Most Americans read food labels to help them decide what to buy but few people actually understand what they're reading or consuming, according to two U.S. studies published on Tuesday.

An online poll of 2,706 adults found half of Americans - or 51 percent - regularly read food labels to help them make informed choices about food with particular interested in fat, calorie and sugar content.

The poll, conducted jointly by Harris Interactive and the Wall Street Journal, found another third said they read food labels sometimes while 17 percent almost never read labels.

But while people read labels regularly, a study by the Vanderbilt University Medical Center, published in the November issue of the American Journal of Preventive Medicine, found most people lacked the skills to understand them.

Overall, we found that pretty much everybody had some difficulty with at least some of the labels, one of the study's authors, Russell Rothman, from the Vanderbilt Center for Health Services Research, told Reuters.

People who had particular trouble were people who had low literacy or math skills.

The Vanderbilt study was based on a survey of 200 primary care patients from a wide socioeconomic range, giving them standardized reading and math tests before quizzing them on how they understood food labels.

Overall, patients correctly answered 69 percent of the questions, but they had problems understanding serving sizes and extraneous information and completing simple calculations.

For instance, only 32 percent of patients could correctly calculate the amount of carbohydrates in a 20-ounce bottle of soda that had 2.5 servings in the bottle, and only 23 percent could determine the amount of net carbohydrates in a serving of low-carbohydrate spaghetti.

We also found, I think, of concern that patients who were overweight and who had chronic illnesses where understanding labels might be particularly important, actually had a harder time reading labels than other people, said Rothman.