What are you feeling?

Muscle cramps

• Magnesium runs muscle contraction and relaxation→ mechanism
• The potassium-sodium gradient drives the muscle's electrical signal→ mechanism
• Calcium is the trigger that fires contraction→ mechanism

Low back pain from sitting

Red flag — If accompanied by loss of bladder/bowel control, saddle numbness, or progressive leg weakness, this can be cauda equina syndrome — go to the ER now, don't wait.

• Most back pain is non-specific and self-limiting; imaging changes ≠ the pain source→ mechanism
• Sitting's harm is the stillness, not the posture→ mechanism

Knee pain

• Pain comes from synovium/subchondral bone — cartilage itself has no nerves→ mechanism
• Progressive loading beats immobilizing for the joint→ mechanism

Heel pain on the first morning step

• The plantar fascia is like a bowstring; the first morning step hurts most→ mechanism

Morning joint stiffness · sudden big-toe attack

• Urate super-saturates and crystallizes in the joint→ mechanism
• How joints handle load and nutrition→ mechanism

Stiff neck · aching neck and shoulders

Red flag — Neck pain after an injury, hands getting clumsy or walking becoming unsteady, a sudden thunderclap headache, or fever with a stiff neck — get medical care now.

• Most neck pain is mechanical; changes on a scan are not the same as the source of pain→ mechanism
• The shoulder trades stability for range, which is why it hurts easily→ mechanism
• Sitting's harm is the stillness, not an imperfect posture→ mechanism

Sore the day after a workout

Red flag — Pain and swelling far out of proportion to the workout, weakness so bad you can't move, and especially cola- or tea-colored urine can be rhabdomyolysis — go to the emergency department.

• Soreness on day two or three isn't lactic acid; it's the muscle repairing→ mechanism

Sprained ankle · hip pain · tingling fingers

• How long a sprain takes, and why it happens again→ mechanism
• Outer-hip pain is often a compressed tendon→ mechanism
• Tingling in the thumb, index and middle fingers at night can be a squeezed nerve in the wrist→ mechanism

Can't fall asleep

• The first-line treatment for insomnia is CBT-I, not a pill→ mechanism
• Start with the architecture of a night's sleep→ mechanism
• Melatonin sets the clock, it isn't a sleeping pill (dose debunk)→ mechanism

Sleepy by day despite enough sleep

• Enough hours ≠ good sleep; broken architecture still leaves you tired→ mechanism
• Repeated apneas chop sleep into fragments→ mechanism

Always tired

• Fatigue is multi-factorial — rule things out before concluding→ mechanism
• Iron deficiency is a common, testable cause→ mechanism
• Low B12 also causes fatigue, esp. vegans / metformin users→ mechanism

After an all-nighter

• What happens hour by hour after a night without sleep, and how to get through the day→ mechanism

Snoring and stopping breathing in sleep

Red flag — If you are so sleepy by day that you nod off while driving, stop driving and get a sleep study soon.

• Loud snoring plus waking tired is often the throat collapsing again and again in sleep, stopping your breathing→ mechanism
• Whether to get tested, and at home or in a sleep lab→ mechanism

Restless legs at night

• The brain's first step in making dopamine needs iron, so restless legs start with a ferritin test→ mechanism
• Trouble sleeping with uncomfortable legs you must move signals another condition; see a doctor→ mechanism

Stomach ache · abdominal pain

Red flag — Belly pain that keeps getting worse and won't ease, or comes with vomiting blood, black or bloody stools, a persistent high fever, vomiting so much you can't keep water down, or severe pain with a positive pregnancy test — go to the emergency department now; so does upper-belly pain with sweating or breathlessness, which can be the heart. Pain with unexplained weight loss, or pain or diarrhea that wakes you at night, needs a doctor soon.

