While moderate amounts of saturated fats are not bad, there is no doubt that it is recommended that in the eaten fat the most abundant fatty acid must be oleic acid (i.e. the fat must contain mostly MUFA, mono-unsaturated fatty acids).

The reason is simply that oleic acid is the most abundant fatty acid in human fat. When the ingested fat consists mostly of oleic acid, it can be used as such, while when the ingested fat contains either mostly saturated fats, like many animal fats or mostly poly-unsaturated fatty acids, e.g. linoleic acid, like most cheap vegetable oils, the ingested fatty acids must be converted by the liver into the fatty acids preferred inside the human body, so that is extra work for the liver and the liver becomes less efficient at old ages. If the liver does not succeed to convert all the ingested fatty acids that should have been converted, than the composition of the fat used for various purposes in the body may be suboptimal.

Examples of foods with optimal fatty acid composition are olive oil, high-oleic sunflower oil (not classic sunflower oil, which contains mostly linoleic acid), hazelnuts, almonds, pistachios, cashews, peanuts.