Hello and welcome to Nutrition Facts. I’m your host, Dr. Michael Greger. Today, we’re going to explore smart nutrition choices based, naturally, on facts. Have a history of high blood pressure in your family? How about heart disease? Diabetes? There are foods we can eat that may not only help prevent many of these chronic diseases but even stop them in their tracks.
Smoothies are one of the most requested topics, but for years there seemed to be little pertinent research. So, I was surprised when I checked back to find hundreds of studies. So, should we drink them or not?
I don’t want you to ever do anything just because I or anyone else told you so. That’s the problem with the field of nutrition. Everyone seems to listen to their respective gurus who can sometimes just make pronouncements from on high without explaining their reasoning.
Can you imagine that flying in any other field of science? It’s not what he said or she said; it’s that the best available balance of evidence bears out. Two plus two equals four, no matter what your favorite mathematician says.
So, about smoothies. Do they make us healthier? Do they cause weight gain? Here’s the story.
A famous study in 2000 compared the impact of soda versus jelly beans. They had people add 28 extra spoonfuls of sugar to their daily diet in the form of jelly beans or soda pop. Then, they measured how many calories they ate over the rest of the day to see if their bodies would compensate for all that extra sugar. So, even adding the jelly bean calories, they were eating pretty much the same number of calories before and after adding jelly beans to their diet. But in the soda group, and despite all the added calories from the cans of soda they were drinking every day, they kept eating about the same amount. So, with the soda calories added in, no wonder they gained weight after a month of drinking soda. Their bodies didn’t seem to recognize the extra calories when they were in liquid form, so didn’t compensate for them by reducing their appetite so they’d eat less the rest of the day. This lack of regulation may be used to advantage, the researchers suggest, if you want to get fat. But what if you don’t?
If we drink a smoothie for breakfast instead of a solid meal, will our body think we skipped breakfast and make us so ravenous at lunch we’d eat more than we normally would and end up gaining weight? Okay, well, first, is this solid versus liquid calorie effect real? Soda and jelly beans don’t just differ by physical form—they have different ingredients. That’s a problem with a lot of these kinds of studies. They used dissimilar foods.
Soup, puréed, blended soup, essentially a hot green smoothie of blended vegetables, is more satiating than the same veggies in solid form. The same meal in liquid form was more filling than in solid form. So, it can’t be the chewing—in fact, there doesn’t appear to be a solid versus liquid effect at all, since cold smoothies appear to be less filling, but hot smoothies appear to be more filling. So filling, that when people have soup as a first course, they eat so much less of the main course, that even when you add in the calories of the soup, they eat fewer calories overall.
So, how can we explain this paradox? Maybe puréed fruit is less filling than solid, but puréed vegetables are more filling? I guess you could try making apple soup or something, but who’s going to do that? Purdue University. To prepare apple soup, they mixed about a cup of apple juice with two cups of applesauce, liquefied it in a blender, and heated it up. If you have people eat three actual apples instead, they start out pretty hungry, but within 15 minutes of apple eating, they were hardly hungry at all. Drinking three cups of apple juice didn’t cut hunger much at all, but what about the soup, which was pretty much just hot apple juice with applesauce mixed in? It cut hunger almost as much as the whole apples, even more than an hour later, and even beat out whole apples for decreasing overall calorie intake for the day. What’s so special about soup? What does eating soup have in common with prolonged chewing that differentiates them from smoothie drinking? Time. It took about twice as long to chew that many times, and think how long it takes to eat a bowl of soup compared to just drinking a smoothie? Eating slower reduces calorie intake.
Or, maybe we just imagine the soup to be filling, it’s like a placebo effect. Feelings like hunger and fullness are subjective. People tend to report hunger more in accordance with how many calories they think something has rather than the actual caloric content. If you study people with no short-term memory, like in that movie Memento, where they don’t remember what happened more than a minute ago, they can overdose on food, because they forgot they just ate, which shows what poor judges we are of our own hunger. And it’s not just subjective effects. In this famous study, “Mind Over Milkshakes,” if you offer people two milkshakes, one described as indulgent—decadence you deserve, the other sensible—guilt-free satisfaction, people have different hormonal responses to them, even though they were being fooled and given the exact same milkshake.
And finally, maybe it was just that the soup was hot, and warmer foods may be more satiating. So, how do we figure out if the solution to the soup mystery was time, thought, or temperature? If only this study had a third group. They had a solid-eating group, and a liquid-drinking group. If only they had a liquid-eating group too—and they did. They also offered the fruit smoothie in a bowl, cold, to be eaten with a spoon—very unsoup-like. So if it was thought or temperature, the fullness rating would be down by the liquid drinking—the smoothie. But if it was just the slowed eating rate that made soup as filling as solid food, then the number would be up closer to the solid-eating group. And it was exactly as high, meaning the only real reason smoothies aren’t as filling is because we gulp them down. But if we sip them slowly over time, they can be just as filling as if we ate the fruits and veggies solid.
Wow, this study thought of everything! You don’t know the half of it. They also wanted to see if it would work with high-fat smoothies. So, what, almond butter or walnuts? No, the LF drink was a liquefied fat smoothie of steamed pork belly. I guess sometimes smoothies can suppress your appetite.
Smoothies (and blended soups and sauces) offer a convenient way to boost both the quantity and quality of fruit and vegetable intake by reducing particle size to help maximize nutrient absorption. Here’s more.
