Health: quotes, facts and reflections - Page 3

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Delayed-onset muscle soreness, the ache that follows hard or unfamiliar exercise, usually peaks 24 to 72 hours later. It is caused mostly by movements in which a muscle lengthens while working against a load, such as walking downhill or slowly lowering a weight.

Movements in which a muscle lengthens under load are called eccentric contractions. The leading explanation is that they cause tiny damage in the muscle fibres, and the ache comes from the repair and inflammation that follow. The American physiologist Theodore Hough first described this delayed soreness in 1902. The popular idea that it is caused by lactic acid is wrong: lactic acid returns to normal within about an hour of exercise, long before the soreness starts. The pain usually fades within a week. The muscle also adapts quickly, so the same workout causes much less soreness the next time, which is known as the repeated-bout effect. Soreness is therefore not a measure of a good workout, and a session that leaves you pain-free may still have done its job.

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The adult heart beats roughly 100,000 times a day at a resting rate of about 70 beats a minute. Each beat pushes out around 70 millilitres of blood, which adds up to about 7,000 litres a day at rest, and more during exercise.

The heart is a muscle about the size of a fist, weighing 250 to 350 grams, with four chambers: two atria that receive blood and two ventricles that pump it out. The right side sends blood to the lungs to collect oxygen; the left side sends it round the rest of the body. Each beat is started by the sinoatrial node, a small cluster of cells that acts as the heart's natural pacemaker. It fires an electrical signal that spreads through the heart muscle and makes it contract. The rate changes all day: it falls during sleep and rises with exercise, stress, fever and illness. Over an 80-year life, 100,000 beats a day adds up to nearly three billion heartbeats.

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Water makes up about 60 percent of an average adult's body weight, and around two thirds of it is inside the body's cells. The share is up to 75 percent in newborn babies and lower in people with more body fat, because muscle holds far more water than fat does.

In a 72 kg adult that is about 40 litres of water. Roughly 25 litres sits inside cells; the other 15 are outside them, in the fluid that bathes the tissues and in blood plasma, the liquid part of blood, which holds only about 3 litres. Muscle is about 75 percent water and fat tissue only about 10 percent, so the share varies with build: one large study measured an average of 58 percent in men and 48 percent in women, who on average carry more body fat. Salts help decide where the water sits: sodium holds it mainly outside cells and potassium inside them, and water moves across cell membranes to keep the balance.

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Mild dehydration, a fluid loss of about 1 to 2 percent of body weight, can bring tiredness, headache and low mood, and in some tests it makes people less accurate at attention tasks. The effects show most clearly in the heat or during exercise.

Two controlled studies at the University of Connecticut, published in 2011 and 2012, made healthy young adults slightly dehydrated by having them walk on a treadmill, in some sessions after taking a diuretic, a pill that makes the kidneys pass more water. In 25 women, a loss of about 1.4 percent of body weight lowered mood, made tasks feel harder and brought on headaches and poorer concentration, although most thinking tests were unaffected. In 26 men, a loss of about 1.6 percent increased fatigue and tension and led to more errors on tests of attention and working memory, the short-term mental notepad we use to hold information while using it. For a 70 kg person, 1.5 percent is about one litre, an amount that can be lost as sweat in an hour of hard exercise in the heat.

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The gut has its own network of around 500 million nerve cells in its walls, called the enteric nervous system. It runs digestion largely by itself, moving food along and controlling digestive juices, while staying in constant contact with the brain.

The network runs from the oesophagus to the anus, and it can keep working even when the vagus nerve, the main nerve linking gut and brain, is cut. That independence is why the American scientist Michael Gershon of Columbia University titled his 1998 book about it The Second Brain. The gut uses many of the same chemical messengers as the brain: about 90 percent of the body's serotonin is made there, where it helps drive the movements of the intestines. Signals travel both ways, through nerves, hormones and the immune system. This two-way traffic, known as the gut-brain axis, helps explain why worry can be felt as butterflies or an upset stomach.

