Water-Soluble Vitamins (B, C)

Thiamine and Wernicke's: The Vitamin You Give Before Glucose

Most vitamin deficiencies are slow, quiet, forgiving. Thiamine is not. A body store measured in a few weeks, a coenzyme sitting at the exact centre of how cells burn sugar, and a brain that runs on almost nothing but glucose — put those together and a small shortfall becomes a neurological emergency. Worse, the obvious first move in a collapsing patient — a bag of dextrose — can be the very thing that tips them over the edge. This is the vitamin whose whole story reduces to a single rule you can never afford to forget: thiamine before glucose.

The B-Complex Vitamins: Riboflavin, Niacin and Pyridoxine

The B vitamins share a first name and almost nothing else. Each one is a different coenzyme — the working part that a specific enzyme cannot run without — and each fails in its own unmistakable way: a cracked mouth, a photosensitive rash with dementia, a burning peripheral neuropathy. What makes them pharmacology and not just nutrition is that several tie straight to a drug. One antibiotic can strip out two of them at once; one B vitamin, given in gram doses, becomes a lipid-lowering agent; another is the specific antidote for a poisoning that stops the brain from making its own brake.

B12 and Folate: One-Carbon Metabolism and the Order of Replacement

Two vitamins share a single job — handing round the one-carbon fragments a cell needs to build DNA — and so their deficiencies look almost identical in the blood: big, immature red cells, a megaloblastic anaemia. But they are not interchangeable, and the trap is famous. Give folate to a patient who is actually short of B12 and the anaemia melts away — while the spinal cord, quietly, is destroyed. The whole clinical art of these two vitamins is knowing that they overlap in the marrow but diverge in the nervous system, and getting the order of replacement right.

Vitamin C, Scurvy and the Antioxidant-Supplement Myth

Vitamin C has two reputations, and only one of them is deserved. As the molecule the body cannot build its own collagen without, it is genuinely essential — deny it and the connective tissue of the whole body quietly unravels into scurvy. As the flagship of the antioxidant-supplement industry — megadose capsules promised to fend off colds, cancer and heart disease — it is one of the most instructive failures in modern medicine. The gap between those two stories is the real lesson: fix a true deficiency and the transformation is dramatic; chase megadoses in the well-fed and you gain, at best, expensive urine, and at worst, a kidney stone.