How Hydrogenated Oils Changed the American Diet

The rise of hydrogenated vegetable oils reflects more than a century of changes in food manufacturing, restaurant operations, and nutritional science. Their affordability, stability, and versatility made partially hydrogenated fats commercially attractive, but the trans fatty acids they produced were eventually linked to increased cardiovascular risk. Scientific research and public-health regulation prompted their removal from much of the food supply, demonstrating that widespread dietary exposure can be reduced through changes in manufacturing and food-service practices. The experience also reinforces the importance of distinguishing industrial trans fats from ordinary non-hydrogenated vegetable oils, evaluating the effects of repeated heating, and considering ingredient quality within the broader diet. For restaurants, responsible sourcing remains a practical decision with consequences that extend well beyond cost and convenience.

In 1911, Procter & Gamble introduced Crisco, a revolutionary new shortening that promised to improve American cooking by replacing butter and lard with vegetable oil that had been chemically altered to behave like butter and lard. It was inexpensive, remarkably stable, and could sit on a shelf for months without becoming rancid. Naturally, everyone was thrilled. Procter & Gamble launched an enormous advertising campaign, distributed free cookbooks, and assured American households that this modern alternative was everything they had been missing. By 1916, annual sales had reached 60 million pounds. It was a tremendous commercial success, and an early demonstration of the food industry's considerable talent for convincing consumers that something engineered primarily for convenience was also an improvement in quality.¹

There was, however, a detail that would become rather important. The partial hydrogenation process responsible for producing these convenient fats could also create trans fatty acids, which subsequent research established as harmful to cardiovascular health. By the time scientists had assembled convincing evidence, partially hydrogenated oils were everywhere. They had become standard ingredients in commercial pastries, packaged foods, restaurant fryers, and countless products whose manufacturers had built entire production systems around them. The industry had spent decades making these fats indispensable, and reversing that decision was going to require rather more than another cookbook.

How We Learned to Manufacture Cooking Fat

Before the twentieth century, the fats used in cooking were largely determined by what agriculture could produce. Butter came from milk, lard from pigs, and olive oil from olives. Each had its own characteristics, limitations, and price. Butter required refrigeration and could be expensive. Lard had a distinctive texture and flavor. Olive oil was particularly suited to Mediterranean cooking, although its availability varied considerably outside the regions where olives were grown. These were familiar ingredients, but they were not necessarily convenient for an emerging food industry interested in manufacturing millions of identical products and shipping them across increasingly large distances.

Vegetable oils offered an attractive alternative, particularly as industrial agriculture and refining made them available in substantial quantities. Unfortunately, many were liquid at room temperature, and certain highly unsaturated oils deteriorated relatively quickly when exposed to oxygen and heat. Neither characteristic was particularly useful when manufacturing shortening for commercial pastry dough or producing packaged food intended to remain on a shelf for several months. Hydrogenation offered a solution. By reacting vegetable oils with hydrogen under controlled conditions, manufacturers could change their fatty-acid composition and produce fats with the consistency, melting characteristics, and stability they wanted.²

It was an extraordinary development in food chemistry. Manufacturers could take an inexpensive liquid oil and turn it into something that performed like a traditional solid fat, often with greater consistency and a longer storage life. The technology also allowed them to tailor those characteristics for particular applications. A commercial bakery needed shortening that created tender pastries. A manufacturer producing cookies needed fat that remained stable during storage. A restaurant needed something that performed reliably in a fryer through repeated cooking cycles. Hydrogenation helped solve these problems, allowing the same agricultural commodities to be transformed into ingredients suitable for an enormous range of industrial applications.

The complication was that partial hydrogenation could also alter the configuration of fatty acids. Naturally occurring unsaturated fats generally contain double bonds in a cis arrangement. Under partial hydrogenation, some of these bonds can rearrange into a trans configuration, creating fatty acids whose physiological effects differ substantially from those of the original oil. Manufacturers had successfully changed the way vegetable oil behaved in food. Unfortunately, they had also changed the way some of its components behaved in the human body. That distinction would take considerably longer to become part of the conversation.

Restaurants Had Their Reasons

It would be convenient to explain the widespread adoption of hydrogenated oils as the result of restaurants simply choosing cheaper ingredients. Price was certainly important, but the commercial advantages were more extensive than that. Anyone who has worked in food service understands that the cost of an ingredient rarely ends with its purchase. There is storage, preparation, labor, equipment maintenance, waste, and the constant challenge of producing the same dish hundreds of times without discovering that today's version bears only a passing resemblance to yesterday's.

