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How did we evolve to solve the daunting task of selecting a diet that supplies all of the essential nutrients? Dietary diversity. By eating a variety of foods, we increase our chances of hitting all the bases. If we only ate for pleasure, we might just stick with our favorite food to the exclusion of all others. But we have an innate tendency to switch things up.
We end up eating more calories when provided with three different yogurt flavors than just one, even if that one is our chosen favorite. So, variation can trump sensation. They don’t call it the spice of life for nothing.
It appears to be something we’re born with. Studies on newly-weaned infants dating back nearly a century show that babies naturally choose a variety of foods even over their preferred food. This tendency seems to be driven by a phenomenon known as “sensory-specific satiety.”
“Within two minutes [of digging into a] meal, the pleasantness of the taste, smell, texture, and appearance of the eaten food [drops off compared to] uneaten foods.” It’s like how the first bite of chocolate tastes better than then the last bite. Our body tires of the same sensations, and seeks out novelty by rekindling our appetite every time we’re presented with new foods. This helps explain the “dessert effect,” where we can be stuffed to the gills but gain a second wind when dessert arrives. What was adaptive for our ancient ancestors to maintain nutritional adequacy may be maladaptive in the age of obesity.
Feed people a four-course meal, and they eat 60 percent more calories than when presented with the same food at each course. It’s not just that we’re getting bored; our body has a different physiological reaction. Give people a squirt of lemon juice, and their salivary glands respond with a squirt of saliva. But give someone lemon juice 10 times in a row, and they salivate less and less each time. Switch to the same amount of lime juice, though, and their salivation jumps right back up. We’re hard-wired to respond differently to new foods.
On the same plate… at the same meal… or even on subsequent days, the greater the variety the more we tend to eat. Give kids the same mac and cheese dinner five days in a row, and they end up eating hundreds of calories less by the fifth day, compared to kids who got a variety of different meals. Even just switching the shape of food can lead to overeating. Give kids a second bowl of mac and cheese, and they eat significantly more if you change from elbow macaroni to spirals. People allegedly eat up to 77 percent more M&Ms if you present them with ten different colors instead of seven, even though all the colors taste the same. The greater the difference, though, the greater the effect. Alternating between sweet and savory foods can have a particularly appetite-stimulating effect. Do you see how, in this way, adding even a diet soda to a fast-food meal can lead to overconsumption?
The staggering array of modern food choices may be one of the factors conspiring to undermine our appetite control. There are now tens of thousands of different foods being sold.
The so-called “supermarket diet” is actually one of the most successful ways to make rats fat. Researchers tried high-calorie food pellets, but the rats just ate less to compensate. So, they “therefore used a more extreme diet” by feeding foods they bought at a nearby supermarket, like cookies, candy, bacon, and cheese, and the animals ballooned right up. The human equivalent to maximize experimental weight gain has been dubbed the “cafeteria diet.”
It’s kind of like the opposite of the original food dispensing device I’ve talked about before. Instead of all-you-can-eat bland liquid, researchers offered free all-you-can-eat access to elaborate vending machines, stocked with 40 trays with a dizzying array of foods like pastries and french fries. Participants found it impossible to maintain energy balance, consistently consuming more than 120 percent of their calorie requirements.
Our understanding of sensory-specific satiety can be used to get people to gain weight, but how can we use it to our advantage? For example, would limiting the variety of unhealthy snacks help people lose weight? Two randomized controlled trials made the attempt, and failed to show significantly more weight loss in the reduced variety diet, but they also failed to get people to make much of a dent in their diets. Just cutting down on one or two snack types seems insufficient to make much of a difference. A more drastic change may be needed, which we’ll cover next.
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