In fact, based on the researchers' own data, the obesity rate may have even risen rather than declined.
"You need to have a healthy degree of skepticism about the validity of this finding," said Dr. Lee Kaplan, director of the weight center at Massachusetts General Hospital in Boston.
No evidence of the kinds of major shifts in the behavior among preschoolers aged 2 to 5 exists which would explain a 43 percent drop in their obesity rates, he said.
The latest study is based a well-respected data set taken from the National Health and Nutrition Examination Survey, or NHANES, which has been conducted annually since the 1960s and involves in-person interviews and physical exams.
The CDC defines obesity in adults as having a body mass index - a ratio of weight to height - above 30, but in children it is defined by where the individual falls on age- and sex-specific growth charts.
The 2011-2012 version of the survey included 9,120 people; 871 of them were 2 to 5 years old.
In some research 871 would be considered a large number. But when the obesity rate is fairly low, having a sample of a few hundred makes it easier for errors to creep in through random chance.
"In small samples like this, you are going to have chance fluctuations," said epidemiologist Geoffrey Kabat of the Albert Einstein College of Medicine in New York City.
To be sure, the CDC scientists were aware of the statistical limitations of their data, and their paper clearly stated that the findings were imprecise.
The 43 percent headline figure refers to the drop from the 13.9 percent rate in 2003-04 to the 8.4 percent rate in 2011-2012. The change of 5.5 points represents a decline of 40 percent from the original 13.9 percent. (The 43 percent trumpeted by a CDC press release comes from rounding those numbers to 14 and 8, respectively.)
For obesity rates to drop, researchers reckon, young children have to eat differently and become more active. But research shows little sign of such changes among 2-to-5-year olds, casting more doubt on the 43 percent claim.
Such a decline would require changes in exercise, food consumption and sleep patterns, said Mass General's Kaplan "There is no evidence of that," he said.
In 2010 [WIC (Women, Infants and Children) program researcher Shannon] Whaley and her colleagues examined the effectiveness of WIC classes and counseling to encourage healthy eating and activities for women and children in the program.
Their findings were discouraging: Television watching and consumption of sweet or salty snacks actually rose, while fruit and vegetable consumption fell - changes that could lead to weight gain. One positive was a rise in physical activity.
Apart from the WIC program, few anti-obesity efforts target preschoolers, Kaplan pointed out. That makes a precipitous decline in obesity in that group highly unlikely.
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