Calculation of Prevalence and Incidence of Nutrition-Related Diseases in the Population PDF

by Reza Hashemi Calculation of Prevalence and Incidence of Nutrition-Related Diseases in the Population 1. Introduction to Nutrition-Related Diseases Although malnutrition status can be categorized into undernutrition, obesity, and micronutrient deficiencies, interest in the correlation be...

by Reza Hashemi Calculation of Prevalence and Incidence of Nutrition-Related Diseases in the Population 1. Introduction to Nutrition-Related Diseases Although malnutrition status can be categorized into undernutrition, obesity, and micronutrient deficiencies, interest in the correlation between diet and lifestyle has shifted with noticeable changes in disease epidemiology in developed countries. Western lifestyles have been modified through rapidly extending life expectancy, changes in living conditions, urbanization, and technological advances, causing obesity, diabetes mellitus, hypertension, hyperlipidemia, heart disease, stroke, cancer, and other chronic diseases related to lifestyle. Nutritional and lifestyle approaches involving weight management, physical exercise, smoking control, and stress management are beneficial strategies to prevent such diseases among the population. The connection between chronic diseases and behaviors associated with food, physical activities, and other lifestyle factors has drawn the attention of the scientific community. Both nutritional deficiencies and excesses in the population are considered nutrition-related diseases despite the primary focus of nutritional care on physiological and metabolic risk factors for the most widely recognized chronic diseases. As an underlying cause, the concept involves malnutrition status. The concept of malnutrition includes both the multiple etiological syndromes of malnourished children from deprived living conditions, without access to adequate health care, including both undernutrition and chronic undernutrition disorders, as well as the multiple etiological syndromes found in obese individuals, associated with several chronic diseases. 1.1. Definition and Classification of Nutrition-Related Diseases Undernutrition, overnutrition, and malnutrition may all result in major clinical problems. While much has been previously made of undernutrition and its relationship to such clinically important topics as wound healing, aging, mobility, hospital length of stay, readmission rates, and mortality, as well as more standard 1 outcome metrics such as rates of infection, length of ventilation, and pressure sores, lesser emphasis may have been placed on the potential relationship between overnutrition or obesity and these same outcomes. The field of nutritional science attempts to understand the relationship between diet, health, and physiological performance. The concept of nutrition-related diseases is a key term within the model. In certain circumstances, individuals have sufficient food in terms of quantity or calories yet still may suffer from malnutrition or nutrition-related diseases because the dietary content is inadequate in terms of the required nutrients such as macronutrients, micronutrients, vitamins, or minerals. Nutrition-related diseases include undernutrition or undernourishment and overnutrition. Undernutrition may occur due to a diet inadequate in energy or nutrients such as protein, vitamins, and minerals, or from problems with absorption of nutrients like diarrheal diseases, parasitism, or the use of specific medications. Undernutrition is the term for conditions in which there is a shortage of calories, nucleotides, essential amino acids, or essential fatty acids. In its early stages, this results in energy being provided by the body's stores, which are often first depleted. Later, physical features such as a greater reduction in muscle mass and subcutaneous fat occur so that the general appearance of the patient becomes thin, and muscle wasting results in poor physical strength, lowered immune function, vulnerability to infection, susceptibility to diseases, slow healing, and so on. 1.2. Importance of Studying Prevalence and Incidence Prevalence and incidence are two important epidemiological indicators that assess the distribution of the disease in the general population. Together, these parameters provide information that is useful for planning both the prevention of diseases and the provision of health services. Understanding the prevalence and distribution of nutritional diseases can be useful in certain stages of program planning, as this information will help to identify, locate, and describe the target population of those programs. Prevalence is the measure used to express the magnitude of a disease or risk factor in a geographic area and at a specific time. In general, when prevalence is used as a measure of the magnitude of the problem, incidence and duration are not considered. Prevalence is the data generally used both for the logistic planning of prevention and control services and to evaluate the effectiveness of the services. The duration of an episode of disease is the most profoundly misunderstood aspect of the disease concept. Failure to recognize the importance of the duration of a health event is a frequent source of avoidable waste in studies of the magnitude and distribution of health phenomena. When the incidence of a mental dysfunction, 2 signs, or symptoms is high, the prevalence may reach very high levels because the duration of the signs, symptoms, or dysfunctions is short. In the course of disease and malnutrition in a population, the variability of the duration is greater than that of the incidence or the prevalence. 