Some foods to avoid while nursing include fish, alcohol and caffeine, according to Mayo Clinic. Other foods can cause an allergic reaction or fussiness, most commonly cow's milk, soy, eggs, peanuts and corn, so nursing mothers may choose to avoid those foods as well.
Certain varieties of fish may contain high levels of mercury that can be passed to the baby through breast milk, reports Mayo Clinic. Exposure to mercury can harm a baby's nervous system. Fish types that can be high in mercury include king mackerel, tilefish and swordfish. Nursing mothers should limit fish caught in local waters to 6 ounces per week and eat no more fish during that time period.
No level of alcohol in breast milk is safe, warns Mayo Clinic. Mothers who drink should not breastfeed for a minimum of two to three hours after consuming one drink, and that time period can vary depending on her weight. Pumping milk and disposing of it does not speed up the process of removing the alcohol from the breast milk.
Caffeine can affect the baby, disturbing her sleep or creating agitation, states Mayo Clinic. Mothers are advised to drink no more than two to three cups, or 16 to 24 ounces, of caffeinated beverages a day.