Eating

• 1. Introduction

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  Eating is the ingestion of solid food for digestion, and it sits at the center of a quiet biological emergency. Heterotrophic organisms cannot make their own nutrition the way autotrophs can. They must ingest external organic matter or they will not survive. That single constraint shapes everything that follows. A newborn human cannot manage solid food at all. A laboratory rat with the wrong neurons severed cannot approach a meal. A hormone you have never heard of can quietly tell your brain you are hungry. How did one survival task split into carnivores and herbivores, into banquets and hunger strikes, into the difference between a meal and a snack? And what is actually happening inside the body when a person decides it is time to start, and time to stop?

• 2. Forms Of Feeding

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  Carnivores and scavengers eat flesh, the meat of other animals, while herbivores and algivores turn to plants and algae. Omnivores consume a mixture of both plant and animal matter. Detritivores and coprophages eat detritus and feces, surviving on what other organisms leave behind. These categories describe the different forms of eating that animals have evolved. Each one is a strategy for solving the same heterotrophic problem from a different angle.

  Animals and phagotrophs eat and digest food internally, taking the organic matter inside the body before breaking it down. Decomposers such as fungi and microbes do the opposite. They secrete enzymes to digest organic matter externally, then absorb the nutrients that result. Because that digestion happens outside the organism, decomposers are not said to eat at all. The word draws a line, and fungi fall on the far side of it.

• 3. Meals And Banquets

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  For humans, eating becomes an activity of daily living, organized into routine sessions known as meals where proper courses of cooked food are consumed with a decent quantity of staples. Snacks are the looser cousin: small quantities of typically non-staple food, often convenience food or raw items like fruits and nuts, eaten for the purpose of degustation rather than to satiate hunger. The distinction is not about quantity alone. It is about why a person reaches for the plate.

  Formal sessions of eating, at a gathering or a date, are known as dining. Large dining events where many people are invited to eat together are banquets or feasts. The craft of preparing and presenting meals for formal eating is called culinary arts. The person who performs it is a cook, a chef, or a cater, depending on the nature of the food service. At their highest level of sophistication, the restaurants and food courts where people eat away from home become, in the source's striking phrase, "theatrical spectacles of global cosmopolitanism and myth." At picnics, potlucks, and food festivals, eating is no longer incidental to the gathering. It is the entire reason for it.

• 4. Rhythms And Postures

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  Doctors in the UK recommend three meals a day, each between 400 and 600 kcal, spaced four to six hours apart. Three well-balanced meals, with half the plate given to vegetables, a quarter to protein such as meat, and a quarter to carbohydrates such as pasta or rice, add up to roughly 1800 to 2000 kcal. That figure is the average requirement for a regular person. People usually have two or three meals a day, with smaller snacks consumed in between.

  Eating positions vary by region, because culture shapes the act itself. Across most Middle Eastern countries, eating while sitting on the floor is most common, and it is believed to be healthier than eating at a table. The Ancient Greeks preferred to eat reclining, at a celebration they called a symposium, and the Ancient Romans adopted the same custom. Ancient Hebrews took up the reclining posture too, using it for traditional celebrations of Passover. In jurisdictions under Sharia law, eating may be proscribed for Muslim adults during the daylight hours of Ramadan, a reminder that when a person eats can be as governed as what they eat.

• 5. Learning To Eat

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  Newborn humans cannot chew, so they survive solely on liquid breast milk or infant formula. Small amounts of puréed baby food are sometimes fed to babies as young as two or three months old. Most infants, though, do not eat anything solid until they are between six and eight months old. In many undeveloped countries, premastication serves as an improvised alternative: adults chew up food and then kiss-feed a softened bolus into the infant's mouth. The reason for all this liquid and semi-liquid feeding is simple. Young infants have few teeth and an immature digestive system.

