17.1 Growth and Strength

Biologically, the years from ages 18 to 25 are prime time for hard physical work and safe reproduction. However, as you will see, the ability of an emerging adult to move stones, plow fields, or haul water better than older adults is no longer admired, and if a contemporary young couple had a baby every year, their neighbors would be more appalled than approving.

Strong and Active Bodies

Maximum height is usually reached by age 16 for girls and 18 for boys, except for a few late-maturing boys who gain another inch or two by age 21. Maximum strength soon follows. During emerging adulthood, muscles grow, bones strengthen, and shape changes, with males gaining more arm muscle and females more fat (Whitbourne & Whitbourne, 2011). By age 22, women have developed adult breasts and hips and men have reached full shoulder width and upper-arm strength.

Peak Performance Because this is a soccer match, of course we see skilled feet and strong legs—but also notice the arms, torsos, and feats of balance. Deniz Naki and Luis Gustavo, now in their early twenties, are German soccer team members in better shape than most emerging adults, but imagine these two a decade earlier or later and you will realize why, physiologically, the early 20s are considered the prime of life.

AP PHOTO/AXEL HEIMKEN

For both sexes, muscles can be powerful. Emerging adults are more able than people of any other age to race up a flight of stairs, lift a heavy load, or grip an object with maximum force. Strength gradually decreases over the decades of adulthood, with some muscles weakening more quickly than others. Back and leg muscles shrink faster than the arm muscles, for instance (McCarter, 2006). This is apparent in older baseball players who still hit home runs long after they no longer steal bases.

Every body system—including the digestive, respiratory, circulatory, and sexual-reproductive systems—functions optimally at the beginning of adulthood. Serious diseases are not yet apparent, and some childhood ailments are outgrown.

In a large survey, 96.1 percent of young adults (aged 18 to 24) in the United States rated their health as good, very good, or excellent, whereas only 3.9 percent rated it as fair or poor (National Center for Health Statistics, 2012). Similarly, 95.3 percent of 18- to 24-year-olds reported no limitations on their activities due to chronic health conditions, a rate far better than that of any other age group (see Figure 17.1).

Strong and Independent Looking at this graph, do you wonder why twice as many 5- to 17-year-olds as 18- to 24-year-olds are said to be limited in daily activities? The answer relates to who reports the limitations. Parents answer for children; adults answer for themselves. Parents tend to be more protective, reporting that chronic conditions (mostly ADHD and asthma) limit what their children can do. Emerging adults are strong and capable—and they say they are even when they are not.

Source: National Center for Health Statistics, 2010.

However, some emerging adults have health problems that they may ignore. Doctors have told 15 percent of 18- to 29-year-olds that they have a chronic disease—most often asthma, arthritis, diabetes, or high blood pressure (National Center for Health Statistics, 2010). These diseases are not usually severe until middle age or later, but their prevalence suggests that health for many emerging adults is not as good as it might be.

Dirty Hands Once men avoided hand washing—it was too ladylike. Then scientists proved that hand contact spread every virus, from the common cold to SARS. Here famed Ferrari driver Fernando Alonso joins a UNICEF hand-washing day, showing a young lad in India that he should wash his knuckles too.

PUNIT PARANJPE/AFP/GETTY IMAGES

Lifelong, many serious conditions can be avoided or ameliorated with preventive medicine. If this were the only way to be healthy, then most emerging adults would be sick, because they avoid medical professionals unless they are injured or pregnant. Thirty percent of this age group in the United States has no usual source of medical care (National Center for Health Statistics, 2012). Perhaps as a result, the average emerging adult sees a health professional once a year, compared with about 10 annual medical visits for the typical adult aged 75 or older. In fact, one-fifth of all young men do not see a doctor or other health professional even once (National Center for Health Statistics, 2012).

Emblematic of the carelessness of emerging adults regarding health is hand washing among students at a college in Ontario, where a viral epidemic led the school’s administration to advocate—by using signs in bathrooms, public announcements, and so on—frequent, careful, time-consuming, hand washing. Most students (85 percent) claimed they practiced proper hand hygiene, but observers discovered that only 17 percent actually did (Surgeoner et al., 2009).

Similarly, while the Centers for Disease Control recommends the flu shot each year for everyone over 6 months of age, only about one-fourth of adults have had the shot in the past year (National Center for Health Statistics, 2012). Young adults especially avoid the flu shot, unless their college requires it. Their reasons? It’s not guaranteed and it takes time (ignoring the time saved by preventing the flu, which it does for most vaccinated young adults).

Bodies in Balance

Fortunately, bodies are naturally healthy during emerging adulthood. The immune system is strong, fighting off everything from the sniffles to cancer and responding well to vaccines (Grubeck-Loebenstein, 2010). Usually, blood pressure is normal, teeth have no new cavities, heart rate is steady, the brain functions well, and lung capacity is sufficient.

