Obese children seem to have gotten fatter around the middle over time, and that development may account in part for an observed increase in suspected nonalcoholic fatty liver disease among adolescents.
Nonalcoholic fatty liver disease (NAFLD) in adolescents has nearly tripled from 1998 to 2008, Dr. Miriam Vos said during a teleconference reporting the results of an observational study that she will present on Monday, May 21, at Digestive Disease Week 2012. In a review of nationally representative data from the National Health and Nutrition Examination Survey (NHANES), the tripling of NAFLD cases outpaced a near doubling of adolescent obesity during the same period.
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From 1988 to 2008, the percentage of overweight adolescents who were obese increased significantly (from 11.2% to 20.6%).
Thus, "our findings suggest that obesity alone does not explain the growing prevalence of the liver disease," she said.
The most common cause of chronic pediatric liver disease, NAFLD has been associated with hypertension, type 2 diabetes, metabolic abnormalities, liver damage, and cancer. Anecdotal data have previously suggested a risk in NAFLD that was linked to obesity in children, but "this finding has not been confirmed in previous studies," said Dr. Vos of Children’s Healthcare of Atlanta. "We wanted to know whether the rates seem high because clinicians are looking more closely [for NAFLD] or because there really are more cases."
The researchers examined the NHANES data sets from 1988 to 2008, which account for 10,359 12- to 18-year-olds after those with incomplete information or known liver disease were excluded.
More conservative cutoff parameters for suspected NAFLD were implemented during the period of the study, so the researchers conducted their analyses using both cutoffs to allow for comparisons with earlier studies, Dr. Vos explained. "Based on the earlier cut-off, [NAFLD] was suspected in adolescents with a BMI in the 85th percentile or higher, and elevated [ALT] levels (defined as greater than 30)," she said. The newly recommended ALT cutoffs are sex-specific; NAFLD is suspected in adolescents in the same BMI range, but at ALT levels greater than 25.8 for boys and 22.1 for girls (Gastroenterology 2010;138:1357-64).
When the sex-specific cutoffs were used, NAFLD rates "increased among all adolescents, from 3.6% to 9.9%," she said.
Dr. Vos said that age, sex, race, and percentage of overweight adolescents did not differ from 1988 to 2008; however, the percentage of overweight adolescents who were obese increased significantly (from 11.2% to 20.6%).
Among overweight adolescents, the prevalence of elevated ALT levels was 13.2% in 2007-2008, which did not represent a significant linear increase over time. Among obese adolescents, however, elevated ALT levels rose from 16.7% to 36.9% from 1988 to 2008. Similar increases were observed in this group when the previous ALT cutoff of 30 was used, as well.
The findings may be limited somewhat by the study’s inclusion criteria, according to Dr. Vos. "It’s tricky to identify NAFLD using population data like this, so we set our definition to look at overweight children who also have elevated serum ALT. By choosing to look only at the overweight children, we might have missed some cases."
Even so, the findings are important from a public health standpoint. "We need to know this kind of information to plan programs that tackle the prevention and treatment of NAFLD, and it also helps us look for clues about why so many children are getting fatty liver disease," Dr. Vos said.
"We need to look beyond just the increase in obesity among children." For example, a further analysis of the cross-sectional data found a parallel increase between NAFLD prevalence and waist circumference. "While the cross-sectional design of our study can’t point to causation, we can hypothesize that the increase in NAFLD may be linked to an increase in visceral adiposity or centrally located fat in kids today," she said, noting that what might be causing such increases is fodder for additional investigation.
Dr. Vos has received financial support in the form of a career award from the National Institute of Diabetes and Digestive and Kidney Diseases, and from the Children’s Digestive Health and Nutrition Foundation. She is the author of "The No-Diet Obesity Solution for Kids" (Bethesda, Md.: AGA Institute Press, 2009), for which she receives royalties.