2. Epidemiology Basics Public examination of the situation in the field of dietology is seen in frequently fixed violations of the diet and weight mode. However, irrespective of the improvement of diagnostic and treatment facilities of practitioners and specialists, it does not guarantee a decrease in the rate of primary diseases of a dietary nature. Rationalization of nutrition organization is possible, providing public health through the obligatory application of a complex approach to dietology. Carrying out an organization-diagnosed measure with the aim of assessing the degree of satisfaction of consumers of dietary services, the condition of clinic-nutrition of the population, and the estimation of the correlation between dietarians providing presumed alimentary dysfunctions and their real weight of disease is a basic moment. Disease statistics continue to grow and make up 60% of the population. Assessment of the role of prevalence, distribution, and germogenity of anthropogenic diet- related constituents in the structure of general morbidity is essential for the foundation of an organization directing dietary activities for the population. The conspicuousness and activeness in growing nosology related to a manner of healthy and regular supply of medical advice – pernicious to the point of properly stating the functions of nutritional services of many doctors of dietary nature, either in the appearance of diseases or in their deterioration of movement. 2.1. Key Concepts in Epidemiology Prevalence and incidence are key concepts in epidemiology and describe different features of the occurrence of a disease in a population. Prevalence is defined as the proportion of the population with a specific characteristic or disease at a specific time or the percentage of a specific population that is affected by a particular condition at a specific time. P is the common symbol to represent prevalence. Incidence is a measure of the risk of a disease in a particular population over a duration of time or the number of new cases developing in a given span of time. Prevalence is the proportion of the population in which the event has already occurred (the proportion of current cases), while incidence is the proportion of cases that occur in the population within a specific time span. In general, incidence is expressed as rates, which are measures of the frequency with which an event occurs in a defined population in a defined period of time. The incidence rate is the 3 number of new cases of a disease in a defined period of time, divided by the number of people in the affected population at the start of the period. 2.2. Measures of Disease Frequency The calculation of prevalence and, particularly, of incidence is easy when the data are from secondary sources, but it can be complicated or even difficult when the data must be obtained during a specific period. These calculations provide important information for public health. Currently, the prevalence of obesity can be easily obtained from many different databases. Moreover, technology and biochemistry allow for obtaining data about lipid status and blood glucose at a rate previously unimagined. However, although some estimates of iron deficiency are available, the assessment of the prevalence of deficiency of many other vitamins or minerals is extremely difficult. Furthermore, the health impact of many nutrition- related diseases, especially those that present as signs or symptoms unspecific to the disease, is underestimated, simply because their prevalence is not known. Though mortality data related to protein-energy malnutrition are easier to obtain in underdeveloped countries than in more developed ones, they are not good measures of the health impact of undernutrition. Likely, data on mortality and incidence morbidity rates would be much more informative. In addition to the calculation of incidence and prevalence measures, the evaluation of the methodology used can increase the knowledge that investigators could obtain, or the accuracy of the results. Measures concerning the concordance of the diagnosis are of particular importance. In practical terms, these results are obtained by the repetition of the evaluation, for example, by the same observer in different time periods, different observers in the same persons for prevalence evaluation, or in relation to a gold standard to achieve their accuracy. They are the measures that objectively show the classification rates into true or false positives or negatives, which are so important in public health. Accurately classifying an individual as unhealthy, healthy, or at risk makes it possible to direct specific food/nutrient interventions to those who can really benefit. 3. Prevalence Studies Prevalence or incidence data are essential for planning both healthcare and primary prevention. This chapter provides some guidelines for the design, planning, and implementation of a survey addressing the clarification of the prevalence and incidence of some chronic diseases related to nutrient intake. Such information is useful for prioritization in the allocation of financial and human resources and to assess the impact of preventive measures. Having some reference to the background risk of chronic and degenerative diseases, which can potentially be modified by an 4 appropriate intake of food constituents, it is possible to evaluate the effects of other determinants, such as nutrient and energetic intakes and physical activity levels. In other words, this type of study allows the evaluation of the strength of associations between chronic disease