The Trick Smoothies Play on the Stomach

Strategic blending can delay hunger.

Matthew Mead / AP

Imagine you have a glass of water and a plate of chicken, broccoli, and mushrooms.

If you were already imagining that, keep imagining it. Then consider a non-trivial question: Would it be better to eat it as it is, or blend it all together and drink it?

Not many people choose the latter. But among them would be hunger expert Robin Spiller, director of biomedical research at the Nottingham Digestive Diseases Centre in the U.K. And he has data and a deeply considered health argument on his side. Spiller and his team compared the two options head to head in a study, and they found that when people who drank the blended “soup,” it kept them from feeling hungry for about an hour longer than the whole-food meal.

So should everyone be blending everything?

I asked Spiller over Skype as I searched online for used blenders. You can save a lot of money by buying a used blender—especially if you’ve been buying smoothies at one of New York’s fad smoothie shops. It’s tough to walk two blocks in Manhattan or Brooklyn without passing one of these places. Ten dollars gets you half a banana, a scoop of peanut butter, a cup of almond milk, and ice, all under the pretense that smoothies are a healthy option for the human-on-the-go.

Could that actually be … true?

“All other things being equal, if you took a meal and blended it, you’re likely to feel fuller longer,” Spiller said.

The body is very sophisticated, of course, but initially when you take a meal it's the size of your stomach that determines how full you feel—regardless of what's in it. After a bit, when the body has analyzed what’s coming out through the duodenum, it’ll start to take a view on whether what you took had nourishment or not. And if it didn't, you start to feel hungry again a lot quicker.

“That’s why if you drank several glasses of water, you feel full, but only for about ten minutes,” said Spiller. “Whereas if you took the same volume of soup you'd feel full for a couple of hours.”

Though it was late evening in England when we spoke, he was boiling with enthusiasm for hunger science, pointing to study after study. It’s been known for a long time that if you eat liquids and solids in a mixed meal, the liquids will empty faster than the solids. This was established decades ago using radiotracers on food, where liquid and solid elements would have different appearances on X-ray images. The technology was quite crude; all you could see was that the food in the liquid phase emptied faster than the solid food. But in the physics department at the University of Nottingham was Nobel Prize winner Peter Mansfield. He developed very rapid sequencing that enabled cine pictures of the stomach using magnetic resonance imaging (MRI). At the time, MRI took ages and wouldn't be suitable for something that moved very fast, like the digestive tract. But this rapid-sequencing led to—among numerous medical breakthroughs—a new era  in smoothie research.

“What we showed is that food separates in layers in the stomach,” said Spiller. Until pretty recently, that was only an assumption. “If, for example, you take a dense material like rice and a glass of water, the rice will sink into the dependent part of the stomach. Then the water will seep out. That means that when you stop ingesting your meal, the size of your stomach will go down much faster than had you mixed the rice and the water up into a homogenous gruel.”

Gravity is important in the layering, as is the position of your body. You will digest differently in upright and prone positions, not to mention hanging upside down. The rate of stomach emptying is regulated by feedback from the duodenum, which has receptors that can tell the body about nutrient value of a meal. It uses this to adjust so that, more or less, you deliver one to two calories per minute into the small intestine. This ensures efficient digestion. If you overwhelm the intestine, it can’t cope. But at a regulated rate it can be very efficient at absorbing energy.

“So if you eat a mixed meal, the water exits the stomach rapidly and the stomach shrinks,” Spiller explained. This is known as gastric sieving. “If, by contrast, you had made that separation of liquid and solid impossible—by blending it into a smoothie or whatever—then that couldn't happen. The liquid that would come out would contain some calories.”

That would signal to the receptors, slowing up emptying.

