Like many people, I've long wanted to overhaul my eating habits and shift to a diet that includes more fresh vegetables and fewer processed foods. But as a mother of three young children, I have found this to be challenging. So the idea of getting vegetable nutrients — including those found in such greens as kale, spinach and Swiss chard — in pureed juice or smoothie form is appealing. But are these popular drinks an effective way for your body to get what it needs?
They can be, experts say, but liquid greens shouldn't take the place of whole fruits and vegetables. Also, what is in the smoothie makes a big difference. Some smoothies might be an adequate meal substitute; others are more of a healthful snack. In some cases, these drinks are just a glorified dessert. "There's a huge spectrum of how filling and nutrient-dense you can make smoothies," says certified clinical nutritionist Gena Hamshaw, author of the Choosing Raw blog.
For instance, a smoothie of bananas with a few leaves of spinach is fit for a snack, whereas a smoothie made from banana blended with almond milk, a cup of fresh spinach or kale, a tablespoon of almond butter, a bit of brown rice protein powder and some berries could serve as an adequate meal replacement.
"Having a balance of carbs, fat and protein is important to building any kind of healthy meal," Hamshaw says. Some good sources of healthful fats include avocados, hemp seeds and nut butters. Protein options include chia seeds, protein powders such as those made from brown rice or yellow peas, and flaxseed. Fruit provides healthful carbohydrates, and diets rich in fruit and vegetables have been linked with lower body mass index, reduced risk of Type 2 diabetes and reduced risk of heart disease and mortality.
Still, people too often make the mistake of replacing breakfast or lunch with a smoothie or juice that doesn't have enough nutrients, Hamshaw says. "Then they're more likely to seek out foods that … aren't healthy or to go overboard with dinner and crave foods that are much heavier."
Moreover, even though smoothies contain the pulp and fibers of fruits and vegetables, consuming those foods whole is generally more nutritious and filling than drinking them as liquids. Chewing and digesting whole foods slows and stabilizes the entry of nutrients into the bloodstream and helps keep insulin and other hormones balanced, says David Katz, founding director of the Yale-Griffin Prevention Research Center and author of "Disease-Proof: The Remarkable Truth About What Makes Us Well."
Why eating the same amount of food in whole form might curb hunger more than when it is blended is a major question in food science, says Barbara Rolls, a nutrition professor at Pennsylvania State University and author of the Volumetrics diet books series. In a 2009 study, Rolls and co-author Julie Flood-Obbagy found that study participants who ate a whole apple as a first course consumed 15 percent fewer calories throughout their meal compared with those who ate only the test meal, whereas people who ate applesauce before their main meal reduced calorie intake by 6 percent; drinking apple juice with or without fiber did not have a measurable effect on calorie intake. Other studies have come to similar conclusions, although several have suggested that such variables as composition of nutrients in the food source, timing of the food or liquid intake, and individual differences in hormones might play a more important role in hunger suppression than the physical form of the food.
Still, Rolls says, "most of us are not eating enough fruits and vegetables, so if smoothies are encouraging people to eat more fruits and vegetables, they're keeping the other ingredients pretty healthy and they're finding them satisfying, I think that could be a good thing."