occurrence and its potential risk factors. The majority of studies to establish the prevalence of malnutrition in different settings are characterized by the use of a cut-off point of nutritional status derived from the distribution of values in large surveys of the general population. Unfortunately, this approach violates one of the principles of epidemiology: that the cut point should be taken from an independent data set. Indeed, a low level of nutrition is a sensitive indicator of risks, showing an increasing trend with decreasing consumption of various dietary components. Due to the natural history of malnutrition, there is often a long and complex latency period between the potential cause and the actual symptoms of a diet-related disease becoming clinically evident. Prevalence and incidence data can be used to infer causation only when they have been derived from randomized control trials and other intervention studies, or from comparisons of a patient study group with a suitable control group. The criteria for diagnosing nutrition-related diseases are not always straightforward, and explicit criteria for the diagnoses will have to be decided in advance. 3.1. Types of Prevalence Studies Nutritional epidemiology is a relatively recent branch of epidemiology that studies the relationship between diet, nutritional status, and health, and the means by which diet, food, and nutritional status may be antecedent to the etiology of diseases. This means that prevalence can be calculated in terms of any disease for which the corresponding parameters of incidence and survival are known. This is because, at any time, the survival rate is multiplied by the incidence rate. This is the basis of the prevalence-incidence proportion and rate formulas. For nutrition- related diseases, it is not always possible to calculate prevalence because the data required to estimate either the number of new cases or the duration of the disease in other than a fraction of the population are often not available. This chapter shows how these basic rates can be used to calculate prevalence and incidence of nutrition- related diseases using an equation that links the respective parameters of disease frequency. There are two types of epidemiological studies: observational and experimental, and two of each type: cross-sectional and case-control for the first, and cohort and experimental for the second. The two types correspond to the prospective and retrospective approaches. Cross-sectional studies, also known as prevalence studies, are designed to describe the frequency that certain diseases or conditions or their 5 determinants are distributed in the population. When the disease frequency parameter is prevalence, a cross-sectional study is usually performed that involves selection of a sample that is thought to be representative of the population, based on which implications for the whole population are made. Randomly means that the individuals invited to take part in the research are selected by chance from a list, while chance represents an equal likelihood for each individual to be included in the study. 3.2. Calculating Prevalence Rates How can you calculate the percentage of the population to which a society's rate of malnutrition relates? The initial time or month of illness is the initial time in the calculation period, and the initial time of the last disease registration or the time of death that occurred in the study period is the final time. All the members of the closed cohort have the duration of observation equal to the final time, with the endpoints set by the initial and final time. The observed prevalence is calculated by dividing the sum of the number of ill-affected individuals by the sum of the duration of observation of all the members of the cohort. The study population, with missing observation for some time during the study period, has the duration of observation equal to the final observation minus the last date of the last time until disease registration. After the estimation of the definite time duration, the case definition before and after adjustment of loss to follow-up should be included to calculate the observed prevalence. 3.3. Factors Influencing Prevalence Estimates The process of estimating the prevalence and incidence of nutrition-related diseases in a population includes several important steps that are frequently neglected, leading to biased results. These steps include problem definition and design of data collection, choice of study population, appropriate statistical methods, and taking into account such factors as temporal and spatial heterogeneity. We will start this chapter with a discussion of ways in which the four main parameters of the prevalence of any chronic disease—incidence, average duration, the cure rate, and mortality—affect the magnitude of prevalence estimates across countries and over time. There are many well-known factors that influence cross-country or cross-time comparisons of clinical markers of disease: non-comparability of definitions, differences in rates of diagnosis and severity levels, differences in risk factors, and response error in epidemiological data. Much less attention has been given, however, to the potential influence of these differences in quantitative disease estimates. Epidemiologists tend to control for other possible confounders but tend 6 not to call into question the estimation of the actual prevalence or incidence of the disease as measured in different populations or at different times. 