EPI242: Cancer Epidemiology 2009 Nutritional Epidemiology Overview Role of nutritional epidemiology is to: - provide the best possible scientific evidence to support an understanding of the role of nutrition in the causes and prevention of ill health - ensure that the information upon which public health decisions are made is of the highest quality Focus of the study may be etiological, prevention, intervention Following is the fra mework provided by Willett8 for classifying the vast number of constituents of foods consumed by humans. The categories are not necessarily exclusive. Energy sources. Energy sources, including proteins, carbohydrates, fats, and alcohol, are transformed into energy to sustain life . Nutrients. Nutrients, including vitamins, minerals, lipids, and amino acids, are required for good health. Other natural compounds. Countless enzymes and enzyme inhibitors, specialized lipids, genetic material, and other substances are required to sustain animal and plant cell structure and function. Cholesterol, for example, is an important structural component of animal cell membranes. Natural toxins. Although most people consider pesticides to be chemical food contaminants, some pesticides are produced naturally by plants as a defen se mechanism. The toxins that make some plants poisonous to humans provide an obvious example of natural toxins. Microbial contamination. Foods are easily contaminated, especially during processing or storage, by fungi. Aflatoxins such as aspergillus have received much attention because of their etiologic link to liver cancer. Food additives. Substances are added to foods to preserve them or to improve their color, taste, or consistency. These chemicals attract a great deal of public attention and concern, yet they are among the best characterized and most intensively regulated. Agricultural contaminants. T hese substances include fungicides, herbicides, pesticides, and growth hormones. Inorganic contaminants. Inorganic substances, including metals, such as lead and cadmium, and synthetic compounds, such as polychlorinated biphenyls, can contaminate our food supply. Chemicals formed by cooking. Numerous substances are created by cooking foods. Some specific agents created by cooking or charring meats have been hypothesized to cause cancer of the gastrointestinal system. History Relatively new as a formal area of research Long history of understanding of how foods relate to diseases 1600; Beri-beri 1753; Scurvy 1980; Keshan disease Consideration of nutrients in balance 1. Excess- western/affluent societies; long latent period; multiple causes; penetrance variable; unknown critical period; may not be reversible 2. Deficiency- anemia, wasting, stunting; short latent period; single/few causes; high penetrance 3. Insufficiency- may have characteristics of either excess or deficiency; osteoporosis 4. Unknown- interactions among nutrients and other factors Complexities in studying nutrients and disease Relationship between food and health is affected by acute or chronic illnesses; access to health care; lifestyle Therefore, nutritional data needs to be collected with purpose in mind and clear hypotheses to enable reliable estimate of the relationship of interest - clear and testable research questions - consideration of the effects of other factors - exposures and outcomes of interest are measured with sufficient precision The need for practical methods to measure diet Methods need to be: Accurate Relatively inexpensive Have high compliance Debate Are useful measurements of the diet of individual subjects within free-living populations possible to collect? Are diets of persons living within one country too homogeneous to detect relationship with disease? Development of nutritional epidemiology Development and evaluation of methods for dietary assessment in epidemiological applications is growing Many aspects of diet can be measured readily with sufficient accuracy to provide useful information Finding that meaningful between person variations within populations do exist Identification of ways to deal with errors/limitations 1. Identify the principle sources of error 2. Explore the impact of these errors on the results 3. Design studies which prevent or control these errors Gains in the field Defining patterns of food consumption vs.. individual foods Discovery that the ways foods are stored and prepared influence impact Study of whole foods in addition to specific nutrients Causal pathway of nutrients Development of measurement instruments Knowing the sources, characteristics and effects of errors in the measurement of dietary exposures Use of biomarkers to identify unreliable dietary intake reporting Need for improvement: Determining the relevance of biomarkers or proxies for dietary intake Identification of susceptible subgroups Study Designs: Correlation or Ecologic The exposure variable is the population per capita consumption of specific dietary factors Relies on disappearance data (food produced and imported minus food exported and fed to animals) Some correlations strong (0.85 for meat intake and colon cancer incidence) Strengths: contrasts in dietary intake are typically large (fat intake between US and nonwestern nations); diets at a regional level are stable over time; values are derived from large populations and are subject to only small random errors