Beyond the headline, the study offers a careful look at how and when body fat may intersect with the biology of puberty—particularly testosterone—to influence anatomy that becomes fixed by adulthood.
What the researchers examined
The study focused on Vietnamese men who visited Hanoi Medical University Hospital for reproductive health checks. Researchers recorded standard penile measurements—taken in flaccid and stretched states—and compared those dimensions with participants’ reported weight status in childhood and their current body composition in adulthood.
Penile size in clinical studies is typically documented using several complementary measures. Length is often measured from the pubic bone to the tip of the glans in a flaccid state and again after gentle stretching; some studies also note a skin-to-tip length, which can be influenced by the thickness of the fat pad above the pubic bone. Girth (or diameter) is commonly measured at the mid-shaft and at the glans.
In this Vietnamese cohort, the average flaccid pubic-to-tip length was 8.9 centimeters, increasing to 14.4 centimeters in the stretched state. Average diameter measured 2.93 centimeters at the glans and 2.83 centimeters at mid-shaft. These figures fall within the range reported in international literature, where average erect length generally clusters around 13–14 centimeters.
Key findings: timing matters more than current BMI
The standout result was the link between prepubertal obesity and smaller penis size in adulthood. Men who had been obese as children tended to have shorter flaccid lengths (by both pubic-to-tip and skin-to-tip measures) and shorter stretched lengths compared with men who had a normal weight in childhood.
In contrast, the men’s current BMI in adulthood was not associated with penis length. The researchers did, however, detect subtle associations with body shape: a larger adult waist circumference correlated with shorter skin-to-tip flaccid length—likely reflecting a thicker suprapubic fat pad that can obscure visible length—while a larger hip circumference showed a slight association with longer stretched pubic-to-tip length. Obese men also had slightly wider mid-shaft diameters than overweight men, though not necessarily compared with normal-weight men.
Taken together, the pattern suggests that what happens hormonally and metabolically before and during puberty may exert more lasting effects on penile development than weight status later in life.
The biology: hormones, adipose tissue, and puberty
Penile growth is tightly coupled to the surge of androgens—chiefly testosterone—that occurs during puberty. Excess adipose tissue can alter that hormone environment. Fat cells express aromatase, an enzyme that converts testosterone into estrogens, which can reduce available androgens during a critical window of genital development. Obesity in youth has also been linked with changes in the timing of puberty and with lower circulating testosterone in adolescent males.
That biological backdrop provides a plausible mechanism for the study’s association between childhood obesity and smaller adult penile length. It also helps explain why adult BMI, long after puberty has concluded, appears less relevant to final dimensions, even though adult waist fat can make the flaccid penis look shorter by burying part of the shaft.
Childhood obesity is one of the most serious public health challenges of the 21st century.
—World Health Organization
The new findings underscore that those challenges extend beyond cardiometabolic risk, reaching into aspects of sexual development that are rarely discussed in clinical settings or public health messaging.
How the measurements were taken—and why that matters
Small methodological differences can change what a ruler records. Measuring from the pubic bone (after compressing the fat pad) typically yields a longer reading than measuring from the skin surface. Stretched length is widely used as a surrogate for erect length because it is easier to standardize in a clinical setting. The Vietnamese team reported both pubic-to-tip and skin-to-tip measurements across flaccid and stretched states, giving a fuller picture of anatomy and the potential visual effects of subcutaneous fat.
Those details matter when interpreting the waist-circumference finding: a thicker suprapubic fat pad shortens the skin-to-tip measurement without changing the actual anatomic length of the corpora cavernosa. In other words, appearance and measurement can be influenced by body fat distribution, even if developmental size is set.
Limitations and lingering questions
As with any single study, there are caveats. The participants were men seeking reproductive health assessments at one hospital, which may not perfectly represent the broader Vietnamese male population or other countries. Childhood weight status was reported retrospectively, introducing the possibility of recall bias. And while the associations align with known hormonal pathways, the study design doesn’t prove causation.
Population differences, nutrition patterns, and environmental exposures could also shape outcomes. Further research that tracks boys prospectively through puberty, measures hormones directly, and uses standardized growth and body composition data would help test the timing-and-hormones hypothesis more rigorously.
Why the findings matter
At a population level, the study adds another dimension to the stakes of preventing childhood obesity. The consequences already include elevated risks for diabetes, cardiovascular disease, orthopedic problems, and psychosocial harm. This research suggests a potential impact on sexual development as well—one that hinges on the timing of excess weight rather than adult size.
At the same time, penis size varies widely and most men fall within a broad, normal range. Anatomy is not a measure of worth, desirability, or sexual function. The take-home message is not to stigmatize boys’ bodies but to reduce the early-life metabolic and hormonal disruptions that obesity can trigger.
For clinicians and parents, the study’s nuance is critical: the window before and during puberty appears especially sensitive. Supporting balanced nutrition, routine physical activity, adequate sleep, and regular pediatric care can help promote healthy development—goals that benefit far more than future sexual anatomy.