Meat products contain both saturated and unsaturated fats. Various foods contain different proportions of saturated and unsaturated fat. The hydrocarbon group may be saturated or unsaturated. Hydrogenation is a useful means for converting unsaturated compounds into saturated derivatives. Each variety has its own unique levels of monounsaturated, saturated, and polyunsaturated fats. Aliphatic compounds can be saturated, like hexane, or unsaturated, like hexene and hexyne. Hydrogenated vegetable oils differ in two major ways from other oils which are equally saturated. The cholesterol is a lipid associated with the kind of saturated fat found in meat. Gallstones form when the bile is saturated, usually with either cholesterol or bilirubin. See saturated fat and cardiovascular disease. Worldwide, dietary guidelines recommend a reduction in saturated fat. They form when urine is persistently saturated with calcium and oxalate. Because of limited particle surface area, these soils can become saturated with phosphates. The thallus must be saturated with liquid water for cyanobionts to photosynthesize. If the air is already saturated with moisture, perspiration will not evaporate.