the chemical composition of the human body, and on the nutritional value of different flours used to make bread. Widdowson also studied the impact of infant diet on human growth. They studied the differing effects from deficiencies of salt and of water and produced the first tables to compare the nutritional contents of foods before and after cooking. They co-authored The Chemical Composition of Foods, first published in 1940 by the Medical Research Council. Their book, "McCance and Widdowson", became known as the dietician's bible and formed the basis for modern nutritional thinking. In 1939, they tested whether the United Kingdom could survive with only domestic food production if U-boats ended all imports. Using 1938 food-production data, they fed themselves and other volunteers a limited diet, while simulating the strenuous wartime physical work Britons would likely have to perform. The scientists found that the subjects' health and performance remained very good after three months. They also headed the first ever mandated addition of vitamins and minerals to food, beginning with adding calcium to bread. Their work became the basis of the wartime austerity diet promoted by the Minister of Food, Lord Woolton. The British public's wartime diet was never as severe as in the Cambridge study because German U-boats failed to halt US convoys, but rationing improved the health of British people: infant mortality declined and life expectancy rose. This was because everyone had access to a varied diet with enough nutrients. The first commodity to be controlled was petrol. On 8 January 1940, bacon, butter and sugar were rationed. This was followed by successive rationing schemes for meat, tea, jam, biscuits, breakfast cereals, cheese, eggs, lard, milk, and canned and dried fruit. Fresh vegetables and fruit were not rationed, but supplies were limited. Many people grew their own vegetables, greatly encouraged by the highly successful "Digging for Victory" campaign. Most controversial was bread; it was not rationed until after the war ended, but the "national loaf" of wholemeal bread replaced the ordinary white variety, to the distaste of most housewives who found it mushy, grey, and easy to blame for digestive problems. Fish was not rationed, but fish prices increased considerably as the war progressed. In summer 1941, the British appealed to Americans to conserve food to provide more to go to Britons fighting in the Second World War. The Office of Price Administration (OPA) warned Americans of potential