The thrifty phenotype hypothesis and the epidemiological observations
focused on impaired fetal nutrition
and therefore a reduction in fetal growth as a central concept to their
hypothesis. Size at birth serves
as a marker of fetal nutrition. Growth is only one manifestation of the im=
pact
of the environment on the
developing organism. There may be also responses of altered maturation and=
/or
altered timing of fetal
development. The fetus in an impaired environment has fewer options. These
options can even include=
death.
In evolutionary terms, the fetus’ sacrifice will allow the mother to repro=
duce
her genes again. In less
dramatic situations, the fetus can change its maturation, gestational leng=
th
and growth rate and be b=
orn
smaller.
•<=
/span>Prematurity itself is a marker of an =
adverse
uterine environment, PARs can be induced in utero by the same stimul=
i that
induces premature delivery.
•<=
/span>Studies in sheep have shown that gest=
ational
length can depend on periconceptual undernutrition. This has been linked to changes in placent=
al and
fetal endocrine functions, including premature activation of HPA (hypothalamic-pituitary-adrenal) =
axis.
•=
span>The long-term effects of prematurity =
may also
be a consequence of being born premature.
All these aspects play a role in inducing long-term consequences for
those being born small.
The fact is that prematurity is not a problem exclusive of the poor.
Despite current levels of nutrition in developed countries, the nutrition of the fetus (a=
nd the
infant) is often unbalanced because of tobacco smoke, unbalanced diets or because of the long and
vulnerable fetal supply line.
•<=
/span>Since the plastic period extends into=
the
neonatal period, altered nutrition in this period has long-term consequences. Rapid growth in childho=
od
compensates for low birth weight. It is not known yet whether the catch-up phase is an independent =
risk
factor or whether it is the biological/sociological (parental overfeeding) consequence of being a
growth-impaired infant. However, compensatory growth can have high costs. In animals, their life-sp=
an is
reduced. Barker’s suggestion is that “a higher rate of cell division causes more rapid shortening=
of the
protective ends of the chromosomes (telomeres) and hastens cell death and degradation. There are a nu=
mber
of other possible processes by which, in humans, undernutrition and small size at birth fol=
lowed
by rapid childhood growth could lead to cardiovascular disease, type 2 diabetes and hypert=
ension
in later life.”
We
must remember that nutrition is only one of the many factors that affect f=
etal
maturation and growth. T=
here are
environmental factors that have adverse consequences and last for long on =
the
organism but have no eff=
ect on
size at birth. For example, during the Dutch winter famine (1944), women w=
ho
ate less than 800 calore=
s per
day had babies with normal birth weight but that became obese later on in
life.
Refs:
•<=
/span>Barker, Fetal origins of adult dis=
ease:
strengths of effects and biological basis, International Journal of =
span>Epidemiology(2002) 31:1235.
=
i>
•<=
/span>Gluckman, The fetal, neonatal and =
infant
environments – the long-term consequences for disease risk, Early Human Development (2005) 81:=
51.
=
13;