The scientists from Penn State university presented the results of two different studies said their findngs demonstrated the role of energy density on body weight.
But they also show that adjusting energy density of the diet, rather than calorie counting, could be a successful weight loss strategy. The results have implications for products targeted at weight loss, and suggest that reducing portion sizes - called for by health campaigners - may not be the most important element when switching to a healthier diet.
In the first study, the US researchers examined the eating patterns of 7,500 men and women who constituted a representative sample of American adults. In the other study, 101 obese women were advised to increase their intake of water-rich foods and to select reduced-fat foods rather than full-fat ones.
"In both cases eating more low-energy-dense, water-rich foods, such as fruits and vegetables, was associated with lower body weights," said Dr Barbara Rolls, director of the studies.
"Decreasing the energy density of your diet by choosing more low-energy-dense foods, such as fruits and vegetables, can be a successful strategy to lose weight without counting calories or fat grams," she added.
Earlier this year, a report from the UK's House of Commons Health Committee cited a combination of too much cheap, energy-dense food and too little exercise as the principal cause of expanding waistlines in the country. There has been a 400 per cent increase in obesity in the UK over the last 25 years, with England witnessing the fastest growth in obesity in Europe.
In the new study in which women were counselled, 101 obese women were divided into two groups. One group, the energy density group, was counselled to eat more water-rich foods and to choose fat-reduced foods as ways to lower the energy density of their diet.
The second group, the reduced-fat group, was counselled with more restrictive messages focusing on eating less fat and limiting portions.
The women in both groups received individual counselling for six months and a follow-up period of six additional months of individual and group counseling. No calorie or fat gram goals were assigned in either group. The women could eat whatever they wanted while following the principles they learned in their counseling sessions.
After the first six-month period, the women in the energy density group had lost 21 pounds while the women in the reduced fat group had lost only 15 pounds.
The women in the energy density group also significantly lowered the energy density of their diet versus the reduced fat group but there was no difference in fat intake.
Study leader Julie Ello-Martin said: "This is the first long-term study to look at how a low energy density diet can affect body weight. It's important because it shows that a healthy diet pattern can result in significant weight loss without counting calories or fat grams."
Both studies were presented at the annual meeting of the North American Association for the Study of Obesity in Las Vegas, Nevada.