Smoothies are often marketed as a good-for-you beverage, and sometimes even as a meal-replacement. Many restaurants and smoothie chains market milkshakes, which used to be a treat equated more with an ice-cream float or malt beverage, as smoothies! Although I enjoy an occasional smoothie, I would never recommend having them regularly and I always check the ingredients or make my own.
Like juice and soda, smoothies liquid calories: therefore they are not going to satisfy your hunger as much as an equivalent amount of calories of solid food. To make things worse, you can drink them quickly without realizing that you’ve had enough until you’ve had too much. Worst of all, they often come in a jumbo cup that contains multiple servings; yet people tend to drink their smoothie in one sitting.
Most smoothies have a high glycemic load because they contain sugars and other additives that are high on the glycemic index despite already being made from sugary fruits and starchy vegetables.  Not to mention that they’re also not a balanced meal.
Smoothies sometimes come with “boosters” of protein, antioxidants or other nutrients that we may think are beneficial and healthy. However, these are usually dietary supplements that have not been approved by the Federal Drug Administration and are therefore unregulated apart from marketing and labeling standards.  Many dietary supplements have not been studied, let alone proven to be beneficial for our bodies, and some studies have found negative side effects from these or similar substances. So you may be paying extra for something that could hurt you in the long term.
By definition, smoothies are blended. Because it’s already partly processed, your body does not have to work as hard to break down the nutrients that are left and, therefore, you burn fewer calories during digestion. Also, the pulp is often filtered out or left at the bottom of the blender, meaning that you won’t get the benefits of the fiber from the fruit or vegetables. This is the main reason why I don’t consider a smoothie a nutritious food that would be suitable as a meal replacement. If you choose to have a smoothie be sure to read the ingredients or make it yourself so that you know what you’re drinking.