Commercial frying illustrates the problem particularly well. Frying oil deteriorates with use, and replacing it costs money. A kitchen must purchase fresh oil, dispose of the old supply, clean the equipment, and absorb the labor and interruption involved. Partially hydrogenated shortenings generally offered greater oxidative stability than the highly unsaturated oils from which they were manufactured. Under suitable conditions, that stability could extend their useful life and help maintain consistent frying performance. For a restaurant preparing large quantities of food, even modest improvements in oil longevity could translate into meaningful operational savings.

Their advantages extended into commercial baking. The physical properties of shortening influence the structure, texture, and consistency of pastries, biscuits, doughnuts, and countless other products. Hydrogenated fats could be formulated with predictable melting characteristics, reducing the variation associated with some traditional ingredients. Food manufacturers could produce doughs and finished products that behaved consistently during storage, transportation, and preparation. As restaurant chains expanded and centralized suppliers became increasingly important, that predictability was enormously valuable. A product needed to look and taste approximately the same whether it was prepared in one kitchen or a thousand.

By the middle of the twentieth century, partially hydrogenated vegetable fats were deeply embedded in commercial food production. They were relatively inexpensive, convenient to store, and adaptable to different manufacturing requirements. They also benefited from the growing perception that vegetable-based fats were healthier alternatives to animal fats. Concern about saturated fat and cholesterol encouraged manufacturers to develop and promote vegetable-based substitutes for butter and lard. The difficulty was that a partially hydrogenated vegetable oil was no longer nutritionally identical to the liquid oil from which it had been produced. The marketing distinction between animal and vegetable fat had become considerably simpler than the underlying chemistry.²

None of this means restaurants deliberately set out to harm their customers. They adopted ingredients that were commercially practical, widely available, and, for much of the period, legally acceptable. But it does demonstrate how easily the priorities of industrial food production can become confused with the priorities of human nutrition. An ingredient that reduces costs and improves consistency may be an excellent business decision. Whether it is an excellent dietary decision requires an entirely different set of evidence.

The Part Where Scientists Became Concerned

By the latter decades of the twentieth century, researchers were finding increasingly convincing evidence that trans fatty acids had unfavorable effects on blood cholesterol. Industrial trans fats could increase concentrations of low-density lipoprotein cholesterol, or LDL, while also lowering high-density lipoprotein cholesterol relative to some alternative fats. Elevated levels of atherogenic lipoproteins contribute to the development of atherosclerosis, the process through which cholesterol accumulates in arterial walls and increases the risk of coronary artery disease. Other research identified potential effects on inflammatory pathways and vascular function. The evidence was becoming difficult to dismiss, particularly for an ingredient that offered no essential nutritional benefit.²

A systematic review published in The BMJ in 2015 examined the relationship between dietary saturated and trans fats, mortality, cardiovascular disease, and type 2 diabetes. The researchers found that higher trans fat consumption was associated with a 34 percent increase in relative all-cause mortality risk, a 28 percent increase in coronary heart disease mortality, and a 21 percent increase in coronary heart disease incidence. These figures represent associations from observational studies, rather than proof that an individual person's risk changes by a fixed amount after eating a particular food. Nevertheless, when considered alongside controlled metabolic research, the findings provided compelling evidence that industrial trans fats were contributing to preventable cardiovascular disease.³

Research into other possible consequences, including diabetes, inflammation, and additional chronic conditions, produced less consistent results. That distinction matters because there is a tendency, particularly in food marketing, to take one legitimate scientific concern and expand it until practically every disease can be attributed to the same ingredient. The cardiovascular evidence was already substantial. There was no need to claim that industrial trans fats caused every conceivable health problem to justify removing them from the food supply.

The more troubling question was why the change took so long. Research examining the effects of hydrogenated fats had been developing for decades, yet partially hydrogenated oils remained common in commercial foods well into the twenty-first century. Manufacturers had invested in production methods, recipes, equipment, and supply chains designed around their properties. Replacing them meant reformulating products, testing alternatives, and sometimes accepting additional costs. The industry's original enthusiasm had created a system that was substantially easier to establish than to dismantle.

