Leptin tells the brain how much fat you carry and should suppress appetite - but obesity makes the brain deaf to it. Myostatin caps muscle growth, and blocking it builds muscle in mice and animals. Why leptin injections fail for ordinary obesity and myostatin drugs add mass without matching strength.

Insulin stores fuel and glucagon releases it; insulin resistance is the muscle, liver and fat going deaf to insulin, and it precedes type 2 diabetes by 10-15 years. A 7 percent weight loss cut diabetes onset 58 percent - and exercise raises glucose uptake by a second route insulin cannot block.

Thyroid hormone (T3/T4) sets basal metabolic rate and thermogenesis; parathyroid hormone and calcitonin manage blood calcium through bone, kidney and vitamin D. What each does, what goes wrong when they're too high or low, and why a normal thyroid can't be pushed to burn more fat.

Oestrogen builds the first half of the cycle, an LH surge ten times baseline triggers ovulation, and progesterone rules the second half. PCOS, thyroid disease and under-fuelling break the rhythm. What the cycle does - and mostly doesn't - to strength, endurance and flexibility.

Adrenaline and noradrenaline mobilise fuel and drive the heart within seconds of a hard effort; cortisol - 8-30 mg a day on a morning-peaking rhythm - raises blood sugar and, chronically, breaks down muscle and piles on visceral fat. How the adrenal glands power training and why unmanaged stress works against it.

A pea-sized gland under the brain commands growth, the thyroid, the adrenals, the gonads, milk, water balance and childbirth. What each pituitary hormone does, how the hypothalamus controls it, and why growth hormone pulses with sleep and hard exercise - plus why injecting it is a different story.

Low testosterone splits into two kinds: the testes failing (LH high) or the brain not signalling (LH low or normal) - and the second is often obesity, opioids, steroids or sleep apnoea. Diagnosis needs two morning tests below 300 ng/dL. A meta-analysis of 41 studies found soy has no effect at all.

The best equation for athletes predicted 80 percent of people within 10 percent of measured metabolic rate. Most managed 41 to 64 percent. Mifflin-St Jeor - the standard in every online calculator - significantly misestimated athletes' rates, though it performed best across BMI categories in a general population. Why the number is a starting point, not an answer.

A 2024 meta-analysis of 24 studies found the majority of variation in VO2max change scores is measurement error, with no strong evidence for stable individual trainability. And in a crossover trial, every one of 21 people who failed to respond to one protocol responded to the other. What actually determines adaptation, and why the unfit gain fastest.

A systematic review of 33 studies and a two-year follow-up of 171 dieters put the 'starvation effect' in proportion: it exists, it is small when measured properly, and it fades once weight stabilises.

A health age questionnaire adds and subtracts years from your birthday based on habits and risk factors. Here is how the estimate is built, what measured fitness-age research shows, and what the number can and cannot tell you.

A fitness test hands you a number in METs or millilitres per kilogram per minute. Here is how to read it against age and sex, why the test method changes it, and why each extra MET matters more than most people realise.

Resting heart rate is the cheapest vital sign there is, and one of the most predictive. A meta-analysis of 46 cohorts puts a number on every ten beats, and a living review of wearables says how much to trust the watch. Here is how to measure it and read it.

Muscle generates force, signals to the rest of the body, and reports its own length and tension back to the nervous system. What the physiology reviews say about each part, and why lifters should care.

A meta-analysis of 37 cohorts on cardiorespiratory fitness and a 15-cohort meta-analysis on daily steps put numbers on how much fitness and everyday movement change the risk of dying, and where the returns flatten.