NEW YORK (Reuters Health) - People who want to cut down on calories might be wise to trade in their regular soda for a diet version, a study suggests.

In mealtime tests with 33 adults, researchers at Pennsylvania State University found that diners ate as much food when they were given a sugary soft drink as they did when they drank water or diet soda.

As a result, their calorie count jumped when they had the sugar-sweetened drink. On average, volunteers downed 128 calories from a "small" 12-ounce soda and 151 calories from an 18-ounce portion.

Other studies have found that people fail to "compensate" for calorie-laden drinks by eating less food than they otherwise would have. And sugary soft drinks aren't the only culprits.

"It's not just soda," said Dr. Barbara J. Rolls, a professor of nutritional sciences at Penn State and the senior author on the new study. "A caloric beverage is going to add calories to your meal."

She told Reuters Health that people looking to cut calories should first keep a record of their diet for a few days, then see where they can trim beverage calories. This is a good first step, according to Rolls, because cutting out beverages can be easier than eating less of your favorite foods.

Water and diet soft drinks are some calorie-free options, but people can also reduce calories by adding water to their juice for a "spritzer," or simply having a small soda instead of a large, Rolls noted.

The study findings, published in the American Journal of Clinical Nutrition, are based on experiments with 18 women and 15 men who had lunch in the research lab once a week for six weeks. Julie E. Flood, a doctoral student in nutritional sciences, led the research.

Each week, the researchers gave their lunch guests the same food, but the beverage alternated between water, diet soda and regular soda. Each was served in either a 12-ounce or 18-ounce portion.

In general, Rolls and her colleagues found, the diners ate roughly the same amount at each meal, regardless of which drink was served. And the bigger the beverage, the more they drank.

When men were given the 18-ounce regular soda, they downed 26 percent more liquid calories than they did when they had the smaller soda. For women, the larger drink increased their liquid calories by 10 percent.

According to Rolls, a "big question" right now in nutrition research is how the body's regulation of liquid calories may differ from that of solid food. Hunger and thirst are controlled by separate mechanisms in the body, she noted.

In addition, the mindset people have about eating and drinking -- drinking to quench their thirst, without thinking of it as a source of calories, for example -- may affect their intake.