Patterns in Health and Disease: Epidemiology and Physiology

Objectives Define or describe the science of epidemiology. Contrast infectious with degenerative diseases as causes of death. Identify three major categories of risk factors and examples of specific risk factors in each. Compare the epidemiologic triad with the web of causation as models to study infectious and degenerative disease, respectively.

Objectives Describe the difference between primary and secondary risk factors for coronary heart disease (CHD). Describe the steps an epidemiologist must follow to show that a risk factor is causally connected to a disease. Describe the hypothesis linking resistance to insulin as a cause of hypertension.

Outline Epidemiology Coronary Heart Disease Physiology Synthesis Physical Inactivity as a Risk Factor Physiology Synthesis

Epidemiology Epidemiology The study of the distribution and determinants of health states and the use of this information in the control of disease Uses of epidemiology: Establish the cause of disease Trace the natural history of disease Describe the health status of populations Evaluate an intervention Epidemiologic triad Shows connections between the environment, agent, and host that cause disease

The Epidemiologic Triad Epidemiology The Epidemiologic Triad Figure 14.1

Disease Control Destroying or removing the agent at its source Epidemiology Disease Control Destroying or removing the agent at its source Altering the environment to reduce transmission of the agent Improving the host’s resistance to the agent Altering the host’s behaviors Improved nutrition Immunization Exercise

Epidemiology In Summary Epidemiology is the study of the distribution and determinants of health states and the use of this information in the control of disease. Disease control can be achieved by: (1) destroying or removing the agent at its source, (2) altering the environment to reduce transmission of the agent, or improving the host’s resistance to the agent, and (3) altering the host’s behaviors such as improved nutrition, immunization, and exercise.

Infectious and Degenerative Diseases Epidemiology Infectious and Degenerative Diseases Infectious diseases Tuberculosis Pneumonia Chronic (degenerative) diseases Cancer, heart disease, and stroke Responsible for 56% of all deaths in U.S. in 2004 Difficult to establish cause of disease Due to genetic, environmental, and behavioral factors

Web of Causation Difficult to establish the cause of chronic diseases Epidemiology Web of Causation Difficult to establish the cause of chronic diseases Atherosclerosis Complex involvement of factors Risk factors Genetic Gender, race Environmental Access to high-fat foods, places to exercise Behavioral Diet Smoking Physical activity/inactivity

Web of Causation: Titanic Disaster Epidemiology Web of Causation: Titanic Disaster Figure 14.2

Web of Causation: Atherosclerosis Epidemiology Web of Causation: Atherosclerosis Figure 14.3

Major Risk Factor Categories Epidemiology Major Risk Factor Categories Figure 14.4

Degenerative Diseases Epidemiology Degenerative Diseases Result from interaction of genetics and environment Inherited/biological factors Age, gender, race, ease of developing disease Cannot be changed Environmental and behavioral risk factors Most susceptible to change Importance of personal responsibility

A Closer Look 14.1 Leading and Actual Causes of Death Epidemiology A Closer Look 14.1 Leading and Actual Causes of Death Leading causes of death Heart disease Cancer Stroke Chronic lung disease Unintentional injuries Diabetes Actual causes of death Tobacco Diet and physical inactivity Alcohol consumption Infection Toxic agents Motor vehicles Firearms

Epidemiology In Summary Epidemiologists show that the major causes of death in the United States are degenerative diseases such as heart disease and cancer. The development and progress of these diseases are affected by the interaction of environmental and behavioral risk factors.

Coronary Heart Disease (CHD) Associated with atherosclerosis Thickening of the inner lining of arteries Contributor to heart attack and stroke death Diseases of the heart and blood vessels are the leading cause of death in the U.S. Associated with risk factors Each risk factor magnifies the risk of CHD Eliminating a risk factor causes a reduction in risk

A Closer Look 14.2 Risk Factors for Coronary Heart Disease Primary Increases risk of CHD in and of itself Secondary Increases risk of CHD if primary risk factor is present Negative risk factor Reduces risk of CHD HDL-C Some can’t be changed Some can be changed

A Closer Look 14.2 Risk Factors for Coronary Heart Disease Can’t be changed Heredity Gender Age Race Can be changed Cigarette smoking High serum cholesterol High blood pressure Physical inactivity Blood glucose Obesity Stress

Coronary Heart Disease In Summary Risk factors can be divided into three categories: genetic/biological, environmental, and behavioral. The risk factors of smoking, high cholesterol, and hypertension interact to magnify the risk of CHD. Similarly, elimination of one of them causes a disproportionate reduction in the risk of CHD.

