Experts have challenged two basic assumptions behind food choices and the causes of obesity.
Drs. Jeff Brunstrom and Peter Rogers of the University of Bristol, UK, investigated the theory that we eat larger amounts of tasty foods, and that these foods tend to be energy-dense.
They explain that highly palatable food is more widely available than ever before. This is one aspect of the so-called “obesogenic environment,” thought to contribute to rising levels of obesity. But they ask: does this necessarily lead to the selection of larger meals?
To answer the question, they carried out a study to measure ideal portion sizes of several popular and well-liked foods. Crucially, they also measured “expected satiation,” that is, to what degree each food would satisfy diners.
In the experiment, 28 normal-weight men and women assessed images of 16 foods at lunchtime, having not eaten for three hours. The foods were: fish fingers, pasta and tomato sauce, raw banana, crackers, chicken tikka masala, Jaffa cakes (chocolate covered sponge snack), pretzels, fries, Pringles, peanut M&M’s, cashew nuts, Crunchie bar (honeycomb covered in chocolate), KitKat, potato salad, chicken chow mein, and baguette with cheese.
Participants chose their ideal portion size for each food, as if they were eating only that food for lunch. They then selected an equally filling portion of a comparison food (pizza). Next they rated their anticipated liking for each of the foods, and reported how much money they would spend on a typical portion.
Results showed that foods with a high expected satiation tended to be selected in smaller portions (fewer calories). The actual energy density of a food was not linked to its ideal portion size. But high energy-dense foods were expected to be less filling per calorie than low energy-dense foods.
Surprisingly, better-liked foods were not chosen in greater quantities. The lack of a relationship between liking and ideal portion size is “particularly striking,” the researchers state.
Participants were willing to spend more money on foods they thought would be more filling. They chose larger portions of foods they would spend less money on because they were regarded as less filling.
Overall, low expected satiation was a much better predictor of choosing a bigger portion than was how much the participant liked the food.
The authors write, “For many, the reason why energy-dense foods promote weight gain is so obvious that it barely merits investigation. This is perhaps because two basic facts are widely accepted: energy-dense foods are more palatable and are therefore more rewarding (desired), and these palatable foods tend to be selected in relatively larger portions.
“Neither assumption was supported by our data.”
This study is unique in that it compares palatability and reward (measured as monetary value) across foods. It also compared foods on a like-for-like (calorie-for-calorie) basis, to understand their relative effect on energy intake.
“It is now widely assumed that palatability plays a key role in decisions about meal size,” they researchers write. “However, an alternative, and previously unexplored possibility, is that decisions are largely motivated by non-affective beliefs, such as those relating to the post-ingestive consequences of consuming food.”
Foods differ considerably in their expected satiation. Per calorie, some foods have been rated as five to six times more filling than other foods. “In particular, foods with low expected satiety tended to be highly energy-dense snack foods (e.g., cashew nuts, chocolate, potato chips, and cakes),” say the experts.
Given the magnitude of these differences, could such expectations play a role in portion size decisions? Dr. Brunstrom’s previous work suggests this is true; high energy-dense foods are indeed consumed in larger portions (more calories) because they have low expected satiation.
The authors conclude that high energy-dense foods are selected in larger portions not because they are especially liked, but because they are expected to be less filling per calorie. This is backed up by earlier studies that found relatively low feelings of satiation after consuming energy-dense foods.
They add, “We are not claiming that liking plays no role in energy intake and other aspects of dietary behavior. Clearly, hedonic responses can be highly relevant. When an unlimited amount of food is available, palatability appears to be closely associated with the weight of food that is consumed.
“By contrast, our analysis relates to everyday decisions about portion size, across a range of foods, and before a meal begins. These findings challenge the role of palatability in meal size selection, and they highlight the importance of expected satiation, a ‘non-affective’ component of food reward.”
Brunstrom, J. M. and Rogers, P. J. How Many Calories Are on Our Plate? Expected Fullness, Not Liking, Determines Meal-size Selection. Obesity, Vol. 17, October 2009, pp. 1884-90.