Perceptions of normal portion size have ballooned over the past 20 years -- increasing by 20 - 40% and adding substantial calories to our daily totals. And according to a recent study, the larger the size of a fast-food meal, the more likely that you will underestimate its calorie content.

Permanent alterations in your lifelong attitudes toward diet and exercise are the keys to successful weight management. You must be motivated enough to change habits not for a few weeks or months, but for a lifetime.

The first step is to determine how many calories you should eat each day. To do this, calculate the number of calories needed to maintain your current weight. This is roughly 15 calories per pound of body weight for a moderately active person (someone who gets at least 30 minutes of moderate to intense physical activity every day). A completely sedentary person requires just 12 calories per pound to maintain weight.

So how many calories do you consume in a meal? In a recent study reported in the Annals of Internal Medicine (Volume 145, page 326 ), researchers found that the larger the size of a fast-food meal, the more likely that you will underestimate its calorie content -- by as much as 500 calories.

The researchers conducted two experiments. One involved 105 people at fast-food restaurants, who were asked to estimate the calorie content of the meals they ordered and ate. The other, a laboratory study, enrolled 40 college students who were shown 15 different sizes of the same fast-food meal and asked to estimate the calorie content.

The results showed that the size of the meal was critical for getting the calorie count right: At the fast-food restaurant, those eating smaller meals estimated they were eating 419 calories when they consumed 514 calories. Those eating larger meals estimated 675 calories but actually ate 1,188 calories. In the laboratory study, students guessed large meals contained an average of 1,000 calories when they really had 1,382 calories and that small meals contained 631 calories when they had 655 calories. In the two experiments, both overweight and normal-weight participants underestimated the calories in larger meals.

The bottom line: The next time you eat a super-size meal, be aware that its calorie count is probably twice what you think it is.