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.”