Noncommunicable diseases are usually thought of as chronic conditions that do not result from an acute infectious process. These conditions cause death, dysfunction, or impairment in the quality of life, and they usually develop over relatively long periods—at first without causing symptoms; but after disease manifestations develop, there may be a protracted period of impaired health. Generally, these conditions or diseases result from prolonged exposure to causative agents, many associated with personal behaviors and environmental factors. The major noncommunicable diseases are listed in Tables 1 and 2. Noncommunicable diseases also include injuries, which have an acute onset, but may be followed by prolonged convalescence and impaired function, as well as chronic mental diseases.
Noncommunicable diseases are the leading cause of functionary impairment and death worldwide. These conditions have been the leading cause of death in the United States and other high-income countries over the last fifty years, and they are emerging as a leading cause of death in low- to middle-income countries. Table 1 depicts the leading causes of death worldwide showing that noncommunicable diseases and injuries account for over two-thirds of deaths. In addition, these diseases cause pain, disability, loss of income, disruption of family stability, and an impaired quality of life.
Disability-adjusted life years (DALYs) constitutes one means of assessing the effect of disease, as shown in Table 2. (DALYs are a measure accounting for years of life spent with diminished function resulting from health conditions of varying severity.) Over the past century, dynamic changes have occurred in the worldwide prevalence of noncommunicable diseases, and even more rapid transitions are expected in the twenty-first century. These changes have been driven by social, economic, and public health progress, and the strategies for change have been illuminated by research.
Ascendency Of Noncommunicable Diseases
In 1900, the average life expectancy worldwide was about forty-seven years, and there were few differences among different countries. The leading causes of death in the United States and worldwide were communicable diseases and nutritionally related conditions (see Tables 1 and 2). Infant and childhood mortality was high because of infections and poor nutrition—the short average life span mainly reflected high mortality in the early years of life. During the first half of the twentieth century, the high-income, industrialized countries of the world made major advances against infectious and childhood diseases through improved public health measures, nutrition, vaccines, and, to a lesser degree, antibiotics. Childhood mortality declined in these countries, survival to middle and late adult life increased, and noncommunicable diseases emerged in the middle of the twentieth century as the major threat to health. This “epidemiologic transition” from communicable to noncommunicable diseases as the major threats to health did not begin in low- and middle-income countries (LMICs) until the last half of the twentieth century. In many countries with rapidly developing economies, such as Malaysia and Korea, the transition was rapid in the latter half of the century. The public health experience of high-income countries was applied successfully by these countries. Currently, many middle-income countries have health profiles that resemble those of high-income countries. But this epidemiologic transition to noncommunicable disease not only differs across countries but has been interrupted by recent events.
Unfortunately, the trend in many countries have been impacted by a new epidemic plague— HIV/AIDS (human immunodeficiency virus/acquired immunodeficiency syndrome). Many countries in Africa and southern Asia have high infection rates for HIV, and the disease principally affects young adults and newborn infants in these areas. From 1980 onward, the rapid transmission of this fatal disease has not been preventable except by behavioral change; there is no vaccine for prevention, and affordable treatment has not been available in low-income countries. This leaves many countries with a double burden of health problems—a new epidemic of infectious disease and unresolved infectious conditions, as well as a growing set of noncommunicable diseases.
The state of the epidemiologic transition varies around the world, reflecting the social, cultural, economic, and health resource factors in various countries. The health status of the nations of the world is reported by the World Health Organization, and models have been developed to predict future disease patterns by region and country. These models are based on observed trends in disease prevalence (frequency) for countries and regions of the world. The book Global Burden of Disease, edited by Christopher Murray and Allen Lopez provides a prediction of health status in 2020. Rather than taking cause of death and age at death as the principal measures of health, Murray and Lopez have combined measures of morbidity (impaired health) and mortality to assess health status (see Table 2).