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Humans lack the enzymes to digest many kinds of dietary fibre, so it reaches the large intestine intact. There, gut bacteria ferment some of it into short-chain fatty acids, small molecules that feed the cells lining the colon and send signals to the rest of the body.

Fibre is the part of plant foods, such as whole grains, beans, lentils, vegetables, fruit, nuts and seeds, that our own digestive enzymes cannot break down. The three main short-chain fatty acids that bacteria make from it are acetate, propionate and butyrate. Butyrate is the main source of energy for the cells lining the colon, and these acids also make the colon slightly more acidic and influence blood sugar, cholesterol and immune signals. Researchers are also studying links between the gut and the brain, but it is too early to treat a bowl of oats as a mood treatment. UK guidelines recommend 30 grams of fibre a day for adults, while most adults eat only about 20 grams.

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The familiar guideline of about 0.8 g of protein per kilogram of body weight a day is set for whole populations, not tailored to individuals. EFSA, the EU's food safety agency, uses 0.83 g for healthy adults, the UK about 0.75 g and the US 0.8 g: roughly 53 to 58 g a day for a 70 kg adult.

These figures are set so that they cover the needs of nearly all healthy people, about 97 or 98 in every 100, and they assume that the diet provides enough energy overall. Needs are higher in pregnancy and while breastfeeding. An international expert group, PROT-AGE, advised in 2013 that people over 65 should aim for at least 1.0 to 1.2 g per kilo to help protect their muscles. People who train regularly for strength or endurance may benefit from about 1.2 to 2.0 g per kilo. Some kidney and liver conditions call for different amounts, so anyone with a long-term illness should ask a doctor or dietitian rather than rely on an online calculator.

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In a 2019 study at the US National Institutes of Health, 20 adults ate an ultra-processed and an unprocessed diet for two weeks each. The meals offered the same calories, sugar, fat, salt and fibre, yet on the ultra-processed diet people ate about 500 more calories a day and gained weight.

'Ultra-processed' comes from the Nova system, first proposed in 2009 by researchers led by Carlos Monteiro at the University of São Paulo in Brazil. It describes industrial products made mostly from refined ingredients and additives rarely used in home cooking, such as flavourings, emulsifiers and modified starches: many packaged snacks, sweetened cereals, ready meals and soft drinks. In the trial, led by the NIH scientist Kevin Hall, the volunteers lived at the NIH Clinical Center in Maryland for four weeks, so everything they ate could be measured, and they could eat as much or as little as they liked. On the ultra-processed diet they ate about 508 extra calories a day and gained about 0.9 kg; on the unprocessed diet they lost about the same. Why these foods lead people to eat more is still being studied.

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Fullness builds from several signals at once, from the stretching stomach, gut hormones, nutrients and the brain, and they begin to change while the meal is still under way. There is no fixed rule that the brain only learns you are full 20 or 30 minutes later.

Physiologists distinguish satiation, the process that makes you stop eating, from satiety, the fullness that lasts until the next meal. As the stomach stretches, sensors in its wall send messages along the vagus nerve to the hypothalamus, the brain's appetite centre. When food reaches the small intestine, the gut releases hormones such as cholecystokinin (CCK) and GLP-1, which slow digestion and add to the feeling of fullness; GLP-1 is the hormone copied by weight-loss drugs such as semaglutide. Eating slowly may help: in a 2008 US study, 30 women ate about 67 fewer calories when they ate the same meal slowly, and felt fuller at the end. The effect varies from person to person.

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Skin makes vitamin D when the sun's ultraviolet B (UVB) rays reach it. At UK latitudes, the NHS says, the sun is only strong enough for this from about late March or early April to the end of September, so autumn and winter sunshine cannot be relied on.

