Fatty acids are fundamental organic molecules that form the building blocks of fats in the human body. Comprising a hydrocarbon chain with a carboxyl group, these essential compounds play vital roles in energy storage, cell membrane structure, and inflammatory regulation. Understanding what fatty acids are—and how saturated, unsaturated, and trans fats differ—is crucial for making informed dietary choices that support cardiovascular health and metabolic function. UK guidance from the NHS and NICE emphasises the importance of fatty acid balance in preventing chronic disease, particularly heart disease and metabolic disorders.

What Are Fatty Acids? Definition and Basic Structure

Fatty acids are fundamental organic molecules that serve as essential building blocks for lipids (fats) in the human body. Chemically, they consist of a hydrocarbon chain with a carboxyl group (-COOH) at one end, which gives them their acidic properties. The length of the carbon chain typically ranges from 4 to 28 carbon atoms, though most dietary fatty acids contain between 12 and 22 carbons.

The structure of fatty acids determines their physical properties and biological functions. The hydrocarbon chain can be either saturated (containing only single bonds between carbon atoms) or unsaturated (containing one or more double bonds). This structural variation significantly influences how fatty acids behave in the body, affecting everything from cell membrane fluidity to inflammatory responses.

Fatty acids exist in various forms within the body. They are typically found esterified (chemically bonded) to glycerol molecules, forming triglycerides, which represent the primary storage form of energy in adipose tissue. Non-esterified (free) fatty acids also circulate in the bloodstream bound to albumin. Fatty acids also combine with other molecules to create phospholipids, essential components of cell membranes, and cholesterol esters.

Understanding fatty acid structure is clinically relevant because it helps explain why different dietary fats have varying effects on cardiovascular health, inflammation, and metabolic function. The NHS Eatwell Guide and NICE guidelines on cardiovascular disease prevention emphasise the importance of fatty acid composition in the diet, particularly the balance between saturated and unsaturated fats, as this directly influences cholesterol levels and heart disease risk.

Types of Fatty Acids: Saturated, Unsaturated and Trans Fats

Saturated fatty acids (SFAs) contain no double bonds in their carbon chain, meaning each carbon atom is 'saturated' with hydrogen atoms. Common examples include palmitic acid (16 carbons) and stearic acid (18 carbons), found predominantly in animal fats, dairy products, and tropical oils like coconut and palm oil. At room temperature, saturated fats are typically solid due to their straight molecular structure, which allows tight packing. UK government dietary guidelines, endorsed by NICE, recommend limiting saturated fat intake to no more than 10% of total energy intake, as excessive consumption is associated with elevated LDL cholesterol levels and increased cardiovascular disease risk.

Unsaturated fatty acids contain one or more double bonds, creating 'kinks' in the molecular structure. These are further classified into:

• Monounsaturated fatty acids (MUFAs), containing one double bond (e.g., oleic acid found in olive oil)

• Polyunsaturated fatty acids (PUFAs), containing multiple double bonds, including omega-3 and omega-6 fatty acids

The double bonds in unsaturated fats prevent tight molecular packing, making them liquid at room temperature. These fats are generally considered cardioprotective, with evidence supporting their role in reducing LDL cholesterol when they replace saturated fats in the diet.

Trans fatty acids are unsaturated fats with a specific molecular configuration where hydrogen atoms sit on opposite sides of the double bond. Whilst small amounts occur naturally in ruminant animals, most dietary trans fats have historically been industrially produced through partial hydrogenation of vegetable oils. In the UK, population intakes of industrial trans fats are now low, largely due to voluntary reformulation by food manufacturers. Northern Ireland follows the EU limit of 2g per 100g of fat, while no general legal limit exists in Great Britain. There is strong evidence linking trans fat consumption to adverse health outcomes, including increased cardiovascular disease risk by raising LDL cholesterol whilst lowering beneficial HDL cholesterol. When shopping, check ingredient lists for 'partially hydrogenated oils' which indicate the presence of industrial trans fats.

