Barbara Rolls conducted at least 75 experiments with soup as a weight loss tool. Her methodology was very simple. In most of her experiments, she recruited two groups of volunteers and provided them with a free all you can eat buffet, the cost of the buffet only being that food was measured as it was doled out onto the plates.
Typically she asked half of her volunteers to begin their meal with a cup (240 ml) of soup, and let the other volunteers eat crackers or nuts or dry appetizers. She then measured how much food the volunteers took. On average, the volunteers who started their meals with a cup of soup ate 150 to 300 calories less in total. Even if the reduction in intake is just 150 calories and even if volunteers ate soup just once a day, they tended to lose about a pound (450 grams) of fat every month, without consciously counting calories, just effortlessly eating less.