have enough body fat!” Your brain then tells
you to eat more and move less, which serves
to change your behavior until your body fat is
within a safer range. You become hungrier
(and probably eat more), your metabolism
slows down (thanks in part to changes in your
thyroid hormone levels) and you start to gain
body fat.
As body fat continues to accumulate,
leptin levels rise, and your fat cells start to
send more messages to your brain—“OK,
we’ve got enough energy stored now!” If that
message is properly received (i.e., you are
sensitive to the leptin message), your brain
then tells you to increase your activity and
makes you less hungry, so you move a little
more and eat a little less, and don’t gain too
much weight.
Although it’s much more complicated
than this simple summary, this energy-
balance system is naturally designed to keep
your body fat levels “just right.” The trouble