Traditional shortening made from partially hydrogenated oils is genuinely harmful, primarily because of its trans fat content, which raises bad cholesterol, lowers good cholesterol, promotes inflammation, and increases the risk of heart disease. The story gets more complicated with modern shortening, though. Since the FDA effectively banned partially hydrogenated oils in 2018, most shortening sold today relies on palm oil, fully hydrogenated oils processed through interesterification, or blends of the two. These reformulated products avoid the worst offender, but they come with their own set of questions that researchers are still working through.
Why Partially Hydrogenated Shortening Was So Damaging
For most of the twentieth century, shortening was made by pumping hydrogen gas into liquid vegetable oil under pressure, a process called partial hydrogenation. This turned the oil into a solid fat that was cheap, shelf-stable, and useful for everything from pie crusts to deep fryers. The side effect was that it created large amounts of industrially produced trans fatty acids, particularly a molecule called elaidic acid. Manufacturers relied on hydrogenation specifically because it extended the shelf life of baked and fried foods.1PubMed Central. Replacements for trans fats-will there be an oil shortage?
The problem with trans fats goes beyond being “just another unhealthy fat.” In a landmark trial, subjects eating a diet high in trans fatty acids saw their LDL (“bad”) cholesterol climb about 14 mg/dL higher and their HDL (“good”) cholesterol drop about 7 mg/dL lower compared to a diet using oleic acid, a common monounsaturated fat found in olive oil.2PubMed. Effect of dietary trans fatty acids on high-density and low-density lipoprotein cholesterol levels in healthy subjects That double hit, raising LDL while simultaneously lowering HDL, is unusual among dietary fats and makes trans fats especially dangerous for cardiovascular health. Even saturated fat, which also raises LDL, doesn’t typically suppress HDL at the same time.
How Trans Fats Damage Blood Vessels
The cholesterol shift is only part of the picture. At the cellular level, the industrial trans fats found in old-style shortening actively inflame and injure the lining of your blood vessels. Lab studies on endothelial cells, the cells that line your arteries, show that elaidic acid and a related trans fat called linoelaidic acid both trigger an inflammatory signaling cascade. They ramp up production of molecules that drive inflammation and simultaneously reduce nitric oxide, a substance your blood vessels need to stay relaxed and open.3PubMed Central. Trans fatty acids induce vascular inflammation and reduce vascular nitric oxide production in endothelial cells Both trans fats also boosted production of superoxide, a reactive oxygen species that appears to be part of the mechanism driving the inflammatory response.
Evidence from both observational and experimental studies broadly supports the conclusion that trans fats are pro-inflammatory and induce endothelial dysfunction, the stiffening and impairment of artery walls that precedes full-blown heart disease.4PubMed. Trans fatty acids – effects on systemic inflammation and endothelial function One mechanism behind the cholesterol disruption involves trans fats tricking liver cells into ramping up cholesterol production. Elaidic acid activates a pathway in liver cells that turns on cholesterol-making genes, an effect linked to its ability to lower intracellular free cholesterol and reduce the sensitivity of a key regulatory protein.5Advances in Nutrition. Mechanisms of Action of trans Fatty Acids So the liver essentially starts overproducing cholesterol in response to the trans fat signal, flooding the bloodstream with more LDL particles.
Heart Disease, Diabetes, and Liver Damage
Given those cellular effects, it’s no surprise that trans fat consumption tracks with real-world disease. A meta-analysis of cohort studies found that industrial trans fat intake showed a trend toward increased coronary heart disease risk, though the authors noted the limited number of available studies prevented completely firm conclusions.6European Journal of Clinical Nutrition. Consumption of industrial and ruminant trans fatty acids and risk of coronary heart disease: a systematic review and meta-analysis of cohort studies Interestingly, the same analysis found no significant association between naturally occurring trans fats from ruminant animals (the tiny amounts in butter and beef) and heart disease, reinforcing that the danger is specific to the industrial variety that dominated shortening for decades.
Trans fats also appear to interfere with how your body handles blood sugar. Research in a human population found that trans fat intake was independently associated with insulin resistance, even after accounting for other dietary factors like saturated fat and added sugar.7PubMed Central. Trans fatty acid intake is associated with insulin sensitivity but independently of inflammation That said, the broader evidence on whether trans fats directly cause type 2 diabetes remains mixed, with inconsistencies across studies preventing definitive conclusions.8PubMed. Trans fatty acids, insulin resistance, and type 2 diabetes
The liver may be another casualty. Animal research has shown that diets high in trans fats can promote nonalcoholic fatty liver disease. Rats fed margarine with high trans fat levels showed greater lipid peroxidation and liver damage, suggesting that trans fats act as a direct source of oxidative stress.9PubMed Central. The intake of high fat diet with different trans fatty acid levels differentially induces oxidative stress and non alcoholic fatty liver disease (NAFLD) in rats A separate mouse study found that animals on a high-fat trans fat diet developed severe fat accumulation in the liver along with increased expression of fat-producing genes, while animals on a lower-fat version of the same trans fat diet did not, suggesting that total fat intake and trans fat content interact.10PubMed. Possible involvement and the mechanisms of excess trans-fatty acid consumption in severe NAFLD in mice
What About Your Brain?
