Nature and Causes of the Burden of Disease in Young People
At first glance, adolescence appears to be a relatively healthy—although not hazard-free—period of life, given the relatively low mortality rates of young people.1 Nevertheless, adolescents and young adults engage in a range of behaviors that can affect the quality of their health and the probability of their survival in the short term as well as affect their lifetime health and survival.
Health Challenges of Adolescents
If we look only at disability-adjusted life years (DALYs) for the adolescent age group, adolescents appear to be relatively healthy. Nonetheless, more than 33 percent of the disease burden and almost 60 percent of premature deaths among adults can be associated with behaviors or conditions that began or occurred during adolescence—for example, tobacco and alcohol use, poor eating habits, sexual abuse, and risky sex (WHO 2002). Adolescence-related risk factors are a greater problem in wealthier countries, largely because of the relatively greater impact of smoking and diet-related risks in those countries, though the prevalence of these risks is expanding rapidly in many low- and middle-income countries (LMICs). Thus, although adolescents are apparently healthy, they are practicing unhealthy behaviors that will ultimately result in much death and disability. This is an immense public health issue. Therefore, focusing attention both on diseases experienced during adolescence and on risk factors with their roots in adolescence makes sense. Adolescent health efforts should emphasize prevention because so much of the disease burden is preventable and because prevention is a particularly cost-effective strategy in relation to adolescents, given the long duration over which benefits will be reaped and adolescents' greater openness to change than adults.
Burden of Disease in Adolescence
The global burden-of-disease approach used to calculate DALYs is an imperfect representation of the prevalence, morbidity, and mortality of conditions that adolescents face. DALYs fail to capture fully the complexity of adolescent health concerns. Nonetheless, no better comprehensive and comparative measure currently exists; thus, the discussion in this section will rely primarily on available DALY data.
The World Health Organization (WHO), in 1999, commissioned a special analysis of the burden of disease in adolescence, which examined the 10 to 14 and the 15 to 19 age groups. The study found that young people age 10 to 19, who constitute 19 percent of the world's population, account for 15 percent of the disease and injury burden worldwide. It also found that more than 1 million people in that age group die each year (WHO 1999). The top three causes of DALYs were found to be unipolar major depression, transportation accidents, and falls. The profile of disease burden was significantly different for younger and older adolescents. In the 10 to 14 age group, injuries and communicable diseases were prominent causes of DALYs. For the 15 to 19 age group, the disease burden shifted to outcomes of sexual behaviors and mental health.
Using 2002 data, WHO has made more detailed calculations of DALYs by sex for the 5 to 14 and 15 to 29 age groups (table 59.1). These age ranges overlap adolescence and are, thus, broadly indicative of the 10 to 19 age group. Notably, table 59.1 shows large differences by sex in the pattern and level of DALYs. These differences are important, because they relate to the different needs of young women and young men for interventions and services. Particular interventions also potentially have different costs and benefits because of the different proportions of females and males.
[Table .]
Worldwide, among young men age 15 to 29, injuries and neuropsychiatric illnesses account for a high proportion of DALYs (33 percent and 32 percent, respectively). By comparison, among young women age 15 to 29, injuries account for 14 percent of DALYs, and neuropsychiatric illnesses account for about the same percentage of DALYs as among young men. However, sexual and reproductive health conditions account for 33 percent of young women's DALYs, much higher than the 10 percent for young men. For both young men and young women, all other communicable and noncommunicable diseases account for moderate proportions of DALYs (7 to 11 percent, depending on sex and disease group).
The disease burden among 5- to 14-year-olds is markedly different from that for the 15- to 29-year-olds, and differences between males and females are quite small. Communicable diseases and respiratory illnesses account for much larger proportions of DALYs for this age group compared with the 15 to 29 age group, whereas neuropsychiatric and sexual and reproductive conditions account for much smaller proportions. HIV/AIDS accounts for less than 4 percent of DALYs for both males and females age 5 to 14.
Health Risk Behaviors among Adolescents and Young People
Young people's vulnerability to risky or unwanted sex and other unhealthy behaviors is tied to a host of individual, family, and community factors that influence their behavior and that are closely related to their economic and educational opportunities. Good health and other physical, moral, and intellectual development outcomes are often mutually reinforcing. For example, healthy children do better in school. Similarly, having more years of schooling provides essential information and skills that are linked to more protective and less risky behaviors.
