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I'm wondering what's the relation between nitrogen factor, protein factor and energy from total protein. i.e

Feta Cheese 
Nitrogen Factor: 6.38
Protein Factor: 4.27
Total Protein: 21.3 g

and I know that:

Calories From Protein: 21.3 * 4.27 = 90.95 kcal

But I don't know how nitrogen factor affects calculation and where. I googled it but I didn't find what I was looking for(maybe use of bad keywords), I appreciate recommendation of any articles for my further reading.

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Isn't the information you need printed on the label? – Mien Jun 24 '13 at 19:18
The calculated values are printed on the labels, it's raw data upon which nutritionists calculate calories. – neelix Jun 25 '13 at 6:04
Why the close vote? This question asks about facts of certain components in a food, without direct reference to health issues. It seems on topic. – SAJ14SAJ Jun 25 '13 at 11:58

Nitrogen factor appears to be a method of estimating protein content in a specific foodstuff based on the amount of nitrogen (since proteins are nitrogen rich). It is not exact, and there are different factors for different types of foods, to provide better estimates based on the ratios and types of proteins in those foods.

This 1931 USDA document gives an excellent background, even though it is somewhat of a bad scan, and challenging to read.

See also, for example, the FAO Conversion Table showing different factors for different foods.

As proteins are a calorific food, the nitrogen factor correlates with calories from protein because it helps you estimate the amount of protein in a food, and therefore the number of calories from protein based on the fact that protein has about 17 kJ or 4 kcal per gram.

Therefore, if you know you have g grams of a food, with a nitrogen factor of f, the estimated number of calories from protein c is:

  c = g * f * 4

I was unable to find a definitive clear meaning of the phrase "protein factor" as it is a common phrase in biochemistry, evidently.

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Thanks a lot for answer, but I can't get when there's protein factor to calculate calculate calories from protein why bother using nitrogen factor? I think I should read the challenging document. – neelix Jun 24 '13 at 18:07
I was not able to find a definition of "protein factor" so I cannot speculate on that. Sorry. – SAJ14SAJ Jun 24 '13 at 18:26
I appriciate your help. proetin calories = gram of protein * protein factor. Your formula yeilds to quite different result: 21.3*6.38*4=543 while true value is 90.95kcal – neelix Jun 25 '13 at 6:05

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