In winter the sun sits low in the sky, so its light passes through more of the atmosphere and most of the UVB is filtered out before it reaches the ground. Vitamin D helps the body control calcium and phosphate, which keep bones, teeth and muscles healthy; a serious lack can cause rickets in children and bone pain from softened bones in adults. The NHS therefore advises everyone, including pregnant and breastfeeding women, to consider a daily 10-microgram supplement in autumn and winter. People who rarely go outdoors, usually cover most of their skin or have dark skin are advised to take one all year, as are babies and young children in most cases. More is not better: adults should not take more than 100 micrograms a day.

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Box breathing is a simple calming exercise: breathe in for a count of four, hold for four, breathe out for four and hold again for four before starting over. It slows the breath and gives the mind a steady rhythm to follow.

The four equal steps are why it is also called square breathing: each count is one side of the box. It resembles the breath-control exercises of yoga and is taught to US Navy SEALs, the navy's special forces, as a way to stay calm under pressure. Slow breathing tends to lower the heart rate by shifting the autonomic nervous system, the part that runs the body without conscious effort, towards its calming 'rest and digest' mode. A 2023 Stanford University study had volunteers spend five minutes a day for a month on one of three breathing exercises, box breathing among them, or on mindfulness meditation. The breathing groups gained more in positive mood than the meditation group, and the exercise built around long, slow breaths out did best. If holding your breath feels uncomfortable, shorten the holds rather than forcing the count.

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The 5-4-3-2-1 exercise is a simple way to come back to the present: name five things you can see, four you can physically feel, three you can hear, two you can smell and one you can taste. If a step isn't available or feels uncomfortable, skip it and stay with something you can notice.

Exercises like this are called grounding techniques. Therapists teach them to people who struggle with panic, spiralling anxious thoughts, flashbacks after a trauma or dissociation, the detached feeling that you or the world around you are not quite real. The idea is simple: attention is limited, and deliberately noticing concrete things through your senses draws it away from worrying about the future or replaying the past. Counting down also gives the mind a small task with a clear end. Few formal studies have tested this exact sequence, so it is best seen as a practical way to get through a difficult moment rather than a treatment. If anxiety is frequent or hard to manage, talking to a GP or a therapist can help.

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Progressive muscle relaxation works through the body one muscle group at a time: you tense the muscles for a few seconds, then release them and notice the change. Repeating this in order, from the feet up to the face or the other way round, teaches you to recognise tension and let it go.

The technique was developed by the American physician Edmund Jacobson, who first presented it at Harvard University in 1908 and described it in detail in his 1929 book Progressive Relaxation. His original method needed weekly lessons and daily practice. The South African psychiatrist Joseph Wolpe later shortened it to about 20 minutes and used it to help patients face their fears step by step, and many shorter versions followed. The idea behind it is that stress and anxiety tighten the muscles, often without our noticing, and learning the feel of real release helps reverse that. It has been used for anxiety, stress, insomnia, low mood and pain, needs no equipment and can be done lying down or sitting in a chair.

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Skin is the body's largest organ, covering 1.5 to 2 square metres (16 to 22 sq ft) in an average adult. On the forearm it is only about 1.3 millimetres thick, yet it keeps water in and germs out and helps control body temperature.

Skin has three main layers. The outer layer, the epidermis, is a waterproof barrier made of flattened cells filled with a tough protein called keratin. New cells form at its base and move up to the surface, and the whole layer is replaced about every seven weeks. Beneath it, the dermis holds blood vessels, nerve endings, hair roots and sweat glands. The deepest layer, the hypodermis, stores fat that cushions and insulates the body. Skin senses touch, pressure, heat and pain, and it makes vitamin D when the sun's UVB rays reach it. Sweating, and the widening or narrowing of blood vessels near the surface, help keep body temperature steady.

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The stomach produces hydrochloric acid, giving its contents a pH of roughly 1.5 to 3.5, on a scale where 7 is neutral and lower numbers are more acidic. That is acidic enough to help break down food and kill many of the microbes swallowed with it.

