Chapter 16: Descriptive Research in Physical Activity Epidemiology

Epidemic of Heart Disease in the 20^th Century

• Epidemiological studies done to find the determinants of heart disease so that prevention could be undertaken

• Studies done between 1940 and 1960 to study physical activity and heart disease; i.e., Framingham Heart Study

• By early 1980’s low levels of PA were associated with increased risk for heart disease.

• By the 1990’s low levels of PA were associated with increased risk for a number of important health conditions,

– including all-cause and cause-specific mortality,

– Cardiovascular diseasee

– Osteoporosis

– Some forms of cancer

– And mental health and quality of life

Initiatives to Increase the amount of Physical Activity – Healthy People 2010

Increase the proportion of adolescents who engage in vigorous physical activity that promotes cardiorespiratory fitness three or more times per week for 20 or more minutes per session

Increase the proportion of adults who engage regularly, preferably daily, in moderate PA for at least 30 minutes per day.

Status of Healthy people 2010

                   1997 levels           2010 Target

Adolescents      64%                     15%

Adults               15%                     30%

Epidemiological Methods

Are used to provide the scientific data for public health endeavors, including:

– Quantifying the magnitude of health problems

–  identifying factors that cause disease (risk factors)

– Providing quantitative guidance for the allocation of public health resources

– Monitoring the effectiveness of prevention strategies using population-wide surveillance programs.

Observational vs. Experimental Research

Observational research uses existing differences in factors that may cause disease within a population, such as physical activity, dietary habits, or smoking.  A portion of the population chooses to be physically active while others do not.  These naturally occurring differences in a population are observed to understand the effect on specific disease outcomes.

It would be unethical to conduct true experimental research on health behaviors such as physical inactivity.

What is Epidemiology?

“The study of the distribution and determinants of health related states or events in specified populations, and the application of this study to the control of health problems.” (Last, 1988)

Distribution of Disease

Relates to the frequency and patterns of disease occurrence in a population.

Frequency – prevalence, incidence, mortality rate

1996 the prevalence of all CV diseases in US ~ 58.8 million/25%

Mortality rate for CV disease was 401/100K for males and 197/100K for females

Can be used for comparisons to other populations

Patterns – person, place, time

• Helpful in developing hypotheses about risk factors for the disease

• Personal characteristics include demographic factors including age, sex, or socioeconomic status.

• Place include geographic differences, urban-rural variation, types of occupations

• Time refers to annual, seasonal, or daily patterns of occurrence. Example: Colds increased in runners within a 14 day period following a 56km ultramarathon

Determinants

• “Is any factor, whether event, characteristic, or other definable entity, that brings about change in a health conditoin, or other defined characteristic.” (Last, 1988)

• Determinants of disease are often called risk factors. Example: CV disease risk factors include, high BP, high LDL cholesterol, low HDL cholesterol, obesity, inactivity, etc.

• Once a determinant is identified health promotion campaigns educate the public

Application of Established Understanding of the causal factors of disease

• This is the major goal of public health

• Also called dissemination, or translation

• Health Promotion Strategies

Definitions of Physical Activity Measurement Concepts

• Definitions are necessary to increase consistency of measurement and to reduce variability across studies

• Physical Activity all movement that is produced by the contraction of skeletal muscle and that substantially increases energy expenditure

• Includes all form of movement in occupation, exercise, home, leisure and transportation settings.

• Exercise is planned, structured, and repetitive bodily movement done to improve or maintain one or more components of physical fitness

• Physical fitness is a multidimensional concept associated with a set of attributes that people have or achieve that relates to the ability to perform physical activity.

• Leisure is a concept that includes the elements of free choice, freedom from constraints, intrinsic motivation, enjoyment, relaxation, personal involvement, and opportunity for self-expression.

• Volitional physical activity refers to activities done for a purpose, either in a structured or unstructured setting

• Spontaneous physical activity refers to brief periods of movement that results in energy expenditure, including fidgeting, or unintentional, short, accumulated periods of movement.

Measurement of Physical Activity

Frequency – number of times per week or per year that one is physically active

According to the ACSM the minimum frequency of vigorous exercise needed to increase CV fitness is 3 days/week

Duration – time spent in a specific activity represented by minutes or hours per session

Intensity – the difficulty of an activity and is generally classified as light, moderate, or vigorous. 

