Cooking Oils: What to Use & What to Lose
- Paula Grubb Nutrition
- Aug 6
- 6 min read
When it comes to cooking oils, not all fats are created equal. Many of the oils commonly found in kitchens and processed foods—like canola, soybean, sunflower, and corn oil—are highly processed seed oils that may contribute to inflammation and digestive discomfort. But it’s not all bad news! Understanding which oils to avoid and which to embrace can make a significant difference for your gut health, immune balance, and overall well-being.
In this blog, we’ll dive into what seed oils are, how they’re made, why they might be causing inflammation and other health issues, and most importantly, which cooking oils you should be using to support your body instead.
Whether you’re managing digestive challenges, autoimmune conditions, or just want to eat healthier fats, this guide will help you make informed, practical choices in the kitchen.
What Are Seed Oils?
Seed oils—often labeled simply as “vegetable oils”—are oils extracted from the seeds of plants such as soybean, corn, sunflower, canola (rapeseed), safflower, cottonseed, and grapeseed. These oils became widespread in the 20th century due to their low cost, neutral flavor, and long shelf life, which made them appealing to food manufacturers.
Unlike traditional oils such as olive oil or coconut oil—which come from the fruit or nut of the plant—seed oils come from seeds that are generally low in fat naturally. Because of this, extracting oil from them requires industrial processing, making these oils very different in quality and stability compared to traditional cold-pressed oils.
Seed oils are high in omega-6 polyunsaturated fatty acids (PUFAs), particularly linoleic acid. While these fats are essential in small amounts, modern diets have pushed consumption far beyond healthy levels.
How Are Seed Oils Made?
Seed oils don’t just come straight from the seed like cold-pressed olive oil. Instead, their extraction involves an intensive industrial process:
Cleaning and Grinding: Seeds are cleaned to remove dirt and debris, then crushed into a paste or meal to release oils.
Heat Treatment: The crushed seed is heated to high temperatures (often 120–200°C or more) to increase oil yield. This heat exposure begins to break down natural fats.
Chemical Extraction: Instead of just pressing, solvents like hexane (a petroleum derivative) are added to extract the remaining oil efficiently from the paste. This process allows manufacturers to get more oil than pressing alone.
Refining: The crude oil contains impurities, free fatty acids, pigments, and odors, so it undergoes refining steps. This includes bleaching (to remove color), deodorizing (to remove smell), and filtering, often at very high temperatures. These steps strip away beneficial compounds like antioxidants and vitamins and create unstable molecules prone to oxidation.
Packaging and Storage: The refined oil is then bottled, often in clear plastic or glass, and shipped out. Exposure to light and air during storage can further oxidize the oil.
This process transforms the original seed oil into a highly refined, chemically altered fat that behaves differently in your body than minimally processed oils.
Why Are Seed Oils a Concern?
High in Omega-6 Fatty Acids
Omega-6 fatty acids, like linoleic acid found abundantly in seed oils, are essential fats that the body cannot make on its own and must obtain from food. They play roles in skin health, brain function, and hormone production.
However, the problem lies in the ratio of omega-6 to omega-3 fats in the diet. Historically, humans ate these fats in a roughly 1:1 or 2:1 ratio. Today, Western diets often contain 15:1 or even 20:1 omega-6 to omega-3, largely due to seed oils and processed foods.
An excess of omega-6 shifts your body's balance toward producing more pro-inflammatory compounds called eicosanoids, which promote chronic low-grade inflammation. This persistent inflammation is a root driver in many chronic diseases, including digestive disorders, autoimmune conditions, cardiovascular disease, and metabolic syndrome.
Oxidation and Harmful Compounds
The refining and high-heat cooking of seed oils causes their unstable polyunsaturated fats to oxidize. Oxidation is a chemical reaction where fats react with oxygen, producing harmful by-products such as:
Lipid peroxides: Reactive molecules that damage cell membranes and DNA.
Aldehydes: Toxic compounds linked to inflammation, cell damage, and even cancer risk.
Trans fats: Though mostly reduced in modern oils, small amounts may still form during processing.
These oxidation products are harmful not only when consumed but can also damage the cells lining your gut, increasing intestinal permeability or “leaky gut,” which can activate the immune system unnecessarily and worsen autoimmune or inflammatory conditions.
Impact on Gut and Immune Health
A sensitive or inflamed gut lining can lead to digestive symptoms like bloating, pain, reflux, and irregular bowel habits. Oxidized seed oils may exacerbate these issues by disrupting the delicate gut barrier.
Additionally, when the gut barrier is compromised, unwanted substances pass into the bloodstream, triggering immune responses that increase systemic inflammation. This inflammation fuels symptoms and flare-ups in autoimmune diseases such as rheumatoid arthritis, Hashimoto’s thyroiditis, and lupus.