According to estimates of the burden of disease, high-income countries in 1990 experienced a preponderance of noncommunicable disease compared to the burden worldwide, as measured by disability-adjusted life years (DALYS), a measure that portrays years of life lost due to disabling conditions, and thus represents the impact of these conditions. Worldwide in 1990, infectious diseases and perinatal conditions top the list. However, by the year 2020 the worldwide burden is projected to shift to noncommunicable diseases, as indicated in the right column. There is an important dimension not obvious from these tables. High-income countries contain 15 percent of the world’s population but account for only 8 percent of the disease burden. Comparisons of high-income countries and LMICs indicate that noncommunicable diseases rates were similar across the two levels of economic income in 1990. A major burden of noncommunicable disease does exist in LMICs, but is overwhelmed by the burden of communicable diseases. If the communicable diseases are controlled successfully, there will be a new worldwide “epidemic” of noncommunicable disease. Already, many middle-income countries that are managing the epidemic of HIV/AIDS are experiencing increased survival to middle adult life and increasing rates of chronic disease and injuries. This change in disease patterns is useful to the health planner who needs to evaluate resource needs, and to the economist who links health, productivity, and economic development as an argument to allocate resources to health services.
There was an alarming rise in chronic diseases and injuries in high-income countries (the United States, Canada, Australia, European nations, and Japan) during the middle of the twentieth century. This increase was especially striking for ischemic heart disease, which became the leading cause of death in these countries in the 1950s and 1960s. This disease results from the formation of plaques containing cholesterol and blood clots in the arteries that supply oxygen and nutrients to the heart muscle. The concerns about this epidemic prompted studies that explored the causes and prevention of the plaques and resultant disease. From 1960 to the present there has been a striking decline in ischemic heart disease deaths in the high-income countries, representing another epidemiologic transition to prevention and management of chronic diseases and increasing survival. Life expectancy has increased in these countries, principally from adding to the years of life after the age of fifty. Somewhat later, in the 1990s, cancer deaths began to decline. This “postindustrialization” improvement in health and survival has important implications beyond the increased life expectancy for these countries. First, noncommunicable diseases can be controlled with prevention and treatment when the modifiable environmental causes are identified and controlled and effective health services are available. Second, research studies can identify the risks and the means to modify them. This information can be applied to individuals and to groups to improve health. Third, the lessons learned from high-income countries might be applied to low income countries to stanch the anticipated epidemic of noncommunicable diseases.
Causes And Prevention Of Noncommunicable Diseases
The causes of noncommunicable diseases are often divided into modifiable and nonmodifiable factors, although these distinctions are blurring with greater knowledge. Chronic diseases result from genetic, behavioral, and environmental factors and the interactions between them. These factors, generally termed “risk factors,” produce molecular and structural changes in organs and tissues but produce few if any early symptoms or signs of disease. After relatively long periods of time, usually decades, disease manifestations and impairment of health result. Risk factors place an individual at a greater likelihood of developing disease, but do not predict disease with absolute certainty. For most chronic diseases, several risk factors contribute.
At the population level, a high prevalence of risk factors can put populations or communities at greater risk and result in more disease. Risk factors for future disease development and early structural changes may be found during the “silent” period before disease becomes manifest.
The prevention of noncommunicable diseases entails a definition of the risk factors and application of interventions to favorably alter risk before overt symptoms or signs develop. At an even earlier level, it may be possible to prevent the development of risk factors through changes in the environment and personal health behaviors. Primary prevention of chronic disease is therefore an important goal, as morbidity and mortality may be averted or forestalled, and promotion of health, or “primordial prevention,” is perhaps the foremost goal. Prevention research seeks to identify risks and to test interventions that modify risk and thereby prevent disease. Chronic diseases result from multiple factors that often interact in an additive or multiplicative fashion to increase risk, but there are also factors that can obviate or decrease disease risk. Because most chronic diseases take years to develop, with overt manifestations occurring in middle to late adult life, there is considerable potential for early identification and modification of risk in childhood, adolescence, and early adulthood.
Injuries include trauma from unintentional causes, such as vehicular and occupational injuries, as well as intentional injuries, such as interpersonal violence and self-inflicted trauma. With increased industrialization and the growth of vehicular traffic, injuries are a major cause of death and DALYs (see Tables 1 and 2), and the estimated worldwide burden in 2020 suggests an even greater increase (see Table 3). Homicides, suicides, and wars are also regrettably important sources of injury-related mortality and morbidity.
