It could be. The process of refining vegetable oil is long and complicated. www.technocheminc.com/oil_refining.htm
I often get a sore tummy after I've eaten out because I usually eat very clean at home. I choose not to eat out very much because of it.
I use first cold-pressed olive oil, cold-pressed virgin coconut oil, butter and occasionally bacon fat to cook with at home.
Just found this link. www.fathead-movie.com/index.php/2011/02/17