Low Dose effects
One area of endocrine disruption which
is a subject of huge debate is the issue of low dose effects.
What a 'low dose' is is not always clear, but it's generally
treated as a dose much lower than would normally be expected
to have an effect.
This page provides a very brief introduction
to the debate. For more information, look at the papers themselves
and look in Journals such as Environmental
Health Perspectives and Environmental
Science and Technology (e.g. Renner, 1998).
There are two particularly important aspects
to low dose effects:
1) The inherent sensitivity of the endocrine
system and development
As described on the complexity
page, the interacting endocrine and developmental systems
are extremely complex and sensitive. Science does not yet properly
understand the operation and interaction of these systems, so
it is not possible to properly evaluate the impact of chemicals
on them. It is quite possible that very low doses of a chemicals
could have very significant effects.
2) Additivity and synergism
The addition of a small quantity of a chemical
to the body may have effects because it acts in addition to another
chemical which is already there. For example, an oestrogen such
as nonylphenol could act in addition to a natural oestrogen already
In synergism the combination of
two chemicals results in a more than additive response - for
example one chemical might lead to an increase in the number
of receptors in a cell, whilst the second chemical binds and
Additivity and synergism can lead to no
threshold effects, were even the tiniest amount of chemical
added could have an effect, because its activity adds to a chemical
which is already there. A parallel is:
You are trying to look over a wall which
is higher than you. You have a small brick to stand on - it's
still not high enough by a long way. Then you try a block of
concrete - you can almost see over, but you're not quite there.
You put the brick on top of the concrete block - now you can
just see over the wall.
If the threshhold for activity is equivalent
to seeing over the wall, you can see how if the block of concrete
- the natural oestrogen - is just too small, you cannot see over,
there is no activity. If the brick, an environmental oestrogen,
is very small but just big enough, it can add to the block and
allow you to see over the wall or activate the system. The turtle
experiments below are an example of such a system.
Some of the research evidence
In the red-eared slider turtle gonadal
sex is determined by the temperatue eggs are incubated at, with
higher temperatures leading to the production of endogenous estrogen
and the development of females. Sheehan et al. (1999) incubated
eggs at a temperature which normally generates a minority of
females, then added a single dose of 17beta-oestradiol to the
shells. The lowest oestradiol dose tested, 400 pg/egg (40 ng/kg)
sex reversed 14.4% of the animals. A dose response curve suggested
no threshold dose for oestradiol addition. This is a perfect
illustration of additivity leading to no threshold.
2) Bisphenol a in mice
In contrast to the clear and undisputed
findings of the turtle research, the situation with low dose
effects of bisphenol a is a subject of huge and continuing scientific
debate. Research by a group led by Fred vom Saal has found that
bisphenol a and methoxychlor exposure
of pregnant mice resulted in increases in prostate weight in
their offspring (vom Saal et al., 1998; Welshons et al., 1999).
However, industry-funded studies have failed to repeat these
results (Ashby, 1999; Cagen et al, 1999). In the latest twist,
vom Saal's team has shown low dose effects on female offspring,
a delay in oestrus (see bisphenol a
3) Vinclozolin in rats
The fungicide vinclozolin is a well know
antiandrogen, listed on the pesticides
page. Research by L. Earl Gray's team (Gray et al., 1999)
has established that alterations in sexual differentiation of
developing male rat occur with low doses of vinclozolin given
to their dams during preganancy and just after birth. Alterations
including retention of nipples occurred at the lowest dose tested,
3.125 mg/kg/day. The authors conclude:
"In summary, studies of vinclozolin
that did not include endpoints sensitive to antiandrogenic effects
and were not designed to detect transgenerational efffects have
reported LOEL [lowest observed effect levels] for vinclozolin
of 100 mg/kg/day or higher. In contrast, in the current study
we found that vinclozolin treatment induced reproductive malformations
below 100 mg/kg/day, and ano-genital distance was shortened,
areolas were induced, and other endpoints were affected by treatment
with vincolzolin at 3.125 mg/kg/day, our lowest dosage level"
Some problems with Low Dose research
As low dose research is often looking for
subtle, but potentially significant, changes, there have been
major problems with reproducibility. The dispute over bisphenol
a is one example, another is the research that was done by Richard
Sharpe's team at Edinburgh University, which initially showed
that exposure of rats in the womb to octylphenol (OP) or butylbenzyl
phthalate (BBP) led to reductions in testicular weight (Sharpe
et al., 1995). The BBP experiments were repeated by others, and
the OP experiments by Sharpe et al. - none of the repeated experiments
showed effects. The authors explained the situation in a letter
to Environmental Health Perspectives (Sharpe and Turner, 1998).
