Pretty much anyone who has ever dieted will have been advised, at some stage, to avoid drinking their calories. In Dr Greger's book "How not to Diet" - he explores some of the fascinating science which provides good support for this advice.
For some reason, our bodies do not register calories taken in the form of liquids in the same way as they do when we ingest the same amount of calories in the form of solid foods. In one study, researchers had people add extra calories to their daily diet either in the form of jelly beans, or sugary soda. The jelly bean group seemed to automatically eat somewhat less the rest of the day and maintained their weight over the study period. The soda group, however, gained weight over the same period as the compensatory mechanisms did not kick in.
Another study compared people eating the same amount of calories of raw apples, apricots and bananas with those taking the same fruits in the form of a smoothie. The people who received the smoothie reported significantly less satiety (satisfaction) than those who had the whole fruit. So - clever scientists thought this might be because people got more satisfaction from actually chewing food, and another study seemed to back this up.
But - just when everything seemed clear and settled, something came up to have the scientists scratching their heads again. Someone had the bright idea to puree and blend soup (essentially a warm vegetable smoothies) and compare that with the satiety effect of eating the same amount of calories in warm veggies. And guess what?! That's right, the soup was often perceived to be more satisfying than the whole vegetables which needed to be chewed.
In fact many people who had soup as a first course, actually ended up eating less calories for the whole meal than people who did not have soup first. Another study was designed to see if it was the fact that soup was warm, that caused a decrease in appetite. There were three groups, one to drink down the fruit smoothie in the usual way, one group to eat the whole fruit and the third group ate the smoothie with a spoon as if it were soup.
This showed that the answer to feeling more satisfied on less calories is to take longer to eat them. The group that ate their smoothie with a spoon felt just as satisfied as when they ate the whole fruit! So the answer is this: smoothies are only not as filling because we tend to gulp them down. Therefore, feel free to enjoy your healthy smoothie as a substitute for a sit down breakfast, but sip it slowly and mindfully, so that your body will fully register the calories taken in, and not trick you by sending you a craving for a cookie by morning coffee time!