during the second world war, yudkin served in the royal army medical corps and was posted to sierra leone. while there he studied a skin disease that was prevalent among local african soldiers and discovered that it was due not to an infection, as had been believed, but to riboflavin deficiency. he found that the army had devised a uniform diet for its soldiers in the four british west african colonies (gambia, sierra leone, gold coast and nigeria). this diet was, on paper, adequate in all nutrients – including riboflavin, which was supplied predominantly from millet. but it turned out that millet, although a staple in the gold coast and nigeria, was loathed by soldiers from sierra leone, who would not eat it even if they were hungry. this experience must have brought home to yudkin the importance of custom and upbringing in determining a person's choice of foods.