The values may be calculated on the whole package contents, then divided by the number of servings/slices in the package - leading to, as others have stated, rounding of the figures. That is why the numbers won't add up on the per serving basis, but if you do the math on the whole package contents it will be much closer to making the equation example work.
My understanding is they do them separately. I'm not exactly sure why. But rounding is a big thing with labeling
So, take your label here. Your label says 1/19/5 for fat, carbs & protien respectively. Assume that this is really .5/18.5/4.5 and the figures are rounded up. That gives you a total of 96.5.
The other thing that I just thought of, and could also cause a discrepency, is insoluable fiber. Since insoluable fiber won't convert to any energy in the body, a manufacturer can subtract that from the caloric total. So in the scenario I created above, assume 3.51 of or fiber grams are insoluable, so you can subtract off 14.04 calories from your total, giving you 82.46 calories. Again, you can round calories to the nearst 5 giving you 80 calories.
That is a pretty far fetched example, but not completly outside the realm of possibily. You can see how complex the world of food labeling really is.
Okay, so of course I"M no help since I quiver at the very sight of the word "math".
On the subject of bread I have been eating Brownberry's Double Protein bread 100 cals/slice .5 fat carbs ? 6 grams of protein
Fitness Minutes: (12,713)
4,114 4/12/10 9:52 A
Interesting. So they measure calories and nutrients at two different points in the process. Well that would explain why they don't add up. Stupid question, but you can't measure nutrients after the fact?
1) When the nutrients are calculated, they are most likely added up from the nutrients within the raw materials (flour, shortening, etc.). There is natrually some calorie loss during the baking process, there by reducing the number of calories. 2) Calories and nutrients are calculated by 2 different methods. Like above, the nutrients are calculated from a raw state. Calories are actually calculated by labratory testing, in which they will "burn" the food and calculate the heat created by the food. 3) Rounding. By law, food companies have a little play in the values. For example, if an item has .45 grams of carbs, then they can put 0 on the label. If they add two items with .45 and 1.05 grams each (1.5 total), then they have to put 2 on the label. Neither one are correct. Same applies for fat and protien, except I believe fat must be expressed in 1/2 gram incriments. You add enough ingredients and you could have some pretty signifigant differences.
Hope that helps.
Fitness Minutes: (12,713)
4,114 4/11/10 3:58 P
Hey guys, I'm trying to build some meals with macro-nutrient breakdowns. I think I'm missing something. For example, 2 slices of Natures Own Wheat bread. You can see their label here.
They claim 2 slices contains 80 calories. 19 grams of carbs, 1 gram of fat and 5 grams of protein. The way I understand it, each gram of carbs = 4 calories, each gram of protein = 4 calories and each gram of fat = 9 calories.
So if I take (19 x 4) + (1 x 9) + (5 x 4) = 105. So doesn't this item *REALLY* have 105 calories? Or is there something I'm completely missing? Or is my math wrong?