People are irrational.

I've observed over the years that many people assume that the term LOW FAT means NON-FATTENING or HEALTHY. That they can eat as much of that stuff as they want, and it will only make them healthier. I blame the simplistic "fat bad, carbs good" education campaign for much of this bizarre behavior.

The overwhelming evidence shows that the best way to improved health and longevity is through overall caloric reduction (note that I'm not advocating a simplistic low-fat or low-carb lifestyle, only that reducing overall caloric intake seems to be the key component of improving fitness.)

I can't tell you how many times I've heard someone say "It's fat free! and proceed to eat an economy-sized helping. Reading the ingredients label usually shows sugar or high fructose corn syrup as the second or third ingredient.

I'm not the only one who has concluded that people are just plain nuts when it comes to food packaging and labeling. Check out the following stories:

How Supersizing Seduces

Do people underestimate the amount of food (and calories) in a larger package? Seems to be the case:

Whichever set of bags you saw, the correct answers were the same. There were 40 ounces of popcorn in the Medium bag, 67 in the Large, and a whopping 112 in the Extra Large. Just as Dr. Chandon and Ms. Ordabayeva had predicted, people systematically underestimated how much extra popcorn was in the larger sizes: the larger the bag, the more people underestimated its contents (emphasis mine -- DH). And, just as predicted, the errors were more pronounced for the bags that expanded in all three dimensions
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The Perils of 'Healthy' Food

Do 'healthy' labels on food mislead people? More food for thought:

Those “Trans Fat-Free” crackers pictured at left seem to have magical calorie-reducing properties on the whole meal, as I found in an experiment described in my Findings column. Foods or restaurants with virtuous reputations seem to be imbued with a “health halo” that makes us underestimate calories, and I’d like to hear Lab readers’ thoughts on how to deal with this effect.
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‘Good Karma’ Adds Calories

And if you aren't convinced, try this on for size:

Dr. Chandon and his collaborator, Brian Wansink, found that this effect wasn’t limited merely to well-known national brands. In one experiment, they created menus from two fictional restaurants. Although the menus were different, they both included an Italian sandwich containing salami, pepperoni and bologna. Some people in the experiment were given a chance to order that sandwich from one menu; other people were given a chance to order it from the other menu. Here’s Dr. Chandon’s description of what happened:

One of these restaurants was called “Good Karma Healthy Foods” and its menu included healthy choices such as cream of carrot soup (90 calories). The other restaurant was called “Jim’s Hearty Sandwiches” and its menu included high-calorie foods such as “Beef on a Wick” (800 calories).

We found that, just as in the other studies, people underestimated the number of calories of the same meal (a sandwich and a soda) when it was on the menu of the healthy restaurant compared to when it was on the menu of the unhealthy restaurant, and consequently were more likely to order chips in the healthy restaurant condition.

In another experiment, Dr. Wansink and Dr. Chandon showed people cups of granola and cups of M&Ms, two snacks containing about the same amount of calories per ounce. Some of the cups were labeled “Regular” and some were labeled “Low Fat” (even though there’s no such thing as low-fat M&Ms). As predicted, people significantly underestimated the calories in the “low-fat” varieties, and they compensated by raising their estimates of the “appropriate” serving size of each snack.
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Add food, eat less?

Can adding food to the plate reduce caloric intake for the entire meal? Sure, if the food added is 'fat free'. Or at least that's what some people 'see'.

Can it be that adding food makes people believe they are eating less?

[snip]

Half of the people were shown pictures of a meal consisting of an Applebee’s Oriental Chicken Salad and a 20-ounce cup of regular Pepsi and they were asked to estimate the amount of calories in the entire meal. The other participants were shown the same salad and drink plus two Fortt’s crackers prominently labeled “Trans Fat Free.” The crackers added 100 calories to the meal, but given that they were “diet” how will their presence influence the estimated amount of calories in the entire meal?

The first group estimated that the meal contained 1,011 calories, which was a little high. The meal actually contained 934 calories — 714 from the salad and 220 from the drink. But, the second group estimated the total amount of calories to go down. Now the average estimate for the whole meal was only 835 calories — 199 calories less than the actual calorie count, and 176 calories less than the average estimate by the other group for the same meal without crackers.
Trying to Diet and Eating Too Much.

Predictably Irrational

If you'd like to know more about this phenomenon, I recommend you read Predictably Irrational: The Hidden Forces That Shape Our Decisions . The author, Dan Ariely, maintains a blog containing examples of ongoing irrationality. Worth a visit now and then.

Carb Wars: Sugar is the New Fat

And, just for fun, here's a book about the overuse of sugar in our diets: Carb Wars: Sugar is the New Fat