A new study that compares weight status with timing of puberty in boys finds a mixed picture with evidence of overweight boys starting earlier and obese boys starting later.

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The study found that – compared with normal weight boys – overweight boys had an earlier onset of puberty while obese boys experienced a later onset of puberty.

The study – led by the University of Michigan’s C.S. Mott Children’s Hospital in Ann Arbor – is published in the journal Pediatrics.

Lead author Joyce Lee, associate professor and pediatric endocrinologist at the University of Michigan Medical School, notes how their findings suggest excess weight appears to have different effects on boys compared with girls:

“In girls, excess weight is associated with an earlier onset of puberty, but for boys we saw a mixed picture. Overweight boys had an earlier onset of puberty while obese boys experienced a later onset of puberty, compared with normal weight boys.”

For their study, the team reanalyzed recent community-based data on puberty in more than 3,600 boys aged 6-16. The data covered white (49.9%), black (25.8%) and Hispanic (24.3%) boys, and came from assessments by trained clinicians from the American Academy of Pediatrics.

The data included measures of weight, height and puberty, such as testicular volume and Tanner stages – a system of rating normal sexual maturity that was first described by James Mourilyan Tanner, a British pediatric endocrinologist.

Tanner stages rate different levels of sexual maturity according to the development of primary (i.e. genitalia) and secondary (i.e. pubic hair) sex characteristics.

The researchers grouped the boys – based on their body mass index (BMI) – into normal weight, overweight or obese and compared age against Tanner stage in each group.

All the boys in the study experienced puberty within the normal age range.

However, for white and black boys, puberty occurred earlier in overweight compared with normal weight boys, and occurred later in obese compared with overweight boys.

The study found no significant differences for Hispanic boys. Also, based on testicular volume, the findings support earlier puberty for overweight compared with normal weight white boys, note the authors.

The study did not investigate the causes of these differences. However, the researchers say previous studies have found that carrying too much body fat can lead to over-production of the female sex hormone estrogen in boys, and they speculate that perhaps an excess of estrogen delays puberty in obese, but not overweight boys.

They call for further studies to “understand the possible relationships among race/ethnicity, gender, BMI, and the timing of pubertal development.”

For the time being, Prof. Lee suggests:

This is important information for pediatricians who are monitoring children for healthy growth and development. Pediatricians should consider the possibility that delayed puberty in boys may be due to obesity.”

Meanwhile, Medical News Today recently learned of another study that followed 20,000 families in the UK and found that at as early as 5 years of age, poor children were nearly twice as likely to be obese, compared with peers from wealthier families.