The "super-size" deals at fast-food restaurants aren't such a bargain once the costs of weight gain are considered, according to a new study.

Researchers at the University of Wisconsin-Madison found that while the average "upsized" fast-food meal costs a mere 67 cents more than a regular meal, those bonus calories could translate into substantial daily costs due to weight gain.

When people put on weight, the study authors say, their grocery bills, healthcare costs and even gasoline expenses climb as well.

"These calculated costs exceed the value of upsized meals and may provide motivation to some consumers not to upsize their meals," Rachel N. Close and Dr. Dale A. Schoeller write in the Journal of the American College of Nutrition.

Based on their estimates, each fast-food "value" meal would cost an adult 5 cents more in fuel expenses -- as heavier passengers reduce a car's fuel efficiency - and about 35 cents in overall food costs, since heavier people need more calories.

Add to that the healthcare cost per super-size meal -- which ranges from 82 cents to $6.64 -- and these fast-food deals are no deal at all, Close and Schoeller assert.

"In essence," they write, "the more a person overeats, the greater the financial cost."

About one third of American adults are considered obese, and critics have long accused the fast-food industry of helping to fuel the problem with their swelling portion sizes. The new study took a different tack and sought to highlight the potential financial effects of voluminous fast-food meals.

"This is another way to present the costs associated with weight gain, and might help convince people that upsizing a meal is no bargain at all," Close said in a statement.

Using nutrition information from several major fast-food chains, the researchers estimated that super-sizing a soda and fries costs consumers only 67 cents, on average. But those cents buy about 400 extra calories, which may carry their own price tag.

For every 100 calories a person eats beyond his daily needs, Close and Schoeller calculate, the price in terms of food, medical care and gasoline rises anywhere from 48 cents to nearly $2. The heavier a person is, the greater the cost.

So while there may be immediate savings in choosing a super-size meal, the researchers conclude, consumers should be aware of the potential "hidden costs" that they will pay later.

SOURCE: Journal of the American College of Nutrition, June 2006.