• Cramps with sudden diarrhea: rehydrate first, and know when to see a doctor→ mechanism
• Upper-belly pain or burning, or fullness after a few bites, with a normal scope: possibly functional dyspepsia→ mechanism
• Mostly bloating and trapped gas, with a swelling belly→ mechanism
• Burning behind the breastbone or in the pit of the stomach, worse lying down after meals, can be reflux→ mechanism
• Recurring upper-belly discomfort is worth one question: have you been tested for H. pylori?→ mechanism
• Lower-belly pain eased by a bowel movement, with stools that swing loose and hard, can be IBS→ mechanism
• Cramps and diarrhea within hours to a day or two of eating something suspect→ mechanism
• Crampy ache with several days of hard, effortful stools→ mechanism
• Lower-belly pain around each period, and where it comes from→ mechanism

Upper-belly fullness · stomach pain · full after a few bites

Red flag — Trouble swallowing, unexplained weight loss or repeated vomiting — don't treat it as ordinary indigestion; see a doctor soon. Vomiting blood or black stools is bleeding in the gut: go to the emergency department the same day. Crushing chest pain with sweating or breathlessness can be the heart — go to the emergency department now.

• A normal scope yet full after a few bites, or pain after meals: often a fundus that won't relax or a stomach too sensitive to stretch→ mechanism
• Start with what the stomach normally does: hold, grind, release in batches→ mechanism
• For recurring upper-belly pain, a test for H. pylori is a common first step→ mechanism
• If it is mostly heartburn and sour regurgitation, reflux fits better→ mechanism

Gas & bloating · a swollen belly

Red flag — Bloating with blood in the stool, unexplained weight loss, anemia or a persistent fever; bloating or pain that won't go away or worsens at night, or a lump in the belly; bloating or a change in bowel habits that first appears after 50; a family history of colorectal cancer or inflammatory bowel disease; or a newly growing belly in a woman over 40 — see a doctor soon. Black stools or vomiting blood: go to the emergency department the same day. If you suddenly cannot pass any gas or stool, with severe belly pain and vomiting, go to the emergency department now.

• People who always feel bloated often have no more gas than others; a swelling belly is often the diaphragm pushing down→ mechanism
• Gut bacteria ferment fiber and make gas; adding fiber too fast is the classic cause→ mechanism
• Which foods ferment most, and how a low-FODMAP trial works→ mechanism
• What your gut microbes ferment and what they make→ mechanism

Diarrhea

Red flag — Bloody or black stools, a persistent high fever, vomiting so much you can't keep water down, much less urine, a dry mouth, dizziness on standing, or diarrhea lasting more than 3 days — see a doctor; don't wait at home.

• What matters most is replacing water and salt; rehydration salts don't stop diarrhea, so keep going until it stops→ mechanism
• After a bad meal: what you can do at home is mostly fluids, and when you must see a doctor→ mechanism
• Why oral rehydration salts can pull water back into the body→ mechanism
• Recurring diarrhea that tracks stress can be IBS→ mechanism

Milk upsets my stomach

• Most adults with lactose intolerance can handle about a cup of milk at once→ mechanism
• Too little of the enzyme that splits lactose; the unsplit lactose reaches the colon and bacteria ferment it — the milk isn't the problem→ mechanism
• Yogurt's fermenting cultures (not probiotic supplements) help people with lactose intolerance digest lactose→ mechanism

Heartburn / acid reflux

Red flag — Trouble swallowing or unexplained weight loss — don't treat as ordinary heartburn; seek care promptly. Vomiting blood or black stools — go to the emergency department the same day. Crushing chest pain with sweating/breathlessness can be cardiac — go to the ER now.

• Stomach contents travel up and burn the esophagus→ mechanism

Bloating · irregular bowel habits

Red flag — Blood in stool, diarrhea waking you at night, unexplained weight loss, anemia, or a family history of colorectal cancer need a doctor and colonoscopy — not ordinary IBS.

• Disordered gut motility and sensing, with no visible structural disease→ mechanism
• How the gut microbiome shapes digestion and mood→ mechanism

Nausea and vomiting

Red flag — Sudden nausea with tight or heavy chest pain spreading to the arm, back, neck or jaw, or shortness of breath: call emergency services now. Vomiting blood or coffee-ground material, green vomit, a severe headache or tummy pain, a stiff neck, possible poisoning, blue or grey lips, severe breathlessness or confusion: emergency department now. Vomiting that won't stop, can't keep fluids down, or lasts over 2 days: contact a doctor the same day; for a vomiting baby under 1, or a young child who stops feeding or shows dehydration, get medical advice now.

