**1. Introduction**

Lipids are water insoluble organic biomolecules that have several important biological functions within the cell, providing energy storage, participating in the formation of cell membranes, and exerting regulatory functions in transduction and signaling processes in multiple metabolic pathways [1]. Through these actions, dietary lipids can affect health, well-being, and the risk of developing disease, such as cardiovascular, inflammatory, and cognitive disorders, among many others [2].

The term lipid is known to describe fatty acids, their esters, and different lipophilic structures. Most dietary lipids consist of triglycerides, but there is also little amount of other lipid classes, such as phospholipids, present in the cell membranes of all food that we eat [3].

Fatty acids (FA) are carboxylic acids with an aliphatic chain of varying lengths: short chain (C < 6), medium chain (6C-12C), long chain (13C-22C), and very long chain (C > 22). The most common chain length range for fatty acids is between C12 and C22 and they can be characterized by saturated and unsaturated (mono or poly) chains [4]. Most of the FA existing in nature have an even number of carbon

atoms and linear hydrocarbon chains, although some of them, found primarily in bacteria, may contain branched or cyclic structures [5–7]. Fatty acids containing a carbocyclic unit naturally occur in specific genera of bacteria and in plants.

In some cases, alicyclic fatty acids, such as cyclopropane (CPFA) and omegacyclohexyl fatty acids (CHFA), are essential for cell survival, as they could affect the membrane fluidity that enables certain microorganisms to survive under extreme environmental conditions [8]. In plants, CPFA are usually minor components, where cyclopropene fatty acids are the most abundant. *Sterculia foetida* seed oil contains 65–78% of cyclopropene fatty acids (principally sterculic acid), suggested to have antifungal and enzyme inhibitor activities [9].

CPFA, especially dihydrosterculic (9,10-methylene-octadecanoic acid) and lactobacillic (11,12-methylene-octadecanoic acid) acids, have been identified as minor component of lipid profile in a wide range of milk and dairy products [10, 11] and, more recently, in meat and fish [12] representing important foodstuffs in human diet.

CPFA concentration ranges from 200 to 1000 mg/kg fat in dairy products and bovine meat [13]; therefore, their dietary intake may not be negligible, and their potential role in human health should not be underestimated.

However, due to their recent identification, so far CPFA have not been yet considered for their occurrence in humans, and several aspects related to their bioavailability and putative bioactivity as well as the bacterial strains producing CPFA in feeds and in which conditions still must be explored.

ω-Cyclohexyl fatty acids (CHFAs), mainly cyclohexyl-undecanoic and tridecanoic acids, occur in several acido-thermophilic bacteria such as *Alicyclobacillus acidocaldarius* and can be biosynthesized by these bacteria species, even by adding cyclohexyl acid to the bacteria culture [11]. 11-cyclohexyl undecanoic acid was first isolated as a minor component of butter fat, then in sheep fat but it is almost certainly produced by bacteria in the rumen. 13-cyclohexyltridecanoic acid has been considered as a potential marker of ruminal acidosis in cow [11]. Recently, both ω-cyclohexyl fatty acids, 11-cyclohexylundecanoic acid and 13-cyclohexyltridecanoic acid, were detected in meat fat, especially in bovine meat but not in pork and horse meat. Therefore, the presence of ω-cyclohexyl fatty acids in foods was related to a ruminal origin and, combined with other fatty acids as branched chain fatty acids, could be proposed as marker of species [14].

This chapter reviews the literature data about the origin and natural occurrence of cyclic fatty acids, their presence in foods, especially in meat and dairy products, and their potential bioavailability and bioactivity in mammals. Finally, the application of some cyclic fatty acids as molecular markers for food authenticity will be provided.