The acid is made by parietal cells in the stomach lining, and an adult stomach releases about 1.5 litres of gastric juice a day. The acid unfolds proteins and turns an inactive substance, pepsinogen, into pepsin, the enzyme that starts protein digestion. Other cells coat the lining with thick mucus and bicarbonate, a natural antacid, so the acid does not reach the wall; drugs such as aspirin can weaken this barrier. Early understanding of gastric juice owed much to an accident. In 1822 a young fur trader, Alexis St. Martin, was accidentally shot on Mackinac Island in Michigan, and his wound healed leaving an opening into his stomach. The army surgeon William Beaumont studied digestion through it for years, and his 1833 book showed that gastric juice breaks food down chemically.

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Exercise sets off chemical signals that tell muscles to adapt, but the repair work, building new muscle protein and refilling energy stores, carries on for hours and days afterwards. That is why rest days are part of training rather than a break from it.

After resistance training, the rate at which muscles build new protein stays raised for up to about two days. Muscles also refill their store of glycogen, the form in which they keep carbohydrate as fuel, and after hard exercise that can take a day or more. Sports scientists describe the pattern with a model called supercompensation, set out by the Soviet scientist Nikolai Yakovlev between 1949 and 1959: fitness dips just after a hard session, climbs back to its starting level during recovery, then rises for a while above it. Train hard again too soon and the next dip begins before recovery is complete; wait too long and the gain fades. Enough food, especially protein and carbohydrate, and good sleep support these processes.

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Wound healing moves through four overlapping stages: clotting to stop the bleeding, inflammation to clean the area, growth of new tissue to fill the gap, and remodelling, in which the new tissue slowly strengthens and settles into a scar.

Within minutes of an injury, platelets, tiny cell fragments in the blood, stick to the damaged vessel and, together with a protein mesh called fibrin, form a clot. In the inflammation stage, white blood cells clear away bacteria, debris and dead cells, which is why a fresh wound can look red, warm and swollen. Next, new blood vessels grow, cells called fibroblasts lay down collagen, the protein that gives skin its strength, and skin cells creep across to close the surface. In the final stage the collagen is reorganised and the scar slowly fades. This can last a year or longer, and even then a scar reaches only about 80 percent of the strength of unbroken skin.

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Short naps of about 10 to 20 minutes improve alertness and performance for many people. Because they usually end before deep sleep begins, you are less likely to wake up groggy than after a longer nap.

The grogginess after waking from deep sleep is called sleep inertia; it usually lasts 15 to 30 minutes but can go on longer. In a 2006 study at Flinders University in Australia, researchers compared afternoon naps of 5, 10, 20 and 30 minutes after a short night's sleep. The 10-minute nap gave the quickest and best boost; the longer naps caused grogginess at first, and their benefits only showed later. In a NASA study of long-haul airline pilots in the early 1990s, pilots given a planned 40-minute rest in the cockpit slept for 26 minutes on average and were more alert later in the flight. Longer naps are not harmful in themselves, but timing matters: a nap late in the afternoon can make it harder to fall asleep at night.

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The talk test judges exercise intensity by your breath: at moderate intensity you can talk but not sing, and at vigorous intensity you cannot say more than a few words without pausing for breath. It needs no equipment, so you can use it on any walk, ride or run.

The US Centers for Disease Control and Prevention (CDC) recommends the talk test because anyone can use it. Research has found that the point where comfortable talking becomes difficult lies close to the ventilatory threshold, the effort level at which breathing starts to speed up sharply, so it is a reasonable stand-in for measured intensity. The NHS and the World Health Organization advise adults to do at least 150 minutes of moderate activity a week, or 75 minutes of vigorous activity, or a mix of the two. The test is not the same as 'Zone 2', a popular training target, usually a steady and fairly easy effort, that coaches define in different ways: by heart rate, by the level of lactate in the blood or by breathing thresholds.

