Many women may not notice if a few meals contain significantly fewer calories than the meals they are used to, a new study shows.

Researchers at Pennsylvania State University in University Park found that when women ate meals altered to contain 800 fewer calories per day, they said they felt just as full as on higher-calorie days, and did not sneak in extra snacks to compensate for the lost amount.

“They never were eating more than we were giving them,” study author Dr. Barbara Rolls told AMN Health.

In an interview, Rolls explained that she and her colleagues reduced intake by cutting portion size and the amount of calories per portion, known as calorie density. The key to reducing calorie density, she said, is to add more water-based foods, such as fruits and vegetables, and to substitute reduced fat options for fatty foods.

Rolls is the author of the Volumetrics Weight-Control Plan, an eating guideline based on monitoring calorie density.

During the study, the researchers asked 24 women between 19 and 35 years old to eat all of their meals on two days each week for 4 weeks at the research facility.

Rolls explained that the participants received one of four types of meals. One type contained a “standard” portion size and calorie density. On another day, the women ate meals reduced by either calories or portion, or reduced by both calories and portion.

When the researchers cut calorie density by 30 percent, women ate 23 percent fewer calories per day. Reducing portions by 25 percent cut calories by 12 percent. Women ate 800 fewer calories on the days when their meals were reduced by both calories and portion, relative to the so-called “standard” meal days.

Rolls and her colleagues presented these findings on Monday during the annual meeting of the North American Association for the Study of Obesity, held in Las Vegas, Nevada.

Women said they felt equally hungry and full on all days, regardless of how many calories they were served, and did not sneak extra snacks on days when they were given less food. They also gave the same taste ratings to all meals, Rolls noted.

“People are not that sensitive to calories, at least not over a few days,” Rolls said.