The food you eat and the water you drink do not magically go directly to your breast milk. What you eat and drink goes first to your stomach to be broken down and then into your intestines to be absorbed and processed. Your digestive system breaks nutrients into parts small enough for your body to absorb and use for energy, growth, and cell repair. The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine. Special cells in the walls of the small intestine absorb water and the digested nutrients into your bloodstream. Your blood carries molecule-sized components such as simple sugars (carbohydrates), amino acids, white blood cells, enzymes, water, fat, and proteins throughout your body. As blood passes by the breasts, milk glands pull out these nutrients for milk production and pass some of them to your baby. Not all molecules are small enough to pass through into milk. (That’s why some medications are safe to take while breastfeeding and some are not. Molecules that are too big can’t get into the milk while really small molecules can.)
Nuts, seeds, beans, and grains all have plant based proteins. Meat and dairy are animal based proteins. Both plant and animal proteins carried in your blood can make it into your milk. Sometimes these proteins can affect baby’s digestive system, causing symptoms like reflux, gas, colic, and blood or mucus in the poops from iritations to baby’s intestinal lining. Diary proteins are the most common cause of upset in the stomach, however research suggests that the proportion of exclusively breastfed infants who are actually allergic to something in their mother’s milk is very small. Fussiness and gas alone are not enough to diagnose a cow milk protein allergy.
In general, there are NO foods that need to be avoided because you’re breastfeeding. Every baby is different in the foods they are sensitive to. IF your baby always seems to have a reaction when you eat a certain food or a large amount of a certain type of food, cutting back on it or cutting it out temporarily may be helpful.