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A resting adult breathes about 12 to 15 times a minute, which adds up to roughly 20,000 breaths a day. Each quiet breath moves about half a litre of air, so the lungs handle something like 10,000 litres a day, almost all of it without any conscious effort.

Breathing is controlled by respiratory centres in the brainstem, the part of the brain just above the spinal cord. They set the rhythm automatically, even during sleep, and adjust it using sensors that monitor the blood. The main trigger is not a lack of oxygen but a rise in carbon dioxide, which makes the blood more acidic; that build-up is what makes holding your breath feel uncomfortable. Unlike the heartbeat, breathing can also be taken over at will, which is why you can speak, sing, sigh or slow your breath on purpose. Rates are higher in children: a newborn baby breathes 30 to 40 times a minute.

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A newborn baby has somewhere between 270 and 300 bones, depending on how they are counted. Many are still separate pieces linked by cartilage, and as the child grows they fuse together, leaving most adults with 206.

The skull is the clearest example. At birth its bony plates are still separate, with soft gaps between them called fontanelles. This lets the head flex as it passes through the birth canal and leaves room for the brain to grow; the largest gap usually closes between 18 months and two years of age. Lower down, five vertebrae at the base of the spine fuse into a single bone, the sacrum, between the ages of about 18 and 30, and each hip bone forms from three separate bones that join by the end of puberty. Long bones such as the thigh bone grow at plates of cartilage near their ends; once these turn to bone after puberty, a person stops growing taller. Even 206 is a typical count rather than a fixed one, because some people have small extra bones.

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People with strong social relationships tend to live longer. A 2010 review of 148 studies, covering more than 300,000 people followed for seven and a half years on average, found that they had a 50 per cent greater likelihood of survival than people with weaker ties.

The review was led by the psychologist Julianne Holt-Lunstad and published in the journal PLOS Medicine. Researchers separate two ideas: social isolation means having few contacts, while loneliness is the painful feeling that your relationships fall short of what you need. Both are linked with poorer health. Friends and family may help by easing stress, encouraging healthier habits and noticing when something is wrong. Studies like these show a link rather than proof of cause, but the pattern has been found again and again. In 2023 the US Surgeon General, the country's leading public-health official, described loneliness as a public-health concern, and in June 2025 a World Health Organization commission estimated that loneliness is linked to about 871,000 deaths a year and affects one person in six worldwide.

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Sardines, salmon, mackerel and other oily fish are rich in EPA and DHA, two long-chain omega-3 fats. The body builds them into its cell membranes, DHA especially in the brain and the retina of the eye, and both help the heart to work normally.

Omega-3 fats are named after their chemistry: the first double bond in their carbon chain is at the third carbon atom counted from the end of the chain, known as the omega end. EPA (eicosapentaenoic acid) has 20 carbon atoms and DHA (docosahexaenoic acid) has 22. Fish get most of these fats from their food: the fats are made by algae and plankton and build up in the fish that eat them. Plant foods such as flaxseed, chia seeds and walnuts provide a shorter omega-3 called ALA, and the body turns only a small share of it into EPA and DHA. The NHS advises eating at least two portions of fish a week, one of them oily. For people who do not eat fish, oil made from algae is another source of EPA and DHA.

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The nose prepares every breath for the lungs. Its lining warms and moistens the incoming air, while hairs and sticky mucus catch dust and pollen, so by the time the air reaches the lungs it is close to body temperature and almost fully humidified.

Much of this work happens on the nasal conchae, three curled shelves of thin bone on each side of the nose, covered in a lining rich in blood vessels. They split the airflow into narrow channels and give it a large, warm, moist surface to pass over. Tiny hair-like structures called cilia then sweep the trapped dirt, caught in mucus, towards the throat, where it is swallowed. Most people also have a nasal cycle: every few hours one side of the nose swells slightly while the other opens up, so the two sides take turns doing most of the work. Breathing through the mouth skips much of this conditioning, which is one reason it can leave the throat feeling dry.

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