• Nausea comes in through three doors and isn't always the stomach; prevent dehydration first→ mechanism
• How to rehydrate when you're vomiting and have diarrhoea→ mechanism

Piles, rectal bleeding, a sore bottom

Red flag — Bleeding that won't stop, a lot of blood or large clots, or severe pain: go to the emergency department now, and don't drive yourself. Painless rectal bleeding at any age: see a doctor soon, and especially get the bowel checked if you are over 50 or have dark blood, a change in bowel habits, weight loss or anemia. A very high temperature, feeling hot, cold, shivery and unwell, or pus from the piles: see a doctor soon.

• Hemorrhoids are normal anal cushions that swelled and slipped; the typical sign is painless bright red bleeding→ mechanism
• Blood color can't tell piles from a bowel problem; get it seen at any age, especially over 50→ mechanism
• Constipation and long straining are the main forces pushing the cushions down→ mechanism

Blood pressure creeping up

Red flag — BP ≥ 180/120 with chest pain, severe headache, blurred vision, slurred speech, or one-sided weakness is a hypertensive emergency — go to the ER now.

• Blood pressure = flow × resistance — silent but organ-damaging→ mechanism

Blood sugar creeping up

• How insulin resistance walks toward type 2 diabetes→ mechanism
• Fructose detours around blood sugar, straight to the liver→ mechanism

Racing or skipping heartbeat

Red flag — Palpitations that don't stop, or come with chest pain, shortness of breath, feeling faint or fainting: go to the emergency department now and don't drive yourself; if they have stopped, or palpitations come with exercise, a new irregular heartbeat, or a resting heart rate above 120 or below 45: contact a doctor the same day. Recurring episodes, ones lasting more than a few minutes, or a family history of sudden death at a young age: see a doctor soon for an ECG.

• What a skipped beat is, which palpitations need action now, and how to catch one in the act→ mechanism
• Racing that starts and stops suddenly is often mistaken for panic attacks→ mechanism
• How stress and rest change the rhythm of your heartbeat→ mechanism
• Too much caffeine can bring on palpitations, and people clear it at different speeds→ mechanism
• When stress never switches off, palpitations are one of the common body signs→ mechanism

Always cold · cold hands and feet

Red flag — If one hand or foot suddenly turns cold, pale or blue, very painful, numb or weak, an artery may be blocked: go to the emergency department now. Burning leg or foot pain at rest, non-healing wounds or toes turning black: see a doctor the same day.

• Hands and feet go cold first because the body shuts their skin blood shunts to protect core temperature→ mechanism
• Fingers turning white, blue, then red in the cold is Raynaud's phenomenon→ mechanism
• When there aren't enough oxygen-carrying red cells, tiredness, feeling cold and breathlessness tend to come together→ mechanism
• When the thyroid slows down, feeling cold is one of the common signs→ mechanism

Headache / migraine

Red flag — A sudden thunderclap headache (peaks in seconds), or headache with fever and stiff neck, confusion, one-sided weakness or slurred speech — go to the ER now.

• A pressing ache on both sides that activity doesn't worsen is usually tension-type headache→ mechanism
• Migraine is a neurovascular event, not just vessel spasm→ mechanism

Poor focus / brain fog

• How cognition really changes with age, and what's modifiable→ mechanism
• The two-way pathway between gut and brain→ mechanism

Low mood / anxiety

Red flag — If you have thoughts of suicide or harming yourself, get help now: call your local crisis line (US 988), or go to the ER. You are not alone.

• How mood links mechanistically to nutrition, sleep, and microbiome→ mechanism
• How chronic stress leaves marks on the body→ mechanism

Worn out by work · burnout

• Burnout isn't depression, and why a vacation isn't enough→ mechanism

Premenstrual mood / weight shifts

• How the estrogen-progesterone tide moves how you feel→ mechanism
• Why fat migrates to the belly around perimenopause→ mechanism

Hot flashes / perimenopause

• Perimenopausal hormone shifts and treatable symptoms→ mechanism
• Long-term bone, heart, and metabolic care after menopause→ mechanism

After birth: leaking urine or low milk

Red flag — After birth, bleeding that soaks two pads in an hour, a painful swollen leg, sudden breathlessness or chest pain, a headache that will not go away or keeps getting worse, or blurred vision: get care now or call emergency services. A red, hot, painful breast with fever needs a doctor soon. Thoughts of harming yourself or your baby: get help now; in mainland China call the national hotline 12356, or go to the emergency department.

