1 and type 2 diabetes mellitus

	Type 1	Type 2
Age	Younger (usually <30)	Older (usually >30)
Weight	Lean	Overweight
Symptom duration	Weeks	Months/years
Higher risk ethnicity	Northern European	Asian, African, Polynesian and American-Indian
Seasonal onset	Yes	No
Heredity	HLA-DR3 or DR4 in >90%	No HLA links
Pathogenesis	Autoimmune disease	No immune disturbance
Ketonuria	Yes	No
Clinical	Insulin deficiency	Partial insulin deficiency initially
	± ketoacidosis	± hyperosmolar state
	Always need insulin	Need insulin when beta cells fail over time
Biochemical	C-peptide disappears	C-peptide persists

Type 1 diabetes mellitus

Epidemiology

Type 1 diabetes is a disease of insulin deficiency. In western countries almost all patients have the immune-mediated form of the disease, otherwise known as type 1A. Type 1 diabetes is a disease of childhood, reaching a peak incidence around the time of puberty, but can present at any age. A ‘slow-burning’ variant with slower progression to insulin deficiency occurs in later life and is sometimes called latent autoimmune diabetes in adults (LADA). LADA may be difficult to distinguish from type 2 diabetes. Clinical clues are: leaner build, rapid progression to insulin therapy following an initial response to other therapies, and the presence of circulating islet autoantibodies. The highest rates of type 1 diabetes in the world are seen in Finland and other Northern European countries, and on the island of Sardinia, which for unknown reasons, has the second highest rate in the world (Fig. 20.5). The incidence of type 1 diabetes appears to be increasing in most populations. In Europe, the annual increase is of the order of 2–3%, and is most marked in children under the age of 5 years. WHO estimated in 1995 that there were 19.4 million people with type 1 diabetes and that the number will rise to 57.2 million by 2025.

Figure 20.5 Age-standardized incidence rates of type 1 diabetes (onset 0–14 years) in Europe, per 100 000/year.

(Data courtesy of International Diabetes Federation.)

FURTHER READING

Ludvigsson J et al. GAD65 antigen therapy in recently diagnosed type 1 diabetes mellitus. N Engl J Med 2012; 366:433–442.

Nathan DM. Navigating the choices for diabetes prevention. N Engl J Med 2010; 362:1533–1535.

Environmental factors

The incidence of childhood type 1 diabetes is rising across Europe at the rate of 2–3% each year, suggesting that environmental factor(s) are involved in its pathogenesis. Islet autoantibodies (see above) appear in the first few years of life, indicating prenatal or early postnatal interactions with the environment. Exposures to dietary constituents, enteroviruses such as Coxsackie B4 and relative deficiency of vitamin D are possible candidates, but their role in the causation of the disease has yet to be confirmed. A cleaner environment with less early stimulation of the immune system in childhood may increase susceptibility for type 1 diabetes, as for atopic/allergic conditions (the hygiene hypothesis) (see p. 824), and more rapid weight gain in childhood and adolescence leading to increased insulin resistance might accelerate clinical onset (the accelerator hypothesis).

Pre-type 1 diabetes and prevention of type 1 diabetes

Children who test positive for two or more autoantibodies have a >80% risk of progression to diabetes, and the risk approaches 100% in those who additionally lose their first phase insulin response to intravenous glucose and/or develop glucose intolerance. The ability to predict type 1 diabetes with this degree of precision has opened the way to trials of disease prevention, but intervention before clinical onset of diabetes has so far proved unsuccessful.

FURTHER READING

Nolan CJ, Damm P, Prentki M. Type 2 diabetes across generations: from pathophysiology to prevention and management. Lancet 2011; 378:169–181.

