Avoid “vegetable” oils (seed oils)
Main examples:
sunflower oil
common cooking oil in UK
soybean and corn oils
these oils are added to many processed/packaged foods, especially in US
Reasons
They are high in omega-6 polyunsaturated fatty acids, which when consumed in excess (especially relative to omega-3 from fish oils) are reported to promote inflammation.
In Western countries like UK and US, we consume far more omega-6 than omega-3 fats, due to a diet high in added vegetable oils and low in oily fish.
The “ideal ratio” of omega-6-to-3 in the diet is thought to be between 4:1 and 1:4. (These ratios are based on the diets of the longest lived populations at the time of the studies that investigated omega-3s and -6s)
In the UK our diet averages around 14:1 (on the pro-inflammatory side).
Excessive inflammation is associated with insulin resistance, which in turn is a major contributor to the chronic diseases that kill most people today.
Omega-6 is pro-inflammatory and omega-3 (specifically the EPA sub-type) is anti-inflammatory.
See my blog post Understanding Fats for a more comprehensive look at fats in general.
Alternatives
Prefer: extra virgin olive oil, olive oil, coconut oil
Maybe use: rapeseed (“canola” or “vegetable”) oil
Details: Rapeseed oil (“canola” in US; in UK usually just “vegetable oil”) may be acceptable for frying with due to its reported relatively high heat resistance, as compared to other seed oils. It usually still has an unfavourable omega-6-to-3 ratio though, and the polyunsaturated fatty acid component of it will be susceptible to heat damage when frying, which makes me question the resources that show rapeseed oil to have higher heat resistance than, say, coconut oil. I’ll be looking into this more.
Note that Extra Virgin Olive Oil (EVOO) is also susceptible to heat damage, so should not be held at high temperatures for longer than necessary (i.e. don’t let the oil sit in an empty frying pan with the heat on – preheat the empty pan, then add the oil just before the ingredients being fried). The redeeming feature of EVOO is the high antioxidant content, which helps to neutralise the unhealthy chemicals produced by any accidental overheating of the oil. See this video for more.