A nursing mother may produce up to a litre of milk per day amounting to some 600-700 calories/day. In addition to this of course, energy is needed to produce this quantity and so the energy allowance for a lactating woman is about 750 calories above her normal needs.
Not all of this has to be supplied by the diet but can be drawn from the stores put down during pregnancy specifically for this purpose. The extra calories required must also provide the necessary nutrients required for milk production ie. calcium, protein, magnesium, zinc and plenty of fluid.
The effect of nutritional deprivation of the lactating mother is to reduce the quantity, not the quality, of the milk. For protein, carbohydrates and most minerals, the milk of a healthy mother has a fairly constant composition. The levels of fat soluble vitamins in human milk can be altered by excessive or deficient intakes of the mother.
The period of lactation is a natural time to lose the extra body fat accumulated during pregnancy. If food choices are careful, gradual weight loss of 0.5-1.0 kilogram per week can be attained without effect on milk supply. However, if a woman does not breast feed, she may not as easily lose the gained weight.