Full Report - Food, Nutrition, and the Prevention of Cancer

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Box 4.2.4 Foods containing dietary fibre
The concept of dietary fibre arose from observations of the low
prevalence of colon cancer, diabetes, and coronary heart disease
in parts of Africa amongst people whose diets were high in
unrefined carbohydrates and whose stools were typically bulky,
and often or sometimes semisolid. Considerable efforts have
been dedicated to characterising the dietary components of
what has come to be called dietary fibre that might confer
health benefit. Naturally occurring dietary fibre is only derived
from plant foods. Pulses (legumes) and minimally processed
cereals are particularly concentrated sources, but vegetables and
fruits also contain significant amounts. Dietary fibre isolated
from plant cell walls and synthetic forms are increasingly entering
the food supply.
High intakes of dietary fibre, variously defined, have been
associated with reduced risk of cardiovascular disease as well as
of some cancers. Definitions of dietary fibre vary. Some are
based on chemical analyses of the components of plant cell
walls, such as non-starch polysaccharide, others on physiological
effects — the carbohydrates that enter the large bowel having
escaped digestion in the small intestine being defined as
dietary fibre. The latter definition includes oligosaccharides and
resistant starch. The World Health Organization and Food and
Agriculture Organization have recently proposed that only polysaccharides
which form part of plant cell walls should be regarded
as dietary fibre and that the health benefits of resistant
starch and oligosaccharides are more appropriately considered
separately.
This box also appears as box 4.1.2 in the previous section
4.2.2), have been shown to have potentially beneficial effects
in laboratory experiments, as detailed in the evidence in
chapter 4.2.5 (also see Chapter 2). 7-9
Pulses (legumes)
Dry pulses are seeds and are higher in protein than most
other plant foods. Soya beans and peanuts contain 37 g per
100 g and 26 g per 100 g protein dry weight respectively,
although, on average, pulses contain around 20 g per 100 g
protein dry weight. 10 These foods are typically high in carbohydrates
and non-starch polysaccharides (dietary fibre),
and are generally low in fat. Soya beans and peanuts are
exceptions, being relatively high in fat with 8 g per 100 g
and 47 g per 100 g fat, respectively (mostly mono- and
polyunsaturated fatty acids). They also contain oligosaccharides
that are not digested in the gut but are fermented
by bacteria in the colon. Soya beans are distinct from other
legumes in that they have a high content of bioactive
isoflavones, or phytoestrogens, which have hormone-like
effects in the body. They are also good sources of saponins
and phytosterols, which decrease cholesterol absorption.
Many legumes contain deguelin, which has been shown to
have anti-tumour effects in laboratory experiments. 11 Most
pulses are virtually indigestible and inedible before cooking;
immature legumes that are eaten green have higher levels
of sugar and lower levels of non-digestible polysaccharides
than dried pulses.
P ART 2 • EVIDENCE AND JUDGEMENTS
Nuts and seeds
Other seeds and nuts are also relatively high in protein and
fat; some contain as much as 60 g fat per 100 g. They are
therefore energy-dense foods (see Chapter 8), as well as
being nutrient-dense. Weight-for-weight, nuts provide more
calories than either meat or cereals (grains), although chestnuts
are the exception as they are relatively low in fat. Most
nuts contain mainly monounsaturated fatty acids, although
the exceptions are coconuts, which contain a high proportion
of saturated fatty acids, and walnuts and pecans, which
contain mostly polyunsaturated fatty acids (see chapter
4.5.2). Nuts and seeds are high in dietary fibre (box 4.2.4),
especially when they are eaten with their skins or hulls; the
fibre content is typically 5–11 g per 100 g. Nuts and seeds
are also high in vitamins and minerals, particularly the B vitamins,
vitamin E, and folate; and the seed coats contain phenolic
compounds.
Herbs and spices
Nearly all herbs and spices contain aromatic compounds,
which are volatile molecules that are usually fat- rather than
water-soluble. The flavour compounds may make up as much
as 15 g per 100 g of a spice by weight, although herbs contain
much lower levels — typically around 1 g per 100 g.
Many plants have evolved to contain these compounds
because they act as deterrents to herbivores. Some of these
aromatic compounds may be bioactive, although possibly not
at the levels found in most diets. Isothiocyanates are responsible
for the spicy/hot flavour of mustard and horseradish,
produced from glucosinolates in cruciferous plants. Chives
and garlic (allium vegetables) contain the distinctive sulphides
discussed above. Terpenoids are common components
in herbs and spices, providing distinctive flavours. Examples
include monoterpenes, such as geranial in lemon grass, and
linalool in bergamot; sesquiterpenes, such as bisabolene in
ginger; triterpenoids, such as the saponin glycrrhizic acid,
found in liquorice root; and tetraterpenoids, such as the
carotenoid, lycopene.
4.2.3 Consumption patterns
Fruits and vegetables
The global average for vegetable consumption (based on
availability and not including vegetable oils) is 2.6 per cent
of total daily energy intake. 17 It is generally highest in North
Africa, the Middle East, parts of Asia, the USA and Cuba, and
in southern Europe. Although consumption levels are similar
in countries of high and low economic status, vegetables represent
a greater proportion of daily energy intake in the lowincome
countries. Intakes range from 5.3 per cent in parts of
Asia to as little as 0.2 per cent in sub-Saharan Africa. On average,
the availability of vegetables is increasing globally.
The global average for fruit consumption (based on availability)
is 2.7 per cent of total daily energy intake. Fruit consumption
is generally higher than vegetable consumption,
but it shows a greater degree of variability. Fruit consumption
is higher in high-income countries, although it represents
a similar percentage of total available dietary energy