Essential vs Non-essential · the line is drawn on rates必需与非必需 · 那条线画在速度上

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必需 = 造得不够快, 不是造不出来 · 两个速度都会动 · 猫的两个酶相乘 · PKU 一个酶翻两栏 · 早产儿那把还没上线的剪刀

Not a property of the molecule

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必需不是分子的属性

Whether your body can make something, and whether you need to eat it, are two different questions.

Nutrition textbooks sort the 20 amino acids into two columns: 9 essential, 11 non-essential. The table looks like the periodic table, as if essentiality were a property the molecule carries — stamped on it like atomic weight, fixed forever.

It isn't. That line is drawn on rates.

The US National Academy of Medicine's Dietary Reference Intakes report gives a plain definition: an amino acid counts as conditionally indispensable when it requires a dietary source because endogenous synthesis cannot meet metabolic need (IOM 2005).

Not one word in that sentence describes a molecule. Every word describes a rate: the rate of making, the rate of using, and which one falls behind.

So the real classification has three columns, not two. In the IOM's table, six amino acids stand in the middle: arginine, cysteine, glutamine, glycine, proline, tyrosine. Normally the body makes enough of them; in certain states it doesn't, and then you have to eat them.

The crucial part is that both rates move. The making rate can fall: an enzyme is naturally sluggish, hasn't developed yet, or has been broken by a mutation. The using rate can rise: the body is growing, repairing a wound, inflamed. When the two rates cross, the same molecule slides out of the non-essential column and into the essential one.

Essentiality is not a property of the molecule. It is a property of the body's state. The three examples that follow are all about this one thing.

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