The biology words, in plain English.

Getting a nutrient out of your gut and into your bloodstream. Swallowing something is not the same as absorbing it — for some nutrients most of a dose never makes it across.

How much of what you swallow actually reaches your bloodstream in a usable form. A high-dose supplement with low bioavailability can deliver less than a smaller, better-absorbed one.

A fluid your liver makes and your gallbladder stores. It breaks dietary fat into droplets small enough to absorb, which is why fat-soluble vitamins depend on it.

A tiny droplet of fat wrapped in bile that ferries fat-soluble compounds across the gut wall. Without one, a fat-soluble vitamin largely passes straight through you.

Dissolves in fat, not water — vitamins A, D, E, and K. They need fat in the same meal to absorb well, and your body can store them, so excess accumulates rather than flushing out.

Dissolves in water — vitamin C and the B vitamins. They absorb without fat, but your body stores little of them, so surplus is mostly excreted in urine.

A protein that acts as a specific doorway through a cell wall. Many nutrients cannot cross on their own and rely on one — which is also why two nutrients using the same doorway can compete.

The point where a transporter is working flat out and cannot move any more. Past it, a bigger dose does not mean more absorbed — the classic example is vitamin C above about 1g at once.

A helper molecule an enzyme needs to do its job. Many vitamins and minerals work as cofactors, which is why being low in one can stall a reaction that seems unrelated.

A protein that speeds up a specific chemical reaction in your body. Digestion, energy production, and hormone activation all run on them.

The first stretch of your small intestine, right after the stomach. It is where several minerals — iron especially — are absorbed.

Where most nutrient absorption happens. Its lining is folded and fringed to create an enormous surface area for pulling nutrients across.

The iron-containing molecule inside red blood cells that carries oxygen from your lungs to the rest of your body. Most absorbed iron ends up here.

The protein that ferries iron through your blood. Your body keeps almost no iron floating loose, because free iron damages tissue.

Heme iron comes from meat and absorbs relatively well. Non-heme iron comes from plants and supplements and absorbs less — but vitamin C in the same meal improves it substantially.

How long it takes your body to clear half of a substance. It tells you how long an effect lasts — caffeine’s 5–6 hours is why an afternoon coffee can still disrupt sleep.

The active hormone form of vitamin D, made by your kidney. It is what actually tells your gut to absorb more calcium.

The storage form of vitamin D that a blood test measures. It reflects your status over the past few months rather than what you took yesterday.

A chemical messenger released into your blood that tells distant tissue what to do. Vitamin D behaves more like one of these than like a typical vitamin.

Minerals carrying a double positive charge — calcium, iron, zinc, magnesium. They compete for the same gut transporters, which is why large doses taken together absorb worse than spaced apart.

A mineral bonded to an amino acid to help it survive the gut and absorb more reliably. Usually gentler on the stomach than plain mineral salts.

The structural protein that holds skin, tendons, and connective tissue together. Building it requires vitamin C as a cofactor — the step that fails in scurvy.

A molecule that neutralises reactive by-products of normal metabolism. Useful in balance, though blunting all of it around training can actually dampen the adaptation you trained for.

Your immune system’s response to damage or irritation. Short-term inflammation after exercise is part of adaptation; long-running inflammation is the kind linked to disease.

The bacteria living in your intestine. Probiotics add live strains; prebiotics are the fibre that feeds the ones you already have.

Glucose circulating in your blood. Both food and exercise move it, which is why combining several things that lower it warrants care.

The most oxygen your body can use per minute during hard effort. It is the standard single-number measure of aerobic fitness.

The structures inside cells that turn fuel and oxygen into usable energy. Endurance training makes you build more of them, which is much of why cardio gets easier.

Muscle fibres growing larger in response to repeated resistance training plus enough protein and recovery.

The process of building new muscle protein. Resistance training raises it for a day or more; dietary protein supplies the raw material.

How much mineral is packed into your bone, the standard measure of bone strength. Impact and resistance loading raise it; vitamin D and calcium supply what it is built from.

The highest daily intake considered unlikely to cause harm in most people. Exceeding it routinely is where supplements start carrying real risk.

The daily intake that meets the needs of almost everyone healthy. It is a population target for avoiding deficiency, not a personal optimum.

A trial where some participants unknowingly get a dummy treatment. Without one you cannot tell a real effect from expectation and natural recovery.

A study that statistically combines many earlier studies. Generally more reliable than any single trial, though it inherits the flaws of what it pools.

Our four-tier label for how good the human evidence is behind one specific claim. Strong: consistent results across multiple good-quality human trials. Moderate: several trials agree, with real limitations. Limited: small, short, or conflicting trials. Insufficient: no good human evidence, or trials that found nothing. It describes the quality of the evidence, not the size of the effect — and never whether it applies to you personally.

Independent verification that what is in the tub matches the label. Supplements are regulated far more loosely than medicines, so this is the main quality signal available. USP and NSF check identity, potency, and contaminants. NSF Certified for Sport and Informed Sport additionally screen for substances banned in competition. IFOS covers fish oil purity and oxidation.

The weight of the actual mineral, as opposed to the weight of the compound carrying it. "325 mg ferrous sulfate" is about 65 mg of elemental iron. Labels quote both, and comparing the wrong number makes a weak product look strong.

Some upper limits apply to everything you take in (vitamin D, zinc, iron); others apply only to supplements, with food excluded (magnesium, vitamin E, folic acid, niacin). Magnesium is the common trap — the 350 mg limit covers supplements only, and magnesium from food is not capped.

Someone who shows little or no effect from something that works on average. Creatine is the classic case: people whose muscle stores are already near saturation from a meat-heavy diet have little room to gain.