The reason for the lack of reproducibility of the original results
One suggested problem obtaining reproducible
results is influences of the conditions in which lab animals
are kept. For example, John Ashby of AstraZeneca's central toxicology
lab has said that the number of animals in each cage and whether
sexes are mixed in the cages may make a major difference (C&I,
Oestrogen resistant animals
Spearow et al. (1999) examined how responsive
different strains of juvenile mice were to 17beta-estradiol (E2),
the main female hormone. They found a more than 16-fold variability
in response between different strains, with low doses of E2 eliminating
spermatid maturation in some strains, whilst 16 times as much
E2 had no effect on the widely used CD-1 strain. This strain
is widely used in research, and has been bred for large litter
sizes. This research suggests that CD-1 mice may give a very
misleading view of hormone disrupting effects.
This page was last
updated in October 1999
to the hormone disrupting chemicals home page
Ashby, J. 1999. Dose levels of 0.01-0.2µg/kg/day
diethylstilbestrol are not suitable for use as a positive control
in endocrine toxicity studies. Regulatory Toxicology and Pharmacology
Cagen, S. Z., Waechter, J. M., Dimond, S.
S., Breslin, W. J., Butala, J. H., Jekat, F. W., Joiner, R. L.,
Shiotsuka, R. N., Veenstra, G. E. and Harris, L. R. 1999. Normal
reproductive organ development in CF-1 mice following prenatal
exposure to bisphenol a. Toxicological Sciences 50, p36-44.
C&I. 1999. Hormone studies flawed, researcher
warns. Chemistry and Industry, 18th October, p784.
Gray, L. E., Ostby, J., Monosson, E. and
Kelce, W. R. 1999. Environmental antiandrogens: low doses of the
fungicide vinclozolin alter sexual differentiation of the male
rat. Toxicology and Industrial Health 15, p48-64.
Nagel, S. C., vom Saal, F. S., Thayer, C.
A., Dhar, M. G., Boechler, M. and Welshons, W. V. 1997. Relative
binding affinity-serum modified access (RBA-SMA) assay predicts
the relative in vivo bioactivity of the xenoestrogens bisphenol
a and octylphenol. Environmental Health Perspectives 105,
Renner, R. 1998. Results of low-dose exposure
may challenge the theoretical basis of toxicology. Environmental
Science and Technology Nov 1, p485A-486A.
Sharpe, R. M., Fisher, J. S., Millar, M.
M., Jobling, S. and Sumpter, J. P. 1995. Gestational and lactational
exposure of rats to xenoestrogens results in reduced testicular
size and sperm production. Environmental Health Perspectives
Sharpe, R. M. and Turner, K. J. 1998. Endocrine
disrupters and testis development. Environmental Health Perspectives
Sheehan, D. M., Willingham, E., Gaylor,
D., Bergeron, J. M. and Crews, D. 1999. No threshold dose for
estradiol-induced sex reversal of turtle embryos: How little is
too much? Environmental Health Perspectives 107, p155-159.
Spearow, J.L., Doemeny, P., Sera, R., Leffler,
R. and Barkley, M. 1999. Genetic variability to endocrine disruption
by estrogen in mice. Science 285:5431, p1259-1261.
vom Saal, F. S., Cooke, P. S., Buchanan,
D. L., Palanza, P., Thayer, K. A., Nagel, S. C., Parmigiani, S.
and Welshons, W. V. 1998. A physiologically based approach to
the study of bisphenol a and other estrogenic chemicals on the
size of reproductive organs, daily sperm production, and behavior.
Toxicology and Industrial Health 14, p239-260.
Welshons, W. V., Nagel, S. C., Thayer, K.
A., Judy, B. M. and vom Saal, F. S. 1999. Low-dose bioactivity
of xenoestrogens in animals: fetal exposure to low doses of methoxychlor
and other xenoestrogens increases adult prostate size in mice.
Toxicology and Industrial Health 15, p12-25.