Rates of illness are so low that many diagnostic tests, such as PSA (for prostate cancer), mammograms (for breast cancer), and colonoscopies (for colon cancer), are not recommended until middle age or later, unless family history or warning signs suggest otherwise. These tests are more likely to be harmful to young adults because of false positives than to benefit them because of early detection. Fatal disease is rare worldwide during emerging adulthood, as Table 17.1 details for the United States. (see page 495).

This does not mean that emerging adults are unaffected by the passing years. The process of aging, called s enescence, begins in late adolescence. [Lifespan Link: Senescence is discussed in more detail in Chapter 20.] However, because of three biological processes we now describe—organ reserve, homeostasis, and allostasis—few emerging adults are aware that their organs and cells are aging.

VISUALIZING DEVELOPMENT

Highlights in the Journey to Adulthood

DATA FROM: ACS, 2012

Organ reserve refers to the extra power that every organ is capable of producing when needed. That reserve power decreases each year, but it usually does not matter because people rarely need to draw on it. Bodies function well unless major stress, genetic weakness, or aging has caused that extra strength to be used up.

Bodies have a muscle reserve as well, directly related to physical strength. Maximum strength potential typically begins to decline by age 25. However, few adults develop all their possible strength, and even if they did, 50-year-olds retain 90 percent of the muscle they had at age 20 (Rice & Cunningham, 2002). Indeed, if a 50-year-old couch potato begins lifting weights, he or she may become stronger than ever.

The most important muscle of all, the heart, shows a similar pattern. The heart is amazingly strong during emerging adulthood: Only 1 in 50,000 North American young adults dies of heart disease each year. The average maximum heart rate—the number of times the heart can beat per minute under extreme stress—declines as the organ reserve is reduced, beginning at about age 25. But the resting heart rate remains very stable.

All the parts of the body work in harmony. Homeostasis—a balance between various parts of the body systems—keeps every physical function in sync with every other. For example, if the air temperature rises, people sweat, move slowly, and thirst for cold drinks—three aspects of body functioning that allow them to cool off. If it gets chilly, pores constrict and people automatically walk faster and shiver to increase body heat. If they are really cold, their teeth chatter too, a more intense form of shivering.

Homeostasis works most quickly and efficiently during emerging adulthood. Thus, as long as they get enough sleep and nourishment, emerging adults are less likely to be sick, fatigued, or obese than older adults. When they catch a cold, they are down for a day or two; older adults often complain that they cannot “shake” a virus.

Each person’s homeostatic systems are affected by age and past experiences, as well as by genes. For example, reaction to weather depends partly on childhood climate (an African may be cold when her roommate from northern Europe is warm), and younger people are generally warmer than older ones. If two people share a bed, one may want more blankets, which is why modern electric blankets have dual controls. Your mother may tell you to put on a sweater because she is cold.

Related to homeostasis is allostasis, a dynamic body adjustment that affects overall physiology. The main difference between homeostasis and allostasis is time: Homeostasis requires an immediate response from the body systems, whereas allostasis refers to longer-term adjustment.

For example, how much a person eats daily is affected by many factors related to appetite—that is the homeostatic “set point.” An empty stomach triggers hormones, stomach pains, digestion, and so on, all of which lead a person to eat again. If an overweight person begins a serious diet, rapid weight loss soon triggers short-term, homeostatic reactions, making it harder to lose weight (Tremblay & Chaput, 2012).

Eating is related to a broader set of human needs: how emotionally satisfied or distressed a person is. Many people overeat when they are upset, and eat less when they have recently exercised—those responses could be considered part of homeostasis as well. Our bodies are designed to be comfortable, with many mechanisms to relieve the pains of hunger, low oxygen, thirst, and so on. Those reactions are short term—that makes them part of homeostasis.

A Moment or a Lifetime These three in New Delhi enjoy free pizza at the opening of the 600^th Domino’s in India, just one of more than 5,000 outside the United States. Cheese and pepperoni may satisfy the homeostatic drive, but they now increase the allostatic load in every nation.

PRASHANTH VISHWANATHAN/BLOOMBERG VIA GETTY IMAGES

Over the longer term, allostasis adjusts to whatever the person eats, breathes, exercises, and so on. If a person is hungry for several weeks, the body adjusts. That allostatic adjustment is why a heavy meal suddenly consumed by a starving person might result in vomiting or diarrhea. Allostasis for a starving person requires gradual readjustment (small, digestible meals) when food is plentiful. Similarly, if a long-term vegan eats a healthy portion of meat, that might cause indigestion.

Over the years, allostasis becomes more crucial: If a person overeats or starves day after day, the body adjusts, but that takes a toll on health. In medical terminology, that person has an increased allostatic load because the body’s adjustment results in a burden that may impair long-term health. Obesity is one cause of diabetes, heart disease, high blood pressure, and so on—all the result of physiological adjustment (allostasis) to long-term daily overeating (Sterling, 2012).

