The Mediterranean diet, high in fruit and vegetables, whole grains, fish and fats like olive oil and nuts, has been touted as the best diet for your health. But a new study published in the American Journal of Critical Nutrition suggests this diet could harm your fertility and immune system unless you make one change. Researchers at the University of Oslo had 27 British students eat a Western diet, low in fruits and vegetables, for one week. They then switched to a Mediterranean diet for one week. Urine samples contained higher levels of chemicals when the students ate the Mediterranean diet. Specifically, they had more than three times higher levels of insecticide and organophosphate that have been linked to negative effects on fertility and immune function. But the students who followed the Mediterranean diet but ate organic foods cut their levels of these contaminants by 90 percent.