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Food labels can be 20% off
The label is only part of the fuzziness. Food also has metabolizable energy, the portion the body can actually extract. Insoluble fiber is mostly indigestible because the carbohydrate and even some of the protein is bound up in plant structure that digestive enzymes can't reach, which adds bulk to stool and lowers the metabolizable energy. There is some evidence that people's gut microbiomes differ in how well they extract energy from fiber.
A label that is off is likely to be consistently off, so tracking still works if it is done the same way each time. Eventually the numbers show what a person is taking in. A budget is the comparison: nobody can know exactly what they earn when inflation and variable interest rates move the numbers, yet a budget still gives a reasonable idea and workable assumptions.
Why do food labels allow so much error?
US Food and Drug Administration rules treat a label as compliant if lab analysis finds calories no more than 20 percent above the declared value. Foods vary from batch to batch, and the Atwater factors behind label values are averages. Manufacturers often pick conservative numbers to stay inside the limit.
How much do restaurant meals deviate from stated calories?
In a 2010 study in the Journal of the American Dietetic Association, Lorien Urban and colleagues at Tufts measured the calories in 29 quick-serve and sit-down restaurant foods. The measured values averaged 18 percent higher than stated. Frozen supermarket meals averaged 8 percent higher. The study was small, and individual items ranged widely.
How many calories does fermentable fiber yield?
Gut bacteria ferment part of the fiber that reaches the colon and make short-chain fatty acids, which the body absorbs. This yields about 2 calories per gram, against 4 per gram for digestible carbohydrate. Insoluble fibers like cellulose ferment less and yield less still.
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