A friend admitted to me recently that she’d paid $65 into the crowdfunding campaign for Soylent, the “future of nutrition.” In August, she expects to receive a week’s worth of meals in the form of an unflavored beige powder. For her contribution she’ll also get a travel mug with a little wire ball inside, so that while reconstituting her meals in water she can more easily break up the chunks.
“Mostly it was curiosity,” said my friend, who asked me not to use her name. She learned about Soylent from a foodie listserv. “I wanted to see what quote-unquote pure nutrition tastes like.” She acknowledged that seven days’ supply will be more than enough to satisfy that curiosity.
According to the 24-year-old software engineer who dreamed up Soylent, the recipe for pure nutrition starts simply with the standard dietary recommendations from the Agriculture Department. Rob Rhinehart claims his powder includes “only the raw ingredients the body needs for energy,” as he wrote on his blog. (Those ingredients include maltodextrin and oat powder, but not people.) After testing the product on himself and others, he and a few business partners are raising money to mass-produce Soylent.
Rhinehart envisions Soylent as part of a future utopia in which no one has to grocery shop or scrub dishes, people spend less money and time on food, and everyone is healthy, having slurped down exactly the nutrients they need. It’s an engineer’s fantasy. Yet it neglects crucial details about the human machine: The body is not a computer in which a certain input guarantees a certain output. It’s not even a car engine, guzzling well-calibrated liquid fuel and returning energy. If you treat it like one, though, you may find your engine growling with hunger all the time.
We have evolved the ability to chomp and smash our food with our teeth and tongues, liquefy it in our stomachs, and wring the last bits of nutrition from it in our intestines. Our bodies are not adapted to suck food through a straw.
“There is very little that’s similar about how, physiologically, we handle beverages versus solid foods,” says Richard Mattes, a nutrition scientist at Purdue University. Chewing is the first difference, of course. And once swallowed, liquids drain from the stomach faster than solids. They slide through the intestine more quickly, and we absorb their nutrients more easily.
This matters because another machine the human body does not resemble is a coin counter. No mechanism inside us tallies up the calories we consume—tossed back in nickels or dimes or quarter-pounders—and reports the total. If it did, we would always feel hungry when we owed our body calories and full when we’d paid off our debt.
Instead, a complicated stew of hormones regulates the appetite. Various signals come from the pancreas, fatty tissues, or the stomach and intestine themselves. Some increase our appetite, and others decrease it; they may be triggered by a lack of food, by glucose in the bloodstream, or by physical stretching of the stomach. Meanwhile, the brain tries its best to make sense of all the signals.
People may think that a calorie is a calorie no matter what form we swallow it in, but in many studies over the past couple of decades, Mattes and other researchers have found that this isn’t the case. When we pipe in liquid calories—something that would have been rare for most of our evolutionary history—the system seems to lose count. Various studies have found that beverages, even when they provide as many calories as solid foods, don’t quell our appetites as well. This means the calories we drink tend to add to the total calories we consume in a day, rather than replacing solid ones.
It’s not clear exactly which mechanisms leave us unsatisfied after a liquid meal. A recent study by Mattes and his colleagues suggests that the power of expectation has a lot to do with it. They gave subjects a small meal of a cherry-flavored substance, as either a liquid or chewy gelatin cubes. Either way, the meal would rapidly dissolve in the stomach. But the researchers used visual demonstrations to make some subjects believe that the food would sit in their stomachs as a solid block, even if it started as a liquid. People who expected a solid in their stomachs felt fuller and ate less food later on—by about 400 calories—than people who ate the same food and expected it to liquefy.