The first reason is that smoothies can be a vessel for a large volume of fruit, much more than you’d eat as whole fruit in one sitting. While fruit has a lot to offer nutritionally, it is also high in sugar. For a diet oriented toward weight loss (not just VLCD, but also Stepwise and Maintenance), we recommend limiting fruit portions to help manage sugar intake.
The second reason is that some scientists argue that putting fruit through a blender, particularly the high-speed blenders commonly used these days, can pulverize and compromise some of the integrity of the fiber content of the fruit. Consequently, a lot of the work that your digestive system would be responsible for is already done. This results in a quicker and more significant spike in blood sugar and insulin response upon intake.
The third reason (related to the reasons above), is that smoothies generally do not keep people full for very long because they lack fiber and protein. Breakfasts like oatmeal, berries and a tablespoon of natural peanut butter can buy you hours of satiety, keeping you satisfied and energized through your morning. On the other hand, most smoothies will have you craving a mid-morning snack within a couple hours.
So, what’s the take-home point? Whole fruit in moderation is the way to go. For VLCD (and for best results on Stepwise), opt for berries, as they have the highest fiber to sugar ratio. Limit portions (one cup berries on VLCD), and watch out for the giant apples and bananas at Safeway these days.
If you are not on VLCD and feel very strongly about incorporating an occasional smoothie, consult with your dietitian. While you’ll inevitably lose some of the fiber benefits during the blending process, if you limit fruit portions and add in ingredients with more staying power (e.g., Greek yogurt, 1 tbsp flaxseed, 1/4 cup rolled oats), you may be able to prolong the satiety window. BUT, pay attention to how long you’re staying full…you may discover that despite your best efforts, you aren’t able to build a smoothie that is low in calories and sugar, and high in staying power. If you’re looking for alternative quick, portable breakfast options, you might try a couple hard-boiled eggs, a low-carb tortilla + nut butter, or a tupperware of Greek yogurt + berries.