• The pelvic floor has carried a pregnancy and been stretched by birth, so some leaking is common; home exercises may help women who already leak→ mechanism
• Each breast sets its supply by how much milk is removed, so more frequent, effective feeding comes first→ mechanism
• What trials say about drinking extra water and milk-boosting herbs→ mechanism
• The signs of heavy bleeding, clots, high blood pressure, infection and a mood crisis after birth→ mechanism

Dizziness · the room spinning

Red flag — Vertigo with double vision, slurred speech, weakness or numbness on one side, a severe headache, being unable to stand or walk straight, or sudden hearing loss can be a stroke — get emergency care now.

• Dizzy and spinning aren't the same; spinning when you roll over is usually a loose ear crystal→ mechanism
• Tiredness plus light-headedness is worth checking for anemia→ mechanism

Car and sea sickness

• Your eyes say still, your inner ear says moving; look at the horizon, and take medicine before you set off→ mechanism

Dry, tired eyes from screens

• Screen eye strain and blue-light glasses — what the evidence says→ mechanism

Ringing in the ears

Red flag — One ear suddenly feeling blocked and muffled while it starts ringing can be sudden hearing loss — see an ear doctor promptly; don't wait at home.

• Tinnitus is mostly the brain turning up a quiet input→ mechanism
• How the ear turns sound into nerve signals→ mechanism

Sore throat · a cold

Red flag — Trouble breathing or blue lips — go to the emergency department now. A high fever that won't come down, chest pain, or symptoms past 10 days or suddenly worse after getting better — see a doctor.

• Cold, flu or allergy — and what fever and a runny nose are doing→ mechanism

Bleeding, swollen gums

• An unbalanced community of bacteria in the gum pocket keeps inflammation going→ mechanism

Bad breath

• 80 to 90% starts in the mouth: bacteria on the tongue and gums make sulfur gases→ mechanism
• Bad breath doesn't mean H. pylori; check the teeth and tongue first→ mechanism

Sneezing and itchy nose: allergic rhinitis

• Immunity treats pollen or dust mites as a threat and releases histamine within minutes on the next encounter→ mechanism
• Allergic rhinitis is not an infection: bouts of sneezing, clear runny nose, itchy eyes and nose, no fever→ mechanism

A cough that won't go away

Red flag — Coughing up blood or labored breathing: see a doctor soon; a lot of blood, no breath or chest pain: go to the emergency department now. A lingering cough with fever, night sweats or weight loss needs a tuberculosis test. A toddler who suddenly starts choking and coughing may have inhaled something: seek care now, and go to the emergency department now if breathing is labored or the lips turn blue.

• A long cough is usually a cough reflex set too sensitive; the question is what turned it up→ mechanism
• A post-cold cough usually stops on its own within 8 weeks; antibiotics don't help→ mechanism
• Blood pressure drugs ending in -pril can cause a dry cough, sometimes months after starting→ mechanism
• How to tell a cold, flu and allergic rhinitis apart→ mechanism

Mouth ulcers

Red flag — An ulcer that hasn't healed after 3 weeks needs a dentist or doctor; one that won't heal is occasionally a sign of mouth cancer. Recurring ulcers with genital ulcers: see a doctor soon; a red, painful eye, light sensitivity or blurred vision: the same day, and loss of vision: the emergency department now.

