A group of women were split into two groups and told to eat as much soup as they wanted, but half were given big spoons and told to eat quickly, while the other half were given small spoons and told to eat slowly. The slow group ended up feeling more satiated—despite eating less food. Prolonged meal duration can allow more time for our body’s own “I’ve-had-enough” signals to develop before too many calories have been consumed. It makes sense. After all, we evolved for millions of years before cooking, when undomesticated fruits and vegetables were much tougher and fibrous. Our body is built to expect us to take our time when eating.
What about when drinking? There weren’t any blenders on the African savannah. In smoothie form, you can drink fruits and vegetables at about two cups a minute—ten times faster than it might take to eat fruits and vegetables in solid form. Liquid calories can be consumed so quickly they can undermine our body’s ability to regulate food intake at healthy levels. It’s not the liquid texture per se, but the high rate of consumption at which liquids are normally consumed. Blend all the smoothies you want, but sip them slowly for a half hour or so rather than gulping them down.
Even when sipped slowly, though, an all-fruit smoothie may not be as filling as eating a whole fruit, so the more greens you can add to your smoothie the better. You can also add ground flaxseeds. As you can see in my video below, the thicker the smoothie, the less hungry you are one, two and even four hours later—and flaxseeds make for thick, milkshake-type smoothies. Researchers found that one tablespoon of flaxseeds significantly suppresses appetite and calorie intake. You can give someone a meal two hours after the tablespoon of flaxseeds, and they eat significantly less—all the while dropping their cholesterol in only one week when eating about a tablespoon of flaxseeds each day.
The fat naturally found in flaxseeds can also help maximize the absorption of fat-soluble phytonutrients. There’s a threshold for optimal absorption that can be reached with just about three walnuts worth of fat. If we’re trying to reduce added fats, a green smoothie with some nuts, seeds, or avocado can enable us to take full advantage of the healthiest foods on the planet—dark green, leafy vegetables.
Smoothies also allow us to eat parts of fruits and vegetables we might not otherwise. If, instead of the lemon juice called for in Mayo Clinic’s basic green smoothie recipe (shown in the video), you used a little wedge of lemon, you might get some seeds and peel, which in vitro at least, appear to suppress both breast cancer and colon cancer cell growth.
Clinical studies on smoothies show what you’d expect to see from eating great foods like greens and berries—enhanced athletic performance and recovery, boosting the antioxidant power of your bloodstream, and potentially improving arterial function in both the short- and long-term. Kiwifruit smoothies protect against DNA damage, and strawberry smoothies protect against inflammation. Of course, presumably, so would just eating greens, kiwis, and berries intact.
There’s been some concern expressed that drinking green smoothies would bypass the nitrate-reducing bacteria in the mouth, but our body’s way too smart for that; it pumps nitrate back into our salivary glands. Even if we deposited greens directly into our stomach with a tube, we’d still produce the nitric oxide so important for artery health.
Concerns have been raised that the oxalic acid in vegetables might increase kidney stone risk, but, as research shows, the opposite might be the case. So are there any downsides of smoothie consumption?
Whether with lemon juice or a lemon wedge, smoothies can be sour. Any time you’re eating or drinking something sour, you have to be careful about eroding the enamel on your teeth. Researchers found that if you soak teeth in a smoothie for an hour, significant enamel is eroded away. But who soaks their teeth in a smoothie for an hour?
What if you instead study the effects of smoothies in situ (meaning in position), as opposed to in vitro (meaning in glass)? If you make people wear slabs of enamel in their mouths while they drink a smoothie to replicate a typical tooth exposure, researchers find almost as much erosion as drinking Diet Coke. So, it’s recommended that smoothies be consumed through a straw, similar to the advice given for other acidic beverages like soda or hibiscus tea. Drinking juice through a straw has less of an acidic effect than swishing it around in your mouth, so avoid swishing around mouthfuls of smoothie in your mouth. You also want to wait at least an hour before brushing so as not to brush your enamel in a softened state; rinsing your mouth with water after drinking smoothies can help rinse away some of the acids to protect your teeth.
One final caveat for smoothies: When I advocate green smoothies to boost fruit and vegetable consumption, I’m talking about whole food smoothies, not those made from juice or with added sugars.
Michael Greger, M.D.