4. Incidence Studies The incidence of disease describes the occurrence of new cases over a period of time and is an important measure used to describe the burden of disease in a population. Both point and period prevalence measures only give a picture of the scope of a health problem in existence. In contrast, incidence estimates show the dynamic nature of the health problem and can provide evidence of the underlying disease process. Incidence measures are needed not only for description, but they are used to estimate the effect of an exposure, the dose-response association, and the strength of the association, all important components of causal inference. Incidence data are needed for prevention and health care planning. They provide the numerical basis for the benefits that may potentially accrue from implementing new health policies or interventions, and estimates of the costs to be incurred as well. The vast majority of nutrition work uses sources of data that collect information from cross-sectional studies. Period cohort data would also be collected from food consumption surveys. However, point cohort designs would need to be utilized to collect the type of data needed to estimate the incidence of nutrition-related diseases. Therefore, data collected in nutrition research are often predominantly derived from long-term population cohort studies, with some also utilizing case- control and case-cohort study designs. These study designs are used to get as close to incidence without waiting. Although these are very good study designs, the cost and resource allocation to conduct them, especially if using assays, or if the disease of interest is one with a low incidence, would be prohibitive to use exclusively for population research. Furthermore, food consumption survey data are considered a lower level of evidence due to the inherent problems of using self-reported dietary data. Regardless, the original reason long-term population studies were established was to study the etiology of diseases that are rare in the population. The primary goal of conducting an incidence study is to identify new cases of the disease. The most costly type of incidence study design is one that collects complete information on all members of the study population. However, this type of study design is only needed if the population is not currently being measured and the information is not available. Complete information can be utilized to estimate incidence and other measures, such as duration of disease, many times over many times. The most detailed information is available from complete information studies. 7 4.1. Types of Incidence Studies Incidence studies can be retrospective, based on pre-existing clinical or hospital records, or prospective, based on defined samples of a population. Prospective cohort studies can determine the incidence of a certain condition within a defined follow-up time. They usually have an initial condition, known as the baseline, when the defined sample of the population that will undergo follow-up is detected. Descriptive data about the studied sample are collected at baseline. The sample is then visited at defined time points until the end of the study, when the presence of the disease or condition is assessed. Study of such groups allows tests of statistical associations between the population and exposure characteristics, and the event's incidence. Incidence is calculated as the new events that occurred in the specified time period in a defined study population at risk of the disease at the beginning of the period. Any new cases that happened during the study period are recorded through a combination of prospective visits and/or through the use of linkage to existing medical records. A follow-up schedule involving regular contact, especially for chronic and disabling diseases, is necessary to maintain accurate data for all participants. In adults, if the condition is chronic or progressive, a baseline age of 20 to 30 years is suitable, and it is advisable to examine the person at regular intervals until some 70 years of age, or earlier if the endpoint is likely to occur. 4.2. Calculating Incidence Rates Quantifying the magnitude of nutrition-related health problems at the population level is a critical first step of nutritional surveillance required to guide and set priorities for intervention programs. This measure permits the identification of subpopulations at risk, and it is essential to evaluate the impact of interventions on these goals. Incidence measures the rate of occurrence of new cases of a disease or condition in a population at risk during a specified period of time. It is defined as the ratio of the number of new cases developing within a specified time period to the population at risk during the same time period. Incidence measures provide important information to enable assessment of the magnitude and importance of the nutrition-related problem in the population, as well as of the specific physiological, dietary, or lifestyle conditions associated with disease development, information which is fundamental for guiding primary prevention programs. For example, information on the incidence of diabetes throughout the period of growth in childhood at the population level has been important to alert countries to the need to ensure implementation of healthy lifestyle conditions. Information on incidence is also important for setting priorities 8 in assigning funds and other resources to nutritional surveillance activities, such as studies of risk factors for disease or those directed towards assessing disease outcomes in study situations which would eventually be extended to the population level. 