Actually, recent research suggests that shields don't affect supply as much as previously thought - see here
I quote...
Since writing the post, a follow-up study has been published, this time looking at the relationship between shield use and infant weight gain over a much longer period2. 54 mothers who used a nipple shield provided by a nurse or lactation consultant in the period just after the birth of their babies were recruited for the study, and completed interviews when their babies were 2 weeks, 1 month and 2 months old. Over time, the proportion of mothers using shields diminished (at 2 weeks, 69% of the mothers were still doing it, at 1 month 48%, and at 2 months 33%) and at each stage the responses of women who were still using the shields were compared with those who weren?t.
The main aim of the study was to determine whether nipple shields had a negative impact on weight gain ? if babies whose mothers were still using shields grew more slowly than those whose mothers had stopped, then this could be taken as an indication that medium to long term use of shields was causing a real problem. Happily, there was no difference between the groups: whether a mother used a shield made no difference to her baby?s pattern of weight gain.
There were a few complaints about nipple shields: 8 women thought they caused nipple soreness; 2 found them messy; 2 found them inconvenient and 3 had problems with them falling off. In spite of this, 90% of the mothers in the study said that using the shield was a positive experience, and 67% felt it helped prevent them from giving up breastfeeding.
If you are a mother who relies on a nipple shield to breastfeed, these results make reassuring reading.
Not a completely persuasive study, but interesting. Supposedly the previous studies used the older, harder type of shields, which weren't so effective.
It makes sense, really - if women can pump for months, and not lose their supply, one would think that sucking through a shield is a better approximation to ordinary sucking and not so much of a problem either?