Dear lactnetters,
Here again is the very good question that was posted last week by Jan
Barger while I was out of town. In turn, I queried Dr. Tom Webster
at Boston University, who is a dioxin epidemiologist. His reply
appears below, and my translation of his reply appears below that.
>Question here: Sandra seems to say that the fat used in milk production
>during lactation are the fat stores we have BEFORE we get pregnant. Then
>what happens to the fat stores we accumulate DURING pregnancy? Are these not
>used for lactation?
>
>If we lose weight during the 6-12-18-24-36 months we breastfeed, are we
>losing fat stores that we have before we got pregnant, or fat stores we
>accumlated during pregnancy?
>
>If we use those fat stores that we accumulated during pregnancy, then why
>would the first child be more likely to have dioxins in the breastmilk than a
>latter child? Then why couldn't there be some control of diet during the
>pregnancy to decrease the potential of accumulated dioxins?
>
>I don't know -- I'm just asking.
>
>If we don't need those fat stores that we accumulate during pregnancy, why
>did Michelson find that women who gained less than 18 pounds during pregnancy
>were more likely to have a low milk supply?
>
>If we don't need them to support lactation -- then why are we blessed w/ them?
>
>Inquiring minds want to know and LEARN!
>
>Jan Barger, RN, MA, IBCLC -- Wheaton IL
>Lactation Education Consultants
>www.lactationeducationconsultants.com
Tom Webster's reply:
Dioxins and other lipophilic chemicals will tend to
equilibrate between all fat in the body. If this equilibration is
fast enough--which I think is true to first approximation relative to
the rate of clearance--then it won't matter very much whether the fat
is "old" or "new."
Lactation causes rapid clearance because the mother is
excreting fat. Typical pharmacokinetic models of this process assume
that the concentration in the fat is equal to the fat concentration
elsewhere in the body.
So the question, I think, is whether the equilibration is
fast enough to make this approximation work reasonably well. It's a
matter of degree. I'd be interested in any evidence that it makes a
significant difference to the simple picture sketched above. "All
models are wrong; some models are useful."
These sorts of issues are common in pharmacokinetic models:
How many compartments are needed to model behavior to a given level
of accuracy? Is equilibration limited by flow to tissues or
diffusion into tissues? Do we need to take into account binding of
certain dioxin-like congeners to CYP1A2 in the liver? (it's
dose-dependent)
My translation:
The fat you accumulate during pregnancy should--if our current
understanding of the physical and chemical properties of dioxin is
correct--quickly absorb the dioxins dissoved in the fat reserves you
already have. In this way, all your fat, new and old, comes to have
the same contaminant levels. It's roughly the same principle that
causes all the water in the ice tea to quickly assume the same level
of sweetness even though the sugar was added to the bottom of the
glass. The only reason to wonder if there might me more going on
than this is the possibility that certain dioxins (there are about 29
different kinds) bind to certain tissues in the liver, making them
unavailable to diffuse out and contaminate the new fat reserves. If
so, then true equilibrium would never be reached. Tom is saying we
really don't know the effect of this binding phenomomenon, but that
we do know that the forces of equilibration appear to work faster
than the forces of excretion. That is, we can excrete dioxins
through urine and feces, but this is an excrutiatingly slow process.
(my aside: the half life of dioxin is about 8 years in the human
body To determine total residence time based on half life, you
multiple by five. Thus, a dioxin molecule that you receive as an
infant while breastfeeding could conceivably stay inside your body
until you are 40 years old.)
Tom's main point is that breast milk is highly contaminated with all
the industrials poisons we've accumulated over a lifetime, including
those, like PCBs and DDT that have been banned for years, because 1)
these chemicals should roughly equilibrate in the body's fat reserves
and 2) because breast milk has so much fat in it. (postscript from
me: we also know this is true because we can measure these chemicals
in breast milk directly!)
Hope this helps.
Sandra
--
--
Sandra Steingraber, Ph.D.
Visiting Assistant Professor
Program on Breast Cancer and Environmental Risk Factors
110 Rice Hall
Cornell University
Ithaca, NY 14853
[log in to unmask]
www.steingraber.com
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