Extensive research has been conducted on the relationships between calories, amount of food eaten and body weight. The association of this information with the role of fruits and vegetables in weight management can be summarized as follows:
Studies consistently show that over the course of a day or two, a person eats about the same weight of food. On average, the weight of food eaten is more constant than the daily calorie intake. So if you eat the same amount (by weight) of food, but lower the calories in each portion, you will consume fewer calories. Studies also indicate that you don't even miss the calories and feel just as full as the high calorie day.
To lose weight a person must eat fewer calories than what he or she expends.
People may not limit what they consume based on calories alone. Feeling full is one reason that people stop eating. Short-term studies indicate that the volume of food people eat at a meal is what makes them feel full and stop eating, rather than the calorie content of the food.
At the same calorie level, foods with low energy density provide a greater volume of food, which may help people feel full at a meal while consuming fewer calories.
Water and fiber increase the volume of foods and reduce energy density. In their natural state, fruits and vegetables have high water and fiber content and thus are low in calories and energy density.
Fruits and vegetables are good substitutes for foods of high energy density.
In addition to nutrients such as protein and fiber, the energy content of food, gram for gram, also affects how full we feel. The term for this is energy density, which is simply the amount of calories in a gram of food. The relationship between the weight of food and its calories content is largely based upon the amount of water in the food. Water adds weight but not calories, so the higher the water content the lower the energy density.