The prevention of vehicular injuries has focused on improved vehicular engineering for safety, better road design and traffic regulation, and improved driver training and enforcement of impaired driver laws. Occupational hazards are addressed with regulations related to the workplace. Homicidal actions, either individual or national, continue to pose a threat, particularly with the development of weapons that pose a threat to noncombatants. The principal casualties of wars continue to be civilian populations, and the advent of chemical and biological weapons increase further the potential for mass civilian deaths, even in the absence of declared national wars.
A major nonmodifiable individual risk for disease is genetic susceptibility. This has a very important influence on disease development, but does not confer an absolute certainty that a disease will occur. In the overwhelming majority of diseases, inherited susceptibility interacts either adversely or beneficially with environmental exposures and personal behaviors to alter molecular or metabolic processes that increase the likelihood of disease development—or to mitigate such changes if the susceptibilities or the exposures are beneficial. For example, smoking and tobacco use are risks for cancer—the risk of smokers getting lung cancer is ten to forty times that of nonsmokers. However, not all smokers will develop lung cancer at the same exposure levels, as measured by number of cigarettes smoked and the duration of smoking. There is an individual difference in susceptibility, attributable in part to genetic differences or to environmental exposures to substances such as asbestos, which increases the risk further.
The delineation of the human genome will accelerate the description of genetic susceptibilities to disease and clarify the interactions between genetic differences and environmental risks. However, discovery of the individual genetic susceptibility will only provide an alert that an increased risk exists. There is no current safe means of changing the genetic makeup of individuals, and multiple changes might be required. Nevertheless, it might be possible in the future to identify those genetically at risk and have them avoid certain environmental exposures, thereby decreasing the risk for disease. The current preventive strategy, however, is to identify and eliminate or minimize damaging exposures and not to seek out and change the genetic component.
In some instances, a single gene abnormality will predominate over environmental influences. For example, familial hypercholesterolemia is a disorder resulting from a single abnormal gene that results in blood cholesterol levels two to four times above normal, depending on whether one or two alleles are inherited. Ischemic heart disease develops in middle or early adult life. The usual modifiers of blood cholesterol levels, diet and physical activity, have little influence on cholesterol levels or disease development. The genetic risk overwhelms the environmental influences. However, the use of potent new drugs that decrease internal cholesterol synthesis in the body can lower blood cholesterol and the risk for heart disease. However, for the overwhelming majority of people, and for the majority of chronic diseases, alteration of personal and environmental exposures are the most important strategy.
Age represents a nonmodifiable factor in development of chronic disease. The mechanism is assumed to be the cumulative, long-term exposure to factors that alter function and structure, including DNA. However, increasing genetic information suggests that there are individual differences in genetic-environmental damage related to exposure.
General Environmental Exposures
Environmental exposures can be categorized into general environmental exposures and personal environmental exposures (see Table 4). The general environment encompasses the social, cultural, and public health aspects of life over which individuals can exert little or no personal control. For example, the quality of air and of drinking water or the exposures in the workplace environment are managed or regulated by public health or industrial organizations. If toxic exposures exist, the only personal option would be to change location or employment, but this is often not feasible. The public generally depends on public health organizations to monitor and manage the ambient environment and to make and enforce regulations in the public interest. Through public action, however, individuals can express preferences if the health risks are known.
Vehicular accidents and injuries result from increased traffic and speed of traffic, but public regulation of vehicular design, highway engineering, and regulation of driver behavior represent social policy changes that can decrease fatalities and injuries. Occupational exposures to silica, asbestos, and heavy metals can lead to lung disease, cancer, and death, and regulation and monitoring of the workplace can minimize or eliminate the risk. Environmental aspects can also facilitate healthful behaviors. The community environment can be made more conducive to personal behaviors that decrease the risk of chronic disease. For example, provision of safe and pleasant community venues for walking or other physical activity can help individuals to develop and maintain personal commitments to increasing physical activity. Similarly, the availability of fresh fruits and vegetables at reasonable prices in local markets can foster healthful purchases and eating behaviors.
