Transcript: The Downside of Green Smoothies
If you split women into two groups and tell them to eat as much soup as they want, but half are given big spoons and told to eat fast, and the other group is given small spoons and told to eat slow, the slow group ended up feeling more satiated, despite eating less food. The thought is that prolonged meal duration can allow more time for our own body’s I’ve-had-enough signals to develop before too many calories have been consumed. After all, we evolved for millions of years before cooking, when undomesticated fruits and vegetables were much tougher and more fibrous. Our body is built to expect us to take our time to eat.
There weren’t any blenders on the African savannah either. In smoothie form, you can drink fruits and vegetables at about two cups a minute—ten times faster than what it might take to eat them in solid form. Liquid calories can be consumed so quickly they can undermine our body’s capacity 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. And so, blend all the smoothies you want, but better to sip them slowly, over a half hour or so, rather than gulping them down.
Even slowly sipped, though, an all-fruit smoothie may not be as filling as whole fruit; so, the more greens you can add to your smoothie the better, and you can add ground flax seeds. The thicker the smoothie, the less hungry you may be one, two—even four hours later, and flax seeds make for thick milkshakey-type smoothies. One tablespoon of flaxseeds was found to significantly suppress appetite and calorie intake. Less hunger, more satiety, more filling, less prospective food intake—meaning you give someone a meal two hours after the tablespoon of flax, and they eat significantly less. All the while, possibly leading to significant reduction in cholesterol. This is just one week after about a tablespoon a 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 like three walnuts' worth of fat. So, if you’re trying to reduce added fats, a green smoothie with some nuts, seeds, or avocado can enable you 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 lemon juice, here in the Mayo Clinic's basic green smoothie recipe, 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, potentially improving arterial function in the short-term, and the long-term. Kiwifruit smoothies protect against DNA damage and strawberry smoothies against inflammation. Of course so would, presumably, just eating greens, kiwis, and berries intact. There has been concern expressed that drinking green smoothies would bypass the nitrate-reducing bacteria in the mouth, but our body’s way too smart for that, and pumps nitrate back into our salivary glands. So, even if you deposited greens directly in your stomach with a tube, you’d still produce the nitric oxide so important for artery health.
Concerns have been raised about the oxalic acid in vegetables might increase kidney stone risk, but as I’ve explained in the past, if anything the opposite might be the case. So, are there any downsides of smoothie consumption?
Whether lemon juice or wedge, smoothies can be sour, and any time you’re eating or drinking something sour, you have to careful about eroding the enamel on your teeth. 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, they do 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. Compared to control, drinking juice through a straw has less of an acidic effect than swishing it around in your mouth, so avoid swishing smoothies around in your mouth, and you want to wait at least an hour before brushing so as not to brush your enamel in a softened state. But rinsing your mouth out with water after drinking is a good idea, as it can help rinse away some of the acids to protect your teeth.
And one final caveat for smoothies: when I advocate green smoothies to boost fruit and vegetable consumption I’m talking about whole food smoothies, not made from juice, or added sugars, or human organs. Some women choose to consume their afterbirth. Though described as “replenishing and delicious,” the problem with eating your placenta is that one of the functions of the placenta is to filter out toxins, and so, it may be contaminated with heavy metals, as well as pose a food poisoning risk if consumed raw, like in a smoothie. Green smoothies are great, but I’d be cautious about drinking certain types of red smoothies.
To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video. This is just an approximation of the audio contributed by Katie Schloer.
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