A Rather Expensive Lesson in Public Health

In 2006, New York City adopted restrictions on artificial trans fats in food-service establishments, with implementation beginning the following year. Restaurants, bakeries, cafeterias, and other operators were required to replace ingredients containing unacceptable levels of industrial trans fats. The regulation followed an unsuccessful effort to secure sufficient reductions through voluntary education. Apparently, asking the food industry nicely had its limitations. The city eventually concluded that removing the ingredient from commercial kitchens would be more effective than expecting every customer to investigate what was happening inside the fryer.⁴

The results were instructive. Researchers studying purchases at 168 fast-food locations found that average trans fat content declined by approximately 2.4 grams per purchase between 2007 and 2009. Saturated fat increased modestly, but the combined quantity of saturated and trans fat still declined. The proportion of purchases containing no reported trans fat rose from 32 percent to 59 percent. Customers continued buying familiar foods, while restaurants and their suppliers changed the fats used to prepare them. There was no requirement for diners to memorize the chemical structure of fatty acids before ordering lunch.⁵

A subsequent study published in JAMA Cardiology examined cardiovascular hospitalizations in New York State counties with and without restaurant trans fat restrictions. Counties implementing the policies experienced an additional 6.2 percent relative decline in combined heart attack and stroke admissions. Because the research was observational, it could not prove that the restrictions alone caused the entire difference. Other changes in medical care, diet, and cardiovascular risk may also have contributed. Still, the findings were consistent with the broader evidence that removing industrial trans fats from widely consumed foods could produce meaningful population-health benefits.⁶

The federal government eventually followed with a more comprehensive intervention. In 2015, the U.S. Food and Drug Administration determined that partially hydrogenated oils were no longer generally recognized as safe for use in human food. Manufacturers were required to phase out most uses, with compliance periods extending through 2021 for certain products and distribution circumstances. Additional regulatory revisions followed in 2023. By then, the commercial advantages that had sustained these fats for generations were no longer considered sufficient justification for their continued widespread use. The food industry adapted, as industries generally do when they are required to.⁷

The consequences extend well beyond the United States. The World Health Organization estimates that industrial trans fat exposure contributes to more than 278,000 deaths worldwide each year. By the end of 2023, recommended elimination policies covered approximately 3.7 billion people across 53 countries. WHO estimated that effective implementation of these policies could prevent nearly 183,000 deaths annually. These are modeled population estimates, rather than individually confirmed deaths, but they demonstrate why reducing exposure has become an international public-health priority.⁸

The Seed Oil Conversation Gets Complicated

The history of partially hydrogenated oils has recently become tangled in a much broader argument about seed oils. Unfortunately, the terms are often used interchangeably, which makes for lively discussion and rather questionable science. Partially hydrogenated soybean oil is chemically different from ordinary soybean oil. Fully hydrogenated oil is different again. Canola, sunflower, safflower, and other seed-derived oils have varying fatty-acid compositions, and those differences influence their nutritional properties and behavior during cooking. Calling all of them dangerous because partially hydrogenated shortening was harmful is rather like condemning every grape because someone once made terrible wine.

Ordinary non-hydrogenated seed oils often contain substantial quantities of monounsaturated or polyunsaturated fatty acids. Clinical evidence generally supports replacing some dietary saturated fat with suitable unsaturated fats to improve cardiovascular risk factors, particularly LDL cholesterol. A 2020 Cochrane review of randomized trials found that reducing saturated fat intake was associated with fewer combined cardiovascular events, with evidence supporting substitution using polyunsaturated fats. The benefits depend on what replaces the saturated fat, and the effects of a dietary change cannot be evaluated without considering the rest of the diet.⁹

One frequent criticism concerns omega-6 fatty acids, particularly linoleic acid, which is present in several common vegetable oils. Linoleic acid participates in biochemical pathways involved in inflammatory signaling, but this does not mean that consuming it necessarily causes systemic inflammation. A systematic review of randomized controlled trials published in the Journal of the Academy of Nutrition and Dietetics found little evidence that increasing dietary linoleic acid raised measured inflammatory markers in healthy adults. The human evidence is considerably less alarming than some of the claims circulating online, where the distinction between a plausible biological mechanism and a demonstrated clinical outcome is occasionally treated as an unnecessary technicality.¹⁰

There is also the question of why seed oils appear so frequently in foods associated with poor dietary quality. Commercial snacks, fried foods, pastries, and packaged meals often contain inexpensive vegetable oils, but they may also contain substantial amounts of refined starch, sodium, added sugar, or saturated fat. Their nutritional characteristics cannot be attributed to one ingredient without examining the complete product. The fact that an oil is used in an unhealthy food does not establish that the oil is responsible for every adverse effect associated with that food. It does, however, provide another reason to pay attention to how commercial food is formulated and why certain ingredients are selected.

