Short answers, pulled from the story.
The nine essential amino acids humans must obtain from their diet are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. The body cannot synthesize these on its own, and deficiency can lead to protein-energy malnutrition and death. Histidine was debated for years before the consensus settled on nine rather than eight essential amino acids.
US and Canadian guidelines recommend 46 grams per day for women ages 19-70 and 56 grams per day for men in the same range, based on 0.8 grams per kilogram of body weight. Active people and athletes may need 1.2-1.8 grams per kilogram depending on whether they do endurance or strength training.
The 2008 China milk scandal exploited the Kjeldahl protein testing method, which measures nitrogen rather than actual protein. The industrial chemical melamine was added to milk to artificially inflate the nitrogen reading and thus the apparent protein content. The limitations of nitrogen-based crude protein measurement made the adulteration difficult to detect through standard testing.
Crude protein is calculated by multiplying the total nitrogen in a food sample by 6.25, based on the average nitrogen content of proteins being about 16%. True protein measures only actual protein -- in dairy, by directly counting peptide bonds -- and excludes non-protein nitrogen sources like urea that have no nutritional value to humans. The United States, Australia, France, and Hungary have adopted true protein measurement as the standard for dairy payment.
Kwashiorkor is a severe form of protein deficiency characterized by apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs. The edema results from disrupted fluid balance and the action of lipoxygenase on arachidonic acid. Protein-energy malnutrition, of which kwashiorkor is the most severe form, accounts for six million deaths annually worldwide.
Plant protein foods contribute over 60% of the per capita protein supply on a worldwide basis. The pattern differs sharply in North America, where animal-derived foods account for about 70% of protein intake. Insects are also a significant protein source globally, with more than two billion people eating them daily.