Adolescent Nutrition and Fertility

Heather Munro Prescott. Cambridge World History of Food. Editor: Kenneth F Kiple & Kriemhild Conee Ornelas, Volume 2, Cambridge University Press, 2000.

The relationship between nutrition and adolescent fertility has been a topic of much discussion in recent research on human biology. The apparent increase in the incidence of teenage pregnancy in Western societies has led some researchers to wonder whether there are biological as well as cultural factors that influence this phenomenon (Vinovskis 1988). Studies of adolescents in different geographic and socioeconomic settings have demonstrated that the age at which sexual maturity is reached is not fixed but is heavily shaped by numerous influences, such as fatness at adolescence, physique, health status, genetics, degree of physical activity, and socioeconomic status (Maresh 1972; Johnson 1974; Short 1976; Zacharias, Rand, and Wurtman 1976; Frisch 1978; Meyer et al. 1990; Moisan, Meyer, and Gingras 1990; Wellens et al. 1990).

Since the reproductive process requires energy, reproductive ability is curtailed in times of food scarcity or when calories burned through physical exertion or exercise exceed the amount provided by food intake. Undernourished women, for example, reach menarche later and experience menopause earlier than do well-nourished ones. Poorly nourished women also have higher frequencies of irregular menstruation and anovulatory menstrual cycles, with menstruation and ovulation disappearing entirely if malnutrition is severe. During pregnancy, malnourished women have a greater likelihood of miscarriage, and if they do carry the infant to term, they experience a longer period of lactational amenorrhea. In men, severe malnutrition leads to loss of libido, a decrease in prostate fluid and in sperm count and mobility, and, eventually, the loss of sperm production altogether. For children and adolescents, undernutrition delays the onset of puberty in both boys and girls, and limits the fecundity of those who have achieved sexual maturity (Frisch 1978).

Researchers disagree, however, on the precise way to measure adequate nutritional status. Some, most notably Rose Frisch (1978), argue that achievement of a critical weight is the major factor in achievement of sexual maturity. Others argue that body composition, particularly the percentage of body fat, is the most important determinant of sexual maturity (Crawford 1975).

Much more is known about the effects of nutrition on adolescent fertility in girls than in boys (Boyd 1980). Fertility in girls is usually based on age at menarche, although even this event is not a foolproof measure because adolescent girls experience a number of anovulatory cycles before establishing regular menstruation (Short 1976). Nevertheless, researchers have clearly established that poorly nourished girls, female athletes, and girls with anorexia nervosa and other eating disorders reach menarche at later ages, have higher incidences of amenorrhea, and have a greater frequency of anovulatory cycles than do girls who have adequate dietary intake relative to calories expended (Frisch 1978;Wyshak and Frisch 1982).

Fertility in boys, however, is much harder to determine. The main indicator of male fertility is spermato-genesis, determined by measuring the amount of sperm contained in the seminal fluid. But data on the beginning of spermatogenesis is more difficult to obtain because there is no clearly visible sign, as is the case in menarche. Moreover, it is not clear whether the appearance of spermatozoa in the urine is a reliable measure of fertility in adolescent boys (Baldwin 1928; Short 1976). Some researchers have attempted to use age of first ejaculation (Kinsey 1948) as an indicator of male fertility, but this, too, is unreliable (Brown 1966; Short 1976). Consequently, less is known about the factors that influence adolescent male fertility.

The Secular Trend in Sexual Maturity

One of the more controversial issues in the history of adolescent sexuality is whether the age of sexual maturity has declined over time. Nearly all of the discussion in this area centers around the apparent declining age of menarche, particularly among young women in Europe and the United States. G. von Back-man (1948) and J. M. Tanner (1962) were among the first to suggest that the age of menarche has declined precipitously over the past several hundred years, from a high of 17 or 18 in early modern times to an average age of 12 in contemporary Western societies.