Determining Association Between Risk Factor and Disease Physical Inactivity as a Risk Factor Determining Association Between Risk Factor and Disease Temporal Association Plausibility Consistency Strength of association (relative risk) Dose-response relationship Reversibility Study design Judging the evidence

Physical Inactivity as a Risk Factor Independent risk factor for CHD Relative risk of CHD due to inactivity is similar to other risk factors Smoking High cholesterol High blood pressure High population attributable risk Percentage of population at risk Due to large number of inactive individuals

U.S. Population at Risk Physical Inactivity as a Risk Factor Figure 14.5

In Summary Physical inactivity is an independent risk factor for CHD. Physical Inactivity as a Risk Factor In Summary Physical inactivity is an independent risk factor for CHD. The relative risk of CHD due to inactivity (1.9) is similar to that of hypertension (2.1) and high cholesterol (2.4). The fact that about 59% of the population is inactive indicates the enormous impact a change in physical activity habits can have on the nation’s risk for CHD.

Hypertension Often occurs with other metabolic abnormalities Physiology Hypertension Often occurs with other metabolic abnormalities Abdominal obesity Peripheral insulin resistance Dyslipidemia High triglycerides Metabolic syndrome Coexistance of these risk factors

Metabolic Syndrome Insulin resistance Physiology Metabolic Syndrome Insulin resistance Reduced ability of peripheral tissues to take up glucose Leads to hyperinsulinemia Due to higher insulin secretion by pancreas Also increases FFA level in the blood Together with abdominal obesity Hyperinsulinemia associated with: Increased SNS activity Increased sodium and water retention Increased smooth muscle proliferation in blood vessels

The Insulin-Resistance and Hypertension Hypothesis Physiology The Insulin-Resistance and Hypertension Hypothesis Figure 14.6

What Causes the Metabolic Syndrome? Physiology What Causes the Metabolic Syndrome? Several competing hypotheses Insulin resistance Changes in blood vessels and water retention cause hypertension Hypertension Decrease in small blood vessels reduces glucose and insulin delivery to muscle, causing insulin resistance Increased SNS activity Increases blood pressure and blood glucose Obesity Adipose tissue secretions affect insulin resistance

A Closer Look 14.3 The Metabolic Syndrome—Is It a Real Syndrome? Physiology A Closer Look 14.3 The Metabolic Syndrome—Is It a Real Syndrome? Reasons for this question: Only half to two-thirds of people diagnosed with metabolic syndrome have insulin resistance Both hypertension and dyslipidemia can exist without obesity or insulin resistance However: Diagnosis helps direct lifestyle changes Classification as a disease allows insurance coverage

Definition of the Metabolic Syndrome Physiology Definition of the Metabolic Syndrome Three or more of the following risk factors: Abdominal obesity Waist circumference >102 cm (men) and >88 cm (women) Hypertriglyceridemia ≥150 mg/dl Low HDL cholesterol <40 mg/dl (men) and <50 mg/dl (women) High blood pressure ≥130/85 mmHg High fasting blood glucose ≥110 mg/dl

Prevalence of the Metabolic Syndrome Physiology Prevalence of the Metabolic Syndrome Overall, 23.7% of the population 6.7% in those 20–29 years of age 40% in those >60 years of age Highest in Mexican-Americans 31.9% of that population

A Closer Look 14.4 Overweight and Obesity Physiology A Closer Look 14.4 Overweight and Obesity Prevalence in US adults 66.3% are overweight (BMI = 25.0–29.9 kg/m2) 32.2% are obese (BMI ≥30.0 kg/m2) Prevalence in children and adolscents 16% females and 18.2% males overweight Linked to rise in diabetes Recommendations for control of overweight and obesity Surgeon General “Call to Action” Healthy People 2010 2005 Dietary Guidelines for Americans

Physiology In Summary The metabolic syndrome model describes the potential causative connections between and among obesity, peripheral insulin resistance, hypertension, and dyslipidemia. The insulin resistance is located primarily in skeletal muscle, being greater in type II muscles with their limited capillary supply. Exercise can both directly and indirectly decrease the risk of CHD by influencing obesity, insulin resistance, and hypertension.

Healthy People 2010 Nutrition objectives Physical activity objectives Synthesis Healthy People 2010 Nutrition objectives Achieve a healthy body weight Eat <30% total calories from fat and <10% total calories from saturated fat Eat at least five servings of fruits and vegetables and six servings of grains daily Physical activity objectives Engage in any leisure-time activity Participate in sustained activity for at least 30 minutes daily Engage in activity that promotes cardiovascular fitness three or more days per week for 20 minutes or more per occasion Participate in activity to enhance strength and flexibility

Study Questions There is a sudden increase in the number of birth defects in one section of a large city. Describe what an epidemiologist might do to determine what is causing this problem. Why is a web of causation model needed to study the causes of degenerative diseases, in contrast to infectious diseases? Physical inactivity was long considered only a secondary risk factor. What “proof” did investigators have to offer to convince the scientific community otherwise? What is the difference between primary and secondary risks for coronary heart disease (CHD)? Why does a high-fat diet not appear as a risk factor for CHD?

Study Questions Draw a diagram of the hypothesized connections between and among obesity, insulin resistance, hypertension, and dyslipidemia. Indicate the primary site of insulin resistance, and explain how exercise training might reduce the problem.