Fats – acidic and unsuitable, MCT and trans-... what purpose do fats serve in our body?

Despite their widespread reputation as harmful dietary components, lipids fulfill four fundamental physiological roles within the human body. Both saturated and unsaturated fatty acids serve as essential substrates for the biosynthesis of phospholipids—the primary structural constituents of biological membranes, ensuring their fluidity and selective permeability. Stored as triacylglycerides (glycerol esters bonded to three fatty acid residues) within adipocytes of adipose tissue, they function as the body’s principal metabolic energy reserve, released via lipolysis during periods of elevated energy demand. The subcutaneous fat layer acts as a natural thermal insulator, minimizing heat loss through the skin to the external environment. Furthermore, polyunsaturated fatty acids such as arachidonic acid (ω-6) act as precursors for eicosanoids—a class of locally acting mediators that include prostaglandins, which modulate inflammatory responses, smooth muscle contraction, and platelet aggregation. Cholesterol, though endogenously synthesized in the liver, plays an irreplaceable role as a precursor to steroid sex hormones (estrogens, androgens), bile acids that facilitate dietary fat emulsification, and as a structural component of plasma lipoproteins, enabling the transport of hydrophobic lipids through the circulatory system.

Fatty acids may be systematically categorized based on the presence or absence of double bonds between carbon atoms, yielding two primary classifications: **saturated fatty acids** (lacking double bonds) and **unsaturated fatty acids** (containing one or more double bonds). Furthermore, a critical classification parameter is the **number of carbon atoms** within the molecular structure, which facilitates subdivision into three distinct categories: **short-chain fatty acids** (up to 6 carbon atoms in the chain), **medium-chain fatty acids** (ranging from 8 to 14 carbon atoms), and **long-chain fatty acids** (comprising 16 or more carbon atoms, including very-long-chain variants).

Unsaturated fatty acids typically exhibit a liquid consistency at ambient temperatures and demonstrate an elevated susceptibility to oxidative degradation—an attribute derived from the presence of carbon-carbon double bonds that facilitate the addition of hydrogen atoms. These compounds perform a multitude of critical physiological roles, and their prolonged deficiency may precipitate heightened vulnerability to infectious agents, increased fragility of microvascular structures, impaired functionality across diverse tissue systems and organ networks, as well as an array of additional pathological manifestations. Classification into the omega-3 (e.g., alpha-linolenic acid) and omega-6 (e.g., linoleic acid) families is determined by the position of the first double bond relative to the methyl terminus of the hydrocarbon chain; both categories are deemed essential fatty acids, meaning they cannot be synthesized *de novo* by the human body and must therefore be obtained through dietary sources. Within the body, these precursor molecules serve as substrates for the endogenous production of long-chain polyunsaturated fatty acids such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), both members of the omega-3 series. Primary dietary sources of unsaturated fatty acids include plant-based oils—particularly those cold-pressed from flaxseed, rapeseed, and sunflower—as well as fatty marine fish. EPA exhibits vasodilatory, antiplatelet, and antiatherogenic properties, whereas DHA constitutes an integral structural component of cellular membranes, including the photoreceptors within the retinal architecture, thereby underpinning accurate chromatic vision. Additionally, DHA confers neuroprotective benefits; however, advancing age is associated with a decline in Δ6-desaturase enzyme activity—a critical catalyst in DHA biosynthesis—which may contribute to age-related cognitive decline. In accordance with the Polish Fat Consensus guidelines, intake of saturated fatty acids (e.g., palmitic, myristic, lauric acids) should be restricted beginning in early childhood, as these compounds have been empirically demonstrated to exert hypercholesterolemic effects by elevating serum low-density lipoprotein (LDL) concentrations. Notably, even saturated fatty acids that do not directly alter lipid profiles (e.g., behenic acid) may promote atherogenesis through modifications in lipoprotein metabolism. Excessive consumption of saturated fats has been cumulatively linked to an increased incidence of malignant neoplasms, including colorectal, breast, and prostate cancers, as substantiated by extensive epidemiological investigations and meta-analytical reviews.

The industrial modification of liquid vegetable oils through partial hydrogenation results in the generation of unsaturated fatty acids with an atypical *trans* double-bond configuration. This phenomenon occurs exclusively under conditions of incomplete hydrogen saturation, whereas full hydrogenation precludes the formation of *trans* isomers. Fatty acids in the *trans* configuration exhibit physicochemical properties akin to those of saturated fats: they elevate levels of atherogenic low-density lipoprotein (LDL) cholesterol, increase the risk of atherosclerotic vascular changes, and may induce insulin resistance. Consequently, nutrition specialists uniformly advocate for the minimization of their dietary intake. Historically, solid margarines intended for spreading on bread were considered the primary source of *trans* fats due to their production via hydrogenation processes. Currently, however, high-quality products in this category are manufactured through esterification—a method that eliminates the formation of harmful isomers. On food labels, *trans* fats may be concealed under terms such as "partially hydrogenated/hardened vegetable oils/fats" or "vegetable fats with modified structure."

Medium-chain triglycerides (MCTs) constitute a subclass of saturated fatty acids distinguished by their unique chemical configuration, which permits direct absorption from the gastrointestinal tract into the portal circulation without reliance on bile salts or pancreatic lipases. This characteristic renders them an exceptionally efficient energy substrate, particularly in clinical nutrition, where they are routinely incorporated into dietary regimens for individuals with hepatic dysfunction, pancreatic insufficiency, or malabsorptive disorders. Upon ingestion, MCTs undergo rapid hepatic metabolism, yielding ketones that serve as an alternative fuel source for cellular respiration. While coconut oil is frequently cited as a natural source of MCTs—primarily due to its caprylic and capric acid content—the predominant lauric acid (though classified as an MCT) exhibits pro-atherogenic properties, as corroborated by the American Heart Association’s 2018 advisory, which recommends treating coconut oil comparably to other saturated fats (e.g., lard or butter) and thus limiting its intake. It is critical to note that all dietary fats, irrespective of their chain length, possess a high caloric density (9 kcal per gram), exceeding that of carbohydrates or proteins by more than twofold. The contemporary obesity epidemic is largely attributable to chronic positive energy balance—the sustained consumption of calories exceeding metabolic requirements. Consequently, dietary optimization should prioritize the reduction of ultra-processed foods (confectionery, fast food, trans-fat-laden baked goods) while emphasizing the intake of unsaturated fats, particularly long-chain omega-3 polyunsaturated fatty acids abundant in fatty marine fish and premium vegetable oils (e.g., flaxseed, canola).