Most historians and researchers in human biology have accepted this model of a secular trend in age at menarche (Zacharias et al. 1976; Frisch 1978; Shorter 1981). Some scholars, however, have questioned the methodology employed in determining the age of menarche in the past. It has been argued that the evidence for Tanner’s secular trend is based on extremely small samples of young women, and some have questioned whether these findings can be applied to larger populations (Brown 1966; Bullough 1981). In addition, others have pointed out that the way in which age at menarche is usually determined is flawed, for it relies on the recollected memory of women who were interviewed much later in life. It has been suggested that during the nineteenth century, an early age of menarche was often associated with sexual promiscuity and/or working-class status. Therefore, some women may have stated a higher age of menarche than they actually experienced, in order to maintain an image of middle-class respectability (Brown 1966; Bojlén and Bentzon 1968; Diers 1974; Bullough 1981). Even Tanner admitted that “the early studies of age of menarche suffered from disadvantages of both sampling and technique” (Tanner 1968, 1973, 1981b).

Another troubling aspect of historical research on the secular trend is the fact that age of menarche appears to be younger in both ancient and medieval times than in the nineteenth century. Reviews of Greek and Roman textbooks, for example, indicate that the average age of menarche in the ancient world was about 13.5 years (Amundsen and Diers 1969; Diers 1974), and evidence from medieval textbooks indicates that the age of menarche varied from 13 to 15 years (Post 1971; Amundsen and Diers 1973; Diers 1974). Researchers are cautious about this evidence, however, because it comes from medical textbooks, rather than from direct observations of young women (Diers 1974).

Some historians have even suggested that there might be ideological reasons for placing so much faith in a secular trend in age at menarche. Both Vern L. Bullough (1981) and Maris A. Vinovskis (1988), for example, have argued that the secular trend is often linked with the increasing problem of teenage pregnancy in contemporary American society and is used to justify very stringent policies regarding teenage sexuality.

Despite these criticisms, however, most researchers do agree that there has been some decrease in the age of menarche over time. Even Bullough states: “Undoubtedly there has been some drop in menarcheal age in the United States since the 19th century, to under 13 in the 1980s” (Bullough 1981: 366). The consensus in recent research on the secular trend is that there has been a statistically significant decline in age at menarche, but the drop has been much less precipitous than that suggested by Tanner and Backman (Frisch 1978; Wyshak and Frisch 1982; Golub 1983).

The History and Geography of Studies on Adolescents

Scientific interest in the fertility and growth of young men and women dates back to at least ancient Greece. Aristotle was among the first to observe the negative effect of excessive training and undernutrition on young boys. He also warned of the dangers of precocious sexuality, claiming that “the physique of men is also supposed to be stunted when intercourse is begun before the seed has finished its growth” (quoted in Tanner 1981a: 8).

During Roman imperial times, Soranus noted that vigorous exercise could inhibit menstruation, but he stated that this was normal and did not require intervention. Like Aristotle, Soranus noted that precocious “excretion of seed is harmful in females as well as males,” and that “men who remain chaste are stronger and bigger than others” (quoted in Tanner 1981a: 10). As a result of these warnings about precocious sexuality, Roman law linked the age of marriage with the age of puberty, which was believed to be 14 for boys and 12 for girls (Amundsen and Diers 1969; Tanner 1981a).

In the Middle Ages, the Renaissance, and the early modern period, medical texts tended to echo the writings of ancient authors on the subject of adolescent growth and development. Among them, as among the ancient authors, there seems to have been at least some understanding that the onset of puberty, and particularly the onset of menarche, was linked with nutritional status. G. Marinello, for example, observed in the sixteenth century that some girls menstruated earlier than others. He stated that “the cause of variation is [differences in] the natural composition of the body, or complexion or habits; thin and long girls [menstruate] later[,] fat and strong ones earlier” (Marinello 1574, quoted in Tanner 1981a: 21). Similarly, Hippolytus Guarionius (1571-1654), a contemporary of Francis Bacon, noted that peasant girls in seventeenth-century Germany menstruated later than the daughters of townsfolk or aristocracy. Guarionius noted that “the cause seems to be that the inhabitants of the town consume more fat (moist) foods and drink and so their bodies become soft, weak, and fat and come early to menstruation, in the same way as a tree which one waters too early produces earlier but less well-formed fruit than another” (quoted in Tanner 1981a: 29).

