Replacing animal fat in the human diet with vegetable oil seems not to lower heart disease risk, and might even boost it, according to a study published Wednesday that challenges a cornerstone of dietary advice. Switching from saturated to unsaturated Omega-6 fats did result in lower blood cholesterol in a trial with nearly 10,000 participants, it said, but not the expected reduction in heart disease deaths. In fact, those with a greater reduction in cholesterol “had a higher rather than a lower risk of death,” according to the research published by the medical journal BMJ.