Other social and environmental characteristics can have a major influence on health, particularly that of populations. Throughout the world, groups with higher personal income and greater education have better health, less disease, and live longer. This observation was substantiated throughout the twentieth century and remains a fundamental factor for both communicable and noncommunicable diseases. The reasons for health disparities by income are not completely understood, but include access to health services, knowledge about and resources to act on health promoting behaviors, and homes and workplaces that have less hazardous exposures. Differences in health status among countries can be explained in part by differences in per capita income, but there are exceptions—some countries that have low income levels have relatively good health status (e.g., Chile, China, and Sri Lanka). Attention to public health measures and availability of basic health services in low-income countries may account for some of the favorable differences. Within a country like the United States, which has a high prevalence of chronic disease, this income and educational gradient is also present. Three times as many people in low-income families report activity limitations as those in high-income families, and individuals in high-income families can expect to live three years longer than those in low-income families.
Disparities in health across racial and ethnic groups are not fully explained, but require study because of the major differences that exist and the need to address risks in groups with the poorest health. Important elements in such disparities include social, economic, and educational factors, as well as access to care, and the affordability of care. These factors tend to be confounding and have made it difficult to sort out the quantitative contribution of each. Some countries have provided more equitable distribution of public health and personal health care despite overall limitations in financial resources. These investments have been associated with improved health for the entire country. Considerable speculation has focused on the role of genetic differences across populations and groups as an explanation of health differences. However, this seems less important than environmental differences, as evidenced by the many instances of rapid change in disease patterns within a generation. This could not be explained by changes in the genetic pool.
Personal Environmental Factors
Personal environmental factors include behavioral choices that are made every day, though many behaviors are habitual. These personal behaviors are extraordinarily important and have been termed the underlying or true causes of chronic disease. They are outlined in Table 4. Use of tobacco, particularly smoking, is the number one preventable cause of death and disability. It directly contributes to ischemic heart disease, stroke, chronic lung disease, and several common cancers, especially lung cancer. These are the leading causes of death in industrialized countries and will be the leading causes worldwide during the coming century. Even exposure of nonsmokers to secondhand smoking increases risk. The prohibition of smoking in public places is an example of a public health regulation that decreases the risk for nonsmokers.
Dietary intake, which includes both the quantity and quality of foods, is highly important in the development of chronic diseases. An intake of calories in excess of what is expended during daily activity leads to obesity and an increased risk of diabetes, high blood pressure, heart disease, and some cancers. Increased physical activity can offset some excess caloric intake and may have additional healthful benefits as well. Since much of the industrialized world now has a predominance of inexpensive, calorically dense food and too little requirement for physical activity, it is not surprising that an epidemic of obesity, and the corresponding increased risk for disease, has occurred. As low-income countries move toward economic development and industrialization, there are similar pressures toward high caloric intake and decreased physical activity. Dietary patterns have important influences aside from the caloric content. Fruits and vegetables and whole grain products have beneficial effects on health, as does a limitation of fat intake to no more than 30 percent of calories.
Other personal behaviors and circumstances contribute to noncommunicable disease development. Alcohol use has both adverse and beneficial effects. Nonpregnant individuals and groups consuming small amounts of alcohol (about one drink per day) experience less ischemic heart disease. However, large amounts (about four or more drinks per day) contribute to chronic liver disease, depression and suicide, and injuries, especially motor vehicular injuries. Any alcohol use during pregnancy carries a risk for impaired fetal development. Illicit drugs are addictive and impair social and occupational functioning and are associated with impaired mental health, notably depression. Both alcohol and illicit drugs can have long-term effects on intellect.
There is evidence that behavioral risk factors are influenced by the availability and affordability of tobacco, foods, and alcohol. When people immigrate to an industrialized country from an area where chronic diseases are less common, their pattern of disease changes to resemble the resident country over several decades of exposure. There has been no genetic change, but the risk factors and disease patterns change as the lifestyle changes. The influence of the new environment is especially marked in younger individuals. While genetic susceptibility is important, the factors of environment and personal behavior are robust, modifiable, and when changed effectively, can reduce disease.
