Before you slather that mayo on your sandwich, there’s something you should know: Mayonnaise is loaded with omega-6 fatty acids, and—unlike its cousin omega-3—getting too much omega-6 is bad for your intelligence. By looking at studies of the fatty acid profiles of women’s breast milk, researchers at UC Santa Barbara and the University of Pittsburgh examined the estimated content of omega-3 docosahexaenoic acid DHA (fat primarily found in certain fish, nuts and seeds, which makes up a large portion of our brains) and linoleic acid (the main source of omega-6 fatty acids) in diets of women living in 50 countries. The researchers found a couple of interesting things: The amount of DHA in a mother’s milk turned out to be the strongest predictor of test performance. On the other hand, the amount of omega-6 fat in mother’s milk—fats such as linoleic acid that come from vegetable oils such as corn and soybean—predict lower test scores. In countries where mother’s diets contain more omega-6, the beneficial effects of DHA seem to be reduced. For your next sandwich, try avocado or black bean spread instead.