A baby's taste start to develop before baby is actually born. When he gets his first taste of amniotic fluid, he experiences the taste of something salty. He carries on tasting the amniotic fluid, which changes depending on what the mother is eating. By how much varies between women. By the time he is born he will have experienced variety of flavours. And what he has been exposed to in the womb stays with him and he may show preferences to certain foods when weaning. The importance of eating a varied diet continues when we decide to breastfeed. (https://www.whattoexpect.com/pregnancy/fetal-development/fetal-taste/.)
As if breastfeeding isn't hard enough, we're given advice about what to eat and how much we should eat all the time. But is there really any research to back it up? I was asked to look into the possible impact on the breast milk depending on what you eat. I knew it was going to be a difficult subject because we know that on the whole it doesn't actually matter what you eat, nor the amount you eat (Jack Newman is a good source for this). Women in areas of the world where food is scarce still manage to breastfeed their babies, so the body will make that milk unless there is actual starvation where the body goes into survival mode. But I have been reading all the research studies I could get my hands on and this is what I have found.
Out of fat, mineral and vitamins, the only one we seem to have any influence over is vitamins. If you add vitamins to your diet the level of that same vitamins is increased in the breast milk. So it's always worth to be taking vitamins yourself (or better yet, get them though food) as well as giving them to your baby. Currently the recommendation is that all babies are given vitamin D from birth because the levels at which it is found in breast milk can be too low.
What about strong flavours? We already know from what I said above, that babies can taste the different flavours, but do they prefer it when it's strong? I found one study which looked at the effect of garlic on breast milk, both how it smelled and how the baby fed. They found that babies fed MORE and for longer when there was a strong odour (1-2 hours after eating it), so that sort of debunks any claims that women need to avoid strong flavours because clearly babies in this study loved (Julie A. Mennella, Gary K. Beauchamp, 1991) it. It also wasn't perhaps that surprising of a finding as women in many cultures eat very strongly flavoured foods and their babies are just fine. With regards to flavours, this is what is thought to be behind the fact that breastfed babies are easier to wean (L Cooke, A Fildes 2011). Because the taste of breast milk changes with what the mother eats, a breastfed baby will have experienced lots of different flavours before weaning begins and this is reflected in the food preferences they have and will naturally like the food the mother would normally eat. A formula fed baby has the same meal 24/7 and so their taste preferences develop differently and are less accepting of a range of flavours when it comes to weaning. They prefer tastes that are similar to that of the formula brand/type they had been having. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3202923/).
The other aspect is the allergenic one. There have been many studies done, from different perspectives. There is no doubt that cows milk protein gets into the breast milk and can therefore cause a reaction. I'm not going into to detail because there are many very good websites solely dedicated to provide information on allergies. I just wanted to high light one specific study (because I found it particularly interesting) which looked at the impact on avoiding dairy, egg and fish and allergic responses in the baby (G. HATTEVIG B. KJELLMAN N. SIGURS B. BJÖRKSTÉN N. I. M. KJELLMAN 1989).The babies were selected because either one or both parents had allergies. They divided the women into two groups, one eating a normal diet and the other avoiding dairy. in the group who avoided dairy, the babies had less allergic responses such as dermatitis (skin flares) and digestive issues during breastfeeding compared to the other group. However, after weaning, there was no difference at all between the levels of allergies. It shows that while breastfeeding it can be useful to avoid dairy but that there is no long lasting effect of doing so. I think this is really useful information for those mothers who believe they have caused their children allergies by having dairy, which doesn't seem to be the case. The babies who develop an allergy (and have never had any formula, because this can change things) would develop it regardless of the mothers diet. But if you notice an allergy in your baby (beyond colicky symptoms), then going to the doctors and getting some dietary advice is something to consider.
Anecdotally I have seen reactions in babies where women consume high amounts of a certain foods. And I mean HUGE amounts, usually raw vegetables like 3-4 peppers or bowls of strawberries daily. This type of extreme unvaried diet can cause a bad nappy rash. Anything which can upset our adults stomachs if we have too much of it, can upset babies stomachs. Mothers just need to eat a varied diet. That's it really.
So is there something you shouldn't/can't eat which is proven to be harmful? Well yes, keeping down the portions of oily fish to 2/week is the official advice because of the potentially high levels of mercury.
If you eat things which may be upsetting your baby's stomach, simply cut down your consumption for a while and see if it makes a difference. Any time you totally eliminate a food group from your diet rather than cut down, please do this with the support from a doctor.
It's very vast subject but I'm hoping I've collected some interesting and useful information for you all.