Obese teens might not develop sufficient bone mass relative to their body weight, according to a new study from Brazil.
Both body fat and lean body mass have an impact on bone growth, but it's not clear if the bones of the heaviest teens are strong enough for their weight and that could have long- and short-term consequences.
Bone mineral acquisition "rises exponentially in both genders" during the growth spurt following puberty, the researchers write in the journal Nutrition.
Previous studies examining whether obesity interferes with bone development have shown conflicting results - some indicate bone density is fine when compared just to the adolescent's lean mass (muscle).
But others have suggested bone density doesn't increase enough during this important period to support the heavier weight of obese teens, potentially putting them at increased risk for bone fractures.
Since obesity rates in children and teens are high in many parts of the world, the researchers wanted to explore the subject further.
"The study was conducted with Brazilian adolescents where the prevalence of overweight/obesity is around 30 percent, similar to the American teenager population," Dr. Tamara Goldberg told Reuters Health in an email.
Goldberg, a physician and professor in the Department of Pediatrics at the Botucatu School of Medicine of São Paulo State University, was part of the team that worked on the new study.
"In the last decades, an increase in obesity has been observed in developed countries as well as in developing countries," she said.
The new study involved 377 adolescents aged 10 to 19 years old. The researchers excluded kids who took calcium or iron supplements, were taking certain medications or were vegetarians or ate high-fiber diets.
The adolescents were all non-smokers and non-drinkers, and none of them participated in any types of regular physical activity.
The researchers used body mass index to categorize the participants into one of four categories. Most - about 42 percent - fell into the normal weight category. Another 13 percent were overweight, 38 percent were obese and 7 percent were classified as extremely obese.
The researchers used dual-energy X-ray absorptiometry, or DEXA testing, to determine total body composition and bone mineral content and bone mineral density of the thigh bones and lower-back vertebrae.
Bone mineral content is the amount of mineral found in a specific area, while bone mineral density is the bone mineral content divided by the size of that area.
The researchers compared the changes in lean mass and fat percentages across the different weight groups - they found that lean mass stayed about the same while body fat percentage went up as weight went up.
But bone mineral density and bone mineral content didn't necessarily increase as much as body fat percentage.
In general, girls with the highest body fat percentages didn't show a comparable increase in bone densities of the thighbone and vertebrae.
In boys, it appeared that bone density and mineral content didn't increase comparably with body fat percentage in any of the bones tested.
While some excess body fat has been identified as a protective factor in women, extreme obesity is not associated with normal bone density, even in adults, Goldberg said.
"The prevention of obesity is the best protective way to decrease the incidence of metabolic cardiovascular events (high blood pressure, diabetes, dyslipidemia) and other factors which interfere with the increase of bone mass," Goldberg said.
This study doesn't prove that excess fat has a negative effect on bone mass, only that they seem to go hand in hand.
It's also important that the kids in this study did not participate in any regular physical activity, a pediatrician who was not involved in the study pointed out.
"Kids who are not obese are more physically active and physical activity, especially high impact activities like soccer, basketball, gymnastics and other sports which require sprinting and jumping, improve bone. Severely obese children are less likely to participate in these sports," Dr. Amanda Weiss Kelly told Reuters Health.
Weiss Kelly is the division chief of pediatric sports medicine at University Hospitals Rainbow Babies and Children's Hospital of Case Western Reserve University in Cleveland, Ohio.
Whereas the study authors speculate that hormones secreted by fat and inflammation related to adipose may contribute to decreased bone density in overweight kids, Weiss Kelly says that may well be true, but so far it has yet to be proven.
"In the end both physical activity and the fat itself may both play a role," she said.
"Parents of obese (and all) children should encourage regular (DAILY) weight bearing physical activity. Weight training is another good activity for improving bone density," Weiss Kelly wrote.
"Finally, making sure that all children are receiving adequate amounts of calcium and vitamin D for their age group is important," she added.