Injuries
Violence and war account for more than a quarter of injury-related deaths among young men age 15 to 29. Adolescent boys and men in their 20s are an important part of the military forces in all countries that have such forces. As such, they are at high risk, particularly in areas where armed conflict is occurring. The United Nations Children's Fund estimates that approximately 300,000 soldiers under the age of 18 are involved in armed conflicts worldwide (National Research Council and Institute of Medicine 2005). Homicide is also an important cause of death for young men, in particular, and it is the leading cause of death for young men in some Latin American countries (WHO 2001b). In addition, road accidents account for significant proportions of injuries and deaths among young people. Self-inflicted injuries, including suicide, which are often related to mental illness, are also a major health problem for young people, accounting for 4 percent of DALYs in men age 15 to 29 and 3 percent of DALYs in women age 15 to 29.
Mental Health
Depression, schizophrenia, and other mental illnesses are important causes of illness and death among young men and women and account for a significant proportion of DALYs for both men (18 percent) and women (23 percent) age 15 to 29; for 5- to 14-year-olds, it is about 9 percent for boys and 8 percent for girls. The relative importance of mental illnesses is much greater in the high-income countries.
Smoking, Alcohol, and Drug Use
Most adult smokers worldwide begin smoking in adolescence or earlier (World Bank 1999a). An estimated 15 percent of young men and 7 percent of women age 13 to 15 are currently smoking cigarettes, according to more than 100 surveys that have been conducted since 1999 by the Global Youth Tobacco Survey Program and carried out under the auspices of WHO and the U.S. Centers for Disease Control and Prevention (National Research Council and Institute of Medicine 2005). Every day, worldwide, almost 100,000 young people start smoking, more than two-thirds of them in LMICs (World Bank 1999a). Of the 300 million young people smoking today, half will eventually die from tobacco use (WHO 2001b). By 2030, tobacco is expected to be the single biggest cause of death worldwide, accounting for about 10 million deaths per year (World Bank 1999a).
Although discouraging young people from starting to smoke and providing means for them to quit is extremely important, deaths caused by tobacco tend to occur many years later. Therefore, tobacco use as an underlying risk factor accounts for very few DALYs in the 5 to 29 age group (WHO 2002). Alcohol and drug use account for 8 percent of all DALYs for young men age 15 to 29 but for only 2 percent for young women. Evidence indicates that young people are starting to drink at earlier ages. Longitudinal studies have found that the earlier young people start drinking, the more likely they are to experience alcohol-related injuries and alcohol dependence later in life (WHO 2001a).
Nutrition and Exercise
Nutritional deficiencies such as anemia are widespread in both young men and women. Worldwide, these conditions account for almost 5 percent of DALYs among girls age 5 to 14 and almost 4 percent among boys of the same age, with anemia being an important component for both girls and boys. Although nutritional deficiencies are relatively less important among 15- to 29-year-olds (just over 1.0 percent among young men and about 1.5 percent among women), anemia accounts for the bulk of these deficiencies. Chronic undernutrition that causes stunting among young people delays growth and physical maturation, increases risks to pregnant mothers and their newborns, and decreases the capacity to learn and to work (Behrman and others 2004; Hoddinott and Quisumbing 2003). Malnutrition can take other forms, some of which lead to being overweight or obese, thereby increasing the risks for diseases such as diabetes. Such forms of malnutrition are of increasing relevance in middle-income countries such as Brazil, China, the Arab Republic of Egypt, Mexico, and South Africa and at times coexist with undernutrition (see, for example, Doak and others 2000).
Nutritional deficiencies increase the risks that girls and young women face during pregnancy and childbirth (Delisle, Chandra-Mouli, and de Benoist 2001), and evidence is emerging about the connection between poor maternal nutrition and greater risk of transmission of HIV from mothers to their infants (Piwoz and Greble 2000).
Diet and lifestyle-related chronic diseases—many with their roots in childhood and adolescence—are emerging as one of the most important health problems in LMICs. Cardiovascular diseases, which are responsible for 10 percent of DALYs lost in LMICs, typically occur in middle age or later; however, risk factors are determined to a great extent by behaviors learned during childhood and adolescence and continued into adulthood, such as dietary habits and smoking. Throughout the world, these risks are starting to appear earlier. Physical activity has decreased markedly in adolescence, particularly in girls, and obesity has increased substantially (MacKay and Mensah 2004).
