The debate as to whether not fruits and vegetables should be categorized as negative-calorie foods has been in existence for an extended duration.
If you are curious as to what exactly a negative-calorie food is, it is a food that requires more energy, or calories, to chew and consume it than it possesses. An example of a negative-calorie food is celery, because the process of chewing and digesting celery burns more calories than the vegetable itself contains.
The idea of fruits and vegetables being acknowledged as calorie-negative foods is a result of, from what I personally believe, the weight-loss company known as Weight Watchers. From my understanding, Weight Watchers made their meal-plan program unique from others by stating that an individual could consume as much fruit/vegetables as desired while still abiding by their pre-determined meal-plan.
While some fruits and vegetables are, in fact calorie-negative, for example celery, others or not, which is why there is a debate about the topic. Consider a banana. An averaged sized banana contains approximately 150 calories, and possesses the highest levels of carbohydrates out of any other fruit. If a person on a diet is aiming to eliminate carbohydrates from their diet, though believes a banana is a calorie-negative food, they could seriously hinder results of their weight loss by consuming a large amount of them in a single day.
If I were to argue between fruits and vegetables and determine which option out of the two is more likely to be calorie-negative, I would choose vegetables as a safer option. However, vegetables become tricky when considering what they are cooked in or how they are cooked. To avoid unnecessary calories from oils and butter, it is likely better to consume raw vegetables, and I would argue that a majority of raw vegetables are calorie-negative.
I personally consume an astronomical amount of fruit in a single day, and I do not count the calories in them. I figure eating fruits and vegetables as snacks is far more nutritious than consuming crackers or nuts all day, however a nutritional balance is key in regards to maintaining a healthy body weight.