Type 2 diabetes mellitus

Epidemiology

Type 2 diabetes is a common condition in all populations enjoying an affluent lifestyle, and has increased in parallel with the adoption of a western lifestyle and increasing obesity. The four major determinants are increasing age, obesity, ethnicity and family history. In poor countries, diabetes is a disease of the rich, but in rich countries, it is a disease of the poor; obesity being the common factor. Glucose intolerance or frank diabetes may be present in a subclinical or undiagnosed form for years before diagnosis, and 25–50% of patients already have some evidence of vascular complications at the time of diagnosis. Onset may be accelerated by the stress of pregnancy, drug treatment or intercurrent illness. The overall prevalence within the UK is 4–6%, and the lifetime risk is around 15–20%. Type 2 diabetes is 2–4 times as prevalent in people of South Asian, African and Caribbean ancestry who live in the UK, and the life-time risk in these groups exceeds 30%. High rates also affect people of Middle Eastern and Hispanic American origin living western lifestyles. Obesity increases the risk of type 2 diabetes 80–100 fold, and this is reflected by the increasing prevalence of diabetes in different populations. On average, the inhabitants of affluent countries gain almost 1 g daily between the ages of 25 and 55 years. This gain, due to a tiny excess in energy intake over expenditure – 90 kcal or one chocolate-coated digestive biscuit per day – is often due to reduced exercise rather than increased food intake. Further, our sedentary lifestyle means that the proportion of obese young adults is rising rapidly, and epidemic obesity will create a huge public health problem for the future. The increasing numbers of obese adolescents presenting with type 2 diabetes, particularly within high-risk ethnic groups, is a matter for concern.

Type 2 diabetes is associated with central obesity, hypertension, hypertriglyceridaemia, a decreased HDL-cholesterol, disturbed haemostatic variables and modest increases in a number of pro-inflammatory markers. Insulin resistance is strongly associated with many of these variables, as is increased cardiovascular risk. This group of conditions is referred to as the metabolic syndrome (see p. 223). The International Diabetes Federation has proposed criteria based on increased waist circumference (or BMI >30) plus two of the following: diabetes (or fasting glucose >6.0 mmol/L), hypertension, raised triglycerides or low HDL cholesterol. On this definition, about one-third of the adult population has features of the syndrome, not necessarily associated with diabetes. Critics would argue that the metabolic syndrome is not a distinct entity, but one end of a continuum in the relationship between exercise, lifestyle and bodyweight on the one hand, and genetic make-up on the other, and that diagnosis adds little to standard clinical practice in terms of diagnosis, prognosis or therapy.

FURTHER READING

Tirosh A, Shai I, Afek A et al. Adolescent BMI trajectory and risk of diabetes versus coronary disease. N Engl J Med 2011; 364:1315–1325.

Yang W, Lu J, Weng J. Prevalence of diabetes among men and women in China. N Engl J Med 2010; 362:1090–1101.

Causes

Inheritance

Identical twins of patients with type 2 diabetes have >50% chance of developing diabetes; the risk to non-identical twins or siblings is of the order of 25%, confirming a strong inherited component to the disease. Type 2 diabetes is a polygenic disorder, and, as with type 1 diabetes, genome-wide studies of associations between common DNA variants and disease have allowed identification of numerous susceptibility loci. Several of these loci subserve beta-cell development or function, and there is no overlap with the immune function genes identified for type 1 diabetes. There is no major gene susceptibility, involving the HLA region. However, transcription factor-7-like (TCF7-L2) is the most common variant observed in type 2 diabetes in Europeans, and KCNQ1 (a potassium voltage-gated channel) in Asians. TCF7-L2 carries an increased risk of around 35%, while other common variants account for no more than 10–20%. TCF7-L2 has now been shown to modulate pancreatic islet cell function. Paradoxically, the genes for type 2 diabetes account for a relatively small fraction of its observed heritability. They do not allow subtypes of the condition to be identified with any confidence, or provide useful disease prediction.

FURTHER READING

McCarthy MI. Genomics, type 2 diabetes, and obesity. N Engl J Med 2010; 363:2339–2350.

Environmental factors: early and late

An association has been noted between low weight at birth and at 12 months of age and glucose intolerance later in life, particularly in those who gain excess weight as adults. The concept is that poor nutrition early in life impairs beta-cell development and function, predisposing to diabetes in later life. Low birthweight has also been shown to predispose to heart disease and hypertension.

Inflammation

Subclinical inflammatory changes are characteristic of both type 2 diabetes and obesity, and in diabetes, high-sensitivity C-reactive protein (CRP) levels are modestly elevated in association with raised fibrinogen and increased plasminogen activator inhibitor-1 (PAI-1), and contribute to cardiovascular risk. Circulating levels of the pro-inflammatory cytokines TNF-α and IL-6 are elevated in both diabetes and obesity.

