Since the 1970s, the number of new food products available to consumers in developed countries has increased dramatically. For any one type of food, there are often many different brands and varieties, from diet or “lite” versions through to indulgent “luxury” brands. Some of these products contain low-calorie sweeteners and fat substitutes, which means that within a single food category, calorie content can vary considerably.
It is well known that developed countries have a problem with obesity and that this is linked to changes in the food environment, such as larger portion sizes and the increased availability of calorie-dense foods. However, until now, there has been little consideration of the impact of eating different brands and varieties of the same food item.
Psychological research has established that, over time, we learn to associate the flavour of a food with its energy content. For example, sweetness or creaminess are common signals that a food contains calories. This learning is important because it comes to determine how and what we eat. However, complex dietary environments, where there are numerous brands and varieties of the same food, might compromise this learning – and this could result in over-consumption.
Studies with non-human animals provide initial support for this theory. Rats that are repeatedly exposed to low-calorie sweeteners go on to eat more food, gain weight and become obese. This is thought to be because they fail to learn an association between sweetness and the presence of calories.
The pepperoni pizza test
In recent research conducted at the University of Bristol, we considered whether a similar phenomenon might be evident in humans who frequently eat different brands and varieties of a common mass-produced food: pepperoni pizza. Our initial pilot work established that there are more than 70 different brands of pepperoni pizza available to UK consumers and that these vary in calorie content by almost 400%; the lowest was 501 calories per pizza (made by Weight Watchers), and the highest contained 1,909 calories per pizza (made by Domino’s).
We first examined exposure to these different brands and varieties by asking 199 adults to complete a questionnaire about their pepperoni pizza consumption over the past year. We found that while some people tended to consume the same brand of pizza, others were much more inconsistent and tended to eat many different varieties.
In the next phase of the research, we invited 66 of these participants to attend our laboratory for a follow-up experiment on eating behaviour. Participants completed two test sessions on separate days. In one session, they ate a fixed portion of pepperoni pizza and were then offered some snack foods (tortilla chips and broken cookies) to eat freely. In the other session, they consumed no pizza but were given the same access to chips and cookies.
We found that participants who regularly ate different varieties of pepperoni pizza showed poor “compensation” for the calories in the fixed-portion pizza test. That is, they went on to eat more chips and cookies which resulted in them consuming substantially more calories overall, compared to their other test session when no pizza was eaten. By contrast, participants who usually ate the same variety of pizza were much better at compensating for the calories in the fixed portion of pizza. They tended to eat fewer chips and cookies after the pizza, meaning that their total calorie intake was more closely matched to their other test session when no pizza was consumed.
Expectations linked to meal size
We also measured participants’ expectations about the extent to which the fixed portion of pizza would fill them up (“expected satiation”). Intriguingly, we found that participants who regularly ate different pizza varieties expected the fixed portion of pizza to be less filling than the other participants.
This latter finding suggests that high levels of variety in the dietary environment may limit the opportunity to learn about the satiating properties of specific foods. Importantly, our previous research has shown that expected satiation is an excellent predictor of meal size and food intake.
Taken together, these findings provide the first evidence that a highly variable and complex dietary environment can make it difficult for people to learn about food and to control their intake accordingly. Given the potential public health implications, further research is needed to determine whether this feature of a Western diet contributes to overeating and obesity.