• Recurring mouth ulcers are a local immune reaction, not contagious, and most heal in 7 to 10 days→ mechanism
• Bites, stress and quitting smoking set it off; SLS-free toothpaste is worth a try→ mechanism
• If they keep coming back, check iron, folate and B12; they can also be a clue to coeliac or Behçet's disease→ mechanism

Hair shedding

• The hair growth cycle and common shedding mechanisms→ mechanism

Gray hair

• How the follicle's pigment cells and their stem-cell reserve run out→ mechanism
• Gray hair with numbness, tiredness or pale skin patches may relate to B12 or the thyroid→ mechanism
• What B12 deficiency does in the body, and how it is checked→ mechanism

Acne / breakouts

• What actually causes acne, and what works vs doesn't→ mechanism

Picky eater

Red flag — If a child chokes while eating and cannot cough or breathe in, call emergency services and start back blows at once; frequent coughing or choking at meals, food coming back through the nose, or shortness of breath after eating needs an urgent appointment.

• Why fear of new food and dislike of bitter greens is mostly a stage, and how repeated tastes help→ mechanism
• Weight loss, frequent choking at meals or a sudden fear of eating may be more than picky eating→ mechanism
• How babies practice tastes and chewing when starting solids→ mechanism

Child short for age

Red flag — Height that keeps sliding down the growth chart, or signs of puberty before age 8 in girls or age 9 in boys, means a child should see a doctor; with weight loss, repeated headaches or vomiting, see a doctor promptly.

• Bones lengthen at the growth plates, shaped by sleep, enough food and the timing of puberty→ mechanism
• Height sliding down the chart, far below the parents' range, or puberty too early or too late needs a doctor→ mechanism
• Most picky eaters grow fine; iron, zinc and fiber are the nutrients to watch→ mechanism

Older parent eating less

Red flag — Losing 5% to 10% or more of body weight over 3 to 6 months without trying, or frequent coughing or choking while eating or drinking, means an older person should see a doctor; eating and drinking very little and suddenly becoming confused, sleepier than usual or hard to wake: call emergency services now.

• With age people fill up sooner, feel thirst later and taste less, so eating less often has causes inside the body→ mechanism
• Losing a lot of weight over a few months without trying, coughing at meals, or dehydration with confusion needs a doctor→ mechanism
• Eating too little, and too little protein, lets muscle waste away→ mechanism
• Long-term acid reducers or metformin can slowly lower B12→ mechanism

Urinary frequency, night-time peeing, urinary infections

Red flag — If you suddenly cannot pass urine at all and your lower belly is painfully full, go to the emergency department now. Blood you can see in the urine, even once, needs a urologist soon. A urinary infection with fever or back pain may have reached the kidney; see a doctor soon.

• Urinary infections usually start with E. coli gripping the bladder wall; cranberry has evidence only for some groups, such as women with repeat infections→ mechanism
• In men, more night-time peeing and a weaker stream are usually the inner ring of the prostate squeezing the urethra→ mechanism
• After menopause, vaginal dryness and repeat urinary infections are one chain, and they do not heal on their own→ mechanism

Heat illness: dizzy and sick in the heat

Red flag — Confusion, garbled speech, seizures or collapse in the heat mean treating it as heat stroke: call emergency services now and cool the person on the spot; cold-water immersion works best.

• From cramps to heat exhaustion to heat stroke; a change in mental state is the dividing line→ mechanism
• If it happens, cool first: cold-water immersion works best, and call emergency services at the same time→ mechanism
• Losing water hits circulation first; plasma volume is the first thing to fall→ mechanism

Hangover and feeling rough after drinking

Red flag — In someone who drinks heavily, sudden confusion, unsteady walking or abnormal eye movements signal Wernicke's encephalopathy: seek care now.

• The liver clears alcohol at a capped rate; hangover pills, teas and sweating cannot speed it up→ mechanism
• Flushing after one drink means the second enzyme is weak from birth and acetaldehyde builds up→ mechanism
• Don't drink on an empty stomach, match each drink with water, and a product-by-product look at hangover remedies→ mechanism

Swollen legs or feet

Red flag — One leg that suddenly swells, hurts, feels warm or turns red may be a deep-vein clot: see a doctor soon, and go to the emergency department now if you have chest pain or shortness of breath. Both legs swelling with breathlessness may be the heart, kidneys or liver: see a doctor soon.

• Swelling is not from drinking too much: pressure too high, albumin too low, or leaky vessels→ mechanism
• Which swelling to take seriously, and which not to massage→ mechanism
• One swollen, painful leg after long sitting or a long flight needs checking for a deep-vein clot→ mechanism