Noncommunicable Diseases Research
The determination that many attributes and behaviors carry risk for disease development has resulted from large observational studies. Groups and populations with varying personal behaviors and degrees of exposure have been studied for the development of disease, and this has allowed quantitative assessments of risk for the multiple factors associated with disease. The risk for disease is generally proportional to the level of the risk factors. The identification of these risks has provided a basis for studies to determine the effects of change. Numerous interventions to modify risk have been tested in clinical trials and community comparison studies. These provide a scientific basis for effective preventive measures that can modify risk factors and decrease disease and subsequent mortality. There is good evidence that lowering blood pressure and blood cholesterol, stopping smoking, and increasing physical activity can decrease deaths from ischemic heart disease and stroke. The majority of studies to identify and modify risk factors have been conducted in high-income countries where the resources for research are concentrated. As noncommunicable diseases increase in lesser developed countries there is a critical need to utilize findings from these studies and to conduct research that relates to indigenous exposures and cultures of LMICs. While many of the major risk factors are likely to be operative worldwide, there are new and perhaps unique risk factors and novel genetic-environmental interactions to be elucidated. The modification of behavioral risks will be dependent on understanding and integrating cultural aspects into lifestyle changes.
The profile of risk factors is constantly evolving, and in some instances there are infectious or communicable factors. Recent research indicates that infectious organisms, viruses, or bacteria may be resident as chronic infections and can initiate or contribute to the development of chronic disease. For example, the principal cause of peptic ulcer of the stomach or duodenum was long thought to be increased gastric acidity from stress and poor dietary habits. But, studies have clearly demonstrated that Helicobacter pylori, a fairly ubiquitous bacterium causing no acute symptomatology, can infect the stomach and cause peptic ulcer, and perhaps gastric cancer. Antibiotic treatment can eliminate peptic ulcer, and this affords evidence for its causative role. Similarly, human papilloma virus (HPV) is present in the early stages of cancer of the cervix. This association has led the testing of a vaccine to eliminate the infection and possibly prevent the development of cervical cancer. Chlamydia has been found in the atherosclerotic plaques of some persons with heart disease, though the role of the organism is unclear. Studies to eliminate the infection may provide insights that would serve to prevent and treat the condition.
Chronic conditions tend to result from the action of multiple factors, genetic and environmental, that increase the risk of disease. As research broadens the understanding of the multiple risks for disease, it would be expected that risk factors might have quantitatively different effects in different countries and cultures and expand the opportunities for intervention and prevention.
After considering these risk factors at the individual and community level, it is instructive to ask whether national changes in risk factors predict countrywide changes in noncommunicable disease. Observations of disease changes indicate that there is an approximate correlation between changes in risk factors and increases or decreases in the relevant chronic diseases. A striking example is the emergence of chronic diseases in LMICs undergoing social changes associated with adoption of harmful behaviors. Smoking and obesity are becoming more common in countries undergoing rapid economic development, and these countries are experiencing the same epidemic of chronic diseases as the industrialized nations. The rates of heart disease, stroke, and diabetes have increased rapidly from 1950 to 1980 in rapidly industrializing countries. As is true with migrant groups undergoing change in risk and disease, environmental factors are critical. However, genetic propensity for particular disease expression may differ across populations and across countries. Subcontinental Indians who migrate to high-income countries and develop obesity have higher rates of diabetes and heart disease than nonmigrating, native-born with the same levels of obesity. Two explanations have been offered. The first is that there is a greater genetic susceptibility in this population, but it is not expressed until the sufficient environment—excess calories—is available. The second is that in utero and early life experiences, especially food deprivation, change the expression of the genes as a means of promoting survival. A thrifty gene would conserve energy and store fat for survival in times of deprivation, but would be detrimental to health in times of plenty. Because developing countries are making the transition from relative privation to adequacy or excess, this becomes an important consideration in determining modifiable causes of chronic diseases.
The experience of high-income countries indicates that noncommunicable diseases can be prevented and managed with declines in death and disability. This “postindustrial” epidemiologic transition is based on the development and application of scientific information. Risk factors predict disease and their modification through public health and personal health measures can change disease patterns in individuals, groups, and populations. Technological development can assist this change through the development of vaccines, medicines, and foods. However, personal behaviors and public health policies will always be important. These innovations can be transported throughout the world, but at a financial cost and with the need to assure cultural integrity. Cost, education, and cultural diversity will be barriers to be surmounted worldwide. Further investigation of risk will include appreciation of gene-environment interactions and exploration of new exposures. The improvement of health for all will depend on this progress.
- Murray, C. J. L., and Lopez, A. D. The Global Burden of Disease. Geneva: World Health Organization.
- World Health Organization (2000). World Health Report 2000—Health Systems: Improving Performance. Geneva: Author.