Another source of discrepancy is whether or not the fiber calories are calculated into the total. Human beings cannot digest fiber, so for us, the calories "don't count." Food companies are allowed to subtract these calories, but they are not required to, and many do not. And unfortunately, they do not have to state on the label which way they did the math (the only way to check is to double check their math).
Also, they're allowed to round their numbers so one company may round to the nearest 5 and another may round to the nearest 10 or perhaps even more.
The fiber calories really upset me the most, because it makes high-fiber vegetables seem much more caloric than they actually are. I can understand why cow-chow would list the fiber calories, but humans aren't cows, so why are calories we can't access allowed on the label?
Ironically, I find that imported foods (like the canned foods I get from Thailand) have more accurate calorie labels than the USA. From what I understand, they have to subtract the fiber calories (and there seems to be a whole lot less rounding in the math. The calorie count will list 167 or 163 where the equivalent American product will list 160 or 170).
You also can't trust the "number of servings" listed on the USA bags. "About 3 servings," I've learned can mean anything from 2.2 servings to 3.8.
I always use the gram weight listed on the package as my serving size (because it's the measurement the labs would have used to calculate the information on the label).
If the calorie count seems to be unreasonably high, and the food item has a lot of fiber, I will sometimes double check the math (each gram of protein and carbs will have about 4 calories, and each gram of fat will have 9 calories).
If there's no fiber, then there's no reason to double check the math. However, I've often found that high-fiber foods are erroneously labeled. Which is one of the reasons, I use an exchange plan for my calorie-counting rather than trying to count individual calories. The food labels, and online calorie counting resources all estimate, and are prone to including fiber calories (which makes a 50 calorie oreo seem equivalent to 50 calories of vegetables, even though half or more of the veggie's 50 calories may not "count" because they come from fiber, which can't be digested).
It turns out there are other calories that may not "count" the way we expect them to. For example sugar alcohols aren't fully digested, but how much is digested may vary from individual to individual. Likewise "resistant starches" also aren't fully digested, but so far, there's no way (that I've read about, at least) to know how many of the resistant starch calories are absorbed, and how many aren't.
The more I understand about the ways in which "a calorie is a calorie" isn't entirely true, the more happy I am with exchange plans (which do control carb, protein, fat, and calories content, so it counts all four, but the counts are based on estimation).
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