Classify fatty acids. Describe the essential fatty acids and the significance of trans fats.

Definition

• The even number arises because they are synthesised and degraded two carbons at a time
• Amphipathic — a polar carboxyl head and a non-polar hydrocarbon tail

Classification BY Chain Length

Classification BY Saturation

• Nomenclature — 18:2 Δ9,12 means 18 carbons, 2 double bonds, at positions 9 and 12 counting from the carboxyl end
• ω (omega) numbering counts from the methyl end — used nutritionally, because man cannot alter that end
• Naturally occurring unsaturated fatty acids are cis; the cis bond puts a kink in the chain, lowering the melting point and increasing membrane fluidity

Essential Fatty Acids

• Linoleic acid (ω-6) and α-linolenic acid (ω-3) are strictly essential
• Arachidonic acid is semi-essential — it can be made from linoleic acid
• Man lacks desaturases beyond carbon 9, so a double bond cannot be introduced at the ω-3 or ω-6 position
• Functions — eicosanoid precursors, membrane fluidity, retinal and brain development (DHA), lowering plasma cholesterol
• Deficiency — scaly dermatitis, poor wound healing, hair loss, fatty liver, growth retardation

Physical and Chemical Properties

Trans Fatty Acids

• Formed by partial hydrogenation of vegetable oils, as in vanaspati and margarine; also present naturally in small amounts in ruminant fat
• The trans configuration makes the chain straight, so it behaves like a saturated fat
• Raise LDL, lower HDL, raise Lp(a) and triglyceride — a worse profile than saturated fat, which raises LDL but does not lower HDL
• Also promote inflammation and insulin resistance
• Restricted by law in India to 2% of total fat, and banned or limited in many countries

Eicosanoid Precursors

• ω-3 and ω-6 compete for the same desaturases and elongases, and for cyclo-oxygenase — which is why the ratio matters more than the absolute intake of either
• This competition is the biochemical basis for the cardiovascular benefit attributed to fish oil

Applied Aspects

• Dietary recommendation — roughly equal parts saturated, monounsaturated and polyunsaturated fat, with an ω-6 : ω-3 ratio of 5:1 to 10:1. The modern Indian diet is often 15–20:1 because of heavy seed-oil use
• Fish oil (EPA and DHA) lowers triglyceride substantially and is used in severe hypertriglyceridaemia
• MCT oil bypasses the carnitine shuttle and needs no bile salts — valuable in fat malabsorption, chylothorax and carnitine deficiency
• DHA supplementation of infant formula supports retinal and neural development, since breast milk supplies it naturally
• Refrigeration and antioxidants delay rancidity — the more unsaturated the oil, the more readily it oxidises, which is why fish oil spoils fastest
• Peroxisomal disorders — Zellweger syndrome and adrenoleucodystrophy result from failure to oxidise very long chain fatty acids
• Rancid fat is harmful as well as unpalatable — the peroxides formed destroy vitamins A and E and generate free radicals
• Coconut oil is unusual among plant oils in being highly saturated, though largely medium-chain, which alters its metabolic handling
• Mustard oil is rich in erucic acid, which has been linked to myocardial lipidosis in animals; the evidence in man is not established