Thus, overeating and underexercising require not just adjustment for each moment (homeostasis) but also adjustment over decades. One heavy meal reduces appetite for the next few hours (homeostasis); years of obesity put increasing pressure on the allostatic system, so a new stress (such as running up three flights of stairs) may cause a major breakdown (such as a heart attack).

Add organ reserve to homeostasis and allostasis and it is clear why health habits in emerging adulthood affect vitality in old age. Because of organ reserve, a heart attack is unlikely before midlife, but years of physical stresses affect overall body functioning. Thus a person may have a heart attack at age 50 because of obesity and smoking that began at age 20.

Young adults rarely experience serious illness because all three aspects of body functioning—organ reserve, homeostasis, and allostasis—work in harmony. Senescence gradually reduces the capacity of each organ and slows down homeostasis, eventually increasing allostatic load. The increased physiological burden is usually imperceptible (except in laboratory analysis) and rarely affects daily life until old age.

Even in the smaller changes of aging, such as the wearing down of the teeth or loss of cartilage in the knees, serious reductions are not normally evident until later in life. For example, brushing and flossing the teeth reduces bacteria of the mouth, but all three aspects of body functioning prevent gum disease in emerging adulthood even with poor oral hygiene and never seeing a dentist. The consequences appear much later, when tooth loss reflects decades of taking teeth for granted.

For everyone, the immune system is a strong and vital part of homeostasis, which is why emerging adults are so healthy. But don’t make the mistake of thinking that eventual illness is simply the result of age and fate. Daily life makes a difference, as proven by astronauts. Those chosen to fly in space are relatively young and in excellent health, with strong immune systems. However, after a space flight, the immune system shows temporary but severe loss. That proves that the body is affected by much more than genes and aging (Crucian et al., 2013).

Partly because of their overall health, strength, and activity, most emerging adults look vital and attractive. The oily hair, pimpled faces, and awkward limbs of adolescence are gone, and the wrinkles and hair loss of middle adulthood have not yet appeared. Obesity is less common during emerging adulthood than in adulthood.

The organ that protects people from the elements, the skin, is clear and taut, characteristics that “can change quite drastically” with time (Whitbourne & Whitbourne, 2011, p. 66). The attractiveness of youth is one reason that newly prominent fashion models, popular singers, and film stars tend to be in their early 20s, looking fresh and glamorous.

Vanity about personal appearance is not admired, so few emerging adults admit that they are intensely concerned about their looks. That was one conclusion from a study of 19- to 26-year-olds in the United States, New Zealand, India, and China (Durvasula et al., 2001). Yet this age group spends more money on clothes and shoes than adults of any other age. When they exercise, their main reason is to maintain—or attain—fit, slender, attractive bodies, unlike exercising older adults, whose main motivation is to maintain or achieve good health. New students in college, no matter what their ethnicity, usually care a great deal about looking good (Gillen & Lefkowitz, 2012).

Concern about appearance may be connected to sexual drives, since appearance attracts sexual interest, and young adults hope to be attractive. Furthermore, in these years many people seek employment. Attractiveness (in clothing, body, and face) correlates with better jobs and higher pay (Fletcher, 2009). Women particularly focus on appearance, especially weight, because how they look is important for both dating and employment (Fikkan & Rothblum, 2012; Morgan et al., 2012).

No wonder emerging adults try to look their best. Usually they succeed.

Staying Healthy

Emerging adults experiment and select from many options. We focus now on two vital choices that help emerging adults stay healthy: exercise and nutrition.

Boot Camp for Beauties Dangerously Fit offers high-intensity exercise in 20 outdoors locations in Australia, claimed to be good for body and soul. Why are all the souls here women age 18 to 30, and why is developing a “killer shape” also promised?

WILLIAM WEST/AFP/GETTY IMAGES

Exercise at every stage of life protects against serious illness, even if a person smokes and overeats. Exercise reduces blood pressure, strengthens the heart and lungs, and makes depression, osteoporosis, heart disease, arthritis, and even some cancers less likely. Health benefits from exercise are substantial for men and women, old and young, former sports stars and those who never joined an athletic team.

By contrast, sitting for long hours correlates with almost every chronic illness, especially heart disease and diabetes, both of which pose additional health hazards. Even a little movement—gardening, light housework, walking up the stairs or to the bus—helps. Walking briskly for 30 minutes a day, five days a week, is good; more intense exercise (swimming, jogging, bicycling, and the like) is better, and adding muscle-strengthening exercise is best.

