On the USDA database the entry for [this chicken thigh](https://fdc.nal.usda.gov/fdc-app.html#/food-details/1899680/nutrients), calculating the sum of its parts using the 4-9-4 rule (4 cal per gram of carb/protein and 9 cal per gram of fat) is ~20 cal off from it’s listed energy amount. A similar noticeable difference can be found on [this nutrition label for some edamame](https://www.instacart.com/store/items/item_1910923160).
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Why does this happen? I know there are tools like bomb calorimeters. Do those provide different numbers than the 4-9-4 rule? If so, is that what they use to list the total calories on the label?
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>I know there are tools like bomb calorimeters. Do those provide different numbers than the 4-9-4 rule? If so, is that what they use to list the total calories on the label?
Yes they use something different these days. Its called the modified [atwater system](https://en.wikipedia.org/wiki/Atwater_system).
Your 20kcal is likely rounding error, and between two different people maybe even more significant if one lacks the needed gut bacteria to actually digest it(novel meal vs regularly eaten meal).
The standards around nutritional labels are pretty complicated, so it’s difficult to say why some specific thing is labeled in a certain way.
In general, they’re allowed to round, or to estimate the quantity based on averages. Packaged foods tend to be pretty consistent, but even they are not 100% consistent. Natural foods even less so.
The results of this rounding and estimation may cause the label to be somewhat inconsistent between different sections. For example, perhaps sugars are required to be rounded to 1g, while calories can be rounded to the nearest 10 kCals.
They’re even allowed to use different rounding rules based on the number of servings in a container.
Basically, it’s complicated, so much so that companies often buy special software to let them make these labels more easily.
In the end, they’re just guidelines to help consumers make decisions, and not designed to be 100% accurate in all cases. No person is distinguishing between a label that says 200 kCals and one that says 220 kCals to make a decision. Either number is accurate enough to serve its intended purpose.
The 20 kcal is probably rounding. Protein carbs and fat are usually rounded to the nearest gram, and total kcal are rounded to the nearest 5 or 10.
Also not all carbohydrates are 4cal/gram. Fiber is counted as a carbohydrate, but depending on the type, it might actually have 0 or 2 kcal/g.
The rules set by the government for counting and calculating this stuff aren’t all clear or consistent. In general, you should treat nutrition labels as rough estimations, not precise measurements.
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