Though it’s not clear that all smoothies would do exactly the same thing as his chicken slurry soup, Spiller explains, the important thing about the materials is that they shouldn’t “crack.” Cracking is a concept that physicists apply to emulsions, which much of our food becomes. Chew a mouthful of fat and water and protein, and you have an emulsion. Emulsions can be stable or unstable, depending on how easily they crack. Products in groceries tend to have stabilizers in them, because most people don’t like things separating out into layers on the shelf.

If you put an emulsion into acid, though, it tends to crack. Protein helps guard against cracking, but its effects can be nullified by stomach acid. For example, if you drink a glass of milk, it’ll probably curdle in your stomach—the protein will separate out, and you'll be left with a translucent lactose-protein liquid, known as whey. So meals are most effective at prolonging satiety when they don’t crack. According to the Nottingham team, unstable emulsions lead to gastric sieving— the stomach empties much more quickly and you don't feel as full. The things that stay together as a homogeneous mixture will have a lot longer enhancement of satiety.

It’s an elegant theory, and I was about to pick up that used blender. But as with every smoothie story, there’s more than one side to this one. The Nottingham team just finished a study comparing eating apples to eating puree and drinking apple juice (“Effect of physical form of apples on gastrointestinal function and satiety: a MRI study”), and the findings remind us that there’s more to consider when choosing food than simply its effects on gastric emptying rates.

“When you eat an apple, there’s a lot of crunching,” said Spiller. “That's a strong stimulus for the gut to secrete fluid. A lot of the behavior of your intestine is anticipatory –– it has to work ahead of what's happening. It’s no good producing enzymes to digest stuff when the material arrives. That’s too late. You’ve got to produce it ahead of time.”

Just as you start to salivate before you even stick the burrito in your mouth––before you’re even in the presence of the burrito you’re imagining (a “mind burrito”)—so your pancreas and stomach start to secrete fluid ready to digest before the food is upon them.  This is known as the cephalic phase of eating. As Spiller explains it, “It’s where your brain is engaged to feed forward to the gut that food is coming, so get ready.”

Compare a smoothie and whole fruit, and the whole fruit will have more of this preparatory response. This will alter the digestive process and the hunger response in ways that remain to be understood. In the apple study, the researchers compared the apple puree and fruit and fruit juice.

“The most obvious difference between those things is that one takes a hell of a lot longer to consume,” said Spiller. “Eating a bunch of apples took 20 or 30 minutes with a lot of chewing and salivation and swallowing. You can drink the same calories in a glass of apple juice in 30 seconds.”

The former involved a lot more secretion of digestive enzymes in the small bowel.

“So, the overall impact of that secretion is not clear,” said Spiller. “It may make a difference [to how you digest food], and the amount of time you spend over your meal may also make a difference. The one things about a smoothie is it’s very much quicker to eat a lot of calories than it is when you eat whole fruit. It might be good for you to sit down at a table and chew your way through that fruit. It’d probably take you half an hour, wouldn't it?”

I told him I can drink a smoothie in under a minute.

“Well,” he said, “that's part of the trouble of a high-pressure lifestyle, isn’t it?”

I have no idea what gave him the impression I live a high-pressure lifestyle. It’s rarely the case that I drink smoothies quickly because I have someplace to be. I’m just the guy who came to him because I’m often asked if smoothies are actually healthy. Above and beyond satiety, people’s concerns tend to come back to fiber: Does blending fruits and vegetables ruin some of the fibrous benefit?

“Blending won’t have a significant negative impact on fiber,” Spiller reassured me. “Fiber is what’s responsible for the viscosity of a smoothie and its impact on the bacteria of the large bowel. Mashing fiber up into small pieces should only enhance its availability for the bacteria. Its prebiotic effect is definitely unimpaired––it might be enhanced, even.”

The last thing I want is to walk by these smoothie shops and see them advertising a “possibly enhanced prebiotic effect.” But if you could stop in and ask them to blend your dinner, and that meant keeping people fuller longer—saving money, reducing environmental impact of food production, and being healthier—I suppose I could potentially maybe lose less sleep over the smoothie craze.