What Happens to Oil When We Keep Heating It

There is a legitimate concern about cooking oils that has received considerably less careful attention than it deserves: what happens when they are heated repeatedly. Deep-frying exposes fat to prolonged heat, oxygen, and moisture released from food. Under these conditions, oils undergo oxidation, hydrolysis, and polymerization, producing compounds that change their flavor, viscosity, and chemical composition. Certain oxidation products, including reactive aldehydes, have demonstrated biological toxicity in experimental settings. The extent of oil degradation depends on its fatty-acid composition, the temperature, the food being prepared, and how long the oil remains in use.¹¹

Repeatedly heated oil is not chemically equivalent to fresh oil, regardless of whether it began as olive, sunflower, soybean, or another variety. Oils richer in polyunsaturated fatty acids may be more susceptible to certain oxidation reactions, although refining, antioxidants, and specific fatty-acid formulations can significantly affect stability. High-oleic sunflower oil, for example, behaves differently from conventional high-linoleic sunflower oil. These are distinctions that matter when choosing a commercial frying medium, and they are considerably more useful than making broad declarations about every oil extracted from a seed.

The long-term human health consequences of consuming individual compounds generated during repeated frying remain less precisely established than the cardiovascular risks associated with industrial trans fats. Experimental evidence identifies legitimate concerns, but the actual exposure experienced by diners depends on cooking conditions, oil management, and how much degraded fat enters the finished food. Responsible restaurants should therefore monitor frying temperatures, maintain equipment, and replace oils before excessive degradation occurs. None of this is particularly glamorous. Fortunately, good food safety practices rarely are.

Why We Choose Extra-Virgin Olive Oil

For those of us who favor European cooking, extra-virgin olive oil hardly requires an elaborate introduction. It has been central to Mediterranean food traditions for centuries, and its usefulness extends well beyond dressing salads or finishing a dish at the table. Produced through mechanical extraction, it contains predominantly monounsaturated fatty acids alongside naturally occurring phenolic compounds. These characteristics have attracted substantial scientific interest, particularly in research concerning cardiovascular health and Mediterranean dietary patterns.¹²

Among the most influential studies is PREDIMED, a large dietary intervention trial conducted in Spain among adults at elevated cardiovascular risk. Participants assigned to a Mediterranean diet supplemented with extra-virgin olive oil experienced fewer major cardiovascular events than those assigned to the control dietary intervention. The corrected 2018 analysis reported approximately 31 percent lower relative hazard in the olive-oil-supplemented Mediterranean diet group. That finding supports the benefits of the dietary pattern as a whole. It does not prove that olive oil alone produced the observed effect, nor that substituting it for every other unsaturated oil would deliver identical results. Science has a habit of being rather particular about these things.¹²

Extra-virgin olive oil is also subject to degradation when exposed to excessive heat or repeated frying, and its phenolic compounds can decline during cooking. Choosing it does not eliminate the need for appropriate culinary technique. Nor does it make every dish prepared with it inherently healthy. A plate containing substantial amounts of cream, cheese, and salt does not acquire extraordinary cardiovascular properties simply because someone used excellent olive oil along the way. The overall quality of the meal still matters.

At Apericena, we use extra-virgin olive oil because we believe in the ingredient, appreciate its place in European cooking, and prefer knowing exactly what goes into the food we prepare. Our kitchen does not cook with hydrogenated oils. This is a deliberate sourcing and culinary decision, supported by the evidence concerning industrial trans fats and by research on Mediterranean dietary patterns. It is also a straightforward preference for an ingredient whose flavor and character we value. We do not need to declare every other cooking oil a public-health emergency to explain why we choose it.

There is, however, an important distinction between a restaurant's cooking oil and every ingredient used in its kitchen. Commercial pastries, sauces, prepared components, and other purchased products can contain fats introduced before they reach the restaurant. Meaningful ingredient standards therefore depend on reviewing supplier information as well as choosing what goes into the sauté pan. The details are less romantic than discussing olive groves in Tuscany, but they are considerably more useful when establishing what a restaurant actually serves.

What a Century of Cooking Fat Tells Us

The history of hydrogenated oils offers an unusually clear example of what happens when a food ingredient is evaluated primarily for its commercial usefulness. Manufacturers developed a technology that made cooking fats cheaper, more stable, and easier to incorporate into industrial production. Restaurants adopted those products for understandable operational reasons, and consumers were encouraged to regard them as convenient modern alternatives to traditional fats. For decades, the arrangement worked remarkably well for the businesses involved. The cardiovascular consequences became apparent later, after partially hydrogenated oils had become routine components of the food supply.

