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By Dr. Richard Sharpe
Senior scientist at the Medical Research Council Human Reproductive
Sciences Unit in Edinburgh
Soy formula milk (SFM) is used in place of breast milk and cow's
formula milk by some mothers (about two percent) in the UK and by
nearly one-quarter of mothers in North America. Only in a tiny minority
of cases can this use be justified on the grounds of (cow's) milk
intolerance by babies.
Babies fed with SFM gain weight normally but there is concern that
this feeding may expose the baby to abnormally high levels of 'oestrogens.'
This is because soya beans, from which SFM is made, contain very
high levels of chemicals called isoflavones, which are phytoestrogens
(literally, plant-made chemicals that mimic the female sex hormone,
oestradiol).
There is no evidence from humans that these phytoestrogens cause
abnormal development of the babies, but few, if any, direct studies
have been made. Scientists such as myself have concerns that, as
exposure of babies to levels of phytoestrogens as high as in SFM
does not occur normally, even in societies that each soy-rich diets
(e.g. Chinese), then feeding of infants with SFM is an unnecessary
risk to take on behalf of the baby.
It is ethically difficult to undertake studies in human babies
who are being fed with SFM, and it is obviously impossible to collect
tissue samples that would allow doctors to tell if the developing
reproductive system, or other organs, of the baby are being affected
by exposure to the phytoestrogens in SFM.
We have therefore done a feeding study with SFM in marmoset monkeys.
We chose this species in particular because more than 80 percent
of pregnancies are (non-identical) twin pregnancies, and this allowed
us to use one twin as a control (fed with cow's formula milk) and
his brother was fed with SFM.
We chose to study only males because all male primates, including
the human, exhibit a period after birth and lasting up to six months
or more, in which the testes are very active and when levels of
the male sex hormone 'testosterone' in blood can reach adult levels.
This period is called 'the neonatal testosterone surge.' Nothing
like it occurs in females.
To our surprise, we found that twins fed with SFM showed major
suppression of their 'neonatal testosterone surge' when compared
with their (control) twin brothers. This change was associated with
a large increase in the numbers of Leydig cells in the testicles
of the SFM-fed neonatal marmosets, which was surprising as these
are the cells that make the testosterone. There is every reason
to suppose that human male babies fed with SFM will show a similar
suppression of their neonatal testosterone surge.
Is this effect a cause for concern? Unfortunately, it is not yet
possible to give a clear answer to this important question. The
reason is that we do not know what the function(s) of the neonatal
testosterone surge is in boys, though effects on growth of the penis
and prostate gland and effects on the immune system are suspected.
Also we do not yet know whether there will be any long-term changes
in the SFM-fed marmosets when they become adults, though we should
have some answers by the end of this year--we know already that
their fertility is unaffected. My expectation is that there will
be no major or adverse long-term changes in SFM-fed males, but this
is just my hunch and remains to be confirmed by facts.
So what would I recommend to new mothers? I am a very strong believer
in 'breast is best'--all four of my children, including one set
of twins, were breast-fed. If not breast-feeding, I would recommend
feeding with cow's formula milk, which is tried and trusted. I would
not recommend feeding babies with SFM, not because it is proven
to cause harm (because it is not) but because it is dabbling with
the unknown and therefore taking an unknown risk (on behalf of the
baby).
If a baby develops a true intolerance to cow's formula milk, which
is very rare, then SFM can be used in the knowledge that millions
of children worldwide have been reared on it without any major problems
as far as we are aware. Mothers who have to choose SFM by force
of such circumstances should therefore do so without worrying that
they will harm their baby. But where there is a choice I would not
recommend SFM.
Dr. Richard Sharpe is a senior scientist at the
Medical Research Council Human Reproductive Sciences Unit in Edinburgh.
He is an expert on all aspects of male reproductive development
and function. He was a member of the Expert Committee that advised
the Foods Standards Agency in compiling their report on 'Phytoestrogens
and Human Health,' which will be published this summer.
The mentioned study can be viewed at the link
below.
Human
Reproduction July 2002;17(7):1692-703
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