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 mineral 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.