A missing piece of the diet
The claim, first made in 1913, was that an animal fed a diet otherwise complete in protein, carbohydrate and fat could still fail to grow, and that the missing piece was a trace substance carried only in certain fats. Elmer McCollum and Marguerite Davis, working with a small colony of rats at a Wisconsin agricultural station, found that young animals fed lard as their only fat stopped growing after several months even though every other measured nutrient was present in normal amounts. Substituting butter or egg yolk for the lard brought growth back within weeks. They called the substance responsible “factor A,” on the reasoning that it was the first of what might be several such accessory factors, and the name stuck once it was shortened to vitamin A. The wider claim built on top of that single result was that nutrition itself needed a new category, alongside protein, fat and mineral.
Feeding rats for months
The method was patient rather than clever: purified diets built from casein, sugars and a single fat source, fed to rats for months at a time, with growth and reproduction as the measured outcomes rather than any chemical assay of the food itself. McCollum had set up one of the first dedicated rat colonies in American nutrition research specifically because rats were cheap, bred quickly and tolerated a controlled diet, which let him vary one ingredient while holding the rest constant. When lard-fed rats stalled and butter or egg-yolk extract restarted their growth, the fat itself, rather than anything in the base diet, was implicated. Later work by other laboratories characterised the chemistry behind the effect, identifying retinol as the active compound the body stores and uses, and plant carotenoids as precursors the body can convert into it.
What the deficiency proves
The basic experimental result has held up completely: animals deprived of this dietary factor fail to thrive and recover once it is restored, and the same pattern shows up in humans as a set of well-documented deficiency conditions. Night blindness, an early and reversible symptom, gives way if untreated to xerophthalmia, a progressive drying and damage to the surface of the eye that can end in permanent blindness. The chemistry behind these effects is equally solid: retinal, one of the interconvertible forms of the vitamin, combines with a protein in the retina to form the pigment that light-sensing cells depend on, and a related form, retinoic acid, acts inside cells to switch genes on and off during development and tissue maintenance. None of this depended on guesswork; it followed directly from tracing what the original growth factor turned out to be.
One factor, several substances
What did not survive intact was the idea of a single, simple substance. “Factor A” turned out to be several related compounds rather than one: retinol itself, several carotenoids that the body converts before it can use them, and further metabolites each doing a different job. The naming system built around the discovery was also contested from the start, with McCollum arguing against Casimir Funk’s term “vitamines” on chemical grounds and preferring “fat-soluble A,” before a revised spelling eventually won out industry-wide. And the assumption that more of a good nutrient is simply better does not hold for this one: taken in excess, preformed vitamin A causes its own set of symptoms, including liver damage and, in pregnancy, a measurable increase in the risk of birth defects, which makes dosing rather than mere presence the operative question.
From rat cages to rice grains
The practical afterlife of this discovery is large. Public health programmes built directly on it now supply high-dose vitamin A to children in the regions where deficiency remains common, and controlled evaluations of those programmes report substantial reductions in child mortality and in the specific conditions the original research described, alongside measurable falls in night blindness and xerophthalmia. Deficiency is still estimated to affect a large share of the world’s young children, concentrated in places where diets rely heavily on staple grains with little of the vitamin or its plant precursors. That same gap has motivated engineering approaches such as rice varieties bred or modified to carry more beta-carotene in the grain, an attempt to fix by breeding what McCollum’s rats first showed could not be skipped in the diet.
Is it worth your time
This is worth understanding less for the specific chemistry of retinol than for what the story shows about how a nutrient category gets established: not by a flash of insight but by feeding animals a deliberately incomplete diet and watching, patiently, what breaks and what fixes it. Readers curious about the origins of modern nutrition science, or about why food labels list vitamins as if they were settled facts rather than the outcome of contested laboratory work, will find the groundwork here. It is less satisfying as a story of a single credited discovery: the naming was disputed, the substance turned out to be several substances, and the man who ran the defining experiment was, by at least one historian’s account, never given the recognition of a Nobel Prize that several later contributors to the same field received.