Abnormalities of insulin secretion and action

The relative role of secretory failure versus insulin resistance in the pathogenesis of type 2 diabetes has been much debated, but even massively obese individuals with a fully functioning beta-cell mass do not necessarily develop diabetes, which implies that some degree of beta-cell dysfunction is necessary. Insulin binds normally to its receptor on the surface of cells in type 2 diabetes, and the mechanisms of ‘insulin resistance’ are still poorly understood. Insulin resistance is, however, associated with central obesity and accumulation of intracellular triglyceride in muscle and liver in type 2 diabetes, and a high proportion of patients have non-alcoholic fatty liver disease (NAFLD), see page 303. It has long been stated that patients with type 2 diabetes retain up to 50% of their beta-cell mass at the time of diagnosis, as compared with healthy controls, but the shortfall is greater than this when they are matched with healthy individuals who are equally obese. In addition, patients with type 2 diabetes almost all show islet amyloid deposition at autopsy, derived from a peptide known as amylin or islet amyloid polypeptide (IAPP), which is co-secreted with insulin. It is not known if this is a cause or consequence of beta-cell secretory failure.

Abnormalities of insulin secretion manifest early in the course of type 2 diabetes. An early sign is loss of the first phase of the normal biphasic response to intravenous insulin. Established diabetes is associated with hypersecretion of insulin by a depleted beta-cell mass. Circulating insulin levels are therefore higher than in healthy controls, although still inadequate to restore glucose homeostasis. Relative insulin lack is associated with increased glucose production from the liver (owing to inadequate suppression of gluconeogenesis) and reduced glucose uptake by peripheral tissues. Hyperglycaemia and lipid excess are toxic to beta cells, at least in vitro, a phenomenon known as glucotoxicity, and this is thought to result in further beta-cell loss and further deterioration of glucose homeostasis. Circulating insulin levels are typically higher than in non-diabetics following diagnosis and tend to rise further, only to decline again after months or years due to secretory failure, an observation sometimes referred to as the ‘Starling curve’ of the pancreas. Type 2 diabetes is thus a condition in which insulin deficiency relative to increased demand leads to hypersecretion of insulin by a depleted beta-cell mass and progression towards absolute insulin deficiency requiring insulin therapy. Its time course varies widely between individuals.

Overview and prevention

Genetic predisposition determines whether an individual is susceptible to type 2 diabetes; if and when diabetes develops largely depends upon lifestyle. A dramatic reduction in the incidence of new cases of adult-onset diabetes was documented in the Second World War when food was scarce, and clinical trials in individuals with impaired glucose tolerance have shown that diet, exercise or agents such as metformin have a marked effect in deferring the onset of type 2 diabetes. Established diabetes can be reversed, even if temporarily, by successful diet and weight loss or by bariatric surgery. Diabetes is therefore largely preventable, although the most effective measures would be directed at the whole population and implemented early in life. Prevention is well worth while, for diabetes diagnosed in a man between the ages of 40 and 59 reduces life expectancy by 5–10 years. By contrast, type 2 diabetes diagnosed after the age of 70 has limited effect on life expectancy in men.

Monogenic diabetes mellitus

The genetic causes of some rare forms of diabetes are shown in Table 20.3. Considerable progress has been made in understanding these rare variants of diabetes. Genetic defects of beta-cell function (previously called ‘maturity-onset diabetes of the young’, MODY) are dominantly inherited, and several variants have been described, each associated with different clinical phenotypes (Table 20.4). These should be considered in people presenting with early-onset diabetes in association with an affected parent and early-onset diabetes in ~50% of relatives. They can often be treated with a sulfonylurea.

Table 20.3 Rare genetic causes of type 2 diabetes

Disorder	Features
Insulin receptor mutations	Obesity, marked insulin resistance, hyperandrogenism in women, acanthosis nigricans (areas of hyperpigmented skin)
Maternally inherited diabetes and deafness (MIDD)	Mutation in mitochondrial DNA. Diabetes onset before age 40. Variable deafness, neuromuscular and cardiac problems, pigmented retinopathy
Wolfram’s syndrome (DIDMOAD – diabetes insipidus, diabetes mellitus, optic atrophy and deafness)	Recessively inherited. Mutation in the transmembrane gene, WFS1. Insulin-requiring diabetes and optic atrophy in the first decade. Diabetes insipidus and sensorineural deafness in the second decade progressing to multiple neurological problems. Few live beyond middle age
Severe obesity and diabetes	Alström’s, Bardet–Biedl and Prader–Willi syndromes. Retinitis pigmentosa, mental insufficiency and neurological disorders
Disorders of intracellular insulin signalling. All with severe insulin resistance	Leprechaunism, Rabson–Mendenhall syndrome, pseudoacromegaly, partial lipodystrophy: lamin A/C gene mutation
Genetic defects of beta-cell function	See Table 20.4