As a psychologist, I probably shouldn’t be surprised, but I still am. I remember drawing hugely complex diagrams of the nervous system for my masters degree, showing all the nerves going into and out of the brain, making contact with virtually every organ and system of the body. I know, intellectually speaking, that the nervous system, and thereby the brain, is hugely influential in everything the body does, but I still can’t help it. I just figure if you were inventing a machine that needs fuel, you’d have a fairly simple fuel gauge — a little meter that says “stomach full, well done”. Nope.
I’ve written in other posts about how mindful eating can help you know when you’ve had enough, so that you don’t over-eat, but recent evidence from the University of Bristol suggests that your current hunger levels probably have more to do with what you remember about your last meal, than with what food you really ate.
Prof Jeffrey Brunstrom and colleagues showed some volunteers bowls of soup, either large or small, to make them think that was what they were getting, and then allowed them to eat soup from a bowl into which was inserted a hidden tube that either sucked soup out, or added extra in to the bowl. (If any engineers with food-vending experience want to donate time to my lab, do get in touch!) Under these conditions, it’s surprisingly easy to fool people. We’re just not that good at estimating quantities. Immediately after they’d finished eating the soup, the participants who’d eaten more soup, due to the machine pumping extra into their bowls, said they felt full. As expected. The universe makes sense.
After two hours, however, the universe ceased playing by the rules of puny humans. By the two hour mark, it wasn’t the actual amount of food they’d eaten that predicted how hungry they were, it was how much they thought they’d eaten that mattered. Those who had been shown the large bowl right at the start, said they felt less hungry.
Effects like these are found again and again in the research literature. They don’t agree with our common sense view of how the world should work, but then isn’t it the very point of science to check whether our common sense view is actually correct? Suzanne Higgs and Morgan Woodward of the University of Birmingham published a study in 2009 showing that watching TV during lunch does something funky to your appetite later that afternoon. They had volunteers eat a standard lunch (ham sandwiches and potato crisps). Each volunteer came in to the lab twice. One of those days they ate lunch watching a comedy clip on TV, the other day the TV wasn’t on, leaving them to eat their food undistracted. (They were given, and ate, exactly the same lunch both days.) Later that afternoon, the volunteers were asked to rate on a scale how hungry they were, what their mood was like, and so on, and then offered a snack of chocolate cookies. The volunteers weren’t aware of any difference in their hunger levels. Regardless of watching TV or not, they rated their hunger levels, their moods, etc, as being the same. But those whose lunch eating had been distracted by TV ate more cookies anyway.
There are two take-away messages from studies like these in my view.
First, it really matters whether you let yourself be distracted whilst eating. If you don’t want to be overweight, it’s just you and the food. Turn off the TV, the radio, YouTube, and the podcasts. Put down the iPhone, the newspaper, and the book. Stop trying to eat whilst walking. Have you ever tried to approach a dog whilst it’s eating? Even the most mild-mannered little doggie will bare its teeth and growl. Dogs want to spend quality time with their food. (It’s not that they think you’re going to steal it, honest.) Release your inner dog.
Second, human rationality is far from infallible. We like to imagine that our conscious minds control everything, that we only do things because we have decided to do them. Rubbish. Far more processing goes on under the radar, subconsciously, and since we’re not aware of all that stuff, we sometimes struggle to make sense of our own behaviour. If you simply rely on your intuition, you’ll never discover that TV has affects how much you ate later. The volunteers in the Birmingham study weren’t able to notice the difference in their hunger levels. Psychology exists as a profession precisely because sitting down and trying to work out what’s going on in your own body, and head, by sheer force of will, doesn’t work. Never has. Never will.