Several lines of evidence point to a connection between trans fat consumption and cognitive decline. A review of dietary fat and dementia risk found support for the idea that high trans fat intake may increase the risk of dementia, while diets rich in polyunsaturated or monounsaturated fats may lower it.11PubMed Central. Dietary fat composition and dementia risk One particularly striking prospective study found that people whose trans fat intake was in the second quintile or higher had roughly double the age-adjusted risk of developing Alzheimer disease compared to those with the lowest intake, and when the researchers adjusted for other types of dietary fat, the relative risk for higher quintiles ranged from about three to five times higher.12JAMA Neurology. Dietary Fats and the Risk of Incident Alzheimer Disease
That said, a systematic review examining the full body of research on saturated and trans fats and dementia described the results across studies as mixed.13PubMed. Saturated and trans fats and dementia: a systematic review So the signal is there, but not all studies agree on its strength. Cognitive research is difficult to do well because dietary habits and dementia develop over decades, and isolating one nutrient’s effect from everything else a person eats is inherently messy.
Trans Fats and Gut Bacteria
An area of more recent interest is how trans fats affect the gut microbiome. A study in mice found that an industrial trans fat-enriched diet significantly altered the composition of gut bacteria, increasing the abundance of bacteria associated with inflammation and decreasing bacteria generally considered beneficial. The trans fat diet also raised markers of inflammation and tended to reduce certain short-chain fatty acids that play a role in gut health.14PubMed. Effect of industrial trans-fatty acids-enriched diet on gut microbiota of C57BL/6 mice
The picture has some complexity, though. A separate mouse study found that when diets lacked lard and relied on other fat sources including trans fats, the trans fats were less disruptive to the gut microbiome than expected, and they did not trigger the same inflammatory pathway seen with a typical high-fat diet containing lard.15PubMed. Impact of Trans-Fats on Heat-Shock Protein Expression and the Gut Microbiota Profile of Mice These are animal studies, so their translation to human gut health is uncertain, but the idea that trans fats might destabilize the gut ecosystem adds another potential pathway through which shortening-heavy diets could cause harm.
The Regulatory Push That Changed Shortening
The health dangers of partially hydrogenated oils eventually became too well established to ignore. In the United States, the removal of artificial trans fats from the food supply is estimated to prevent roughly 50,000 premature deaths per year.16PubMed Central. The Demise of Artificial Trans Fat: A History of a Public Health Achievement Real-world results have backed that projection. After localized restaurant bans on trans fats went into effect, researchers observed a roughly 4.5% reduction in cardiovascular disease death rates, translating to about 13 fewer cardiovascular deaths per 100,000 people per year. When assessed over a longer period, heart attacks and strokes combined dropped by about 6%, or roughly 43 fewer events per 100,000 people.17PubMed Central. Systematic review of dietary trans-fat reduction interventions
These numbers represent one of the clearest public health wins in modern nutrition policy. The FDA’s 2015 determination that partially hydrogenated oils are no longer “generally recognized as safe,” with full enforcement starting in 2018, effectively reshaped the shortening market. If you buy shortening today, it almost certainly contains no partially hydrogenated oil.
Modern Shortening and the New Questions
So if the trans fats are gone, is today’s shortening safe? The answer is less definitive than you might hope. Most reformulated shortening relies on one or both of two strategies: using palm oil (or palm kernel oil) for its natural solidity, or using interesterified fats, which are made by rearranging the fatty acid molecules on a fat’s glycerol backbone to change its melting point without creating trans fats.
Modeling studies suggest that replacing partially hydrogenated soybean oil with palm-based alternatives in applications requiring solid fat reduces predicted cardiovascular disease risk by a modest amount, though less dramatically than some simpler models had suggested. Replacing partially hydrogenated oils across all applications (both frying and solid-fat uses) was predicted to cut trans fat intake by about 1% of energy and reduce cardiovascular risk by roughly 1.5%.18PubMed Central. Predicted changes in fatty acid intakes, plasma lipids, and cardiovascular disease risk following replacement of trans fatty acid-containing soybean oil with application-appropriate alternatives A direct comparison in human subjects found that a palm oil-based margarine produced significantly better HDL cholesterol and apoA-1 levels than a partially hydrogenated soybean oil margarine, with no significant difference in LDL.19PubMed. Replacement of partially hydrogenated soybean oil by palm oil in margarine without unfavorable effects on serum lipoproteins Palm oil is clearly better than partially hydrogenated oil, but it is still high in palmitic acid, a saturated fat, which means it is not exactly a health food.
