Foods that breastfeeding moms should avoid

Baby Camille (Irene and Darwin Ecleo's 3rd child)

Certainly sons are a gift from the LORD, the fruit of the womb, a reward. Like arrows in the hand of a warrior are the sons born in one’s youth. Blessed is the man who has filled his quiver with them. He will never be shamed for he will destroy his foes at the gate. (Psalm 127:3-5) 

By and large, there are only two foods breastfeeding moms should keep away from or eat in very small amounts: alcohol and fish. 

Alcohol can be passed from the mother to baby in breast milk which affects neurologic development.  One drink or less per day is usually accepted as being "harmless." 

On the other hand, fish (including canned tuna) should be avoided or eaten in moderation, no more than 2 servings a week.  This contains mercury that can likewise harmfuly affect the baby's neurologic development.

In addition, any foods that someone in the family is allergic of should be avoided; the most common ones are eggs, soy, peanuts, corn, wheat, and milk. 

Particular foods may also cause irritation, discomfort, gas, or intestinal irritation in babies and they should be avoided if infant shows negative reactions to them: caffeine (tea, coffee, and chocolate), citrus fruits, broccoli, garlic, and spicy peppers,   

Some herbs, like peppermint and parsley, can decrease milk supply if consumed in large amounts.  (But other supplements, like Fen-U-Greek, can improve milk supply.)

The baby gives the lowdown once something is clashing with her/his system.  One thing to take into account: it can take up to 14 days of cutting a food group entirely out of mom’s diet before it is no longer excreted into the breast milk, so don't decide after only 24 hours if a change is making a difference to the baby.

Reference: 

Dr. Marra Francis , h What foods should I avoid while breastfeeding?, http://www.parents.com/advice/babies/newborn-care/what-foods-should-i-avoid-while-breastfeeding/