Study Designs: Correlation or Ecologic Weaknesses: highly correlated with other factors (GNP); disappearance data may lead to erroneous conclusions (alcohol consumption and breast cancer- who is drinking the alcohol in the population?); cannot be independently reproduced Solutions: collect information on actual dietary intake in a uniform manner from the population subgroups of interest; China has 65 geographical areas with unusually large variation in rates of many cancers; select men vs. women; etc. Study Designs: Correlation or Ecologic Special Exposure Groups Seventh-day Adventists (SDA): vegetarian, have half the expected colon cancer rates of non-SDA in the same geographical region Subject to same strengths/weaknesses as ecological studies, but useful in refuting hypothesis; i.e., SDA abstain from alcohol consumption yet do not have significantly different rates of breast cancer mortality than non-SDA Study Designs: Correlation or Ecologic Migrant Studies and Secular Trends Migrant studies show influence of environmental exposures in contrast to genetic factors Secular trends likewise may reveal non-genetic factors that are influential Study Designs: Case-Control Control for confounding by design (matching, restriction) or in the analysis (multivariate methods) if confounding variables are known However, for dietary studies, bias may seriously effect outcome Due to the limited range of variation in diet, the relative risks are modest (0.5-2) and even a small error in measuring intake (3-4%), which is difficult to avoid (recall bias), can distort relative relationship Inconsistent findings between studies would be predicted and are indeed found in the literature Study Designs: Case-Control Selection of controls with medical conditions may potentially have diet related conditions Inherent biological complexity Nutrient-nutrient interactions may cause inconsistent results, but are nonetheless accurate Even consistent findings may be wrong; i.e., positive association between total energy intake and risk of colon cancer, that in prospective studies found either non or inverse associations Study Designs: Cohort Strength: less susceptible to bias Weakness: may have low frequency of outcome, more expensive, follow-ups Some populations are followed-up (standard of care for cancer patients) AND are inherently very interested in diet, so can find a committed population. Study Designs: Controlled Trials Undertaken only when there is a body of nonexperimental data that justify benefit to humans High probability of finding lack of effect due to diets exerting effects after long period of time; duration of trial must be long and compliance must be high Health conscious people tend to enroll, those at higher risk may not be included in study Imprecise measure of effect because if benefit is observed, ethical considerations require stopping the trial Measures of Exposure Types of measures Dietary habits Food patterns, meals, foods, nutrients Individuals, groups, populations Anthropometry Biological Knowledge/attitudes about food Expression of measures Continuous Total cumulative dose (e.g. lifetime) Average dose (e.g. average per day) Percent of standard Discrete Eat a particular food item Rank (quartiles) Percent above/below standard Measures of Outcome Types of measures Dietary habits Anthropometry Biological Physiological Disease/health status Morbidity Mortality Expression of measures Continuous Categorized into discrete (e.g. cut-points: BP, obese, syndromes) Measures for the relationship between exposure and outcome Average level of group outcome in discrete categories of exposure (e.g. BP levels by thirds of fat intake) Regression (change in outcome per unit change in exposure) Standardized morbidity or mortality ratios Absolute risk or absolute risk reduction Relative risk (ratio of the rates of appearance of outcomes in different categories of exposure) Attributable risk percent (proportion of cases in the exposed population that can be attributed to the risk factor) Number needed to treat (number of subjects in whom the exposure needs to be altered in order to save one person from getting the outcome) Interpretation of null results Variation in diet is insufficient Variation may exist but only within the no effect range of the dose response curve Method of measuring dietary intake is not sufficiently precise to measure differences that truly exist Low statistical power Temporal relationship between the measured exposure the occurrence of the disease did not encompass the true latency period Unmeasured third variable was related to exposure and disease in opposite directions; negative confounding Methodological sources of bias Issues of nutritional epidemiology 1. 2. 3. 4. 