I was lucky enough to have had a great course covering this in nursing school at the U of Washington, with professor Bonnie Worthington-Roberts.
During breastfeeding, metabolism is altered, and the work of BF plus the loss of the product amounts to a metabolic demand on the mother which must be met from somewhere.  Lacking stored energy to burn, mothers will graze or hunt, and I use these words very loosely, for food to meet their energy needs.
We learned that pregnant and lactating women, as all women, should eat as their appetites dictated, and that left to their own devices, their bodies would land at the weight which each woman's lipostat determined.  The lipostat is like a thermostat but for fat, and it is not controlled by wall switch you can operate with your fingers, but by a place in the primitive brain which is notoriously hard to get at even with tweezers, Q-tips, or crochet hooks (don't try this at home).  Most of us never even have a chance to alter the factory setting, more's the pity.
I was then 22 years old, childless, and had never considered a calorie, never experienced conflict about whether to consume something I felt like eating on the spur of the moment or in a planned way, and never tried to alter my body weight one way or the other.  I realize this is like claiming I had never seen a TV set or perhaps spent my formative years on another planet, but it is nonetheless true.  We didn't have a TV either, BTW, do I see a connection here?
My own appetite while pregnant was legendary, and weight gain was absolutely linear at one pound per week times 40 weeks.  While breastfeeding, I had almost no appetite for big meals the first 5 or 6 months, when I suddenly became ravenous again and could eat twice the amount of any two friends put together at a given meal.  Not owning a bathroom scale, it was only by which clothes I could wear that I realized my appetite had returned when I was around my pre-pregnant size.  The really great part was that I could keep on eating to satisfy my continued voracious appetite as long I was lactating, no matter how few meals a day I was supplying  a child and I never needed to buy new clothes except when the old ones wore out.  There is no way I was producing milk amounting to 500 kcals per day when the child was having a cuddle at the breast before sleep once a day, but this remarkable state of grace persisted as long as I was lactating at all.  Between children my appetite went back to normal.  Alas, after weaning the last time, my appetite continued to be hearty while my metabolism went back to fuel economy mode.  This has been of tremendous benefit to the nice woman who runs the clothing shop which keeps my body covered, if not to me personally.
Based on this study with its sample size of one, I can only conclude that I managed to grow two babies to average birth weights at term and then to feed them both on my own milk until we were ready to move on, and never thought about what or how much to add to, or subtract from, my diet.
Rachel Myr
in Kristiansand, Norway, still wantonly ingesting uncounted calories daily, though today I would be willing to sacrifice things quite dear to me for the chance to have that fast-burning metabolism just long enough to get into my clothes from say, 1995.  Is that really asking so much?  

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