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Food labels can be 20% off
Insoluble fiber is mostly indigestible, because carbohydrate and even some protein sit bound up in plant structure where digestive enzymes cannot reach them. That bulks up stool and lowers the metabolizable energy in the food. There is some evidence that people with different gut microbiomes extract different amounts of energy from fiber.
Label error does not make tracking pointless. An error is typically consistent, so a person who tracks consistently can eventually learn roughly what they are taking in.
The energy-out side is more complicated still. Resting metabolic rate is the biggest bucket, at 50 to 70 percent of daily expenditure for most people, higher in sedentary people. The thermic effect of food adds about 5 to 10 percent, and physical activity covers the rest.
Why can food labels be off by 20 percent?
In the United States, the Food and Drug Administration lets the actual calorie content of a product exceed the declared amount by up to 20 percent before it counts as misbranded. Label values are usually calculated from general conversion factors worked out by the chemist Wilbur Atwater in the late 1800s, not measured in each product. Those factors assume an average digestion for each nutrient type. Real foods and real digestive systems vary around that average.
How does plant structure hide calories from digestion?
Plant cell walls are built from cellulose and other tough polymers that human enzymes cannot break down. They act like cages that keep some of the fat, starch, and protein inside from being reached. A US Department of Agriculture study led by David Baer found that whole almonds delivered fewer calories than standard factors predicted, partly for this reason. Grinding or cooking breaks the cages open and raises the energy available.
Why does resting metabolism take such a large share?
Some organs are small but burn a great deal of energy even at rest. The brain uses about 20 percent of resting energy while making up around 2 percent of body weight. The liver, heart, and kidneys also run constantly and account for much of the rest. Skeletal muscle at rest uses far less per pound than those organs.
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