The health consequences of inactivity in early adulthood have been found in dozens of studies. One of the best is CARDIA (Coronary Artery Risk Development in Adulthood) that began with over four thousand healthy 18- to 30-year-olds. Most (3,154) were reexamined 7 and 20 years later. Those who were the least fit at the first assessment were four times more likely to have diabetes and high blood pressure in middle age. Problems began, unnoticed, and continued to worsen unless the unfit person changed habits, which rarely occurred (Camhi et al., 2013).

Fortunately, most emerging adults are quite active, getting aerobic exercise by climbing stairs, jogging to the store, joining intramural college and company athletic teams, playing at local parks, biking, hiking, swimming, and so on. In the United States, emerging adults walk more and drive less than older adults, and 61 percent of them reach the standard of exercising 30 minutes a day, five days a week. That 61 percent is higher than any other age group, and higher than the percentage of young adults who achieved that standard a decade ago (52 percent).

Past generations quit exercising when marriage, parenthood, and career became more demanding. Young adults today, aware of this tendency, can choose friends and communities that support, rather than preclude, staying active. Two factors can encourage activity:

1. Friendship. People exercise more if their friends do so, too. Because social networks typically shrink with age, adults need to maintain, or begin, friendships that include movement, such as meeting a friend for a jog instead of a beer or playing tennis instead of going to a movie.
2. Communities. Some neighborhoods have walking and biking paths, ample fields and parks, and subsidized pools and gyms. Most colleges provide these amenities, which increases the exercise of students. Health experts cite extensive research showing that community design can have a positive effect on the levels of obesity, hypertension, and depression (Bors et al., 2009).

Nutrition is another lifelong habit embedded in culture. At every stage of life, diet affects future development. For example, a program in Guatemala that provided adequate nutrition to pregnant women and children under age 3 had benefits for emerging adults 20 years later—the children of these women had more education and better jobs than a comparable group of emerging adults who had not been such fortunate babies (Martorell et al., 2010).

For body weight, there is a homeostatic set point, or settling point, that makes people eat when hungry and stop eating when full. Of course, extreme dieting may alter the set point: Eating disorders such as anorexia and bulimia nervosa may worsen in early adulthood, and death from such disorders is more likely in the 20s than in the teens. Or people may routinely overeat, again shortening their life span. [Lifespan Link: Eating disorders were discussed in detail in Chapter 14.]

The body mass index (BMI)—the ratio between weight and height (see Table 17.2)—is used to determine whether a person is below, at, or above normal weight. A BMI below 18 is a symptom of anorexia, between 20 and 25 indicates a normal weight, above 25 is considered overweight, and 30 or more is called obese. Emerging adulthood correlates with healthy weight.

About half of all emerging U.S. adults are within the normal BMI range, as are less than one-third of adults aged 25 to 65. Emerging adulthood is also the time when the lowest proportion of the daily diet comes from fat: less fat is another sign of good nutrition (National Center for Health Statistics, 2012).

Once emerging adults become independent, they can change childhood eating patterns. Sometimes they do. As a generation, U.S. young adults consume more bottled water, organic foods, and nonmeat diets than do older adults, and many become more fit than their parents were at the same age. A large British study found that about one-half of those who were obese as children become normal-weight young adults, with healthier eating and social patterns (Viner & Cole, 2005).

A study in the United States found that emerging adults who lived on college campuses ate a more balanced, healthier diet than those living with their parents (Laska et al., 2010). The effect of home cooking continues, however, no matter where the person lives: The strongest influence on fruit and vegetable consumption in emerging adulthood is family patterns at home during childhood (Larson et al., 2012).

Obviously, better eating is not automatic. Although some emerging adults lose excess weight, others gain. According to the British study cited above, 12 percent of normal-weight teenagers become obese by age 30 (Viner & Cole, 2005). Further, many nutritionists note that the fruit and vegetable consumption of young adults is about half of what it should be (Larson et al., 2012). Almost everyone could improve. Just as health in emerging adulthood is affected by consumption 20 years earlier, so is health in middle age affected by consumption during early adulthood.

Particular nutritional hazards await young adults who are immigrants or children of immigrants. If they decide to “eat American,” they might avoid curry, hot peppers, or wasabi—each of which has been discovered to have health benefits—and indulge in fast food, which tends to be high in fat, sugar, and salt. Although older immigrants overall are healthier than native-born Americans, their young-adult offspring have significantly higher rates of obesity and diabetes than their parents, particularly if their national origin is African or South Asian (Oza-Frank & Narayan, 2010).

No matter what their ancestry, today’s emerging adults are fatter than past cohorts, and as they age they gain weight—about a pound a year, according to the CARDIA study. Specifics of diet matter: CARDIA found that fast foods, high-fat diets, and diet soda each had independent effects, with the overall result affecting not only body weight but also other health factors indicated by laboratory tests (Duffey et al., 2012). [Lifespan Link: Adult obesity is discussed in Chapter 20.]