olga Yastremska/AlamyWhether you are roasting a chicken in the oven, browning onions in a frying pan or choosing a spread for your toast, oils are at the heart of our culinary activity.We have a dizzying array of choice. From sunflower to flaxseed, avocado to coconut, around 30 varieties of oil are now used for cooking. Your decision on which to use could have a profound effect on your health, with consequences for your cholesterol, blood pressure and risk of cardiovascular disease.If you believe the headlines, then palm oil is out, sunflower oil is on shaky ground and there seems no end to the benefits that extra virgin olive oil brings to the table. But are these claims backed up by solid science? And how do the health effects of these products weigh up against their environmental costs?Saturated or unsaturated?First, some chemistry. Cooking oils contain fats, which are made from long chains of carbon atoms linked together. Saturated fats, found in red meat and dairy, are so named because each carbon atom is linked to the next by a single bond. The remaining electrons of each carbon atom are then available to form bonds with hydrogen atoms making the molecule fully saturated with this element. This structure makes these fats very rigid and stable, which is why butter and lard are solid at room temperature.Unsaturated fats, commonly present in plants and oily fish, have at least one double bond between neighbouring carbon atoms, which reduces the number of bonds that can be