Apple, or chocolate cake?

How does the brain make decisions about what to eat? And how does stress influence this decision? Katie Haylor spoke with neuroeconomist Todd Hare from the University of Zurich, to find out.

Todd - So that will mean different things to different neuroscientists. My primary behaviour of interest is decisions and one thing, just to give an example, a lot of my work focuses on how people decide what to eat. So the behaviour is - I'm going to eat the apple and not the chocolate cake, or maybe I am going to eat the chocolate cake, but that's what I'm interested in.

Katie - Tell me a bit about your research and the kind of studies that you do.

Todd - Dietary choices are a big part of what we do. So we generally set it up such that people are coming into our experiments hungry. That means motivated to get some food, and we give them different options. Apple and chocolate cake is an example I just gave, but we give them hundreds of different combinations of junk food and healthy food. Sometimes they're choosing between two healthy foods, sometimes between two junk foods, sometimes between a junk food and a healthy food. While they're doing this, we are looking at the levels of activity, brain activity, loosely defined using functional magnetic resonance imaging devices.

Katie - Okay, so this is looking at the brain while it is actively engaged in doing something and using proxy markers to indicate which bits of the brain are busy and which bits are not?

Todd - Exactly right. So one of the projects we just recently finished up is looking at how stress will change the way that people make decisions about these different food items. So we brought people into the lab. Half of them we randomly assigned to go through a psychosocial stressor. That meant that they had to stick their hand in an ice cold bucket of water and one of our experimenters is there observing them judging how well they're doing this the whole time. That's the social part of the stressor.

Katie - That sounds hideous. So you've got a physical sensation that's not very nice and you feel like you're being judged by somebody. That sounds hideous. Why would you do this?

Todd - It works very well. It creates stress, sort of along the same levels of having a fight with your spouse or your boss, being late to an important meeting. We get to about that level of stress, so things that occur on a regular basis in daily life.

Katie - OK, so what does that kind of, I guess, kind of chronic lowish level stress do to our decisions to eat a cake or the apple for instance?

Todd - We don't know about the chronic, just to be clear, because we do this one time. It may be that this adds up over time, we just can't answer that with this experiment. But what we see in the acute level is that you seem to shift more toward the immediate benefits and those are the taste of the food items. So I'm much more likely to eat the chocolate cake in that case if I've been through this stress protocol than the apple.

Katie - Is that because it tastes better or is that because it's got way more sugar and fat in it that I can use to run away from my virtual Lion?

Todd - That's a very good question. In this case we don't know for sure. It would make sense that given that you're in a situation you might need to escape from when you're being stressed, you would focus more on things like energy that could help you do that, but we can't definitively establish that with this experiment.

Katie - OK. So what do you think is going on then in the body to encourage us, potentially when we're stressed, to reach for that piece of cake, which I've got to be honest, I've been there. I'm sure everyone has.

Todd - We actually see that there are two different pathways that seem to be influenced following this stress protocol. One of them is most closely associated with the physiological reaction. So we measure this with cortisol, something your body produces whenever you're stressed. There we see that if your cortisol level is higher, there are more interactions between a region of the brain known as the amygdala, which is in a region that has a lot of cortisol receptors. This seems to have a stronger interaction with regions that are involved in making the choice. The amygdala is also something that is reflecting the tastiness of these foods. So it could be that taste is having a stronger impact on choices via the cortisol effect on the connectivity between these regions.

Katie - But taste is subjective, right? I might think that an apple is really tasty and you might think it's horrible.

Todd - So that's a very important part of the experiment. We ask each individual person how tasty they think the different foods are. So I get your rating for how good an apple is, I get your rating for how good broccoli is, I get your rating for how much you like ice cream. All these things, so I know exactly your subjective opinion of taste.

Katie - OK. So I'm kind of my own control in a way.

Todd - So what we've shown is that stress makes you, stress makes it harder for you to overcome taste temptations.

Katie - If we know this about stress, can we apply it in some way to help people change the behaviours that are sometimes quite ingrained and sometimes really difficult to change?

Todd - One possibility is that stress sort of pushes you toward a default pattern of behavior. Your stress, you want to allocate resources in your brain to avoiding the stress or overcoming the stress. So you fall back on a pattern of behavior or choices that you generally make. If before you're stressed you've established a pattern of making healthy choices in the domain of food, this would be one way to use stress or be resilient to stress if your stress causes your default to be a healthy choice in the first place.