Trans fats, or trans-fatty acids, are unsaturated fatty acids containing at least one carbon-carbon double bond in the trans configuration. Their molecular structure distinguishes them from the more common cis unsaturated fatty acids found in most natural vegetable oils and many other foods. Trans fats can arise through industrial processing, particularly the partial hydrogenation of liquid oils, and they also occur naturally in small quantities in meat and dairy products from ruminant animals such as cattle, sheep and goats. The existence of naturally occurring trans fats does not make industrial trans fats harmless; WHO considers both forms relevant to health, while industrially produced trans fats are a major preventable dietary hazard.
Historically, industrial trans fats became widespread because partial hydrogenation could alter liquid vegetable oils into semi-solid fats with useful technological properties, including improved texture, stability and shelf life. Partially hydrogenated oils were used in products such as shortenings, margarines, baked goods and fried foods. The same chemical modification that produced desirable food-manufacturing characteristics also created substantial quantities of trans-fatty acids. As evidence accumulated linking industrial trans fats to cardiovascular disease, many countries introduced restrictions, reformulation requirements or bans on partially hydrogenated oils. Modern food manufacturing can generally replace them with other fats and oils without requiring trans fat for basic food functionality.
Industrial trans fats have an adverse effect on cardiovascular risk factors. They can increase LDL cholesterol and reduce HDL cholesterol, producing a particularly unfavourable lipoprotein profile. WHO reports that high intake of industrially produced trans fat is associated with increased coronary heart disease and mortality risk and estimates that hundreds of thousands of deaths globally each year are attributable to industrial trans-fat intake. Trans fat contributes to atherosclerotic cardiovascular disease by adversely affecting blood lipids and other physiological processes. It is therefore treated differently from many other dietary components: the public-health objective is elimination of industrial trans fat rather than merely moderate consumption.
Potential dietary sources include foods made with partially hydrogenated oils, certain fried foods, some commercially baked goods, shortening, some margarines and certain packaged snacks. The exact prevalence varies greatly between countries because regulation and food reformulation differ. In the UK, partially hydrogenated vegetable oils have largely been removed from supermarket own-brand products, so contemporary UK exposure is generally much lower than it was historically. Nevertheless, checking ingredient lists for terms such as “partially hydrogenated” remains useful, particularly for imported or specialist foods and foods from jurisdictions with weaker restrictions.
Natural ruminant trans fats require a separate distinction. Small amounts occur in beef, lamb, dairy products and other foods derived from ruminants because microorganisms in the animals' digestive systems transform unsaturated fatty acids. These naturally occurring trans fats are chemically trans fats and are included in WHO's overall trans-fat recommendation. However, the quantities consumed in ordinary diets are generally much smaller than the quantities that historically arose from industrial partial hydrogenation. The nutritional lesson is that “natural” and “industrial” describe the source, not whether a fatty acid is chemically a trans fatty acid.
WHO recommends that trans fat contribute less than 1% of total daily energy intake for adults, equivalent to less than approximately 2.2 g per day on a 2,000-calorie diet, while emphasising that industrially produced trans fats should be avoided. This limit is deliberately stringent because trans fats provide no known health benefit and can be replaced technologically by healthier fats. WHO's approach is therefore different from its treatment of saturated fat, for which a maximum of 10% of energy is recommended. The practical aim for industrial trans fat is elimination rather than finding a desirable intake.
Trans fats should not be confused with all hydrogenated fats or with unsaturated fats generally. “Hydrogenated” can refer to different degrees of hydrogenation, while the specific concern is the production of trans-fatty acids during partial hydrogenation. Fully hydrogenated fats are structurally different and do not contain the same substantial trans-fat problem, although they can be high in saturated fat. Likewise, ordinary unsaturated oils are not trans fats merely because they are liquid or processed. The relevant distinction is the molecular configuration of the fatty acids and, for food regulation, the manufacturing process that produced them.
The practical significance of trans fats is that consumers should not need to deliberately obtain them. Foods based on unhydrogenated vegetable oils, nuts, seeds, fish and other sources of unsaturated fats provide preferable alternatives. Reducing deep-fried and heavily processed foods can reduce exposure when ingredient information is uncertain, while reading labels can identify partially hydrogenated oils where they remain present. Current evidence and international guidance make industrial trans fat an unusually clear nutritional target: it has no established health benefit, increases cardiovascular risk and can generally be removed from the food supply through reformulation.
