Protein (nutrient)

• 1. Introduction

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  Protein is the second most abundant molecule in the human body, trailing only water. Every cell contains it. Hair, skin, blood, bone, muscle -- none of these structures exist without protein holding them together and keeping them running. Enzymes that speed up chemical reactions are proteins. So are antibodies, hormones, and the channels built into cell membranes. Without protein, life as we know it stops.

  Yet for all its importance, protein is often reduced to a number on a nutrition label or a scoop of powder in a gym bag. The real story is stranger and richer. Nine amino acids that the human body cannot make on its own must arrive through food or the consequences are severe -- and one of those nine, histidine, was debated for years before scientists agreed it belonged on the essential list. The way food companies measure protein in their products has been at the center of international food safety scandals. And the question of how much protein an individual actually needs remains, in the words of researchers, unresolved.

  This documentary travels through the biology of protein, its sources across the globe, the methods used to test it, and the ways its absence -- or excess -- shapes human health.

• 2. What Protein Does

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  Muscle is the most obvious home for protein in the body, but it is far from the only one. Cellular messengers called hormones are built from protein. Transport molecules that carry materials through the bloodstream are built from protein. Glycoproteins, G proteins, and ion channels -- all proteins -- form the working components of cell membranes. Enzymes and antibodies, essential to digestion and immune defense, are proteins as well.

  The precise shape of any given protein is not random. The types of amino acids it contains, and the order in which they are arranged in the chain, determine the unique three-dimensional structure the protein will fold into. That structure, in turn, determines what the protein can do. A slight change in sequence can produce an entirely different function -- or a non-functional molecule.

  Proteins also feed a secondary manufacturing process inside the body. When proteins are broken down through catabolism, the released amino acids do not simply go to waste. They become raw material for non-protein molecules that are equally essential to life: nucleotides, certain neurotransmitters, and heme -- the iron-containing compound at the core of hemoglobin.

• 3. Where Protein Comes From

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  On a worldwide basis, plant foods supply more than 60% of the protein people eat. The picture shifts sharply in North America, where animal-derived foods account for about 70% of protein intake. Neither pattern reflects the full range of human food cultures, and a third source -- insects -- is easily overlooked by people in wealthier countries.

  More than two billion people eat insects daily. In parts of Africa, insects supply up to 50% of dietary protein. Protein powders made from cricket flour sit alongside more familiar options like casein, whey, egg, rice, and soy in the processed supplement market.

  Among plant sources, the concentrations run higher than most people expect. Soybeans, lentils, kidney beans, mung beans, chickpeas, cowpeas, lupines, and wing beans all carry protein concentrations above 7%. The same threshold is met by a long list of nuts and seeds: almonds, Brazil nuts, cashews, pecans, walnuts, cotton seeds, pumpkin seeds, hemp seeds, sesame seeds, and sunflower seeds. Among cereal staples, buckwheat, oats, rye, millet, maize, rice, wheat, sorghum, amaranth, and quinoa each exceed that 7% mark.

  An emerging source sits outside traditional agriculture entirely. Photovoltaic-driven microbial protein production uses electricity from solar panels and carbon dioxide drawn from the air to feed microbes grown in bioreactor vats. The resulting dry protein powder makes highly efficient use of land, water, and fertilizer -- a consideration that grows more relevant as conventional agriculture strains to meet global demand.

• 4. The Measurement Problem

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  The classic tests for protein in food -- the Kjeldahl method and the Dumas method -- do not actually measure protein. They measure nitrogen. The reasoning is straightforward: fat, carbohydrate, and dietary fiber contain no nitrogen, so in most foods, nitrogen signals protein. Multiply the nitrogen content by a factor based on the expected protein type and you arrive at a figure known as "crude protein".

  On food labels, the standard conversion multiplies nitrogen by 6.25, because the average nitrogen content of proteins is about 16%. The Kjeldahl method is the most widely used because AOAC International, the body that sets global food testing standards, adopted it. But the system has a fundamental flaw: it cannot distinguish real protein from non-protein nitrogen, or NPN.

  NPN occurs in significant amounts in milk, edible insects, and fish. More troublingly, deliberate adulteration of protein meals with NPN compounds that inflate crude protein readings has been documented in the food industry for decades. Quality control checks for urea and ammonium nitrate are now standard practice among buyers of protein meals as a result.

  The limitations of the Kjeldahl method were central to two separate food safety crises originating in China. The 2007 Chinese protein export contamination and the 2008 China milk scandal both involved the industrial chemical melamine, which was added to raise the apparent protein reading without adding any actual protein.

