Now that we’ve seen the good qualities of fat, let’s look at the flip side. Fats are healthy when they come from whole, natural, unprocessed foods. The problem is when fats become damaged. Fats are damaged by heat, light and oxygen. High heat and chemicals used by manufacturers to process oils are the chief sources of damaged fats. One particularly damaging process is partial hydrogenation, which gives oils longer shelf life. This process creates trans fats and other altered molecules that are harmful to the human body.
Polyunsaturated fats are the most fragile. Oils that are high in polyunsaturated fats (such as flaxseed oil) must be refrigerated and kept in a dark container. Cooking with polyunsaturated fats, as well as exposing them to heat, light or oxygen, damages the fats, depleting their nourishment and creating oxidized fats and toxins. Store these oils in a dark airtight container in the fridge, use them immediately after opening and don’t keep them for long periods of time. You can usually taste when they start to change.
Mono- and polyunsaturated fats are unstable, so the cooking process can hydrogenate them, creating harmful trans fats. It’s better not to cook with them at all.