Nutrition

• 1. Introduction

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  Nutrition is the biochemical and physiological process by which an organism uses food and water to support its life. Roughly 30 elements turn up in organic matter, but three carry the heaviest weight: nitrogen, carbon, and phosphorus. Every living thing, from a geothermal-spring bacterium to a human child, must somehow gather these and turn them into energy and structure. Too much of an essential nutrient causes malnutrition. So does too little. How does a plant pull food from dirt and air with no mouth? Why does a cat sicken on a diet that would keep a dog healthy? And how did a process older than life on land become a hard science only in the last hundred years? Those questions sit at the center of what follows.

• 2. The Chemistry Of Eating

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  Approximately 30 elements appear in organic matter, and nutrients are the substances built from them that let an organism survive, grow, and reproduce. They split into two scales. Macronutrients are the primary substances an organism needs in bulk, while micronutrients are needed only in trace amounts. Organic micronutrients are classified as vitamins, and inorganic ones as minerals.

  Essential nutrients have a sharper definition: the body cannot make them on its own. A nonessential nutrient, by contrast, the organism can synthesize internally. This single distinction shapes every diet on Earth, because it decides what an organism is forced to hunt for and what it can build for itself.

  Once absorbed, nutrients enter a chain of metabolic biochemical reactions inside the cell. Fueling reactions create precursor metabolites and energy. Biosynthetic reactions convert those precursors into building-block molecules. Polymerizations combine the blocks into macromolecule polymers, and assembly reactions use those polymers to construct cellular structures. The food disappears into the machinery that the food itself built.

  The same chemistry has a dark side at scale. Over-nutrition of macronutrients is a major cause of obesity and raises the risk of various non-communicable diseases, including type 2 diabetes, stroke, hypertension, coronary heart disease, osteoporosis, and some cancers. The line between fuel and harm is a matter of dose.

• 3. Counting Calories

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  One Calorie is the amount of energy required to raise 1 kilogram of water by 1 degree Celsius, and that humble unit governs the entire economy of animal life. All macronutrients except water can be burned for energy, though that is never their only job. The numbers attached to each are precise and unequal.

  Carbohydrates contain 4 calories per gram and store significant amounts of energy. Plants typically synthesize them during metabolism, so animals, with only a limited ability to generate carbohydrates, must gather most from nature. Broken down, they yield glucose, the simplest carbohydrate, and short-chain fatty acids. For herbivorous land animals they are the most abundant nutrient of all.

  Fats contain 9 calories per gram, more than double the carbohydrate yield, and they are not soluble in water. That density lets lipids store energy for an extended period. Most dietary lipids are triglycerides, made of glycerol and fatty acids, with phospholipids and sterols present in smaller amounts. The body responds to supply: it makes fewer fatty acids as dietary fat rises, and more as carbohydrate intake rises.

  Protein also carries 4 calories per gram, but burning it for fuel wastes its talents. Consumed protein breaks down into amino acids, which the body reassembles into new proteins for cellular structures, fluids, and enzymes. Those enzymes are essential to most metabolic processes, and to DNA replication, repair, and transcription.

• 4. Many Ways To Feed

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  Heterotrophs obtain nutrients by consuming the carbon of other organisms, while autotrophs produce their own from inorganic carbon like carbon dioxide. Between them sit the mixotrophs, organisms that can do both, including some plankton and carnivorous plants. The classification does not stop at carbon.

  Phototrophs draw energy from light, while chemotrophs pull it from the chemical energy stored in matter. A separate split tracks where an organism gets its electrons. Organotrophs take electrons from other organisms; lithotrophs strip them from inorganic substances such as water, hydrogen sulfide, dihydrogen, iron(II), sulfur, or ammonium. A final pair, prototrophs and auxotrophs, divides those that can build essential nutrients from other compounds from those that must consume them preexisting.

  Foraging is the process of seeking out nutrients in the environment, and it too comes in flavors. Animals and bacteria navigate toward food, while plants and fungi extend outward to reach it. The search may be random, with no method, or systematic, going directly to a known source. Optimal foraging theory models this as a cost-benefit analysis, in which an animal maximizes nutrient gain while minimizing the time and energy spent. It was built for animals but stretches to other organisms, and it sorts the living world into specialists adapted to a single food and generalists able to eat many.

• 5. When Nutrients Run Short

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  Malnutrition is the name for nutrient deficiency, the state of an organism that lacks the nutrients it needs. It is not the same as a nutrient inadequacy, which sits above deficiency but below the recommended dietary level, producing hidden symptoms that are hard to identify. Deficiency can come from a sudden drop in intake or from an inability to absorb essential nutrients, and it can arise from other illnesses rather than from diet alone.