  In response to these limitations, several countries have moved toward "true protein" measurement. The United States, Australia, France, and Hungary have adopted it as the payment standard for dairy. Rather than measuring nitrogen, true protein measurement in milk directly counts peptide bonds. For grain testing, Canada, the UK, Australia, Russia, and Argentina use near-infrared reflectance technology -- a form of infrared spectroscopy -- to achieve the same goal.

• 5. Digestion and Absorption

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  Protein digestion begins in the stomach. The enzyme pepsinogen is converted to its active form, pepsin, by the action of hydrochloric acid. Pepsin starts breaking long protein chains into smaller fragments. The process continues in the small intestine, where trypsin and chymotrypsin take over.

  By the time most proteins reach the absorptive cells lining the small intestine, they have been reduced to single amino acids or short peptide chains. Peptides longer than four amino acids are generally not absorbed. Even those that do enter the intestinal cells are typically broken down further into individual amino acids before passing into the bloodstream.

  The rate at which this happens varies considerably depending on the protein source. Studies in humans have documented measurable differences between soy and milk proteins in how quickly and completely their amino acids are absorbed. Among milk proteins, about 50% of ingested protein is absorbed between the stomach and the jejunum -- a section of the small intestine -- and roughly 90% is absorbed by the time digested food reaches the ileum, further along.

  Newborn mammals represent a striking exception to adult protein digestion. At birth, the small intestine can absorb intact proteins whole -- a temporary capacity that serves a crucial purpose. It allows immunoglobulins, the antibody proteins carried in breast milk, to transfer directly from mother to infant, conferring passive immunity before the newborn's own immune system matures.

• 6. How Much Is Enough

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  US and Canadian Dietary Reference Intake guidelines set the recommended daily allowance at 46 grams for women ages 19-70 and 56 grams for men in the same age range. These figures were calculated using a base rate of 0.8 grams of protein per kilogram of body weight, applied to average body weights of 57 kilograms for women and 70 kilograms for men.

  Actual consumption in the United States runs well above those recommendations. Data from the National Health and Nutrition Examination Survey for 2013-2014 found that women 20 and older averaged 69.8 grams per day, while men averaged 98.3 grams.

  The RDA figures address structural needs but do not account for protein used in energy metabolism -- a meaningful gap for people who exercise. Research suggests endurance athletes working out for two to five hours per training session use protein for 5-10% of their total energy expenditure. For them, one review recommended a maximum intake of 1.2-1.4 grams per kilogram of body weight daily. Strength-training athletes face a different demand: building and maintaining greater muscle mass, which may push requirements to 1.4-1.8 grams per kilogram. A proposed ceiling for daily protein intake sits at approximately 2 to 2.5 grams per kilogram, or roughly 25% of total energy intake.

  The requirements shift further for specific life stages. Childhood growth, pregnancy, breastfeeding, recovery from malnutrition, and healing after surgery or injury all raise protein needs above sedentary adult baselines.

• 7. Deficiency, Excess, and Special Cases

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  Protein-energy malnutrition, or PEM, accounts for six million deaths annually worldwide. It affects children and adults and appears across both low-income and industrialized countries, where hospitals and the elderly population carry a disproportionate burden.

  The most recognizable severe form of protein deficiency is kwashiorkor. Its symptoms include apathy, diarrhea, failure to grow, flaky skin, fatty liver, and edema -- fluid accumulation -- in the belly and legs. The edema results from disrupted fluid balance when proteins that normally manage fluid transport are absent, and from the action of the enzyme lipoxygenase on arachidonic acid to form compounds called leukotrienes. Untreated, PEM can cause intellectual disability.

  At the other end of the spectrum, the US and Canadian Dietary Reference Intake review found insufficient evidence to set an upper limit for safe protein consumption. But excess protein is not without consequences. When amino acids exceed the body's needs, the liver deaminates them -- stripping out their nitrogen and converting it to ammonia, then to urea, which the kidneys excrete. The leftover carbon skeletons can be converted to glucose. Excess protein may also increase calcium excretion in urine, raising the risk of kidney stones.

  People with chronic kidney disease are advised to reduce protein intake. A 2009 review, updated in 2018, found that doing so lowered the likelihood of progressing to end-stage kidney disease. A low-protein intake of 0.6-0.8 grams per kilogram per day may help preserve remaining kidney function, though malnutrition is a risk at that level.

  For individuals with phenylketonuria, or PKU, the essential amino acid phenylalanine must be kept extremely low to prevent intellectual disability. Because phenylalanine is a component of the artificial sweetener aspartame, people with PKU must avoid low-calorie foods and beverages that carry this ingredient -- a constraint that extends well beyond whole protein foods into the processed food supply.