The protein figure on a food label is really a nitrogen count
Nobody weighs the protein in your breakfast cereal directly. Standard tests measure the total nitrogen in a sample and multiply it by 6.25, since proteins average about 16 percent nitrogen. It is a clever shortcut with one serious weakness: any nitrogen-rich chemical, protein or not, pushes the number up.
The trick works because fat, carbohydrate and fibre contain no nitrogen, leaving protein as the only big nitrogen source in most foods. The classic procedure, the Kjeldahl method, was adopted by AOAC International and is used by food standards agencies worldwide; a rival approach, the Dumas method, is also approved by some bodies. What neither can do is tell real protein from other nitrogen compounds, which occur naturally in milk, fish and edible insects.
That blind spot has been exploited. Buyers of protein meal routinely screen for urea and ammonium nitrate, cheap additives that fake a higher reading. In 2007, contaminated Chinese protein exports, and in 2008 the China milk scandal, involved melamine, an industrial chemical added to milk or gluten precisely because it inflated the measured protein. Partly in response to such problems, dairy testing in the United States, Australia, France and Hungary now relies on true protein, detected through peptide bonds, the links that join amino acids together. Several grain-exporting countries use near-infrared scanning for the same purpose.
The distinction matters because the body cares about amino acids, not nitrogen. Humans cannot make nine of them, including leucine, lysine and tryptophan, and must get them from food. Digestion begins in the stomach, where acid converts pepsinogen into the enzyme pepsin, and continues in the small intestine. Chains of more than four amino acids are mostly not taken up by the gut lining. Newborn mammals are the exception: their intestines take in whole proteins, letting antibodies pass from mother to infant through milk.
Where people get protein varies enormously. Worldwide, plant foods supply more than 60 percent of it, while in North America animal products provide about 70 percent. More than 2 billion people eat insects daily, and in parts of Africa insects supply up to half of dietary protein. At the experimental frontier, microbes fed on carbon dioxide and electricity from solar panels are grown in vats and dried into protein powder, using very little land, water or fertiliser.
Source: Protein (nutrient)