  Between 8 and 12 months of age, the digestive system improves and deciduous teeth begin to emerge in the process known as teething. Many babies can then begin weaning and eating finger foods, though their diet stays limited because most still lack molars and canines and have only a few incisors. By 18 months, babies often have enough teeth and a mature enough digestive system to eat the same foods as adults. Mastering the act takes far longer. Learning to eat is a messy process, and children often do not manage neatness or eating etiquette until they are five or six years old.

• 6. The Emotional Plate

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  Compulsive overeating, also called emotional eating, is defined as the tendency to eat in response to negative emotions. The relationship between feeling and appetite is not uniform across people. Empirical studies have found that anxiety leads to decreased food consumption in people of normal weight, but increased consumption in the obese. The same emotion pushes two bodies in opposite directions.

  Laboratory studies have repeatedly shown that overweight individuals are more emotionally reactive and more likely to overeat when distressed than people of normal weight. Obese individuals were also found to experience negative emotions more frequently and more intensively. A naturalistic study by Lowe and Fisher compared normal and overweight female college students and confirmed the tendency to overeat, but with a twist. The pattern held only for snacks, not meals. Obese individuals did not eat more at mealtimes. Lowe and Fisher suggest the company explains it: meals are often eaten with others, and the presence of other people reduces distress, while snacks are usually eaten alone.

• 7. Start And Stop

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  Ghrelin is a hormone that increases appetite by signaling to the brain that a person is hungry, and it is released by the stomach to begin the whole cycle. Environmental signals join it: the smell or thought of food, the sight of a plate, the sound of someone talking about a meal. Then come the metabolic signals. As time passes between meals, the body pulls nutrients from long-term reservoirs. When the glucose levels of cells drop, called glucoprivation, hunger arises. A drop in cellular lipid levels, called lipoprivation, does the same. The brain monitors glucoprivation on its side of the blood-brain barrier, since glucose is its fuel, while the liver watches the rest of the body for both.

  Stopping a meal has its own machinery. Cholecystokinin, secreted by the duodenum, controls how fast the stomach empties and is thought to act as a satiety signal to the brain. Peptide YY 3-36, released by the small intestine, signals satiety too, and insulin in the blood tells the brain that nutrients are being absorbed. Long-term satiety comes from fat stored in adipose tissue, which secretes the hormone leptin to suppress appetite. Hormones including bombesin, neurotensin, anorectin, calcitonin, enterostatin, and corticotropin-releasing hormone have all been shown to suppress food intake. Eating rapidly leads to overeating and obesity, probably because the feeling of satiety can be slower to arrive. The source adds a practical note: cessation of eating within two hours of sleeping can reduce body weight.

• 8. Circuits And Disorders

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  Rats whose brain-stem motor neurons have been disconnected from the cerebral hemispheres, a procedure called decerebration, cannot approach and eat food, and must take it in liquid form instead. That experiment showed the brain stem itself plays a role in eating, because it holds neural circuits that read hunger and satiety signals from the rest of the body. The hypothalamus carries its own set of switches. Two peptides there produce hunger: melanin concentrating hormone, or MCH, and orexin. MCH plays the bigger role in hunger, and in mice a mutation causing its overproduction led to overeating and obesity. Orexin governs more of the relationship between eating and sleeping. Neuropeptide Y and agouti-related protein also induce eating, while leptin suppresses MCH and orexin and triggers satiety, aided by two more arcuate-nucleus peptides, CART and α-MSH.

  When these systems are disrupted, eating itself can become a disorder. Depression, food allergies, certain ingested chemicals, pituitary gland malfunction, and other endocrine problems can all distort appetite. In anorexia nervosa, people restrict calorie intake out of fear of gaining weight, and the resulting malnutrition drives an unhealthy weight that damages overall health. Bulimia involves recurrent binge eating, the consumption of a substantial amount of food in a short period, followed by maladaptive compensation such as induced vomiting, excessive physical activity, or laxatives. When eating and drinking are not possible at all, as often happens during recovery from surgery, the body is sustained through enteral nutrition or parenteral nutrition, two routes that keep a person nourished without a single bite ever passing the lips.