Other sugars, sugar substitutes, and corn syrup are also used. The marginal rate of substitution between perfect substitutes is likewise constant. These have much lower fat content and are suitable as milk substitutes. The majority of sugar substitutes approved for food use are artificially synthesized compounds. These inhibitors usually contain electrophilic groups like halogen substitutes and epoxides. It readily substitutes for potassium in minerals, and is therefore fairly widespread. It is also used as a filler in sugar substitutes and other products. A number of commercially available saliva substitutes exist. It substitutes for choline nutritionally, but only partially. The softening process often substitutes sodium cations. The US Sugar Association has launched a campaign to promote sugar over artificial substitutes. The use of cocoa butter substitutes in Canada is not permitted. Dendrimers are also being investigated for use as blood substitutes. Emollients are available as lotions, creams, ointments, bath oils, or soap substitutes. Sugar rationing during World War II led to an interest in sugar substitutes.