Interesterified fats remain something of an open question. Most studies comparing interesterified and non-interesterified versions of the same fat have found no consistent differences in blood lipid markers, suggesting a neutral effect on cardiovascular risk.20Nutrition Bulletin. Interesterified fats: What are they and why are they used? A briefing report from the Roundtable on Interesterified Fats in Foods But there’s a catch: most of those studies used interesterified fats that don’t closely resemble what actually ends up in commercial products. When researchers at King’s College London tested a commercially relevant interesterified palm-based hard stock, they found it increased blood triglyceride levels for several hours after a meal compared to its non-interesterified equivalent.21PubMed Central. What are interesterified fats and should we be worried about them in our diet? That’s a single research group’s finding and not yet conclusive, but it suggests that the “everything is fine now” narrative might be premature.
What Happens When You Fry with Shortening
Even setting aside the type of fat in your shortening, how you use it matters. Deep frying at high temperatures over extended periods breaks down any cooking fat, producing polar compounds, free fatty acids, and other degradation products. The longer an oil or shortening sits in a fryer being reused, the more these breakdown products accumulate, and research has shown a strong correlation between polar compound levels and total frying time.22Journal of the American Oil Chemists’ Society. Quality control in the use of deep frying oils
A study comparing ten vegetable oils during deep frying found that vegetable ghee (a product similar to shortening in many markets) had the highest malondialdehyde and peroxide values of all oils tested, indicating more oxidative breakdown. The levels of total polar compounds, while still within legal limits for all oils tested, were higher in vegetable ghee, palm oil, and refined olive pomace oil than in other options.23Journal of Food Biochemistry. Oxidative Changes in Ten Vegetable Oils Caused by the Deep-Frying Process of Potato So if you’re using shortening for deep frying, the thermal stability of the specific product matters, and repeated use without replacement accelerates the formation of compounds you don’t want in your food.
Butter, Margarine, Olive Oil, and the Bigger Cooking-Fat Picture
Where shortening fits in the hierarchy of cooking fats depends partly on what you’d replace it with. A large prospective study following over 500,000 people found that higher consumption of both butter and margarine was associated with increased cardiovascular death rates, about 8% higher for the top butter consumers and about 10% higher for the top margarine consumers, compared with non-consumers. Olive oil consumption, by contrast, was associated with about 5% lower cardiovascular mortality. Canola oil showed a marginal association with lower risk, and corn oil showed no significant relationship either way.24PubMed Central. Cooking oil/fat consumption and deaths from cardiometabolic diseases and other causes: prospective analysis of 521,120 individuals
These numbers are modest in absolute terms, but they illustrate a consistent theme in fat research: liquid unsaturated oils generally outperform solid fats for cardiovascular health. Shortening, by definition, is a solid fat. Whether it’s made from hydrogenated oil, palm oil, or interesterified blends, its fundamental composition leans toward saturated fatty acids, which is exactly what gives it the texture bakers prize. If you’re baking an occasional pie, the health difference is negligible. If shortening is a daily staple in your cooking, substituting liquid oils where possible is a straightforward way to shift the balance.
Trans Fats in Breast Milk
One underappreciated way trans fats travel through the body involves breastfeeding. A study analyzing the breast milk of 33 lactating women found that industrially produced trans fats, particularly elaidic acid, showed up at measurable levels in colostrum, transitional milk, and mature milk. Total trans fat content averaged about 1.5% of fat in colostrum and just under 1% in mature milk. Perhaps more concerning, the levels of elaidic acid and total trans fats in colostrum were negatively correlated with levels of DHA, an omega-3 fatty acid critical for infant brain development.25PubMed Central. Content of industrially produced trans fatty acids in breast milk: An observational study The researchers found no correlation between the mothers’ estimated trans fat consumption and the trans fat levels in their milk, which suggests the relationship between dietary intake and breast milk composition isn’t straightforward. Still, the inverse relationship between trans fats and DHA in breast milk adds another dimension to why reducing population-level trans fat exposure matters.
Shortening in Practice
For most home cooks and bakers, the practical question comes down to frequency and alternatives. Modern trans fat-free shortening is a significant improvement over what was on shelves twenty years ago, and occasional use in baking is unlikely to pose a meaningful health risk. The saturated fat content is comparable to butter, and for people who need a dairy-free solid fat for baking, shortening fills a real gap. Where the concern sharpens is with heavy, repeated use, particularly in deep frying, where thermal degradation of the fat itself compounds the issue of its baseline fatty acid profile.
If you’re reading ingredient labels, the key things to watch for are the presence of palm oil (common and not necessarily a red flag in moderation, but high in saturated fat), fully hydrogenated oils (different from partially hydrogenated and free of trans fats, though the interesterification question remains open), and any lingering partially hydrogenated oils, which should be rare in the U.S. market but can still appear in imported products or specialty items. Some countries have been slower to enact trans fat bans, so packaged goods from certain markets deserve a closer look at the ingredients list.