During the eighteenth and nineteenth centuries, ideas about adolescent growth and development shifted as a result of contemporary debates about the place of so-called inferior races in the natural order. The egalitarian rhetoric of the Enlightenment called into question earlier ideas of racial inferiority, implying that the “natural rights of man” applied to men and women of all races, classes, and ethnic origins. But many scientists at this time endorsed racist ideas about human difference and looked for physical traits that would demonstrate that working-class Europeans, Africans, Asians, and Native Americans were inferior to white, middle-class Europeans (Stepan 1982; Schiebinger 1993). One of the physical signs that was used to “prove” the inferiority of nonwhite and working-class individuals was early age of menstruation and physical maturity, because such traits seemed to place these groups closer to animals.

Scientists attributed the allegedly early age of puberty in nonwhite and lower-class individuals to two causes: precocious sexual activity and warm climate. Both of these factors, it was argued, caused a buildup of heat in the body. Because many believed that heat was the engine of growth, anything that caused an accumulation of heat would lead to an early age of puberty. Albrecht von Haller (1775), for example, claimed that girls in the southerly regions of Asia, where the climate was warm, were marriageable in their eighth year and gave birth in their ninth or tenth year; conversely, women in Arctic regions did not menstruate until age 23 or 24. This view was shared by other eighteenth-century writers, most notably J. F. Freind (1738), Herman Boerhaave (1744), and Montesquieu (1751). Similarly, Martin Schurig (1729), a Dresden physician, noted that bodily maturation in girls could be accelerated by indulging in conversations with men, kissing, and other sorts of sexual encounters. Schurig claimed that indulgence in sexual activity was why prostitutes and lower-class women had an earlier age of menarche than did gentlewomen (Tanner 1981a).

To be sure, some authors, most notably George-Louis Leclerc de Buffon (1749-1804) and John Roberton (1832, 1845), continued to argue that nutritional status was the most important factor in determining age of puberty. During the middle-to-late nineteenth century, Charles Darwin (1868) also tried to postulate a relationship between food supply and fertility, observing that domestic animals with a regular, plentiful food supply were more fertile than corresponding wild animals. Moreover, reformers who were interested in improving the plight of the laboring classes argued strongly against the idea that lax morals caused an early age of puberty among working-class girls. On the contrary, they argued that difficult living conditions delayed puberty in the working classes (Whitehead 1847).

Climatic theories about puberty persisted well into the twentieth century. As late as the 1950s, some medical writers were still claiming that women from the tropics matured much earlier than those from temperate or cold climates (Peters and Shirkande 1957; Shaw 1959).

As a result of the views described above, middle-class parents in Europe and the United States viewed precocious puberty with a certain degree of alarm. Nineteenth-century medical-advice literature cautioned mothers to prevent all children, but especially daughters, from masturbating, reading romantic novels, and indulging in any activity that might excite the sexual passions (Neuman 1975). Many physicians also believed that too much consumption of meat contributed to precocious sexual longings and sexual development in young girls and could even cause insanity and nymphomania. Doctors, therefore, advised mothers to restrict their daughters’ intake of meat in order to prevent such disasters (Brumberg 1988). In retrospect, this advice may have contributed to the perceived secular trend in the age of menarche.

During the mid-twentieth century, a number of developmental studies uncovered the methodological flaws in earlier studies of the effect of sexual activity and climate on age of puberty. Tanner (1962), for example, demonstrated that many of the studies of adolescents from tropical regions were performed on girls and boys from wealthy families who had access to abundant sources of protein-rich foods and, therefore, did not represent the average age of puberty in these societies. Moreover, researchers noted that the age of menarche in Eskimo girls was actually earlier than that of western European girls, a phenomenon that resulted from the Eskimos’ meat-rich diet (Bojlén and Bentzon 1968).

The most definitive studies on the relationship between nutrition and adolescent fertility were performed during and after World War II. Studies of children who had experienced famine, illness, and other harsh conditions during the war years showed a definite link between poor nutrition and age of physical maturity (Ellis 1945; Markowitz 1955; Krali-Cercek 1956; Maresh 1972; Wellens et al. 1990; Murata and Hibi 1992). Likewise, cross-cultural studies of women in both Western and non-Western countries have demonstrated that socioeconomic status and, hence, nutrition, are much more important than climate in determining age of puberty (Kolata 1974; McBarnette 1988; Brink 1989; Riley, Huffman, and Chowdhury 1989). These discoveries have completely undermined earlier notions about the effect of climate or sexual activity on age of puberty and have established once and for all that nutritional status is the determining factor in age of sexual maturation.