Health Conditions Linked to Seed Oils
Seed oils’ pro-inflammatory and oxidative effects mean they can worsen or contribute to several chronic health issues, especially where inflammation is a key factor:
Autoimmune diseases: Excess omega-6 and oxidative stress can trigger immune system imbalances that worsen conditions like rheumatoid arthritis, lupus, and autoimmune thyroid disease.
Digestive disorders: Conditions like Irritable Bowel Syndrome (IBS), Inflammatory Bowel Disease (IBD), gastroesophageal reflux disease (GERD), and food sensitivities can be aggravated by gut irritation from oxidized oils.
Skin conditions: Chronic inflammation worsened by seed oils can flare eczema, psoriasis, and acne.
Metabolic syndrome and insulin resistance: Inflammatory fats can impair insulin signaling and promote weight gain.
Cardiovascular disease: While controversial, some research links oxidized fats from seed oils to artery damage and heart disease through increased inflammation and oxidative stress.
Why You Should Avoid Cooking with Seed Oils
Many seed oils have a low smoke point (the temperature where they start to smoke and break down), but more importantly, their unstable omega-6 fats are prone to oxidation even at moderate heat.
When you cook with seed oils, especially frying or roasting at high temperatures, you increase the formation of toxic oxidation products in your food. These compounds contribute to:
Oxidative damage to your cells
Increased inflammation throughout your body
Potentially harmful inhalation exposure while cooking
Deep frying takes these risks even further. When oils like canola, soybean, or sunflower are heated to very high temperatures—as in deep fryers—they rapidly break down and form toxic aldehydes, trans fats, and other harmful by-products. These compounds are known to worsen oxidative stress, irritate the gut lining, and trigger systemic inflammation.
What’s worse, deep-fried foods from restaurants or takeaways are often cooked in oils that have been reheated multiple times, further increasing the concentration of these damaging substances.
And here's something to think about: thin foods like French fries (shoestring-style chips) absorb significantly more oil than chunky chips or roasted potatoes. Because thinner fries have a larger surface area relative to their size, they soak up more of the heated, oxidised oil. This means you're not just getting more fat — you're getting more of the harmful by-products produced during the cooking process.
If you’re dealing with gut symptoms, inflammation, or autoimmune issues, avoiding deep-fried foods altogether is one of the most impactful changes you can make. Opt instead for baking, grilling, or air frying using stable fats like olive oil, ghee, or avocado oil for a safer, gut-friendly approach.
How to Spot Low-Quality Seed Oils
Since seed oils are so common and often hidden in processed foods, it’s important to recognize warning signs on labels:
Refined, bleached, deodorized: Indicates heavy processing and loss of nutrients.
Partially hydrogenated: Contains trans fats, which are linked to heart disease and inflammation.
“Vegetable oil” or “vegetable oil blend” without specifics: Often a mix of cheap, refined seed oils.
Clear plastic bottles: Allows light to oxidize the oil faster.
Unnaturally long shelf life: High processing often includes preservatives or antioxidants that mask rancidity.
What Oils Should You Use Instead?
Choosing stable, minimally processed oils rich in antioxidants and balanced fatty acids supports your health and digestion.
Extra Virgin Olive Oil: Cold-pressed and minimally processed, rich in monounsaturated fats and polyphenols. Ideal for dressings, low to medium heat cooking, and drizzling. Its antioxidants help protect against oxidation and inflammation.
Avocado Oil: High smoke point and stable fat profile make it excellent for medium to high heat cooking. Also contains heart-healthy monounsaturated fats.
Coconut Oil: Contains medium-chain triglycerides (MCTs) that are easily digested and may support gut health. Best for moderate heat and baking.
Butter and Ghee: Rich in fat-soluble vitamins (A, D, E, K) and stable saturated fats. Ghee, clarified butter, has a higher smoke point. Use in moderation for cooking and flavor.
Grass-fed Animal Fats: Tallow and lard from grass-fed animals contain beneficial fatty acids and are heat stable.
Nut Oils (Macadamia, Walnut): Use cold-pressed oils for flavor in salads or finishing dishes. Avoid heating to preserve nutrients.
Practical Tips to Reduce Seed Oil Intake
Cook more at home using olive, avocado, or coconut oil.
Read labels carefully, avoiding processed foods with hidden seed oils.
Ask restaurants about the oils used, especially for fried or baked foods.
Replace packaged snacks with whole food options like nuts, seeds, and fresh produce.
Store oils properly in dark bottles, away from heat and light to reduce oxidation.
Balance omega-3 intake by eating fatty fish, flaxseeds, chia seeds, and walnuts to improve your fatty acid ratio.
Final Thoughts
Seed oils have quietly become a staple of modern diets but may be silently contributing to inflammation, digestive issues, and chronic disease. By understanding their risks and choosing healthier, stable fats, you can support your gut, reduce inflammation, and improve your overall health.
If you’d like guidance tailored to your unique health needs, I’m here to help. At Paula Grubb Nutrition, we focus on practical, personalized nutrition strategies to help you feel your best every day.
