Kathy Dettwyler asks for references for this.  I will try to help.
BF does 'cost' energy, but most of us have stores (thunder thighs, pillow
bellies, etc.) from pregnancy which help meet these energy needs.
The metabolic costs of BF are more than just what we put out as milk.  It
takes energy to produce the milk as well.
When I had the authors of the reference work quoted in the PPD article as
instructors in a truly excellent course on maternal-child nutrition at the U
of Washington a couple of decades ago (the course where I got all my basic
knowledge about, and much of my interest for, BF), it was emphasized that
because of the energy stores we gained in pregnancy, the need for daily
increased intake was much less than the total metabolic cost of lactation,
at least until such stores were used up.
Lactation hormones slow the passage of food through our digestive tracts and
non-nutritive suckling increases the secretion of hormones promoting more
complete digestion of food-- by both mother AND baby.  So mooning around
with the baby at her breast benefits the baby and the mother and is good for
the household food budget as well.  (Uvnäs-Moberg K, Widström AM, Marchini G
and Winberg J: Release of GI hormones in mother and infant by sensory
stimulation(Review). in Acta Paediatrica Scandinavica 76(6), 851-860)
Despite the metabolic efficiency which accompanies BF, it does 'cost'
something to produce milk.  Whether these costs are an added burden to
depressed women is another matter, IMO.  Maybe getting slimmer as the baby
thrives and grows, is a help to some women.  Maybe the difficulty in
returning to pre-pregnant body form while artificially feeding can add to
the depression of mothers with PPD.
cheers
Rachel Myr
midwife
Kristiansand, Norway

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