– May be expressed in absolute or relative terms. 

– Unit for intensity is the MET, which is the resting metabolic rate.

– One MET is equivalent to 3.5 ml O₂/kg/min, or written as 3.5 ml O₂•kg^-1•min^-1

– One MET is equivalent to 1 kcal•kg^-1•hour^-1 for a 60kg person (132lb.)

5) Other relative expressions of physical activity include: percentage of maximum oxygen uptake (%VO₂max), percentage of maximal HR, Rating of perceived exertion.

Dose – combination of frequency, intensity and duration of PA and can be expressed as kcal/day, MET-hours per day, minutes on a treadmill GXT.

Figure 16.2 shows a strong inverse dose-response relation between max MET level and all-cause mortality

Assessment of Physical Activity

Can be measured in a variety of ways.

Can measure the amount of heat that directly comes off of the body during exercise, called direct calorimetry.

Can ask people to estimate the amount and type of activity done during the past week or year.

Occupational titles no longer reflect the amount of physical activity required in a job.

Global Questionnaires – are one- to four-item instruments that permit a general classification of one’s habitual activity program:

– Sedentary

– Light

– Moderate

– Heavy

These are easy to administer to thousands of people.

Short-recall questionnaires generally give 5-15 items to reflect on recent activities of the past day or week or month

Quantitative history questionnaires are detailed instruments that have from 15-60 items and reflect the intensity, frequency, and duration of physical activity patterns in various categories. Lengthy; 15-30 minutes, and usually administered by a interviewer.

Epidemiologic Study Designs

Initial Hypotheses are developed

Crudely tested with simple descriptive methods

Use cross-sectional and ecological study designs

Then analytical designs are used to test more specific hypotheses.

Analytical studies use case-control and cohort study designs

After the analytical studies consistently demonstrate an exposure-disease link experimental studies are designed, using the randomized controlled trial. 

Outcomes may be mortality, disease incidence, or an intermediate end point such as blood cholesterol levels or BP

Descriptive Epidemiology

“general observances concerning the relationship  of diseases to basic characteristics such as age, sex, race, occupation, social class, or geographic location.” (Last, 1988)

The major objective of descriptive studies is to quantify the magnitude of specific health problems, identify population subgroups that may have higher rates of disease, and develop hypotheses about specific factors that may be determinant of disease.

Cross-sectional Designs

Dependent variable may be mortality, but usually the risk factors for disease.

Risk factors for Heart disease include: blood cholesterol or blood pressure

Example:  Study of the effects of physical activity, 2 estrogen metabolites, and BMI.

A low ratio of the 2 estrogen metabolites (called 2/16 ratio) was believed to increase risk of breast cancer.   The subjects filled out a questionnaire regarding physical activity, height, weight, etc. 

Results showed interaction between physical activity and adiposity (BMI, kg/m²)

Figure 16.3

Advantage of Cross-sectional Studies

Can control for individual differences that may confound the study.

Example:  If body fat may alter the results, individuals could be selected with the proper activity levels and body fat levels.

Limitation of Cross-sectional Studies

Study outcome and exposure are measured at the same time.

Impossible to know whether physical activity level actually was responsible between the 2 factors studied

They cannot make definitive conclusions as to cause and effect.

A lack of association between two variables does not mean that there is no longitudinal relationship

Ecological Designs

Use existing data sources for both exposure and disease outcomes to compare and contrast rates of disease by specific characteristics of an entire population: i.e., census data, vital statistics records, employment records, or national figures for health information, such as food consumption.

Examples

One study looked at disease rates among occupational groups.

Bus drivers compared to the conductors showed that the drivers had twice the heart disease mortality rates as the conductors

Analytical Designs

The Case-control and Cohort designs are Analytical Designs, which test specific hypotheses regarding the causal links between various exposures and mortality and incidence outcomes using purely observational methods.

Natural History of Disease Model

Chronic diseases include heart disease, osteoporosis, cancer, etc.

Researchers are typically interested in how exposure to physical activity early in a disease process alters the course of the natural history of the disease. 

