A new study found that number of obese kids in kindergarten is increasing over time. One out of eight kindergartens is already considered obese by the time they reach elementary.

Researchers from Emory University's department of global health in Atlanta led by Solveig Cunningham evaluated data of 21,260 kindergarten students enrolled in the Early Childhood Longitudinal Study in 1998. Out of the total number of participants, the researchers followed 9,358 -- a representative sample of 3.8 million kid of the same age in the U.S. -- until they reach their eighth grade.

The researchers used the Centers for Disease Control and Prevention's standard thresholds, which says that kids with a body mass index (BMI) of more than 95 percent are obese and kids with BMI of more than 85 percent are overweight.

During whole course of the study, height and weight of the participants were measured seven times from 1998 to 2007. It was then discovered that, initially, 12.4 percent of the kids were obese and 14. 9 percent were overweight. After nine years, as the kids enter their eighth grade, the number of obese kids increased by 8.4 percent to 20.8 percent, while the number of overweight kids increased by 2.1 percent to 17 percent.

The researchers noticed that the increase mostly happened before the kids enter the fifth grade.

 "We speculate that obesity-prevention efforts that are focused on children who are overweight by the age of 5 years may be a way to target the children who are most susceptible to becoming obese during later childhood and adolescence," Cunningham wrote in the study.

"This cohort is of particular interest because they were growing up during the 1990s and 2000s, when obesity became a major health concern," she added.

However, the researchers admitted that further analysis is needed to fully establish their findings. They had no data on weight during school breaks and after eighth grade, which made it hard for them to monitor the full trajectory of the prevalence of obesity in the study group. Also, the figures and occurrence may not be the same with the kids who entered the school in the following years.

The study was published in the New England Journal of Medicine.