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. Talk about brain food.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.Trouble is, the average American diet doesn’t provide enough beneficial omega-3s—in fact, our DHA levels are among the lowest in the world—and instead, it’s heavy in omega-6s.In general, mayo is the most omega-6-rich food you’ll find in your refrigerator. “If you have too much of one—omega-6—and too little of the other—omega 3—you’re going to end up paying a price cognitively,” notes Steven Gaulin, UCSB professor of anthropology and co-author of the study.“Back in the 1960s, in the middle of the cardiovascular disease epidemic, people got the idea that saturated fats were bad and polyunsaturated fats were good,” explained Gaulin. “That’s one reason margarine became so popular. But the polyunsaturated fats that were increased were the ones with omega-6, not omega-3.“So our message is that not only is it advisable to increase omega-3 intake, it’s highly advisable to decrease omega-6—the very fats that in the 1960s and 70s we were told we should be eating more of.”Gaulin notes that our brains burn up omega-3 quickly—think of it as brain fuel—so we need to replenish it every day. The best sources of omega-3 fatty acids include wild salmon, walnuts, chia seeds, herring, sardines and fish oil.As for your sandwich, ditch the mayo and try these delicious spreads instead.