The disease could be extended by preventive means (active lifestyle), or hastened by adverse means (sedentary lifestyle)

Cohort Studies

The Terms follow-up, prospective, and longitudinal studies have all been used to describe cohort studies

A large disease-free population is defined, and assessment of relevant exposures is obtained.

After a baseline assessment the follow-up period begins; from 2 to 20 years or more.

During the follow-up period the number of deaths due to a certain diagnosed disease are tabulated

Basically calculating disease rates for various levels of exposure; i.e., mortality rates of those who exercise regularly are compared with the mortality rates of non-exercisers

The study could also compare different levels of exercisers.

Measured in Person-Years of follow-up

Relative Risk are calculated as the ratio of a reference exposure level to a different exposure level; no effect would be 1.0, <1.0 indicate reduced risk,> 1.0 indicate increased risk.

Cohort Study of Heart Attacks and physical activity

Harvard Alumni Health Study: 16,936 male alumni aged 35-74 years in 192 and 1966.

Completed mailed questionnaires, which included simple questions about walking, stair climbing, and exercise activities.

Subjects were classified in terms of 6 energy expenditure levels.

Stratified comparisons of these data by age group, smoking status, blood pressure level, and adiposity revealed that these potentially confounding factors did not account for this relationship.

The effect of physical activity on heart attack risk were independent of many important factors that could potentially confound this relationship.

Relative Risk was obtained by dividing the heart attack rates for less-active men (<2000kcal/week, 57.9 attacks per 10,000 person-years) by rates for highly active men (2,000+kcal/week, 35.3 attacks per 10,000 person-years). 

Relative risk was 1.64,  meaning that the less-active men were 1.64 times more likely to have a heart attack.

Advantages of Cohort Studies

Temporal sequence between exposure and outcome is clearly defined.

Cohort studies are good for rare exposures.

Cohort studies are good for understanding the multiple effects of a single exposure; physical activity on a number of health problems like cancers, stroke, hypertension, depression, suicide, CV, all-cause mortality, etc.

Disadvantages of Cohort Studies

Difficult and costly to conduct due to keeping track of large numbers of individuals over long periods

Susceptible to large numbers of losses

Some disease outcomes are rare and even a large cohort may not produce enough cases for meaningful analysis.

i.e., in the Harvard Alumni study it took 6-10 years and 16,936 individuals to collect 572 cases of first heart attack (42.2/10,000)

Case-control Studies

Aims to identify factors that are causally related to a diseasee outcome. 

A population of individuals with disease (i.e., cases) and without disease (i.e., controls) is recruited into the study over the same period of time.

The two groups may be matched on some confounding factor, such are age or ethnicity.

Both groups are interviewed about exposure to certain causal factors.

Retrospective exposure assessment identifies factors that influence the natural history of the disease during its induction period.

The data from the two groups are contrasted and expresses the relationship as an “odds ratio” (OR).  An estimate of the relative risk that would have been calculated in the study group if a cohort study had been conducted.

The Null value of the odds ratio is 1.0; values >1.0 indicate increased risk, <1.0 indicate less risk.

Advantages of Case-Control studies

First line of analytical investigation because it can provide valid estimates of exposure-disease relationships in a shorter period of time with less monetary expense than the cohort design.

Effective for investigating rare diseases

Disadvantages of Case-control studies

Exposure information is obtained after the disease has been diagnosed, and the challenge of recruiting an appropriate control group.

Retrospective recall of activities, intensities, frequencies

The controls need to be representative of the population from which the cases were obtained.

Experimental Designs with Randomized Trials

Experimental designs allow researchers to identify the effects of a specific intervention on a health outcome in a group of people (experimental group) while simultaneously monitoring changes in the same health outcome among people not receiving the intervention (control).

Randomized trials that are focused on changing health at the individual level are called clinical trials.

Due to ethical considerations the control group is given an activity that does not have the same physiological effect as the experimental group.

Randomized trials that are focused on changing behaviors in communities are referred to as community trials

Community interventions are done:

– Targeting everyone may prevent more cases of disease than targeting just high-risk individuals.

– Environmental modifications may be easier to accomplish than large-scale voluntary behavior change.

3) Risk-related behaviors are socially influenced

4) Community interventions reach people in their native habitat

5) Community interventions can be logistically simpler and less costly per person.