Speed Eating

We’ve all heard the advice to eat slowly, but does that really make a difference when it comes to weight loss? Researchers decided to put it to the test with an innovative study. Here’s our first story.

Eating slowly is often advised for weight management, but does it really matter how fast we eat? If you give people a meal, but half are given small spoons and told to eat slowly and the other half are given big spoons and told to eat quickly, an amazing thing happens. The slow group not only ended up feeling more satiated at the end of their meal, but they did so after eating less of it. They felt more full eating less.

Prolonged meal duration can allow for more time for our body’s own “I’m full” satiety signals to develop before an excess of calories have been consumed. The slower we eat, the more time our body has to catch up. As Harvard’s “Healthy Weight Checklist” puts it: “Slowing down at meals…can help avoid overeating by giving the brain time to tell the stomach when it’s had enough food.” We evolved for millions of years trying to extract calories from undomesticated fruits and vegetables, which were much tougher and more fibrous. Our body is built to expect us to take our time when eating.

When we eat, “anorexigenic”–meaning “to suppress appetite”– hormones, such as GLP-1 and PYY, are released from cells lining our intestines into our bloodstream. These hormones then have to travel to our brain to flick the satiety switch to get us to slow down. This process takes time. Preload studies, where you feed people a small first course 1, 5, 15, 30, or 60 minutes before the main meal, show that this fullness feedback loop can take between 15 and 30 minutes to fully tamp down their appetites. This helps explain the eating-fast-or-slow study results. The fast group was done eating in under nine minutes. The slow group, on the other hand, stopped after 29 minutes. Even though they ended up eating less overall, their brain had time to kick in, thereby giving them a stronger sense of satiety.

In a meta-analysis of eating rate and obesity, every one of the population studies found that those who eat faster are at higher risk of obesity (approximately doubling their odds). In a behavioral treatment program for obesity, those who were able to extend their average meal length just four minutes lost more weight over a seven-month period. There’s lots of ways you can extend meal duration, like taking smaller bites, chewing longer, or choosing foods that just take longer to eat.

The “cephalic phase” of digestion starts before food even hits our stomach. Cephalic means in the head. There are nerves traveling straight from your brain to your mouth. That’s how even the thought of food can get us salivating. And the nerves are a two-way street. Signals coming from your mouth can tip off your brain to what’s coming down the pike.

To test the effect of this back-and-forth on appetite, you can insert a tube down someone’s throat to compare regular eating to sneaking the same amount of food directly into their stomach. Removing the experience of the taste, smell, and texture of the food left people feeling significantly less full even though they ended up with the exact same amount of food in their stomach. This wasn’t just a psychological effect. Objective measures, such as slowed stomach emptying times, prove that sensations from the mouth translate into physical fullness.

Another way to study the cephalic phase response is to use “sham feeding,” less delicately known as the “chew-and-spit” technique. You can double the insulin levels in people’s blood within 15 minutes just having people chew on some pizza, even if they don’t swallow any of it. Based on the signals coming from the mouth, your brain can anticipate how much insulin is going to be needed to handle the coming load and tries to get a head start.

You can see how the cephalic phase response could work in concert with the digestive hormone feedback loop. It would take the average person about 20 minutes to eat 10 cups (1 kg) of apple slices, whereas drinking the same number of calories of apple juice would take about two minutes. Not only would the 20 minutes allow time for your “stop-eating” hormones to make it up your brain, the apples offer 18 more minutes of oral exposure, and more than 10 times the signals directly from the mouth to the brain that you’re chowing down.

Next, we look at why taking smaller bites and drinking thicker smoothies through narrower straws might help us feel fuller faster.

I talked about the “cephalic phase” of digestion before food even hits our stomach, thanks in part to nerves traveling back and forth between your brain and your mouth to help regulate the amount of food we eat, the energy in half of the energy balance equation, but it also plays a role in energy out.

The cephalic phase response plays a role in what’s called diet-induced thermogenesis, the calories your body burns just to process the food you eat. As soon as food hits your mouth, your brain starts priming the pump. It costs the body roughly 10% of the calories you eat to get at the other 90%. So, that could add up to hundreds of calories a day, and about half of those calories are all due to contact between food and the inside of your mouth. We know this because if people are tube-fed, diet-induced thermogenesis gets cut in half or even more.

Half of just 10% of your calories you eat may not sound like a lot, but it could add up to thousands of calories a month. What are the practical implications, though? You can increase oral exposure time by slowing down at mealtime. If you have people eat especially fast (consuming in 5 minutes what would normally take them 15), diet-induced thermogenesis gets cut nearly a third within 15 minutes. So, you don’t get to take full advantage of the effect for weight loss. Whereas, having people slow down by chewing each bite “until no lumps remain” significantly boosted thermogenesis compared to a rapid eating group.

But was it the act of chewing itself or the just the extra time the food was present in the mouth? You don’t know until you put it to the test.

A research group in the Netherlands came up with an elegant solution. They had people effectively take a teaspoon of tomato soup every 5 seconds or a tablespoonful every 15 seconds until they were full. Because a tablespoon is three times bigger than a teaspoon, the eating rate was exactly the same—a quarter cup per minute. Note, though, the oral exposure time was completely different. Even though the tablespoon of soup lasted slightly longer in the mouth, because the tablespoon group was only getting 40 spoonfuls a minute, the total oral exposure time every minute was only half that of the teaspoon group—12 seconds versus 24 seconds. So, same eating rate, but the soup was only in the mouths of the tablespoon group a fifth of the time and in the mouths of the teaspoon group nearly half the time. So, who do you think ended up eating more soup?

If chewing was the critical factor, then both groups would presumably eat the same amount of soup, creamy tomato in this case, since there was no chewing in either group. Similarly, since their stomachs were all filling up at the same rate, if it was just the length of the meal, then both groups would get full right around the same time. But if instead it was all the amount of time food is physically in your mouth, then the teaspoon group would get full faster and end up eating significantly less, and that’s exactly what happened. The teaspoon group had enough after about four minutes, but the tablespoon group ate closer to six minutes, ending up consuming a third more soup.

It makes sense that taking larger bites and sips would lead to greater consumption, because you’re simply eating faster, but this line of research suggests that it’s more than that. Eating and drinking larger mouthfuls is bad for fullness signals. Thanks to the cephalic phase response, your brain gets more food-in-residence signals from your mouth. So, taking smaller bites and drinking thicker smoothies through narrower straws may help you feel fuller faster.