For decades, dietary fats were simply "good" or "bad." Science is now revealing a far more fascinating story.
From sizzling bacon to a crisp salad dressing, fats and oils are central to our food experience. Yet, this kitchen staple is also one of nutrition's most controversial topics, surrounded by conflicting advice. Are seed oils toxic? Is olive oil always the healthier choice? The answers lie in the intricate chemistry of these molecules and how our bodies use them. This article cuts through the noise, exploring the science of fats—from their basic structure to the latest research that is reshaping our understanding of how they affect our health.
At their core, fats and oils are chemical compounds known as triglycerides2 . Imagine a single glycerol molecule, which forms the backbone, attached to three fatty acid chains. This structure is the functional unit of both the solid fat from an animal and the liquid oil from a plant.
The carbon atoms in the chain are fully "saturated" with hydrogen atoms, resulting in straight chains that pack tightly together.
This dense packing leads to a solid state at room temperature.
The chains have one or more double bonds between carbon atoms, creating "kinks." These kinks prevent the molecules from packing closely.
This makes them liquid at room temperature.
Glycerol Backbone + 3 Fatty Acid Chains
A vocal movement on social media has demonized seed oils like soybean, canola, and sunflower oil, blaming them for inflammation and chronic disease. However, nutritional scientists argue this is based on a misunderstanding of the evidence1 .
Seed oils are often blamed for inflammation and chronic diseases.
Large-scale studies show linoleic acid in seed oils reduces cardiovascular risk.
Lowers cardiovascular disease risk
Reduces LDL, increases HDL
May help lower blood pressure
Seed oils are rich in linoleic acid, an essential polyunsaturated omega-6 fatty acid that our bodies cannot make on their own1 .
The poor health reputation of seed oils is often conflated with the ultraprocessed foods they are commonly found in1 . It's the high levels of sodium, added sugars, and additives in these foods—not the seed oil itself—that is the likely culprit.
Seed oils get blamed for health issues actually caused by other components in processed foods.
As nutrition scientist Matti Marklund states, "Omega-3 and omega-6 are necessary and are important, and we should probably eat more of both"1 .
The key is increasing intake of omega-3s from sources like walnuts and fatty fish while maintaining a balanced diet.
Some of the most important properties of fats are not about health, but about how they create texture and structure in our food. One key process is emulsification—the mixing of oily and watery liquids9 .
You can characterize an emulsion with a simple experiment using two dyes: one that dissolves in oil (Sudan III) and one that dissolves in water (methylene blue)9 .
If the blue (water-soluble) dye spreads, it's an oil-in-water emulsion. If the red (oil-soluble) dye spreads, it's a water-in-oil emulsion9 .
This simple test reveals why butter (a water-in-oil emulsion) doesn't mix with your bread the same way mayonnaise (an oil-in-water emulsion) mixes with your salad.
While seed oils have been vindicated, new research challenges old assumptions about other "healthy" fats. A 2025 study published in Cell Reports found that oleic acid, a monounsaturated fat abundant in olive oil, has a unique property: it drives the creation of new fat cells5 .
Researchers fed mice specialized diets enriched with specific fatty acids, including those found in coconut oil, peanut oil, and olive oil. They then closely monitored the precursor cells that develop into fat cells (adipocytes)5 .
Oleic acid was the only fatty acid that caused these precursor cells to proliferate more than others. It boosted a signaling protein (AKT2) and reduced the activity of a regulating protein (LXR), resulting in faster growth of new fat cells5 .
Key Insight: The lead researcher, Michael Rudolph, summarized the takeaway: "Moderation and to consume fats from a variety of different sources"5 .
(Percentage of Total Fatty Acids)2
| Fat or Oil | Saturated | Monounsaturated (Oleic) | Polyunsaturated |
|---|---|---|---|
| Butter (cow) | ~53% | ~29% | ~3% |
| Lard | ~42% | ~44% | ~10% |
| Canola Oil | ~6% | ~62% | ~32% |
| Soybean Oil | ~15% | ~24% | ~61% |
| Olive Oil | ~16% | ~71% | ~13% |
| Coconut Oil* | ~87% | ~6% | ~2% |
*Coconut oil is high in saturated fat, but primarily in medium-chain fatty acids like lauric acid.
To understand the quality and composition of fats, food scientists rely on a suite of analytical methods and reagents. Here are some of the key tools.
| Test or Reagent | Primary Function | What It Measures |
|---|---|---|
| Free Fatty Acid (FFA) Test | Measures hydrolytic rancidity | The level of fatty acids that have broken off from triglycerides, indicating spoilage or processing quality4 . |
| Iodine Value | Quantifies unsaturation | The degree of unsaturation in a fat, which correlates with its resistance to oxidation and its physical state3 . |
| Saponification Value | Determines average fatty acid chain length | The amount of alkali needed to saponify a fat, indicating the average molecular weight of its fatty acids3 . |
| Peroxide Value | Measures oxidative rancidity | The concentration of peroxides and hydroperoxides formed in the initial stages of oil oxidation3 . |
| Gas Chromatography | Separates and identifies fatty acids | The precise fatty acid composition of a fat or oil after conversion to methyl esters (FAMEs)3 . |
Modern kits, like the SafTest™ Free Fatty Acid Test Kit, have streamlined this process. They use stabilizing reagents and optical readers to deliver results in about 30 minutes, replacing lengthy titration methods that could take up to eighteen hours4 .
Control solutions are also used to verify the accuracy of these tests, ensuring that the data on our food labels is reliable7 .
Up to 18 hours with titration
Results in about 30 minutes
The science of fats is dynamic and often counterintuitive. The evidence strongly supports including polyunsaturated-rich seed oils as part of a heart-healthy diet, debunking popular myths about their toxicity1 . At the same time, newer research on oleic acid reminds us that even fats traditionally considered healthy are best consumed in moderation5 .
There is no single "best" oil. A healthy diet is not about banning one type of fat and embracing another. It is about variety and context. Use different oils for different purposes, focus on whole foods, and remember that the nutritional profile of any meal comes from the entire plate, not just the cooking fat you use.
The most recent studies continue to refine our understanding. An October 2025 trial from King's College London offered reassurance about certain processed fats, finding that interesterified fats—used to replace harmful trans fats in spreads and baked goods—did not raise cholesterol or cause metabolic harm in healthy adults8 . As Professor Sarah Berry noted, this "highlights that not all food processing is bad for us!"8