In a recent post, about Prof. Richard Wrangham’s theory that the discovery or invention of cooking played a key role in human evolution, I mentioned that standard references on the nutritive value of foods — such as the USDA Bulletin HG-72, Nutritive Value of Foods — are compiled on the assumption that the calories in a particular food item, such as a carrot, remain the same whether the carrot is eaten raw or cooked. As Prof. Wrangham has pointed out, cooking of some foods significantly changes the degree to which calories and other nutrients are absorbed by the body. In some cases, such as proteins or starches, cooking improves the availability of nutrients; in other cases, heat may have the effect of reducing availability, or actually of altering the nutrient itself, as happens with some vitamins.
This is actually one symptom of a larger problem with nutritional labeling as it is done currently. In essence, the composition values (e.g., X mg of vitamin Q-17) are arrived at by a static analysis on a lab bench, and do not take into account the availability of the nutrient to the body (bioavailability). Of course the information provided is a lot better than nothing, but it is potentially misleading in some cases. For example, dairy products are generally an excellent source of calcium; but if your cheese is being eaten in a sauce on some other foods (spinach, or whole grain products), you will not get nearly the normal benefit from the calcium in the cheese. This happens because certain organic compounds in the spinach (oxalic acid) or the grains (phytates) bind with calcium ions in a structure that is indigestible. Tetracycline antibiotics can do the same thing, and in that case the effectiveness of the antibiotic is compromised as well.
There are also cases where the established testing procedures can be fooled, either accidentally or on purpose, by being taken “out of context”. You may remember the incidents of pet food and infant formula from China that were contaminated with melamine. Melamine is a plastic that was added deliberately to these items so that, when they were tested for protein content, the readings would be higher (better) than they really were. This trick works because the standard method of testing foods for protein content is, in effect, a test for nitrogen content. When we know we are testing foods, this works OK, because of the three macro-nutrient groups (carbohydrates, fats, and protein), only protein contains any significant amount of nitrogen. When the test is applied to something else, it still detects nitrogen just fine, but there is no guarantee that the nitrogen is in protein. (This same test applied to a chemical fertilizer, which is rich in nitrogen, will typically show that it contains an enormous amount of protein.)
The moral of the story, I suppose, is that nutrition labels are useful, but they have their limitations. You should probably also keep in mind what your grandmother told you about eating a balanced diet.