Sexual and Reproductive Behaviors
Worldwide, the majority of young people initiate sexual activity during adolescence. Significant proportions—in some regions and countries, the majority—marry and become parents (table 59.2). Globally, the age of onset of puberty has been decreasing progressively for both boys and girls (National Research Council and Institute of Medicine 2005). The age at first marriage has also increased in most parts of the world over recent decades, except in Latin America (Mensch, Singh, and Casterline 2003). The decline in the age at puberty, combined with the general trend toward later marriage, increases the period of time during which adolescents may be sexually active before marriage and may result in sexual initiation at an earlier age (National Research Council and Institute of Medicine 2005).
[Table .]
Young women typically make the transition to marriage and parenthood at an earlier age than young men, and early marriage predisposes girls to HIV infection through unprotected sex, because the partner, by virtue of age, has an elevated risk of being HIV positive. In addition, marriage changes adolescent girls' support systems, thereby limiting their access to knowledge about HIV/AIDS (Bruce and Clark 2003).
All these key transitions to adulthood bring with them the potential for risks to health that may have both immediate and longer-term effects. Among young women age 15 to 29, illnesses related to pregnancy and childbearing account for 16 percent of their DALYs. Some have unwanted pregnancies, and in countries where abortion is legally restricted, unsafe abortion is an important source of mortality and morbidity for young women, with abortion complications accounting for almost 3 percent of DALYs worldwide among females age 15 to 29. (WHO 2004c).
Even though adolescent childbearing has declined in recent years, the proportion of young women who become mothers during adolescence remains high in most LMICs, and very early childbearing remains an issue in some regions (table 59.2). Childbearing before age 16 also brings with it a high risk of health consequences, both for the mother and for the newborn (Save the Children U.S.A. 2004; WHO forthcoming-b).
In the most recent surveys carried out in LMICs, high proportions of adolescents report that they have heard of contraceptive methods; however, little is known about the quality and accuracy of young people's knowledge of contraception. Moreover, substantial proportions of young women appear to have an unmet need for contraception; they are not using contraception even though they are sexually active and do not want to have a child (CDC and ORC Macro 2003; Westoff and Bankole 1995).
In addition to having a risk of early and unwanted pregnancy, adolescents are also at risk of acquiring sexually transmitted infections (STIs), including HIV. HIV/AIDS accounts for most of the sexual and reproductive health DALYs lost by young men age 15 to 29 (almost 9 percent). Among young women age 15 to 29, HIV/AIDS accounts for a higher proportion of DALYs than for young men (almost 12 percent) because of their higher levels of susceptibility. STIs and other sexual and reproductive health disorders together account for just over 5 percent of young women's DALYs, much more than among young men. About half of all HIV infections occur in people under age 25, and for biological, social, and economic reasons, young women are disproportionately affected, especially in Sub-Saharan Africa, where young women have twice the prevalence rate of young men (UNAIDS 2003).
Poverty and Adolescent Health
Poverty and inadequate health systems compound adolescents' vulnerability to sickness and early death. At the same time, poor health exacerbates poverty by disrupting and cutting short school opportunities, by weakening or killing young people in the prime of their working lives, or by placing heavy financial and social burdens on families.
Poor adolescents bear a disproportionate burden of the health problems in their age group. An analysis of data from demographic and health surveys (Macro International 1990-98, unpublished raw data) indicates a strong association between poverty and the health status of adolescents and between poverty and adolescents' use of health services. For example, the poorest 20 percent of young women are between 1.7 and 4.0 times as likely to have an early birth as the richest 20 percent of young women. Similar disparities between rich and poor adolescents are seen for indicators such as early marriage, skilled attendance at birth, use of contraception, and knowledge of HIV/AIDS transmission, and these disparities tend to be greater for adolescents than for older women. For example, surveys in 45 countries show that the poorest 20 percent of women age 15 to 49 have a total fertility rate almost double that of the richest 20 percent, whereas among adolescents age 15 to 19, total fertility among the poorest 20 percent is more than triple that of the richest 20 percent (Macro International 1990-98, unpublished raw data).
