You May Need an Oil Change
The Truth About Cooking Oils, Trans Fats, and the Low-Fat Diet That Backfired
Healthy cooking oils play a crucial role in supporting heart health and reducing harmful fats in your daily diet.
If you grew up in a certain era, you remember it. The blue can of grease that lived near the stove. The block of lard that made the flakiest biscuits, the crispiest fried chicken, the most tender pie crust. The spoonful of bacon fat stirred into fresh-snapped green beans. Butter cascading down a mountain of mashed potatoes.
And then, seemingly overnight — it was all gone.
Butter, lard, and animal fats were declared public health enemies. The study was in. The researchers, the physicians, and the organizations with “Association” attached to their names all agreed: Americans needed fewer saturated fats and more carbohydrates. The low-fat era had begun.
In their place came vegetable shortening, corn oil, soy oil, cottonseed oil, safflower oil, margarine, and canola oil. “Everything is better with Blue Bonnet on it.” “It’s not nice to fool Mother Nature.” “I Can’t Believe It’s Not Butter.”
Decades later, we are still sorting out what that oil change actually cost us.
How We Got Here: The Ancel Keys Story
To understand how dietary fat became the villain, you have to go back to 1955 — and to the White House.
President Dwight D. Eisenhower suffered a highly publicized heart attack in the fall of that year. He was known to enjoy rich food and smoked regularly. The nation took notice. Heart disease was claiming the lives of businessmen and executives at alarming rates, and the medical establishment was under pressure to find a cause.
Around the same time, a researcher named Ancel Keys — a physicist-turned-physiologist at the University of Minnesota — was conducting what would become one of the most influential and most contested studies in nutritional history: the Seven Countries Study (1958–1970). Keys documented a correlation between saturated fat consumption and cardiovascular disease rates across seven countries and concluded that dietary saturated fat was a primary driver of heart disease.
His conclusions became the scientific foundation for four decades of low-fat dietary guidance — influencing the USDA Food Pyramid, non-profit dietary recommendations, and the reformulation of the American food supply.
What the study did not fully account for:
- The role of refined carbohydrates and sugar as independent cardiovascular risk factors
- The confounding effect of smoking (widespread among the populations studied)
- Data from countries that contradicted his hypothesis, which critics later argued were selectively excluded from the final analysis
- The distinction between different types of fat and their differing effects on health
A subsequent meta-analysis by Siri-Tarino et al., published in the American Journal of Clinical Nutrition (2010), analyzed 21 prospective cohort studies and found no statistically significant association between dietary saturated fat intake alone and cardiovascular disease risk — a finding that reignited the debate and contributed to a substantial revision of dietary fat guidance.
📌 Important nuance: The science here is genuinely contested and continues to evolve. The AHA and many cardiologists still recommend limiting saturated fat and replacing it with unsaturated fats for cardiovascular health. What has changed is the recognition that the low-fat/high-carb substitution of the 1970s–1990s was itself harmful — replacing fat with refined carbohydrates did not improve health outcomes and may have made them worse. The lesson is about what replaces fat, not simply whether fat itself is dangerous.
The Real Villain: Partially Hydrogenated Oils and Trans Fats
Here is where the oil change story gets most important — and where a critical distinction must be made.
Not all vegetable oils are created equal. The article’s central concern is not liquid vegetable oils like olive oil or even canola oil — it is partially hydrogenated oils, which were used for decades in margarine, shortening, packaged baked goods, and processed snacks.
Hydrogenation is an industrial process that adds hydrogen to liquid vegetable oil to make it solid at room temperature and extend its shelf life. The byproduct is artificial trans fatty acids — a form of fat that has no safe level of consumption according to the FDA, the WHO, and virtually every major health authority.
Trans fats raise LDL (“bad”) cholesterol, lower HDL (“good”) cholesterol, promote systemic inflammation, and have been directly linked to increased risk of:
- Cardiovascular disease and heart attack
- Stroke
- Type 2 Diabetes
- Systemic inflammation — a contributing factor in many chronic diseases
Critical update: After decades of pressure from researchers and consumer advocates, the FDA officially removed partially hydrogenated oils (PHOs) from the GRAS (Generally Recognized as Safe) list and required their phase-out from the U.S. food supply, completing the process by 2020. The primary source of artificial trans fats in American food has been largely eliminated from manufactured products.
This is significant progress — but it does not mean all processed food is now safe. It means one specific category of harmful fat has been regulated out. Reading labels remains essential.
Liquid Vegetable Oils: A More Nuanced Picture
The remaining question is about the liquid vegetable oils that replaced animal fats — corn, soy, cottonseed, safflower, and canola oils. These do not contain artificial trans fats, but they are not without legitimate nutritional concerns:
- Most are high in omega-6 polyunsaturated fatty acids. While omega-6 fats are essential, the modern diet consumes them at a ratio of roughly 15:1 to 20:1 over omega-3 fats — far above the 4:1 or lower ratio associated with better health outcomes. This imbalance is associated with increased inflammatory signaling.
- Polyunsaturated oils have relatively low smoke points and are prone to oxidation when heated to high temperatures — producing compounds that may contribute to oxidative stress and cellular damage.
- Many are heavily refined and processed, removing naturally occurring antioxidants that would otherwise protect the oil from oxidation.
📌 A note on canola oil: Canola was developed as a food crop in Canada in the 1970s by selectively breeding rapeseed to remove erucic acid, making it safe for human consumption. It is not the same as industrial rapeseed oil. Canola in its cold-pressed form has a reasonable fatty acid profile, though most commercially available canola oil is heavily refined. As with all oils, quality and processing method matter.