4.3. Relationship between Prevalence and Incidence To relate prevalence with incidence, we need to describe the sequence of states or categories through which the individuals in the population associated with the respective study must pass. Let there be an idiosyncratic risk associated with developing a new onset disease that is distributed over the individuals in the population, such that Yi is 1 if an individual i has this idiosyncratic risk, and 0 otherwise. Let Xi be the calendar time until the individual either is censored from the study, dies from a cause other than the disease, or has a new onset disease, if Yi is 1. Let Ii be a binary indicator, such that it is 1 if an individual has a new onset disease in interval t, and 0 otherwise. From this description, it can now be shown that the prevalence of a nutrition-related disease at time t (Prt) is a function of both its incidence as well as its duration. Prt is simply the number of diagnosed individuals in the population at time t divided by the sample size, which can be expressed as the total number of individuals who are not censored from the study on or before t. Based on the data from the surveillance, Prt can also be determined for the normal individuals, such that Pnt differs from Pt. To avoid having a double subscript, let Pii be 1 until I reaches the value 1, and 0 afterwards. If there are a sufficient number of intervals in the follow- up period, Prt can be approximated using data on the number of new onset diseases in the population, such that writing this statistic with a “hat” on top makes Prt-hat. Then, the prevalence of normal individuals at time t can be expressed as: Pnt = 1 - Pd - Pt-hat. If a nutrition-related disease has a very long duration, the prevalence equals the prevalence of normal individuals at some time in the future. For the nutrition-related diseases, Prt can be greater or lower than the prevalence of normal individuals, such that: P, Prt - %, or, P, Prt + %. To represent these relationships, the equation describing the prevalence of a nutrition-related disease at time t during the survival study has three combined parameters: I(Y). The explanations leading to these relationships are as follows. Individuals who have risk of developing a nutrition-related disease cannot have a new or existing diagnosis, thus Ii has no relationship to IYj, and Ii is 0 when Yi = 0. A similar statement can be made that I(Yi) can take on values: I(Y), non-I(Y), and Yi, such that I, II(Yi) = I(Y), non-I(Y), Yi. If Prt = Pnt, these three parameters are independent of each other. If a nutrition- related disease has a median duration that is much less than the available follow-up time for all individuals in the study, Prt < Pnt if I(Y) > non-I(Y), Prt = Pnt if I(Y) = 9 non-I(Y), or Prt > Pnt if I(Y) < non-I(Y). Some nutrition-related diseases have a relatively long median duration, and some have a very long median duration, such that the incidence of the nutrition-related disease is similar to the incidence of the disease itself. For some of these diseases, existing cases have a long duration too. 5. Epidemiological Methods in Nutrition Research Epidemiology is a set of methods used to describe and understand the frequency, distribution, and causes of health problems in human populations. In this chapter, we will explain the methods used to calculate some of the measures that are most often used in nutrition research, and we will discuss the limitations of these methods. As will be seen, a full discussion of the research methods used to study nutrition and health would go well beyond the limits of a single chapter in the present context. Once the methods of surveillance have been described, some of the most important contributions of nutrition surveillance to public health will be outlined, using the results of national nutrition surveys that have been conducted in Western European countries during the 1980s as examples. In order to study the relationships between dietary intake and health status, it is often necessary to calculate the prevalence or incidence of nutrition-related health problems. Prevalence is the nearly ubiquitous measure of the frequency of diagnosis of an ailment that is found in a given period. It is the proportion of a population or a defined subpopulation that is found to have a condition at a specific time. If there is no scientific interest in the duration of the illness or in the natural history of the disease, the concept of incidence is of little importance, and prevalence deserves greater attention. Incidence is the measure of the occurrence of new cases of an ailment that are observed in a given period. Prevalence is the counting of the number of cases of a recognized illness, while incidence measures how the disease actually occurs in time; that is, the probability of occurrence of the disease. Injury and mortality rates may be seen as the incidence of certain specific maladies, but researchers usually make this distinction with special terminology. 