Whole foods that are in their most natural state, such as vegetables, fruits, lean protein, beans, nuts, seeds and whole grains that have not been highly processed are considered clean foods. I personally recommend consuming clean foods that are free of pesticides, antibiotics and growth hormones. I consider clean foods to be foods that nourish our bodies to support optimum health and wellness. Whereas, foods that are highly processed and refined, which have become more common in the American diet have limited nutritional value. In addition, these types of foods often contain unhealthy ingredients, such as artificial flavors, colors and preservatives coupled with excessive amounts of sodium that can be very depleting for our bodies. When foods are altered in this way, they are no longer considered clean foods.
As for your concern about breastfeeding while consuming clean foods, I believe that eating in this way will be beneficial for your baby, and your body will adjust naturally to this positive change. In addition, when you consume nourishing foods free of chemicals that have a high nutritional profile – you will foster a strong foundation of health and wellness for you and your baby.
(On a side note, I recommend while you're breastfeeding to limit your intake of raw vegetables and juices, since they can have a detoxifying effect on your body.)