What the Labels Mean

Where do they get the nutrient values on the labels?

Manufacturers of processed foods have several choices. They can take samples from 12 different batches and have them analyzed for their chemical components. There are one or more officially-approved methods of analysis for each constituent. They can also have the nutrients calculated from the recipe. Since that method depends on someone performing laboratory analyses to provide the information for the recipe's ingredients, the following section will address the problems of getting that analytical information in the first place.

How do they analyze for nutrients?

Protein is estimated based upon the amount of bound nitrogen in the sample. Total fat is estimated from the weight of all individual fatty acids plus the imputed weight of one unit of glycerol for each three fatty acids. Total carbohydrate is calculated by subtracting the weight of protein, total fat, moisture, and ash from the total weight of the sample of food.

Carbohydrates

Carbohydrates aren't the sum of the sugars, starches, etc.?

Chemically they are. However, the label regulations require calculating total carbohydrates by difference rather than by measuring them directly. It is both cheaper and serves as a catch-all for certain nutrients that are not well characterized. On the other hand, should manufacturers choose to report them individually, carbohydrate constituents like sugars, starches, dietary fiber, sugar alcohols, etc. must be determined by chemical analysis. But the value reported on the "Total Carbohydrate" line must still be calculated by difference, not by adding up the individual components.

Does calculating rather measuring carbohydrates lead to any problems?

Yes. This is the reason why it is possible to have a label where the weight of any or all of the carbohydrate constituents reported somewhat exceeds the weight listed for total carbohydrates which can confuse consumers.

But how could it come out with a smaller number?

Because of something called "standard error." Every measurement technique has a degree of imprecision (think about weighing yourself or taking your measurements several times in a row and you'll understand). Add to it the problem that the error of the "total carbohydrates" reported may include five different standard errors. Clearly, it is not improbable that the two methods, calculation versus direct measurement, may not match up.

While it's clear that a label reporting slightly more grams of fiber than total carbohydrates is incorrect, there is no reason to believe that less-bothersome-looking labels are any more accurate.

Please note, however, that an honest difference is small, probably no more than a gram. If the difference is larger, they are likely "presubtracting" the fiber from the Total Carb reported. Clearly that is not in accordance with the regulations, but it is becoming more common as products are being marketed to lowcarbers.

Does standard error lead to any problems?

The effect of standard errors is particularly bothersome in those situations where one large number is subtracted from another. The small number remaining may be very inaccurate.

Calories

How are calories calculated?

Calories may be calculated using any one of several methods. The old-fashioned bomb calorimeter, one of the acceptable methods, is a poor model for the human body. Ideally, calories represent physiological energy -- the energy value remaining after digestive and metabolic losses are deducted from the gross energy. This type of calculation is called the Atwater system for determining energy values.

When you use the USDA Nutrient Database, Atwater values are what you get. In addition, alcohol is 6.93 calories per gram, coffee and tea are estimated from seeds and vegetables, respectively, and the values for fructose and sorbitol came from a different system.

However, for processed foods, manufacturers are permitted to calculate calories from the average values of 4-4-9 kcal/g for protein, carbohydrate, and fat, respectively. The only exception is that, optionally, they may subtract the insoluble fiber from this calculation. In other words, even though alcohol is not reported as a nutrient, its calories must be reported, as must be the 4 calories per gram imputed to soluble fiber, even though it is lumped with insoluble fiber in the nutrient section.

Unprocessed Foods

What about unprocessed foods?

Many companies use standard values that come from the USDA, their suppliers, etc. These standard values are often based on averages.

This can lead to even more discrepancies than for processed foods. For example, there are numerous varieties of apples and their nutrient content will vary among types, by season, possibly by storage method, and so forth. So the probablility that a particular apple has the nutrients listed in the standard is not as good as the probability that a serving of applesauce, made from many different apples, meets its standard.

Next: Nutrition Label Accuracy