I just got a fascinating paper from the business and marketing literature describing studies done on how people's consumption increases when they believe the foods they are eating are low-fat. The paper, published in the Journal of Marketing Research, describes three studies showing that subjects, particularly if they are overweight, tend to overeat snack foods when they believe they are lower in fat.

In the first study, the researchers provided the subjects — who were students and their families visiting a university open house — with gallon containers of unusual colored M&Ms labeled either 'New Colors of Regular M&Ms' or 'New Low-fat M&Ms.' (The M&Ms were exactly the same — there are no low-fat M&Ms.) The only difference in these M&Ms was the labels on the bowls. The subjects were instructed to eat as much as they wanted as they roamed through the display areas. Upon leaving, the subjects gave up their containers of candy and answered a questionnaire. The researchers measured the amount of M&Ms left in the bowls and compared the consumption of the 'Regular' to the 'Low-fat' and correlated the results to the answers on the questionnaire.

It turns out that the people eating the 'low-fat' M&Ms consumed 28.4% more calories than those consuming the 'regular' M&Ms. When broken down by weight of the subjects, the differences were even more dramatic.

Overweight subjects eating the 'low-fat' M&Ms consumed 47% more calories than than did overweight subjects who got the 'regular' M&Ms. Normal weight subjects consumed only 16% more calories than the normal weight subjects eating the 'regular' M&Ms.

All subjects were asked to estimate how many calories of M&Ms they had eaten. All subjects significantly underestimated their consumption, but the overweight subjects way underestimated how many of the 'low-fat' M&Ms they had eaten.

These findings are not just fascinating, they have real world implications. As the authors point out:

"Because low-fat foods are believed to contain fewer calories than regular versions, it might be reasonable for a hungry calorie counter to consume more candy when it is described as low-fat than when it is not. The key question for public health is whether a low-fat claim would lead such a person to eat so much more that it offsets the potentially lower-calorie density of low-fat foods. To determine this, we surveyed the fat and calorie content of all brands of chocolate candies, bars, cookies, milk drinks, and muffins with a least a 5% market share.

We found 17 brands that were sold with both a regular and a low-fat version of the same product. The serving sizes indicated on the products were similar for both versions. Although, on average, the low-fat versions contained 59% less fat per serving than regular versions, they contained only 15% fewer calories (140 verses 170 calories). If participants in [the study] had eaten real low-fat M&Ms (with the market average of 15% fewer calories than regular chocolate candies), they would have consumed 47% less fat but 9% more total calories. This is a conservative estimate. In reality, the increase in calories is likely to be even higher because the ingredients used to replace fat tend to make people hungrier." [My italics]