What a Nutrient Is

The definition is a statement about an enzyme

A nutrient is a substance your body requires for normal function and cannot assemble in sufficient quantity out of what it already holds, so it has to arrive from outside. That is a chemical claim rather than a nutritional opinion, and it can be checked.

Follow the word essential far enough and it narrows to one thing: the body does not carry out that synthesis. Not that it is precious or more important than the rest of the list. Only that the reaction which would build it does not run in you. Making a molecule means forming and breaking bonds in a set order — bonding run forwards — and every one of those steps is done by an enzyme. Take one enzyme out of the chain and the chain stops, and whatever sat at the end of it becomes something you have to eat.

Ascorbic acid is the clearest case there is. Most mammals build their own in the liver, starting from glucose, along a route of four enzyme-catalysed steps. Human cells run the first three. The fourth converts L-gulonolactone into ascorbate, and the enzyme for it is L-gulonolactone oxidase. The gene for that enzyme is still in the human genome, on chromosome 8, carrying mutations that stop a working protein being assembled. The instructions are there and cannot be read.

So vitamin C is essential for you and is not a vitamin at all for a goat, whose liver makes it by the gram. The molecule is identical either way. What differs is one catalyst in one species. The same fault sits in the other haplorrhine primates and in guinea pigs, which is why guinea pigs, and not rats, are the animals used whenever ascorbate is studied.

Non-essential is the worst-named word in the subject: cholesterol is non-essential and sits in every cell membrane you own.

Six classes, sorted by what they are

Everything a human body requires falls into six classes. Sorted by the kind of substance each one is, rather than by which aisle it comes from, they divide first into two.

Four of the six are organic: carbon-based, built by something living, and destroyable — burn them and they leave as carbon dioxide and water. Two are not. Water and the minerals were made by no living thing and can be unmade by none; a mineral atom can only ever be moved. You can watch that division happen. Dry a food and burn it at around 550 °C: the grey residue in the crucible is the mineral fraction, which is literally how a laboratory measures mineral content — by weighing ash. There is no ash for vitamin C. It left with the smoke.

The line that runs across all six is what happens to the substance once it is in. The three macronutrients are dismantled: their atoms become your atoms, or they leave as carbon dioxide and water. Vitamins and minerals largely are not — they work intact, or after one modification, as part of the machinery rather than as material for it. That difference, not importance, sets the quantities.

Where the line falls inside a class

Essential and non-essential do not sort neatly by class. The line runs through the middle of two of them, and where it falls is decided by which reactions your cells can catalyse.

Twenty amino acids build every protein in a human body, and nine of them are essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The other eleven are not essential because their carbon skeletons are already in circulation. Ordinary metabolism produces pyruvate, oxaloacetate and alpha-ketoglutarate, and an enzyme moves an amino group onto one of those — transamination — giving alanine, aspartate and glutamate. Attaching nitrogen to a ready skeleton is easy chemistry. Building the skeleton is not: the branched shape of leucine and the fused ring of tryptophan each need a pathway human cells do not have.

The fatty acids tell the same story in a line. Human cells carry a desaturase that can place a double bond at the ninth carbon of a chain, and no enzyme at all for a bond further along it. Plants have those enzymes. So linoleic acid and alpha-linolenic acid have to be eaten, and the whole omega family downstream of them depends on what arrives. The chemistry is one absent enzyme, again.

No carbohydrate is essential at all, because the liver can build glucose from amino acids and from the glycerol backbone of a fat — a statement about which reactions run, not an argument about what makes a good diet.

Cysteine shows what conditionally essential means chemically. The body does make it, but only by pulling sulphur off methionine, so cysteine can never be more available than methionine is. Glutamine, arginine and glycine have the same shape of constraint: the enzymes are present, the output is capped, and the cap can be reached. Read properly, essentiality is a statement about supply against demand rather than a fixed list — which is why the nutrient library gives each entry a page of its own.

Grams against micrograms

Macronutrient and micronutrient are statements about quantity and nothing else. The prefixes measure how much arrives, not how much it matters.

A gram is a thousand milligrams and a million micrograms. Protein, fat and carbohydrate arrive in grams and tens of grams. Most minerals arrive in milligrams. Vitamin B12, iodine, selenium and vitamin K arrive in micrograms — millionths of a gram, too little to see on a fingertip. Around seven orders of magnitude separate the ends of that range.

