Fair warning: You are almost certainly eating more than you think you are. And probably exercising less.
The "underreporting" phenomenon was discovered in many studies, and describes the fact that practically everyone professional included under-estimate how many calories they are truly eating, and overestimate how much exercise they are getting.
It's actually quite shocking study after study over the last 30 years has shown that on average, people simply cannot or do not accurately report, or record their food intake. In short, it's a scientific fact that people are terrible at knowing how much they eat.
This can happen in several ways:
Missing or forgetting food eaten Incorrect portion sizes Incorrectly describing the foods Under-recording: weighing and recording food can be tedious and it's easy to not be 100% diligent Some people tend to undereat (consciously or subconsciously) when they know they are being monitored; then overeat the rest of the time Deliberately not reporting food that seems trivial but actually isn't Food labels themselves can be inaccurate (sometimes over 200% more calories)
It's interesting that the degree to which underreporting occurs can vary according to a person's weight, income, education, gender, psychological factors.
There have been many, many high quality studies done on this problem. Here are a few...
Studies consistently show that the obese under-report their food intake (by up to 30-50%) and over-report their activity to the same degree. One study compared twins where one twin was obese and the other wasn't obese. The study showed that the obese twin was underreporting by 764 calories per day, and overestimating exercise.
Another study looked at women who claimed to be not losing weight while eating little food ("small eaters"). When they were supplied with the true calorie-intake they claimed to be eating, guess what happened? They lost 1.7 pounds per week! Other similar studies have shown the same result.
The degree to which underreporting occurs can be quite severe: One study found that certain people were underreporting their food intake by over 2000 calories per day!
Do you still think you're the exception?
Consider this study done by researchers at Pennington Biomedical Research Center. They compared the accuracy of dietitians' self-reported food intake to the accuracy of non-dietitians of comparable age and weight. All of the subjects were trained to provide a 7-day food intake record and were told to be as accurate as possible.
Results? On average, dietitians underreported their food intake by an average of 223 calories per day. The non-dietitians underreported their intake by an average of 429 calories per day. This was proved by an method called the "doubly labeled water" technique which can measure 24-hour energy expenditure within + or - 5%.
Now, if dietitians whose profession is to advise people on food intake, and who are trying to be as accurate as possible, still underreport calories ... what does that mean for the rest of us? (Yes, it means we are almost guaranteed to be wrong, too).
Over-Estimating Amount of Exercise
The reporting problem applies to exercise, too, for many of the same reasons.
When you combine underreporting calories, and overreporting exercise, it's easy to see how it can seem like the Energy Balance Equation isn't true.
Bottom line: If you're struggling to lose weight or steadily gaining, you need to take a serious look at your true calorie intake. Many people will swear they're hardly eating anything and not losing, when in reality they are overeating and not expending as many calories as they think.