In January 1917, Germany started using U-boats (submarines) in order to sink Allied and later neutral ships bringing food to the country in an attempt to starve Britain into surrender under their unrestricted submarine warfare programme. One response to this threat was to introduce voluntary rationing in February 1917,[48] a scheme said to have been endorsed by the king and queen themselves.[112] Bread was subsidised from September that year; prompted by local authorities taking matters into their own hands, compulsory rationing was introduced in stages between December 1917 and February 1918,[48] as Britain's supply of wheat stores decreased to just six weeks worth.[113] It is said to have in the most part benefited the health of the country,[48] through the 'levelling of consumption of essential foodstuffs'.[114] To assist with rationing, ration books were introduced on 15 July 1918 for butter, margarine, lard, meat, and sugar.[115] During the war, average calories intake decreased only three percent, but protein intake six percent.[48]