Chemistry Reference
In-Depth Information
2000), and hence the ability to drink fresh milk, which contains both calcium
and components that stimulate its uptake (along with small amounts of
vitamin D) may have provided an advantage to persistent individuals.
In only one case has selection against lactase persistence been proposed.
Anderson and Vullo (1994) suggested that selection had acted in favour of
lactase non-persistence in malarial regions because of the observation that
individuals with flavin deficiency are at a slightly reduced risk of infection by
malaria. The consumption of milk, which is rich in riboflavin was, therefore
said to be unfavourable as it would keep the flavin level in the bloodstream
high. This explanation is not widely supported, and is thought to be an
unlikely mechanism by which to explain the current distribution of lactase
persistence.
6.10.
The Role of Other Factors that Influence
Lactase Expression
While it has been well established that regulation of LCT is predominantly
under transcriptional control, there is evidence to suggest that additional
levels of control over expression of the enzyme exist (Rossi et al., 1997).
Heterogeneity of the lactase non-persistence phenotype has been reported
by a number of research groups. Some have observed individuals who show
slower/abnormal processing (Witte et al., 1990; Sterchi et al., 1990) which
may imply post-translational controls such as glycosylation and/or transpor-
tation, whilst others have made observations suggestive of epigenetic regula-
tion. Although most non-persistent individuals show no immuno-histological
staining for lactase in the jejunal biopsies of the small intestine (concordant
with low lactase activity and transcriptional regulation of LCT), it was found
by Maiuri et al. (1991) that some individuals show patchy expression of the
enzyme in the intestinal epithelia. This mosaic expression pattern may result
from somatic cell changes in methylation or histone acetylation.
6.11.
Present-Day Health and Medical Considerations
Lactose malabsorption is often confused with other illnesses associated with
milk drinking, such as milk protein allergy, which has quite different causes
(reviewed by Crittenden and Bennett, 2005), and in recent times lactose
intolerance has been blamed for causing a variety of systemic conditions,
often without clear evidence (Matthews et al., 2005; Campbell and Matthews,
2005). Nonetheless, it does appear that the consumption of milk and milk
products by those who cannot digest lactose is a relatively common cause of
