Q. I've read many times that eating slowly can help a person eat less and lose weight. I've tried this approach recently, and I do find it helps keep me satisfied, even when eating smaller portions. What is the mechanism behind this -- or is it all in my head?
A. It's true ... and it is all in your head (and your gut). Let me explain.
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Why do we get hungry? And when we eat a meal, why at some point do we feel full? Until recently, we had no idea. We knew that appetite must be lodged in the brain. But how does the brain decide to feel hungry, or to feel full?
Scientific research in the past 25 years has taught us that the stomach and small intestine send signals to the brain. Different signals turn our appetite up or down. Some of the signals are digestive hormones: chemicals that are made in the gut and travel through the blood to the brain. Other signals are hormones made by cells that store fat. Still other signals travel up nerves -- particularly the vagus nerve, running from the gut to the brain.
When it's been three to four hours since your last meal, your stomach makes a hormone call ghrelin (pronounced GRAY-lin) that travels in the blood to the brain. The brain responds by feeling hungry. When you're eating a meal, your stomach and intestine make hormones that tell the brain to start feeling full. One hormone is called leptin, another is cholecystokinin (CCK). Stretch receptors in the stomach also are activated as the stomach fills with food or water. These receptors send signals up the vagus nerve to the brain, which tell the brain to feel full.
It takes time for the various "full" signals to get their message through to our brain. In most of us, the signals don't start getting their message through for 15 to 20 minutes after we start eating. And then it takes still more time before our brain says "Enough!"
So suppose you're just sitting down for a meal. Your plate is full -- there might be 2,000 to 3,000 calories on it. Now suppose, based on the amount of calories you've already burned that day, all you need from this meal to keep in energy balance is 1,500 calories. But you have no way of knowing that. All you know is that you are hungry, so you start eating.
If you eat slowly, your brain will start to feel full after about 15 minutes. A few minutes later, your brain will say "Stop!" and there may still be lots of calories on your plate. Maybe you'll save the rest of the meal as leftovers for the next day.
On the other hand, if you're hungry, it's easy to consume 3,000 calories in 15 to 20 minutes -- more calories than you need before your brain determines that you're full. As a result, the extra calories don't get put in the refrigerator as leftovers. Instead, they get put in your mouth and turn into fat.
So though it's hard to achieve a healthy weight, one simple thing can really help: Slow down when you eat, and savor every bite.
• Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: AskDoctorK.com.