The rest comes from extra foods that you should eat.
Breastmilk is ideally your baby's only food for about half a
year, a major source of nutrition for a year, and still a good
source of nutrition for as long as you continue breastfeeding (see
the page on how long to breastfeed and
the page on introducing solid
foods).
Your body needs to produce milk that contains all these
nutrients. The nutrients have to come out of your body.
If your diet is deficient in some of these nutrients, your
breastmilk will still contain reasonable amounts of them. That
means your own body will be robbed of them.
If you are seriously malnourished, then your breastmilk may be
deficient in some nutrients too. This is generally not a problem
for mothers in industrialized nation, but it is a problem in many
less fortunate areas of the world.
Don't panic! All you need to do is to eat a reasonable variety
of foods (which you should be doing anyway). Just eat a little
extra of everything. Trust your hunger. Eat to satisfy hunger, not
habit.
Important: It is not a good idea to go on a weight-loss
diet while you are pregnant or breastfeeding. Well, most diets are
never a good idea, but this is particularly important now because of
the extra work your body needs to do. And remember: breastfeeding is
helping you lose weight anyway. Just eat
well and all will be fine.