The way in which portion size information is provided on snack packaging could influence how much people consume, although the effects may differ by gender, new research suggests.

According to the Economic and Social Research Institute (ESRI), which carried out the research, portion sizes have "increased substantially" over the last 20 years and this has contributed to increasing rates of obesity.

"Consequently, policies and practices that reduce exposure to larger-sized portions and packages have been proposed. Many manufacturers display recommended portions on packs. However, findings from behavioural science show that written labels are rarely effective, either because they are too complex or because they do not draw attention," the researchers noted.

They decided to carry out two controlled behavioural experiments to ascertain whether consumers notice portion information and whether it alters how much they eat.

In the first study, 369 consumers received 40g cans of crisps in a venue. Half of the cans had the usual portion size and nutritional information in a table on the packaging. However, the other half had additional white stripes that marked where one portion ended and the next began, and a label which read ‘1 portion' between the stripes.

The participants thought they were there for a study on something else and so were unaware their consumption of the crisps was being studied until they were informed at the end.

The crisps were the same in each can.

This study found that the additional portion size markings influenced how much men consumed.

"Over 60% of men in the control group ate more than one portion of crisps, but only 40% of men who saw the portion size markings - a reduction of one-third," the researchers said.

In the second study, 800 households received gift packs containing packets of chocolate biscuits. Again, half of the biscuits had the usual nutritional information on their packaging, while half had additional portion markings.

The households were contacted a few days later and asked to take part in a survey and were then told that the biscuits were part of the study.

This study found that when woman received the packages, there was a 26% drop in the number of households eating more than the recommended portion serving, when the additional white stripes were used.

The study also noted that households with children were less likely to open packs with additional portion size markings.

The researchers said that there were two consistent findings associated with both studies:
-The additional markings reduced consumption among people inclined to consume the most
-Consumers were much more likely to notice the additional markings than the portion sizes that are written in standard nutritional information tables.

The researchers also found that there was lots of confusion about where information on portions comes from, with one-third of consumers thinking it is a health recommendation from the Government. However, portion sizes are actually determined by the manufacturer. Just 15% of people were aware of this.

The researchers concluded that the format, colour and placement of information "alters how people respond to it" and as a result, food labeling policies and regulations need to consider how information is presented to consumers.

"Changes to the format of information can change how it is perceived. Displaying portion size information as highly visible stripes influenced how much attention consumers paid to it and how much they ate.

"Controlled experiments like these can provide objective evidence about how people understand, attend to and act on nutritional information," commented the study's lead researcher, Dr Deirdre Robertson.

Details of these findings are published in the journal, Annals of Behavioral Medicine.