  A starving organism does not simply fail. It adapts by reducing energy consumption and expenditure to stretch its stored nutrients, drawing on energy reserves until they are depleted. A lack of just one essential nutrient can cause bodily harm, and an overabundance can cause toxicity, so a balanced diet must hold appropriate amounts of all essential and non-essential nutrients. Those amounts shift with age, weight, sex, and physical activity levels.

  Guardrails exist to keep most people inside the safe range. The Daily Reference Values keep the majority of people from nutrient deficiencies. DRVs are not recommendations but a combination of nutrient references that tell professionals and policymakers the maximum and minimum intakes for an average person. Food labels lean on the same values to set safe nutritional guidelines for the average healthy person.

• 6. A Science Born In Revolution

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  Scientific analysis of food and nutrients began during the chemical revolution in the late 18th century, when chemists started experimenting with elements and food sources to build theories of nutrition. That work ran through the 18th and 19th centuries, but modern nutrition science did not truly begin until the 1910s, when individual micronutrients started to be identified.

  The first vitamin to be chemically identified was thiamine, in 1926. Six years later, in 1932, vitamin C was identified as a protection against scurvy, and the role of vitamins in nutrition was studied across the following decades. The science had a social engine behind it. The first recommended dietary allowances for humans grew out of fears of disease caused by food deficiencies during the Great Depression and the Second World War.

  For humans specifically, the timeline reaches far deeper. Early human nutrition meant foraging, like any other animal, until it diverged at the start of the Holocene with the Neolithic Revolution, when humans developed agriculture. The Chemical Revolution of the 18th century let people study the nutrients in food and refine its preparation. In the 20th century, advances in economics and technology brought mass production and food fortification to better meet human needs.

• 7. Life's Recycling System

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  A nutrient cycle is a biogeochemical cycle, the movement of inorganic matter through soil, organisms, air, or water, where it is exchanged in organic matter. It runs on an asymmetry: energy flow is a unidirectional, noncyclic pathway, while the movement of mineral nutrients is cyclic. Energy passes through and is gone, but minerals come back around.

  The living planet runs several of these loops at once. The biogeochemical cycles performed by organisms and natural processes include the water, carbon, nitrogen, phosphorus, and sulfur cycles, alongside others such as the oxygen cycle. Each returns its essential element to the environment after it has been absorbed or consumed, so the same atoms can be used again.

  The stakes of keeping these loops turning are systemic. Without proper nutrient cycling, there would be a risk of change in oxygen levels, climate, and ecosystem function. The cycles are what stop the world from running down.

• 8. The Strategies Of Organisms

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  Animals are heterotrophs that consume other organisms, and they sort by what they eat: herbivores eat plants, carnivores eat other animals, and omnivores eat both. The strategy carries hidden costs. Many herbivores rely on bacterial fermentation to wring digestible nutrients from indigestible plant cellulose, while obligate carnivores must eat meat to get vitamins and nutrients their bodies cannot synthesize. Much of animal behavior bends to this need. Migration and seasonal breeding track food availability, courtship displays advertise health, and through conditioned food aversion an animal can instinctively avoid a food that once caused toxic injury.

  Most plants take their nutrients from inorganic substances in the soil and atmosphere, including carbon, hydrogen, oxygen, nitrogen, and sulfur. Their roots pull cations from the soil through cation exchange, pumping hydrogen ions into the soil with proton pumps to displace cations from negatively charged particles. In the leaves, stomata open to take in carbon dioxide and release oxygen. Atmospheric nitrogen is plentiful but unusable to almost all plants, so bacteria perform nitrogen fixation to turn inert atmospheric nitrogen into biologically usable forms in the soil. Since none of this supplies energy, green plants gather sunlight in chloroplasts and convert it through photosynthesis.

  Fungi are chemoheterotrophs that feed on external matter, most of them absorbing it through a root-like mycelium that can extend indefinitely. The fungus secretes extracellular enzymes to break down surrounding matter, then absorbs the nutrients through its cell wall. Its way of life can be parasitic, feeding on living hosts, saprophytic, feeding on dead and decomposing organisms, or symbiotic, exchanging nutrients with a partner. Specialized pet food, manufactured since 1860, brought this logic into human care: cats are sensitive to nutrients such as taurine and need extra ones from meat, and large-breed puppies risk overnutrition because small-breed food is more energy dense than they can absorb.