Doing a lemon detox while nursing can really harm your nursing relationship. Breastfeeding requires an *additional* 600 calories per day. If you severely cut down on your caloric needs, the first thing that will happen is that your body will take what it needs from you in order to feed your baby, which means you will lose nutrients and vitamins that are essential to your survival and well-being. After that, your body just won't have enough building blocks (fat, protein, vitamins, minerals) to make breast-milk at all. I think it would be highly likely that after five days your breast-milk would be seriously diminished, if not gone completely.
I understand that you are worried about passing toxins on to your baby, but the breastmilk that your body makes is pure. I'm not sure exactly what toxins you are talking about (it sounds like you mean from foods you are eating or from the environment) but it's not likely that anything bad in your body can pass through to your breast milk. For instance, even if you have mastitis, your milk is unharmed and will not have any infectious materials in it.
And what's more than that, breastmilk is so important and perfectly formulated for your baby's health needs. Also, it's cold and flu season right now. But you pass antibodies and white blood cells through your breastmilk to your baby to help him fight off whatever is going around.
Now, something like the toxins in a cigarette can be passed through the breastmilk to the baby. However, as I've talked to lactation consultants, they have assured me that it's more important for a smoking mom to breastfeed because of the immeasurable importance and health benefits of breastfeeding. Sure, there is danger in the nasty stuff from cigarettes in breastmilk, but that the good things in breastmilk-- antibodies, white blood cells, personalized nutrition, "good" bacteria--are so important as to outweight the negativity of smoking.
And also, remember that formula doesn't have any of those great things either plus your breastmilk is free. A detox diet that will limit your body's ability to make breastmilk will mean that you will rely more heavily on formula. Breastmilk is full of ingredients and living substances that formula could never replicate!
Just some thoughts to consider.
A., CCLD, CCBE
Childbirth educator, doula