Hydrogenated and partially hydrogenated oils

What are hydrogenated oils?

Hydrogenated oils are liquid vegetable oils that have been chemically treated with hydrogen gas under pressure, in the presence of a nickel or palladium catalyst, to convert double bonds in unsaturated fatty acids into single bonds. The result is a fat that is solid or semi-solid at room temperature, more shelf-stable, and cheaper than butter.

The process was patented by Wilhelm Normann in 1902 and industrialised by Procter & Gamble as Crisco in 1911. It became the backbone of the twentieth-century industrial baking supply — margarine, shortening, coffee whitener, ready-made pastry, biscuits, microwave popcorn — because it gave manufacturers cheap fats with butter-like performance.

Partial vs full hydrogenation

Partial hydrogenation leaves some double bonds intact but rearranges their geometry from the natural cis configuration to the trans configuration. These industrial trans fats behave in the body like saturated fats but with additional pro-inflammatory and lipoprotein effects.

Full hydrogenation converts every double bond to a single bond, producing a saturated fat with no trans isomers. Fully hydrogenated oils are then interesterified — rearranged with another liquid oil — to produce spreadable products without partial hydrogenation.

Health evidence

The evidence that industrial trans fats cause cardiovascular disease is among the strongest for any dietary factor. The 2015 Cochrane review estimated that a 2% increase in energy from trans fats increases coronary heart disease risk by roughly 23%. The 2019 WHO REPLACE technical documents summarised the mortality attributable to industrial trans fats at approximately 500,000 deaths per year globally before the elimination programmes began.

Regulatory action followed:

• Denmark banned industrial trans fats in 2003.
• New York City banned them in restaurants in 2006.
• The FDA revoked GRAS status in 2015, effective 2018 with a limited use extension to 2020.
• The EU capped trans fats at 2 g per 100 g of fat in 2019, effective 2021.
• WHO REPLACE aimed for global elimination by 2023; achieved partially, ongoing.

How to spot them on labels

• "Partially hydrogenated" oil — banned in most jurisdictions but still legal in some.
• "Hydrogenated" or "fully hydrogenated" oil — legal, still a Group 4 marker.
• "Shortening" — usually contains hydrogenated or interesterified fat.
• Nutrition panels reporting 0 g trans fat can legally contain up to 0.5 g per serving under FDA rules; small servings can add up.

How NakedCalorie flags hydrogenated oils

Partially hydrogenated oils are a Tier 1 flag (highest severity) with an inline explanation of the industrial trans fat context. Fully hydrogenated oils are a Tier 2 marker and trigger NOVA Group 4 classification. See palm oil for the most common trans-fat replacement in industrial baking, and ultra-processed foods for the wider category these ingredients define.