Better Choices: What to Cook With
For everyday cooking, the oils best supported by current evidence are those that are minimally processed, stable at cooking temperatures, and recognized by the body as close to whole food sources:
| Oil | Best Use | Key Benefit |
| Extra virgin olive oil | Low-to-medium heat, dressings | Rich in oleic acid and antioxidants; Mediterranean diet staple |
| Coconut oil | Medium heat | Stable saturated fat; recognized by the body; use in moderation |
| Avocado oil | Medium-to-high heat | High smoke point; rich in oleic acid |
| Grass-fed butter / ghee | Low-to-medium heat, finishing | Contains fat-soluble vitamins; short and medium-chain fatty acids |
| Lard (from pasture-raised pigs) | Medium heat | Largely monounsaturated; stable for cooking; traditional use |
Oils to minimize, particularly for high-heat cooking: Corn oil, soybean oil, cottonseed oil, generic “vegetable oil,” and any product listing “partially hydrogenated” in the ingredients — though the latter should now be largely absent from U.S. manufactured foods.
And yes — if you are still regularly deep-frying in refined vegetable oils at high temperatures, that is a conversation worth having with your doctor. The combination of refined omega-6 oils and high heat is where the oxidation concern is most significant.
The Cholesterol Connection
One reason the low-fat narrative persisted so long is the association between saturated fat, LDL cholesterol, and heart disease. This relationship is real but more nuanced than originally presented.
The liver produces cholesterol — it is essential to every cell in the body, to hormone production, to vitamin D synthesis, and to the formation of bile. The question is not whether cholesterol exists but whether specific dietary patterns drive the kind of LDL cholesterol particle distribution associated with arterial plaque.
Current research distinguishes between large, buoyant LDL particles (less associated with cardiovascular risk) and small, dense LDL particles (more strongly associated with risk) — a distinction the original Keys hypothesis did not make. Refined carbohydrates and excess sugar are now understood to drive small, dense LDL production, while the relationship between saturated fat and this specific particle type is more complex than early guidance suggested.
This does not mean saturated fat is unlimited or risk-free. It means the picture is more complicated than “fat clogs arteries” — and that the low-fat dietary shift, which replaced fat with refined carbohydrates and sugar, may have addressed one variable while worsening another.
What You Can Do Right Now
- Check your pantry. Look for “partially hydrogenated” in any ingredient list and phase those products out. As of 2020 they should be rare, but some older stock and imported products may still contain them.
- Understand the difference between fat types. Not all fat is harmful. Monounsaturated and omega-3 fats are well-supported by research. Artificial trans fats are harmful. The saturated fat question is nuanced and depends on the overall dietary context.
- Consider your omega-6 to omega-3 ratio. Reducing processed seed oils and increasing oily fish, walnuts, and flaxseed moves the ratio in a healthier direction.
- Cook at appropriate temperatures. Use oils with suitable smoke points for the method. Save extra virgin olive oil for dressings and finishing; use avocado oil for higher-heat cooking.
- Talk to your doctor about your specific lipid panel. Total cholesterol is a less useful marker than the breakdown of LDL particle type, HDL, and triglycerides. Ask for a comprehensive lipid panel if cardiovascular risk is a concern.
The oil change of the 20th century was made with good intentions and incomplete data. The oil change of the 21st century should be made with better information — and that starts with reading labels, asking questions, and understanding what is actually in the bottle.
I’m just the messenger.
About the Author
Lynn Peterson, B.A. (English/Communications), M.S. & D.Sc. (Education), is a Certified Post-Secondary Instructor and Certified Health Coach, writer, and health advocate.
Please note: Lynn is a Certified Health Coach, not a licensed physician. This content reflects her research and educational background and is not a substitute for advice from a qualified healthcare provider.
Her mission: “We are developing and dying from preventable, chronic diseases.” Her goal is to demystify medicine, illuminate the diet–disease connection, and help people make informed choices about what they eat. Food: The Final Frontier.
References & Further Reading
- Siri-Tarino, P.W., et al. (2010). Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease. American Journal of Clinical Nutrition, 91(3), 535–546. https://doi.org/10.3945/ajcn.2009.27725
- U.S. FDA. (2018). Final Determination Regarding Partially Hydrogenated Oils. https://www.fda.gov/food/food-additives-petitions/final-determination-regarding-partially-hydrogenated-oils
- WHO. (2023). Eliminating Industrially-Produced Trans Fats. https://www.who.int/news-room/questions-and-answers/item/nutrition-trans-fat
- Simopoulos, A.P. (2016). An increase in the omega-6/omega-3 fatty acid ratio increases the risk for obesity. Nutrients, 8(3), 128. https://doi.org/10.3390/nu8030128
- Chowdhury, R., et al. (2014). Association of dietary, circulating, and supplement fatty acids with coronary risk. Annals of Internal Medicine, 160(6), 398–406. https://doi.org/10.7326/M13-1788
- American Heart Association. (2021). Dietary Fats and Cardiovascular Disease. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/dietary-fats
- Weil, A. Is Lard Healthy? https://www.drweil.com/diet-nutrition/nutrition/is-lard-healthy/
- NIH National Library of Medicine. Trans Fatty Acids and Cardiovascular Disease. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2824611/
- Keys, A. (1970). Coronary heart disease in seven countries. Circulation, 41(4 Suppl), I1-211. https://doi.org/10.1161/01.CIR.41.4S1.I-1
⚕️ MEDICAL DISCLAIMER
This article is for general informational and educational purposes only. It does not constitute medical advice and should not replace consultation with a licensed physician or qualified healthcare provider. The author is a Certified Health Coach, not a licensed medical doctor. Dietary fat recommendations vary by individual health status. Always consult your doctor or a registered dietitian before making significant changes to your diet, particularly if you have cardiovascular disease, diabetes, or high cholesterol.