Are seed oils such as sunflower oil bad for you?
Based on current research, seed oils are not bad for you; on the contrary, they lower LDL cholesterol and are associated with a lower risk of cardiovascular disease, making them a better choice than butter or palm oil.
Seed oils such as sunflower oil are high in linoleic acid, a polyunsaturated fatty acid. The idea that they cause inflammation is a popular claim, but clinical studies in humans do not confirm this. Higher intake of this type of fat does not raise markers of inflammation or oxidative stress.
For your heart, sunflower oil actually has a favourable effect compared with saturated fats. Seed oils and other unsaturated vegetable oils lower LDL cholesterol (the 'bad' cholesterol) and total cholesterol, and higher intake is associated with a lower risk of cardiovascular disease. If you replace butter or palm oil with a seed oil, your cholesterol profile improves further. The certainty of this evidence is moderate, but not watertight.
The picture for diabetes points in the same direction. Higher intake of polyunsaturated fats from vegetable oils is associated with a lower risk of type 2 diabetes. This is an association from population studies, not a proven causal effect, but it is consistent with what we know about saturated fats as the counterpart.
By comparison, butter, palm oil and coconut oil raise LDL cholesterol and thereby the risk of atherosclerosis. Tropical oils such as coconut also raise HDL ('good' cholesterol), but whether that outweighs the LDL increase is uncertain. Major health organisations and European diabetes guidelines therefore explicitly advise choosing non-tropical vegetable oils, including seed oils, over saturated fats.
Based on epidemiological and clinical research and multiple reviews; the certainty per outcome ranges from moderate to low. No large RCTs with hard endpoints (such as heart attacks) specifically for sunflower oil.