5. Measurements of diet lack precision and specificity Nutrient intakes are highly correlated; attribution of causation to one nutrient considered to be acting on its own may be misleading Biological measurements of nutrients in tissues may not accurately or reliably reflect dietary intake; biological regulation of these measurements is complex, may be influenced by other other nutrients, homeostasis Lack of consideration of the effects of the way food is prepared (whole, processed, extracts) Everyone is exposed to causal/preventive factors, i.e. everyone eats fats, fiber, vitamins over a lifetime Issues in study design and interpretation of results If null finding is observed, you should define the conditions and limitations of the null finding Demonstrate that true variation exists in the diet and that method of measuring diet is adequate for detecting variation No study instrument is without some potential source of error; use valid instrument of study Report confidence intervals Issues in study design and interpretation of results (cont.) Range of latency period should be discussed. Case-control study of cancer describe over what time period dietary intake was reported; Prospective study may allow analysis over different times Describe the dietary and non-dietary correlates of the primary exposure what has been evaluated as potential confounding variables Types of Measurements Twenty-four-hour Recall is an attempt to define and quantify food intake during a specific day. Dietary Records are detailed descriptions of types and amounts of foods and beverages consumed, meal by meal, over a prescribed period (3-7 days). Food Frequency Methods collect long-term diet over months or years, not just a few days. Different levels of information concerning diet Food Item/Food group level red meat, fish, butter, vegetable oil, cruciferous vegetables, coffee, beer salted, smoked, pickled, fried, cured foods Nutrient Level energy, animal protein, fat, vitamin c, selenium Other Factors nitrates/nitrites, food additives, contaminants, mutagens and carcinogens, antioxidants Measurement Techniques of Food Consumption Food Recording Techniques Weighing method: (1) precise weighing (weighing both before and after cooking); (2) weighed inventory (weighing only after cooking) Recording in household measures Interview methods Recall Methods (usually 24 H) Dietary history and food frequency methods Food Frequency Questionnaire based on Dietary History Measures the habitual diet of the subject at least qualitatively and half-quantitatively Frequency and portion size of use 100-200 food items are asked as well as the usual eating pattern. Refers to the previous month or year Seasonal difference are asked 60-90 minutes interview The best method of estimating the habitual diet of an individual over a long time period Idea in diet and cancer studies New Challenge for Nutritional Epidemiology: Cancer and Chronic Disease Occur with a low frequency with multiple causes acting alone or in combination (nutrition, smoking, drinking, occupational exposure, physical activity, etc.) Occur only among exposed but also among unexposed. (everyone eats fat, fiber, vitamins). New Challenge for Nutritional Epidemiology: Cancer and Chronic Disease Have a longer latent time (20-40 years for cancer) or unknown. Not readily reversible, may result from excessive or insufficient intake of dietary factors, which makes the intervention difficult Percentage of Cancer Deaths Attributed to Various Factors (Doll R and Peto R, JNCI, 1981) Tobacco Use: 30% (25%-40%) Diet: 35% (10%-70%) Infection: 10%? (1%-?) Reproductive & Sexual Behavior: 7% (1%-13%) Occupation: 4% (<2%-8%) Alcohol: 3% (2%-4%) Geophysical factors (natural radiation): 3% (2-4%) Pollution: 2% (<1% – 5%) Food additives: <1% (-5% – 2%) Medicines/Medical Procedures: 1% (0.5%-3%) Industrial consumer products: <1% (<1%-2%) Unknown: ? Harvard Center for Cancer Prevention (www.hsph.harvard.edu/cancer) Tobacco: 30% Diet in adult life, including obesity: 30% Sedentary lifestyle: 5% Infectious agents: 5% Defects in single genes that run in family: 5-10% Exhibit 16–1 American Cancer Society (ACS) Guidelines on Nutrition and Physical Activity for Cancer Prevention ACS Recommendations for Individual Choices 1. Eat a variety of healthful foods, with an emphasis on plant sources. Eat five or more servings of a variety of vegetables and fruits each day. Choose whole grains in preference to processed (refined) grains and sugars. Limit consumption of red meats, especially those high in fat and processed. Choose foods that help maintain a healthful weight. 2. Adopt a physically active lifestyle. Adults: engage in at least moderate activity for 30 minutes or more on five or more days of the week; 45 minutes or more of moderate-to-vigorous activity on five or more days per weekmay further enhance reductions in the risk of breast and colon cancer. Children and adolescents: engage in at least 60 minutes per day of moderate-to-vigorous physical activity at least five days per week. 3. Maintain a healthful weight throughout life. Balance caloric intake with physical activity. Lose weight if currently overweight or obese. 4. If you drink alcoholic beverages, limit consumption. ACS Recommendations for Community Action Public, private, and community organizations should work to create social and physical environments that support the adoption and maintenance of healthful nutrition and physical activity behaviors. Increase access to healthful foods in schools, worksites, and communities Provide safe, enjoyable, and accessible environments for physical activity in schools, and for transportation and recreation in communities. Source: Reprinted with permission from T Byers, et al., American Cancer Society Guidelines on Nutrition and Physical Activity for Cancer Prevention: Reducing the Risk of Cancer with Healthy Food Choices and Physical Activity. CA: A Cancer Journal for Clinicians , Vol 52, pp. 92-119, © 2002, Lippincott Williams & Wilkins.