Milk-and-Dairy-Products-in-Human-Nutrition-FAO

Chapter 4 – Milk and dairy products as part of the diet 159
4.10.1 Lactose intolerance and malabsorption
Lactose is the principal carbohydrate in milk. Cow, goat and buffalo milk contain
less lactose than human milk. Lactose is a disaccharide composed of the two simple
sugars, glucose and galactose. An enzyme, lactase (a β-galactosidase), is required
to hydrolyse lactose into the simpler sugars in order for humans to digest and
then absorb the sugars. In adults with lactase deficiency (also called lactase nonpersistance,
LNP), lactose is not digested in the upper bowel and reaches the lower
bowel, where it is fermented by gut micro-organisms, which produces hydrogen,
carbon dioxide and methane gas. Undigested lactose also draws water into the
intestinal lumen through its osmotic effect, which increases motility and can cause
diarrhoea. Symptoms include abdominal pain, bloating and flatulence. Thus, low
lactase levels cause lactose malabsorption (or lactose maldigestion). When lactose
malabsorption gives rise to symptoms, this is called “lactose intolerance”, i.e. lactose
malabsorption is the physiologic problem that manifests as lactose intolerance. The
definitions used by the American Academy of Pediatrics Committee on Nutrition
(Heyman, 2006) are given in Box 4.1. Lactose maldigestion does not lead to
symptoms of lactose intolerance in all LNP subjects, and a small percentage of LNP
subjects remain free of symptoms even after ingestion of large amounts of lactose
(Scrimshaw and Murray, 1988). Although rarely life-threatening, the symptoms of
lactose intolerance can lead to significant discomfort and disrupted quality of life
(Heyman, 2006).
Lactase deficiency in adults is a normal developmental phenomenon characterized
by the down-regulation of lactase activity, which occurs soon after weaning in
most ethnic groups (EFSA, 2010). Lactose maldigestion increases with age during
adulthood (Goulding et al., 1999). The lactase persistence trait is more common in
populations that practice cattle herding and dairying (Swallow, 2003), and is related
to genetic selection of individuals with the ability to digest lactose (Heyman, 2006).
Children of some ethnic groups commonly lose lactase at one to two years of age
(e.g. Thai children) while in others lactase persists until later in life (10–20 years
of age) (e.g. Finnish children) (Sahi, 1994; Wang et al., 1998). According to some
estimates, approximately 70 percent of the world’s population has primary lactase
deficiency (Heyman, 2006). The frequency of lactose maldigestion varies widely
among populations but is high in nearly all but those of Northern European origin
(Table 4.6) (Scrimshaw and Murray, 1988; Heyman, 2006). Lactase deficiency in
Europe has been reported to vary between 4 percent (in Denmark and Ireland) and
56 percent (in Italy) (Ingram et al., 2009a, cited in EFSA, 2010). In South America,
Africa and Asia, over 50 percent of the population are reported to have lactase
nonpersistence, and in some Asian countries this rate is almost 100 percent (Lomer,
Parkes and Sanderson, 2008). However, because definitions vary from study to
study and subjects are not generally representative of the whole population, the
exact incidence is unknown. The symptoms attributed to lactose intolerance are also
common in the absence of lactose ingestion as they can sometimes be attributed to
other components of the diet, and are highly susceptible to a placebo effect (Shaukat
et al., 2010, cited in EFSA, 2010).
In individuals who are diagnosed with lactose intolerance, avoidance of foods
that contain lactose, such as milk, will relieve symptoms. However, most individuals
can tolerate some dairy products and can progressively increase tolerance because