Early puberty is a risk indicator for adult diseases such as depression, obesity, type 2 diabetes, cardiovascular diseases and cancer. There is therefore considerable interest in understanding why children trend towards beginning puberty earlier. The new study is one of the first major studies to indicate that overweight is also associated with the timing of puberty among boys. Researchers hope to learn why childhood obesity is associated with attaining puberty earlier so that those affected can avoid future negative health effects.
Many parents want to hang on to their sweet little kids as long as possible rather than having to be dragged along by unruly teenagers. However, there are also good health reasons for children remaining in childhood until the right time. Major studies have shown that children who enter puberty too early or too late develop diseases in adulthood more often and earlier. New research has also indicated possible causes, with overweight being an important factor among girls – and now also boys.
“Unlike previously, the new study shows that overweight is not only associated with earlier puberty among girls. We found that girls who were overweight attained pubertal milestones on average 5 months earlier than normal-weight girls versus 3 months for boys who were overweight. To ensure that these trends were not caused by genetic and social factors, we examined 850 sibling pairs and observed identical trends in this group. Our theory is that being overweight shifts the balance in fat tissue by increasing the bioavailability of sex hormones, but it is still too early to say for sure,” explains Cecilia Ramlau-Hansen, Professor, Department of Public Health, Aarhus University.
The association is convincing
The study is based on data from the Danish National Birth Cohort, in which 92,000 pregnant women who gave birth in 1996–2003 participated. The current study extracted data for 11,046 of the children born between 2000 and 2003. The parents reported the children’s weight and height at age 7 years. Then the children completed a questionnaire every 6 months from 11 until 18 years old about their pubertal development, including pubic hair, acne, voice break and breast development.
“Based on the information on puberty the children provided, we determined when overweight children and normal-weight children reached the various stages of puberty. We were thus able to compare body mass index data with puberty and observed a definite correlation. Overweight girls attained pubertal milestones on average 5 months earlier than normal-weight girls versus 3 months for overweight boys,” explains Nis Brix, PhD, Department of Public Health, Aarhus University.
The differences naturally vary enormously. The boys in the study had fully developed pubic hair (Tanner stage 5) when they turn 15 years. Boys who were overweight reached this on average 3–4 months earlier and boys with obesity 5–6 months earlier. Among the girls, the difference was even greater. Normal-weight girls usually have full pubic hair shortly before they turn 16 years, but this was 6–7 months earlier among girls with overweight and 9–10 months earlier among girls with obesity.
“These are clear associations, but to ensure that overweight is the cause rather than genetic factors, social class or something completely different, we conducted a sibling-matched analysis in which we compared children who had grown up under the same conditions and with similar DNA. The numbers naturally decrease, and so does the statistical significance, but the associations were the same, so we feel reasonably confident that the link between overweight and earlier puberty did not result from other factors associated with both overweight and early puberty,” says Nis Brix.
Mechanism remains unknown
The new study indicates that body mass index and puberty are clearly linked – even after adjusting for confounders such as genetics, parents’ highest educational level, maternal smoking in the first trimester, mother’s body mass index during pregnancy, mother’s age at menarche, birthweight, childhood diet and childhood physical activity. The researchers therefore believe that the physiological effects of childhood overweight may cause earlier puberty.
“We have considerable research ahead before we can explain the exact mechanisms that lead to the altered puberty profile. Previous studies have shown that childhood obesity is associated with elevated insulin levels and insulin resistance, which is thought to trigger earlier puberty because it disturbs the balance between the hypothalamus, pituitary gland and the body’s glands,” explains Nis Brix.
Much evidence suggests that the effect of insulin on the liver, adrenal glands, ovaries and fat tissue earlier in life also greatly affects the availability of steroids and sex hormones, which in turn can lead to earlier onset of puberty. In any case, the new results are worrying.
“Although previous studies suggest that earlier puberty leads to an increased risk of depression, cardiovascular diseases, diabetes and some types of cancer, such as breast cancer, it is extremely important to understand what mechanisms are affected and whether that influence can be prevented and then to try to prevent obesity in childhood,” concludes Cecilia Ramlau-Hansen.