Cancer

Meat consumption has been implicated in many cancers, as being either protective or causative, depending on the type of cancer. Meat consumption has been shown to protect against cancers of the stomach (Hirayama, 1990; Tuyns et al, 1992; Azevedo et al, 1999), liver and the oesophagus (Zeigler et al, 1981; Tuyns et al, 1987; Nakachi et al, 1988). These are three of the top five cancers globally. On the other hand, meat consumption has been implicated as a cause of colorectal (colon and rectal), breast and prostate cancer, with the main emphasis being on CRC. CRC is the fourth most common cancer in the world, but in Europe and other Western countries it is second in terms of incidence and mortality (after lung cancer in men and breast cancer in women) with 190000 new cases per year in Europe (Black et al, 1997; Bingham, 1996). There is strong evidence from epi-demiological studies showing that diet plays an important role in most large bowel cancers, implying that it is a potentially preventable disease (Higginson, 1966; COMA, 1998). The precise dietary components that influence CRC risk have not been fully elucidated. However, epidemiological studies suggest that high intakes of fat, meat and alcohol increase risk, whereas vegetables, cereals and non-starch polysaccharides, found in fruit and many other foods, decrease the risk (Bingham, 1996). For many of these dietary factors the evidence is equivocal. In the case of meat, the evidence is conflicting, early cross-sectional comparisons attributed much of the world-wide variation in CRC incidence to fat and animal protein consumption (Armstrong and Doll, 1975). In contrast, subsequent case-control and cohort studies are much less consistent (Hill, 1999a).

Meat consumption and CRC became a high profile issue during 1997 and 1998 with the global launch of the World Cancer Research Fund report (WCRF, 1997), timed to coincide with the publication of the British COMA report, both on diet and cancer. The WCRF report was particularly negative towards red meat, which fuelled the launch publicity. This stimulated several critical appraisals of the report, all challenging the conclusions regarding meat (Hill, 1999b). The scientific evidence is not sufficiently robust to recommend a maximum of 80g/day red meat as pronounced by the WCRF and the initial announcement by COMA for a similar recommendation was subsequently revised. Most of the data showing an association between meat consumption and CRC are American, whereas several studies conducted outside the US (many in Europe) have shown no such relationship (Hill, 1999a). On final publication, COMA (1998) reassured UK consumers that average consumption levels (90g/day of cooked red meat) were acceptable. COMA suggests that high consumers, less than 15% of the UK popu lation, eating above 140g/day might benefit from a reduction. Equally importantly, this report acknowledged that meat and meat products remain a valuable source of a number of nutrients including iron and that for many a moderate intake makes an important contribution to micronutrient status. The potential effect on iron status of further reductions to red meat intakes was subsequently investigated, as recommended within the COMA report. Given that a 50% reduction in intake would result in a third of women having low iron intakes (below 8 mg/d), the appropriateness of public health messages concerning meat consumption should be carefully considered prior to reaching the media (Gibson and Ashwell, 2001).

Various components of meat (protein, iron, and heterocyclic amines) have been suspected of contributing to the development of CRC. Dietary protein is broken down in the body to amino acids, which are further degraded to ammonia, which may have cancer-initiating effects. The human colon is also rich in amides and amines that are substrates for bacterial nitrosation by nitric oxide (NO) to N-nitroso compounds that are found in human faeces. There is no conclusive evidence that protein derived compounds can increase cancer risk in humans. It is hypothesised, but not yet established, that the intake of iron from meat and other iron-rich foods may increase the risk of cancer via the production of free radicals in the body. Heterocyclic amines are formed by the Maillard reactions that involve amino acids, sugars and creatine during cooking. They are usually produced on the surface of meat during cooking at very high temperatures, such as in frying, grilling or barbecuing but they are minimal when meat is steamed, microwaved or marinated. The heterocyclic amines are known mutagens in vitro and carcinogens in rodents. The most abundant heterocyclic amine produced in meat is phenylimadazo pyridine (PhIP), which is a relatively weak carcinogen compared to other heterocyclic amines such as IQ and MeIQ. The role of hete-rocyclic amines in causing CRC is not fully elucidated in humans.

