U.S. schools are doing a little better to limit the amount of junk food students can buy in vending machines or elsewhere, the U.S. Centers for Disease Control and Prevention said on Monday.

The CDC's survey of middle- and high-school principals in 40 U.S. states found that the number of schools limiting carbonated soft drinks was a median of 63 percent in 2008. That compared to 38 percent in 2006.

A median of nearly 44 percent limited sports drinks, compared to 28 percent in 2006, the survey showed.

Schools should implement nutrition standards that provide students with healthy choices throughout the school day and throughout the school campus, the CDC wrote in an e-mailed update.

The CDC estimates that 16 percent of U.S. children and young adults aged 2 to 19 are obese. Obesity raises the risk for heart disease, diabetes and asthma and obese children are very likely to remain obese as adults.

Federal and state governments have asked schools to help control how much access students have to junk food that can add calories without boosting nutrition. In the United States, such issues are often controlled by local school boards.

From 2004 to 2009, the number of states with nutrition standards for foods outside of school meal programs increased from six to 27, the CDC report reads.

Despite these improvements, greater efforts are needed to ensure that all foods and beverages offered or sold outside of school meal programs meet nutrition standards, it added.

The CDC survey found large variations from one state to another in controlling junk food access in public schools.

For example, in Connecticut, Hawaii, and Maine, in more than 80 percent of schools, students could not purchase candy and salty snacks in 2008; however, this was true in only 18.2 percent of schools in Utah, the report reads.

About one-third of U.S. adults over age 20 are obese and another one-third are considered overweight. Adult obesity rates have increased dramatically over the past two decades.

The U.S. Institute of Medicine, an independent body that advises the U.S. government on medical and health matters, says Americans should eat at least nine servings of fruits and vegetables a day, and at least three servings of whole grains and notes that people who eat this much will have little room left for sugar or fatty foods without overeating.

The Institute last month also said local governments should consider zoning laws to limit access to junk food near schools.

The American Heart Association now recommends that U.S. adults eat less than one soft drink's worth of sugar a day.

We know that states with laws regulating the competitive food environment are doing well and those that are holding schools accountable are doing better, American Heart Association President Dr. Clyde Yancy said in a statement. 

Strong public policy initiatives could close the gap in areas that have yet to improve nutrition standards and minimize access to these less healthy food and beverage options.