Unhealthy lifestyle behaviors associated with increased risk of premature death include things like smoking, and excessive drinking, and not eating enough greens. The best way to get your greens is in whichever way you’ll eat the most of them, and one way to sneak extra greens into your daily diet is with whole food smoothies, a potent blend of good nutrition in a quick, portable, delicious form. The Mayo Clinic offers this as a basic green smoothie recipe, combining the healthiest of fruits—berries—with the healthiest of vegetables, dark green leafies. Two ounces of baby spinach is about a cup and a half. Curly parsley is another mild beginner green to start with. Surprisingly, the sweetness of the fruit masks the bitterness of the greens, such that the pickiest of children love them, along with any adults who would otherwise not be consuming dark green leafy vegetables for breakfast.
Or even fruit for that matter. The average teen may only be getting about 1/20th of a serving of fruit, otherwise—and Loops don’t count. But offering smoothies can have a dramatic effect on fruit consumption for students who do not want to take time peeling or chewing fruits. Who doesn’t have time to chew a fruit? But the milkshake-y texture of smoothies may not just boost the quantity of fruit and vegetable consumption, but also the quality.
Carotenoid phytonutrients, like beta-carotene and lycopene, can exist as microscopic crystals trapped within the cell walls of fruits and vegetables, and they’re only released when the cells are disrupted; that’s why we need to chew really well. (“Mastication” is doctor-speak for chewing.) We either have to chew better or choose plants that are easier to chew. For example, while tomatoes have more beta-carotene than watermelon does, the watermelon’s beta-carotene is more bioaccessible, because it has kind of wimpy cell walls. But the cells of other fruits and vegetables are smaller and tougher. To maximize nutrient release, food particle size would ideally be reduced down to smaller than the width of the individual plant cells, but you can’t do that with chewing. Most vegetable particles end up greater than two millimeters when you chew them, whereas if we broke open all the cells we could release much more nutrition. We can never chew as well as a blender. The particle size distribution from chewing is about what you’d get blending in a food processor for about five seconds, or one of those high speed blenders for maybe half a second. 40 seconds in a blender and you can break spinach down to a subcellular level.
Why does that matter? Well, take folate, for example: the B vitamin in greens especially important for women of childbearing age. Feed people a cup of spinach a day for three weeks, and their folate goes up compared to control. But even just chop it up finely with a knife first before chewing it, and you end up with more than twice as much ending up in your bloodstream—and the same absorption boosting effect with lutein, the greens nutrient so important for our eyesight. It’s not what you eat; it’s what you absorb.
But for lutein, the boost was only 14%; so, a few extra bites of the whole leaf greens would have gotten you just as much. And some other nutrients, such as vitamin C, aren’t affected by pre-chopping at all. And this is less of an issue with cooked vegetables. This is for raw carrots. Boil the carrots for three minutes first, and even just regular chewing can release like ten times more, but not as much as blended. Intense cooking—boiling for 25 minutes—so damages the cell walls that even gulping down large particles can result in significant absorption. But even then, blending may double carotenoid availability, explaining why we may be able to absorb three times the alpha- and beta-carotene from puréed cooked carrots compared to mashed cooked carrots. So, blending vegetables—raw or cooked—into soups, sauces, and smoothies can maximize nutrient absorption. You went to the store and bought it, or toiled in your garden to grow it; you might as well take full advantage of it.
Might there be a downside to enhanced absorption, though? We’ll find out, next.
Eating intact grains, beans, and nuts (as opposed to bread, hummus, and nut butters) may have certain advantages for our gut flora and blood sugar control, raising questions about blending fruits and vegetables. Here’s the research.
Fruits and vegetables are the most nutrient-dense foods on the planet, and dark green leafy vegetables lead the pack. Each of the top five so-called “powerhouse fruits and vegetables” were greens. And so, if we blend them up in a smoothie (or soup or sauce), we’re taking the food with the most nutrition and breaking all the cell walls to dump that nutrition into our bloodstream. Chewing is good, but blending is better, in terms of digestive efficiency and absorbing nutrients.
But if we suck up all that nutrition such that none of it makes it down to our colon, might we be starving our microbial selves? The reason intact grains, beans, and nuts are better than bread, hummus, and nut butters is that no matter how well we chew, intact food particles make it down to your colon where they can offer a smorgasbord for your good bacteria. But if your grains, beans, and nuts are finely ground up into flour or paste before you eat it, you may be leaving your gut flora high and dry. Would the same be true for fruits and vegetables?
There are special classes of phytonutrients in fruits and vegetables that appear to protect against colon cancer. They can escape digestion and absorption in our stomach and small intestine, and end up in our colon to act as prebiotics. No matter how much we chew, they stay attached to the fiber. But if we use a blender, might we prematurely detach these nutrients? No. Even if you blend in a high-speed blender for five minutes, they remain bound to the fiber for transport down to your colon bacteria. You can do smoothie experiments on people with ileostomy bags that drain the contents of the small intestine, and show that most of the polyphenol phytonutrients make it out intact. So we don’t have to worry we may be robbing Peter to pay Paul when we blend fruits and vegetables. Is there any downside to smoothies, then?
Well, just as smaller particle size may improve digestive efficiency and gastrointestinal absorption of nutrients from fruits and vegetables, the same may be true for grains. But the concern is that this could boost starch availability and cause a blood sugar spike.
Even just chewing really well can boost the glycemic and insulin response. It’s ironic that there were health crusaders pushing people to chew more to digest their food better, but if what you’re chewing is a five-cheese pizza, maybe it’s better not to digest so well.
To see any graphs, charts, graphics, images, or studies mentioned here, please go to the Nutrition Facts podcast landing page. There, you’ll find all the detailed information you need plus links to all the sources we cite for each of these topics.
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Thanks for listening to Nutrition Facts. I’m Dr. Michael Greger.
This is an approximation of the audio content, contributed by Allyson Burnett.