Truswell summarised the evidence in 2000 and showed that 20 out of 30 case-control studies and 10 out of 14 prospective studies showed no relationship between meat intake and CRC with some of the results of the remaining studies being confused and one prospective study showing an inverse correlation between meat consumption and CRC risk (Hill, 2000). If meat consumption were associated with increased risk for cancer, one would expect mortality from cancer to be much lower among vegetarians. In a recent meta-analysis of five cohort studies, results have shown no significant differences in mortality from cancer in general, and more specifically mortality in stomach, breast, lung, prostate and colorectal cancer between vegetarians and omnivores (Key et al, 1998, 1999). If red meat consumption were associated with increased risk for CRC, one would expect a decrease in the incidence of CRC to occur over time as a result of decreasing meat consumption trends. During the past 30 years, red meat consumption in the UK has decreased by approximately 25%, while during the same time the incidence of CRC has increased by about 50% (Hill, 1999b). Similarly, if meat consumption were associated with increased risk for CRC, one would expect the rates of CRC to be higher in countries with high meat consumption and lower in countries with low meat consumption. People in the Mediterranean countries eat more red meat than do, for instance, the inhabitants of the UK, yet these countries have lower CRC rates (Hill, 2000). Such paradoxical findings are further evidence that, at current levels, meat consumption is not a risk factor for CRC incidence.

Epidemiological associations between dietary components, specific foods or food groups and chronic disease, such as cancer, can identify risk factors, but are generally insufficient to establish cause and effect relationships. Findings from epidemiological studies must be combined with other types of evidence (e.g. animal experiments, human clinical trials) before a persuasive causal relationship can be established. CRC is multi-factorial; it is confounded by diet, smoking, alcohol, physical activity, obesity, aspirin use, age and family history. There are known protective and causative factors. It is well-known that daily consumption of vegetables and meat reduces the risk of cancer at many sites, whereas daily meat consumption with less frequent vegetable consumption increases risk (Hirayama, 1986; Kohlmeier et al, 1995; Cox and Whichelow, 1997). Evidence suggests that it is the reduced intakes of the protective factors such as vegetables and cereals that are the main determinants of CRC risk with meat being coinci-dentally related.

There is a need to assess the role of meat when consumed in normal quantities, by normal cooking methods, and within the context of a mixed, balanced, diet. The method of cooking meat and the degree of browning are of particular importance to this whole issue. A major effort by International Meat Industry partners has attempted to raise awareness of the complexities of meat preparation and cooking habits and how these differ between countries. Dietary assessment techniques adopted by nutrition scientists currently do not take full account of the diverse differences between meat products world-wide and the consequent influences these may have on the body. For example, it is well recognised that meat is often cooked more evenly through the muscle within Europe, whereas it tends to be 'blackened' on the outside whilst remaining rare on the inside in North America. This may be one reason for the greater negative findings in American studies of the role of meat in CRC, compared with European studies. This hitherto unexplored facet of meat consumption may have far-reaching implications for interpretation of epidemiological data and ultimately for public health recommendations. Certain marinades applied to meat before cooking will reduce the quantity of potential carcinogenic materials present. The application of knowledge in this area to the production of processed meat products with all the nutritional benefits and none of the potentially harmful components would be progressive indeed.

In summary, it is important not only to examine the relationship between meat consumption and CRC alone, but also to look at meat preparation and cooking differences in conjunction with protective factors, such as vegetables and cereals. At a meat and diet workshop, it was stated:

It is time that the meat CRC story was laid to rest, so that we can get back to recommending that young women of childbearing age eat meat as a ready source of available iron. (Hill, 2000)

Nevertheless, it is sensible to consider that there must be an optimal range for meat intakes in order to ensure a balanced diet is achieved whilst optimal weight is maintained. From this practical perspective COMA's (1998) suggested intake range of 90-140g cooked meat per day is sensible as a public health message. The overemphasis on reducing meat, however, rather than encouraging greater accompanying plant food intake has served only to confuse the public (Hill, 1999b). Evidence suggests that the risk of cancer will be reduced to a greater extent by increasing intakes of fruit and vegetables than by lowering meat intakes. Once again, the move towards pre-prepared meal solutions provides opportunity for manufacturers to develop recipes with a healthy balance of meat and vegetable ingredients such that the nutritional profile of the dish is optimised.

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