The eventual response required scientific research, regulatory action, and substantial changes to commercial production. It also demonstrated that industries capable of engineering ingredients for greater efficiency are capable of engineering alternatives when public-health evidence requires them to. The food industry successfully removed much of the industrial trans fat exposure it had helped create. There is something reassuring about that, although one might reasonably wish the process had moved a little faster.

For restaurants today, the lesson extends beyond a single ingredient. Cooking fats should be selected with attention to nutritional evidence, culinary performance, preparation methods, and the quality of the finished meal. Ingredient sourcing deserves the same scrutiny. The fact that something is inexpensive, convenient, and commercially popular tells us a great deal about why it appears in our food. It tells us considerably less about whether it belongs there.

The responsibility of a restaurant is to make those decisions thoughtfully, long before a plate reaches the table. Customers should be able to enjoy a meal without needing to investigate the chemistry of the cooking oil, decipher a supplier's ingredient list, or conduct a small epidemiological study between courses. They have enough to think about. Deciding what belongs in the kitchen is our job.

References

1. American Oil Chemists' Society. (2019). The battle over hydrogenation (1903–1920). https://www.aocs.org/resource/the-battle-over-hydrogenation-1903-1920/

2. National Research Council. (1989). Diet and health: Implications for reducing chronic disease risk. National Academies Press. https://www.ncbi.nlm.nih.gov/books/NBK218759/

3. de Souza, R. J., Mente, A., Maroleanu, A., et al. (2015). Intake of saturated and trans unsaturated fatty acids and risk of all cause mortality, cardiovascular disease, and type 2 diabetes: Systematic review and meta-analysis of observational studies. BMJ, 351, h3978. https://doi.org/10.1136/bmj.h3978

4. New York City Department of Health and Mental Hygiene. (2006). Board of Health approves regulation to phase out artificial trans fat in New York City's restaurants. New York City Department of Health and Mental Hygiene.

5. Angell, S. Y., Cobb, L. K., Curtis, C. J., Konty, K. J., & Silver, L. D. (2012). Change in trans fatty acid content of fast-food purchases associated with New York City's restaurant regulation: A pre-post study. Annals of Internal Medicine, 157(2), 81–86. https://doi.org/10.7326/0003-4819-157-2-201207170-00004

6. Brandt, E. J., Myerson, R., Perraillon, M. C., & Polonsky, T. S. (2017). Hospital admissions for myocardial infarction and stroke before and after the trans-fatty acid restrictions in New York. JAMA Cardiology, 2(6), 627–634. https://doi.org/10.1001/jamacardio.2017.0491

7. U.S. Food and Drug Administration. (2015). Final determination regarding partially hydrogenated oils (removing trans fat). https://www.fda.gov/food/food-additives-petitions/final-determination-regarding-partially-hydrogenated-oils-removing-trans-fat

8. World Health Organization. (2024). Countdown to 2023: WHO 5-year milestone report on global trans fat elimination. https://www.who.int/publications/i/item/9789240089549

9. Hooper, L., Martin, N., Jimoh, O. F., Kirk, C., Foster, E., & Abdelhamid, A. S. (2020). Reduction in saturated fat intake for cardiovascular disease. Cochrane Database of Systematic Reviews, 2020(5), CD011737. https://doi.org/10.1002/14651858.CD011737.pub3

10. Johnson, G. H., & Fritsche, K. (2012). Effect of dietary linoleic acid on markers of inflammation in healthy persons: A systematic review of randomized controlled trials. Journal of the Academy of Nutrition and Dietetics, 112(7), 1029–1041.e15. https://doi.org/10.1016/j.jand.2012.03.029

11. Bazina, N., Ahmed, T., Almdaaf, M., Abu Hallalah, H. M. O., & Jibia, S. (2025). Chemical changes in deep-fat frying: Reaction mechanisms, oil degradation, and health implications. Food Science & Nutrition, 13(10), e70969. https://doi.org/10.1002/fsn3.70969

12. Estruch, R., Ros, E., Salas-Salvadó, J., et al. (2018). Primary prevention of cardiovascular disease with a Mediterranean diet supplemented with extra-virgin olive oil or nuts. The New England Journal of Medicine, 378, e34. https://doi.org/10.1056/NEJMoa1800389