Role of Fatty Acids in the Body and Health

Fatty acids serve multiple critical physiological functions beyond their well-known role as an energy source. When metabolised through beta-oxidation in mitochondria, fatty acids yield approximately 9 kcal per gram, making them the body's most concentrated energy reserve. During periods of fasting or prolonged exercise, adipose tissue releases stored triglycerides, which are broken down into fatty acids and transported to tissues for energy production.

Cell membrane structure and function depend fundamentally on fatty acids. Phospholipids, which form the bilayer of all cell membranes, contain fatty acid chains that determine membrane fluidity, permeability, and the function of membrane-bound proteins. The ratio of saturated to unsaturated fatty acids in membrane phospholipids influences cellular signalling, receptor function, and inflammatory responses. This has particular relevance in conditions such as insulin resistance and cardiovascular disease.

Certain fatty acids serve as precursors for bioactive signalling molecules. Arachidonic acid (an omega-6 PUFA) is converted into eicosanoids, including prostaglandins, thromboxanes, and leukotrienes, which regulate inflammation, blood clotting, and immune responses. Omega-3 fatty acids (EPA and DHA) produce resolvins and protectins, which have anti-inflammatory properties. This balance between pro-inflammatory and anti-inflammatory mediators has clinical implications for conditions ranging from rheumatoid arthritis to cardiovascular disease.

Essential fatty acids (linoleic acid and alpha-linolenic acid) cannot be synthesised by the body and must be obtained through diet. Deficiency, though rare in developed countries, can lead to dermatitis, impaired wound healing, and growth retardation in children. UK scientific advisory committees do not recommend restricting omega-6 intake, but rather emphasise achieving adequate long-chain omega-3 intake alongside replacing saturated fats with unsaturated alternatives. Patients with cardiovascular disease or inflammatory conditions should discuss fatty acid intake with their GP, as dietary modification may complement pharmacological management.

Dietary Sources of Essential Fatty Acids

Omega-6 fatty acids, particularly linoleic acid, are abundant in the UK diet and found in:

• Vegetable oils (sunflower, corn, soybean)

• Nuts and seeds (walnuts, sunflower seeds)

• Poultry and eggs

• Processed foods containing vegetable oils

Deficiency of omega-6 fatty acids is exceptionally rare in developed countries, as they are widely distributed in commonly consumed foods. The NHS Eatwell Guide does not specifically emphasise increasing omega-6 intake, as most individuals consume adequate amounts.

Omega-3 fatty acids exist in two main forms with distinct dietary sources. Alpha-linolenic acid (ALA), the plant-based omega-3, is found in flaxseeds, chia seeds, walnuts, and rapeseed oil. Whilst ALA is essential, the body converts it to the more biologically active forms—eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—with limited efficiency (approximately <5% to EPA and <1% to DHA, with variation between individuals). EPA and DHA are obtained directly from:

• Oily fish (salmon, mackerel, sardines, herring)

• Fish oil supplements

• Algae-based supplements (suitable for vegetarians and vegans)

NHS guidance recommends consuming at least 2 portions of fish per week, including 1 portion (about 140g) of oily fish for cardiovascular health. Pregnant or breastfeeding women should limit oily fish to 2 portions weekly and avoid shark, marlin and swordfish due to mercury content. Fish liver oil supplements should be avoided during pregnancy due to their vitamin A content.

Practical dietary advice for optimising fatty acid intake includes replacing saturated fats with unsaturated alternatives (using olive or rapeseed oil instead of butter), incorporating oily fish regularly, and choosing whole food sources over processed foods high in omega-6 oils. Patients should discuss omega-3 supplements with a pharmacist or GP, particularly if taking anticoagulants or antiplatelet medications, or if considering supplements providing ≥1g daily of EPA+DHA. Note that prescription omega-3 products (such as icosapent ethyl) differ from over-the-counter food supplements and are prescribed according to specific NICE guidance. If you experience any suspected side effects from supplements or medicines, report them via the MHRA Yellow Card Scheme.

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