5.1. Study Designs in Nutrition Research To plan the prevention and treatment, as well as the education of the public and health policy, it is necessary to estimate the size of the nutrition-related health problems in populations. The calculation of the size and trend of nutrition-related health problems requires the collection of comprehensive data for the estimation of the prevalence and incidence of diseases. The present communication outlines the basic elements and essential techniques for the epidemiologic estimation of the size and pattern of nutrition-related diseases in the population. The estimation of the size of the nutrition-related health problems in populations is based on the 10 information retrieved from two main approaches: the collection of information from all existing cases of recognized diseases in a population and the collection of data from a sample of the existing cases for the estimation of the mean duration and case-fatality rate. 5.2. Data Collection and Analysis Techniques In general, data on the prevalence or incidence of nutrition-related diseases that are derived from self-reporting may need to be enhanced by gathering medical records or questionnaires to the potential respondents' usual medical care providers. Thus, researchers must weigh the potential burden imposed on respondents, their families, their employers, or governmental agencies against the value they gain from using medical records data rather than just relying on self-reported data. The researchers who gather information from medical records must design and complete the data collection form that requests this information in a manner that minimizes potential concerns about privacy and accuracy, including issues related to confidentiality and the nature of the sensitive information that has been collected. In addition to meeting the requirements of the Privacy and Security Rules of relevant regulations, it is important for researchers to be aware of all other relevant local, state, or national human subjects protection regulations that must be followed. In addition, researchers should be aware of any Institutional Review Board requirements to report to these research review committees any issues that may surface regarding the provision of medical care to the survey respondents while these individuals are participating in the survey, as well as any issues that are identified during the process of obtaining requested medical records. In sum, the collection of medical records is a cost-effective means to enhance the accuracy of the data used for population-based health analyses, but it involves a number of steps that researchers need to undertake to minimize the potential concerns of both the medical records custodians and the surveyed individuals. 6. Challenges and Limitations in Epidemiological Studies of Nutrition-Related Diseases The purpose of this chapter is to specifically address the strengths and weaknesses of epidemiological studies on nutrition-related diseases with respect to the identification of associations between diet and disease. This session aimed to examine how studies deal with these points by circulating methodological pointers before the meeting. As a result, the reports from the four workshops can be compared with respect to the pooling of studies, reliability, and comparability of data on dietary intake, on outcomes, and on the reliability of the tools assessing diet 11 and disease. A starting point is the logical flow of epidemiological studies. Together, these problems contribute to the consistency rarely found in association studies, for instance, in the field of nutrition and health. The chosen nutritional status parameter, either calculated by prevalence studies or linked to dietary intake, will in turn influence the outcome. Clearly, the recommendations and the discussions reported below should also be taken into account in the deliberations on developing research strategies and collecting important new data. Within the exploration of the reliability of nutrition and pathology data, the issue of pooling studies to be used in a meta-analysis was addressed. Workshop discussions demonstrated that careful consideration needs to be given to the potential sources of heterogeneity before forming groups. Turning to dietary intake, it was reported that study data were inconsistent in some studies. Except for a study involving middle-aged women, the participants were drawn from an older population such that information on current nutrient intake was not likely to reflect the nutrition of a segment of the population at considerable risk for cognitive decline and degenerative disease. Furthermore, interview data that is collected with less structured techniques or is based on infrequent measurement is subject to unreliable measurement over time. 7. Case Studies and Examples in Nutrition Epidemiology Setting. The large majority of nutritional surveillance programs take place in developing countries. Conditions include deficiency diseases, both lack and excess; epidemiologic data are typically generated through dietary assessments, in some cases in combination with biochemical assessments at the population level. Investigations of nutrition-related chronic diseases in developing countries, where these diseases are becoming important, have utilized the methods in use in Western countries. Example 1: Anemia in Vietnamese Women The results of an anemia survey in Vietnamese women of childbearing age illustrate the estimation of prevalence of a simple clinical sign. A national survey enumeration of subsequent consecutive sampling, designed to be representative at the commune level, of households with women aged 15–45 years as of the set date was carried out, followed by examination of all nonpregnant women in the selected households for hematocrit values. The national prevalence of anemia in this group was 35.3 percent. Economic trends in Vietnam, along with massive social changes and increasingly liberal agricultural policies, have enabled Vietnam to escape from the nutritional crisis the country was experiencing in the mid-1980s. Normative values for hematocrit to serve as the basis for assessing the prevalence of anemia were 37 for a pregnant 12 woman, 36 for a non-pregnant woman, and 35 for a woman who was in her first or second trimester of pregnancy. 13