The reason is catalysis. A catalyst is not consumed by the reaction it enables. A magnesium ion in an enzyme’s active site holds the substrate in position, the reaction happens, the product leaves, and the magnesium is still there for the next one — thousands of times a second. A vitamin-derived coenzyme does the same: altered during the reaction, then restored. Material is spent. A tool is handed back.

The macronutrients have no equivalent trick, because they are the substrate rather than the tool. Their carbon ends up in your tissue or leaves through your lungs as carbon dioxide. There is no catalytic way around needing the mass.

The category with no requirement figure

There is a large class of substances a body is not required to receive and is measurably better for receiving. They are not nutrients by the definition this lesson opened with. Leaving them out would make the list honest and useless.

Phytonutrients are the compounds a plant makes for its own purposes — deterring what would eat it, colouring what would pollinate it, shielding it from sunlight. No requirement figure has been set for any of them and no deficiency state described, which is exactly why they sit outside the list. Nothing in you is built out of them. They arrive, interact with signalling already running, and leave.

You have already spent a whole course on one family of them. Terpenes are phytonutrients — so the grammar you learnt for oils is not a separate grammar. It reads a vitamin just as well.

Ascorbic acid is a five-membered ring carrying four hydroxyl groups, and it dissolves into water on contact for the reason linalool is more at home near water than limonene is: a hydroxyl group is polar wherever it sits, on a terpene skeleton or on a sugar-derived ring. Alpha-tocopherol — vitamin E — is a phenol with a sixteen-carbon tail. The phenolic hydroxyl is the same group carvacrol carries in oregano, and it releases its hydrogen readily for the same structural reason: the aromatic ring behind it spreads the resulting change across itself rather than leaving it on one atom. The tail is plain hydrocarbon, and it is what keeps the molecule inside a fatty membrane rather than in blood.

In laboratory assays — compounds in a tube, or applied to cells in a dish, which is not the same as what happens in a person — ascorbic acid, the tocopherols and the phenolic terpenes all behave as antioxidants, through that shared structural feature rather than through anything they share as categories. Free radicals and antioxidants takes it apart properly, including what a tube can and cannot tell you.

This is why nutrition belongs in a chemistry course. A vitamin is not a special kind of substance with rules of its own. It is a molecule, obeying rules already in your hands: polar or not, small or large, stable or reactive.

The shape of enough

One picture sits underneath every lesson that follows, and it is not a straight line. Plot how well some function works against how much of a nutrient is available to it, and the curve has four distinct regions.

1. 1.Deficiency. Supply is below what the chemistry needs and the function it serves runs under capacity. The only region where the relationship is close to linear, and where a nutrient is easiest to mistake for a remedy: here alone, more of it changes the outcome steeply.
2. 2.Sufficiency. Enough has arrived for the function to run at its normal rate. Nothing is held back for want of material.
3. 3.Plateau. More arrives and nothing further changes. An enzyme has a fixed number of active sites, a transporter a fixed number of carriers, a binding protein a fixed number of seats; once those are occupied, additional material has nowhere to act. Saturation is a measurable ceiling, not a disappointment.
4. 4.Excess. Past some point, more stops being neutral, and what happens then is decided by solubility: a water-soluble molecule is filtered by the kidney and leaves, while a fat-soluble one has somewhere to be stored and accumulates there. Vitamins as molecules works through that difference.

The four regions are the same for every nutrient and the widths are not. For most vitamins the plateau is broad. For the trace minerals it is narrow, because a body uses very little of them and has few ways to hold or release a surplus, so the distance between too little and too much is genuinely short. That sits behind the cautionary wording on a mineral label.

None of this tells you how much of anything to take, and it is not meant to. What it gives you is the shape of the question: the answer is a range with two edges, not a direction. Where a real question exists about your own status, that is the territory of testing and of professional medical care, which nothing here replaces. Reference figures on labels, and which supplements are available at all, differ from market to market.

Nothing here is intended to diagnose, treat, cure or prevent any disease. A dietary supplement does not replace a varied diet, and nothing here replaces professional medical care. If you are pregnant, nursing, under medical care or taking medication, speak to your healthcare practitioner before use. Keep supplements and essential oils out of reach of children. Product availability and label reference figures differ from market to market.

Where this goes

You now have the definition, the six classes, the reason the quantities differ by a factor of millions, and the curve. What follows reads the two classes with the most chemistry in them — a vitamin whose polarity decides its fate, and a mineral whose charge is its whole function.