7.24.2019 2:00 PM

Mind and Body

Researchers Point to One Reason Boys Are Hitting Puberty Earlier Than Ever

Health records spanning 50 years support a controversial idea.

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The timing of puberty appears to be trending younger, which experts say can result in negative psychological and physiological outcomes. This trend has appeared most obviously in girls: The age girls enter puberty has progressively declined over the past 200 years, driven in part by increasing body weight. Studies on boys have been less definitive — yet a study published Monday in JAMA Pediatrics suggests a similar factor could be driving early puberty in boys as well.

Researchers from Sweden’s University of Gothenburg and the Karolinska University Hospital assessed trends in puberty by examining the health records of 4,096 boys born between 1947 and 1996. This study’s marker for puberty is peak height velocity (PHV), the period when a person’s maximum rate of growth occurs.

One of the challenges of studying when boys enter puberty is that they lack a distinct indicator of sex-hormone changes. For girls, scientists use menarche — the first occurrence of menstruation — as that indicator. In girls, menarche happens after PHV is over. Meanwhile, in groups of boys, it’s not until after PHV that other signs of puberty emerge, like the deepening of the voice.

The team examined 50 years’ worth of data on the heights and weights of Swedish boys, determining that with each successive decade, boys reached PHV 1.5 months earlier. In 1947 the age of PHV was 14.2 years, while in 1996 the age of PHV was 13.7 years.

The researchers also show that a higher body mass index (BMI) was associated with a younger age at PHV. Earlier pubertal timing based on PHV was found to be associated in increased childhood weights, based on the BMIs of boys when they were 8. However, this consistent trend towards earlier puberty still clocked in at 1.2 months earlier per decade when they controlled for BMI, indicating that increased BMI can’t explain entirely what’s going on.

Puberty is a milestone in human development, and in some populations of boys, that milestone is happening earlier. Unsplash / Ben White

In an editorial published alongside the study, Vanessa Curtis, M.D., an endocrinologist at the University of Iowa, and David Allen, M.D., a pediatric endocrinologist at the University of Wisconsin, write that “while this study helps to establish the secular trend toward earlier pubertal growth spurt in males, it does not directly show that males are attaining sexual maturity earlier.” They see PHV as a “short-lived parameter” and note that factors like nutrition, activity level, and endocrine-disrupting chemicals could affect the timing of PHV without altering the timing of sexual maturity.

Curtis and Allen write that this study appears to indicate that the trend toward earlier average puberty in Swedes is driven more by a reduction in boys who experience puberty later, then an increase in those who experience it earlier. This could be because of improved living conditions and reduced rates of malnutrition.

Still, they see this study as a good thing: It adds to a fuller understanding of pubertal timing and the spectrum it exists in. Scientists are still figuring out what the markers of an especially “early bloomer” are and what is now the new normal.

The contrasting information that’s emerged from study populations from different countries only adds to this complexity. For example, a 2019 study on Chilean boys showed that both total body obesity and central obesity (excess belly fat) were associated with the greater likelihood of entering puberty before age 9. Meanwhile, Curtis and Allen cite a recent study that evaluated a large, racially diverse group of American boys, for whom obesity was linked to a later appearance of pubic hair and testicular enlargement, compared with boys with normal weight.

It’s likely that race, nutrition, genetics, and environmental factors all impact when a child hits puberty. What’s really needed are more studies that reveal consistent trends across populations. Boys have entered puberty earlier and earlier over the last century — but the exact why is still misunderstood.

Partial Abstract:
Importance: A secular trend for earlier menarcheal age has been established in girls but there are few studies of pubertal timing for boys.
Results: Of the 4090 participants, most were white and the mean (SD) age at PHV was 13.9 (1.1) years. A linear regression model revealed a significant association between year of birth and age at PHV. Age at PHV was 1.5 months earlier for every decade increase in birth year (95% CI, −1.72 to −1.19; P < .001). After adjusting for childhood BMI, age at PHV was 1.2 months earlier per decade increase in birth year (95% CI, −1.41 to −0.89). All analyses were repeated in the subgroup of boys born in Sweden and with parents born in Sweden with similar results, indicating that the secular trend was not explained by demographic changes in the population between 1947 and 1996.
Conclusions and Relevance: We provide evidence of a secular trend for earlier pubertal timing in boys that is partially explained by an increased childhood BMI, but other factors that are unknown contribute.
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