Cancer
Cancer is the second leading
cause of death in Britain, accounting for 25% of all deaths. It has been
estimated that diet may be linked to
30-70% of cancers (Doll, 1990). Certain cancers, such as colon, breast
and prostate are clearly diet related (Cummings & Bingham, 1998).
Sir Kenneth Calman, Chief Medical Officer, has stated (1997) that "there
is a relationship between eating red meat and cancer".
The Oxford Vegetarian Study found cancer mortality to be 39% lower
among vegetarians compared with meat-eaters (Thorogood, 1994).
A study of 23,000 largely vegetarian Seventh Day Adventists found cancer
mortality rates to be 50-70% of those of the general population for several
cancer sites unrelated to smoking or alcohol (Phillips, 1975).
Professor Nick Day of the University of Cambridge and the European
Prospective Study into Cancer has stated that vegetarians may suffer
40% fewer cancers than the general population.
The World Cancer Research Fund's dietary advice to minimise cancer
risk involve reducing the intake of dietary fat and increasing the consumption
of fruits, vegetables and wholegrains.
Colon Cancer
Vegetarians have lower rates
of colon cancer than non-vegetarians (Phillips, 1980). Incidence of colon
cancer has been strongly linked to
the consumption of meat (Armstrong, 1975, Singh & fraser, 1998). Willett
(1990) carried out a study of over 88 000 women aged 34 to 59 years. Women
eating red meat daily ran over twice the risk of developing colon cancer
than women eating red meat less than once a month.
Reduced incidence of colon cancer in vegetarians may be attributed
to dietary differences which include increased fibre intake, increased
consumption of fruit and vegetables, and decreased intake of total fat
and saturated fat. The mechanism by which a vegetarian diet is protective
against colon cancer is unclear and a great deal of research is being
carried out in this area.
It has been suggested that secondary bile acids are carcinogens which
may play an important role in colon cancer. These are derived by bacterial
metabolism from primary bile acids made in the liver and secreted into
the intestine. Vegetarians have lower levels of secondary bile acids
than non-vegetarians (Turjiman, 1984). The differences in bacterial populations
between the intestines of vegetarians and non-vegetarians may also be
important. Bacterial flora in vegetarians has been shown to possess reduced
ability to transform bile acids into potential carcinogens (Johansson,
1990).
The role of dietary fibre in prevention of colon cancer may also be
important. This was first noted in 1971 when it was suggested the high
incidence of colon cancer in Western countries was linked to low fibre
diets. Other dietary components associated with high fibre foods, such
as folate, have also been implicated as having protective effects.
Chen (2002) found that the risk of distal stomach adenocarcinoma was
positively associated with red meat intake.
Breast Cancer
Evidence also suggests a vegetarian diet is protective against breast
cancer (Phillips, 1975). This may be due to the increased fibre and reduced
fat intake of vegetarian diets. Vegetarian diets can alter the levels of
circulating sex hormones which may have a beneficial effect. Fibre is thought
to be protective by modifying circulating oestrogen levels.
Studies of adolescent girls have shown age of menarche to be delayed
in vegetarians (Sabate, 1992). Later age of menarche is believed to lower
the risk of breast cancer in adult life.
Other Cancers
Studies have shown vegetarians to suffer less from various other cancers.
Mills (1989) studied the incidence of prostate cancer amongst 14,000
Seventh Day Adventists and found a relationship between increased risk
and increasing animal product consumption.
Mills (1988) also found pancreatic cancer to be associated with consumption
of animal products. Increasing consumption of fruit, vegetables and pulses
was shown to have a protective effect.
Rao (1989) found a vegetarian diet to be protective against oesophageal
cancer.
Studies have also shown vegetarians to have lower incidence of lung
cancer. This can be largely attributed to vegetarians tending to be non-smokers.
High consumption of fruit has also shown to be protective against lung
cancer (Fraser, 1991).
