Fooling the Brain with Diet Foods
Susie Swithers, Purdue University
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from the show Do Diet Foods Make You Fat?
Ben - If we struggle to change the way that we eat then one solution may be to exchange the foods that we rely on for lower calorie alternatives – using sweeteners instead of sugar or low fat versions that taste almost as good as the full fat original. But could this sort of diet be fooling our brains into thinking that rich, sweet, fatty foods actually don't contain many calories? Professor Susie Swithers of Purdue University believes that this may be the case. Susie, what led you down this avenue of research? Why did you think this might be the case?
Susie - We have lots of evidence that, as Marion talked about, you can learn about the consequences of eating different foods. If that's true, that kind of learning could affect not only preferences as to what you decide to eat, but learning about what the consequences are of eating foods. We know that we typically finish eating long before we could have absorbed or digested all the calories in a meal. So, we’re actually making decisions about when to stop eating in anticipation of what's going to happen. We thought that that kind of experience with food, and the caloric consequences of eating foods, could be an important way to determine things like meal size. If that's true, then giving experiences where the taste of food, like a sweet taste, is sometimes followed by a lot of calories and sometimes followed by not very many calories, that could become problematic. Instead of learning that sweet tastes always predicts a lot of calories, what gets learned is sometimes it predicts and sometimes it doesn’t predict calories, and that may take away a signal, and that could make things harder.
We know that the availability and consumption of foods that are produced with these non-caloric sweeteners has really grown over the last 30 or 40 years at the same time that both the population in the US and now globally, the inability to regulate body weight in the healthy range has increased. So there seemed to be this relationship between the availability and use of these products, and an increase in the inability to regulate body weight. That's what got us interested; could it be the case that these products might actually be interfering with that particular kind of learning?
Ben - How does the body actually count calories? How does it make that link with – I tasted this flavour say, half an hour ago or even an hour ago, and now, I have the following amount of sugar or protein floating around my bloodstream?
Susie - We really don't know the answer to that. There are lots of potential ways that that could happen. There is some evidence in humans for example, using functional imaging data, that the brain can tell the difference between a caloric and a non-caloric sweetener. There's evidence that we have neurons in our brain that are designed to tell us about whether glucose or a particular simple sugar is being utilised. So, that may be one possibility. There are other places in the body that appear to be sensitive to changes in the utilisation or availability of energy - the liver for example. There are also a whole host of hormones that are released in the digestive tract when real sugars actually show up that don't appear to be released when non-caloric sweeteners are involved, and that may be another way we can differentiate between things that deliver calories and those that don't. But we really don't know the answer to what are we tracking.
Ben - So there are obviously lots and lots of different potential mechanisms that we need to look at. What are you actually doing to explore this link?
Susie - What we’re doing is re using a simplified model. One of the challenges to answering these questions in humans is that humans have all kinds of experiences with a variety of foods before they show up into a laboratory and it’s very difficult to understand and control what sorts of experiences they've had. So we actually use an animal model because we have much better control of what they've been exposed to, and we’re trying to use principles that we know about learning in other contexts to understand whether learning is critical and if we can apply principles that we understand from learning to tweak the system and figure out if there are ways to undo any potential negative consequences.
What we typically do is we give our animals experience with diets that are sweet tasting and one group gets a diet that's sweet tasting and that always delivers extra calories. The other group gets diets that are still sweet tasting, but they don't include extra calories. Then we look at the consequences of those diets on things like food intake, body weight, adiposity, how fat they are, and we’re also starting now to try to look at whether those kinds of experiences affect hormone release and other sorts of metabolic processes.
Ben - So, the rats who have been exposed to these low-calorie alternatives, when they are then offered a full sugary drink, how did they react? Did they eat or drink just as much as the other rats or do they eat or drink more, and does that then translate into a weight gain?
Susie - If we take animals that have had experience with no-calorie sweetener and then give them access to foods that have lots of calories in them, they overeat as long as those calories are sweet. In fact, if we do this chronically then the animals overeat enough to gain extra weight. We also have some evidence that their energetic or metabolic responses to sweet tasting foods are altered. They don't seem to produce the same increases in body temperature in response to sweet tasting caloric diets. Our interpretation of that is that they've lost the ability to predict that calories are going to arrive and therefore, they seem to be erring on the side of over-consuming and reducing energy expenditure in case energy is not actually delivered.
Ben - So you are seeing both physiological and behavioural responses?
Susie - That's correct.
Ben - Do you think that translates well into people? We know that different species experience flavours in a different way. Are rats particularly good because we think they have the same sort of flavour profile as humans?
Susie - We certainly know that they respond to sweet tastes in a way that's similar in some ways to what humans do. They show strong preferences for sweet tastes and, as Marion pointed out with human infants, infant rats show preferences for sweet tastes from very early in life. So, we believe that that aspect is similar. We also think that this is a very basic learning process. This is the kind of learning that Pavlov started studying almost 100 years ago. It’s a fundamental and, we think, really elementary form of learning, and we certainly know that humans can show Pavlovian learning about flavours in a very similar way. Most people are probably familiar with this in the context of not learning pleasant associations with food, but learning unpleasant ones. I've heard stories of people, for example, who’ve consumed far too much tequila on one occasion, felt ill afterwards and avoid tequila for a very long time. That's a similar sort of process. They've learned that tequila produces a negative consequence and they avoid it. So we think that these kinds of processes are likely to happen in people, but of course, it’s a much more difficult question to ask in people.