This isn't a technical answer to the request for research on lactational
amenorrhea and exercise, just some general info.  Many women who engage in
extensive physical exercise such as marathon running, tennis, even
gymnastics, find that their body fat levels far very very low.  This is
correlated with anovulation and amenorrhea several mechanisms have been
proposed, but the basic idea is that your body interprets the low levels of
nutritional reserves as a sign that the environmental conditions are not
good for a pregnancy and several years of lactation.  If your body is
sending signals that you don't have the energy reserves to give a pretty
good guarantee of a healthy pregnancy and lactation, then your body cuts its
losses by not ovulating, not menstruating, not getting pregnant.
Breastfeeding will contribute to low body fat stores a little bit (most of
the necessary nutrients are extracted from the food, rather than drawing on
reserves).  If this woman wants to get pregnant again and is having
irregular or no menstrual periods, she probably needs to have her body fat
levels assessed, maybe cut back on the exercise and increase her food intake.

You can do a literature search under "Rose Frisch" and "Sara Quandt" for
more references.

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Katherine A. Dettwyler, Ph.D.                         email: [log in to unmask]
Anthropology Department                               phone: (409) 845-5256
Texas A&M University                                    fax: (409) 845-4070
College Station, TX  77843-4352