Eating Too Much Bacon Could Harm Your Health

Consumption of red and processed meat in the UK has reduced over recent years, however on average men still exceed current recommendations [1]. It is becoming common knowledge that processed meat increases risk of colorectal cancer, however there are inconsistencies within the literature with regards to its role in the pathogenesis of other chronic diseases, particularly cardiovascular disease and type 2 diabetes. After discussing the null observations from a recent study by Lenighan et al. [2] in comparison to wider research, it has been concluded that red meat should remain part of a balanced diet, but ideally intake should be no more than 70g/day, with processed meat intake being minimal. Moreover, substitution with a serving of poultry, fish or legumes, should allow for protein and iron requirements to be met whilst reducing risk of chronic disease.

Meat is a source of high quality protein and provides vitamin B12, zinc and iron [3], so is generally promoted as part of a balanced diet. However, there is often debate regarding the ideal type of meat to consume. Red meat, such as beef and lamb, and processed meat, such as bacon, sausages and salami, have been associated with increased risk of colorectal cancer. As a result, dietary recommendations are for intake to be limited to 70g/day, which is an amount that is deemed to be able to maintain adequate iron and zinc status of the general population but reduces adverse health effects [4].

A recent cross-sectional study published by Lenighan et al. used dietary pattern analysis to investigate the association between processed meat consumption and biomarkers of two other chronic diseases of significant public health concern, cardiovascular disease (CVD) and type 2 diabetes (T2D) [2]. This review will discuss their findings within wider literature and consider the biological mechanisms that may explain any association, to determine whether recommendations to limit red and processed meat intake are appropriate, and if regular consumption of processed meat may be detrimental to long term health.

Method

Study population

Data was used from the National Adult Nutrition Survey (NANS) conducted in the Republic of Ireland on men and women aged 18-90 years.

Dietary assessment and analysis

A semi-weighed food diary was completed over 4 days, with foods coded and aggregated into 33 food groups to represent the overall diet, assigning 4 to meat. Lifestyle habits and anthropometric measurements were obtained, and an Alternative Healthy Eating Index (AHEI) score was calculated.

Biochemical measurements

Glucose, TAG, total and HDL-cholesterol, LDL-cholesterol, insulin, leptin, TNF-α and adiponectin levels were measured.

Statistical analysis

Food groups were converted to percentage of total energy intake and four main dietary patterns using two-step cluster analysis.

Results

Subject characteristics

786 men and women were included in the final sample. Red meat intake was 134g/day for men and 89g/day for women, with processed red meat intakes being 52g/day for men and 29g/day for women.

Results of dietary pattern analysis

Four dietary patterns were identified.

Pattern 1 had the lowest mean daily intake of processed meat, with participants having a better dietary quality and higher mean daily intake of carbohydrates, sugar, fibre, EPA and DHA.

Pattern 2 had high total fat intake, as well as MUFA, PUFA and α-linoleic acid.

Pattern 3 participants had highest energy intake and high contribution of unprocessed meat, alcohol and confectioner.

Pattern 4 had a significantly higher contribution of processed red meat to other patterns, with participants being older with low AHEI scores and greater intakes of total fat and SFA.

No differences were observed in markers of CVD and T2D.

Discussion

The study by Lenighan et al. reported no association between red or processed meat consumption and biomarkers of CVD and T2D [2]. The observation that those following the dietary pattern featuring highest processed meat consumption had lowest AHEI scores [2], may make this result even more surprising and may question the extent of adjustment in the analysis as risk of mortality tends to be inversely associated with HEI score [5]. It may also be that misclassification occurred as a result of changes in dietary pattern over the follow-up period, that any association between processed meat and CVD or T2D was with non-traditional and unmeasured biomarkers, or that the use of cluster analysis caused a significant loss in statistical power. The complexity of the aetiology of chronic diseases also makes it challenging to use dietary patterns analysis to isolate the effects of one food or food group [3].

Prior to considering the wider literature, within which there are some inconsistencies, the biological mechanisms by which high red and processed meat intake may contribute to adverse health should be considered. These effects are frequently attributed to its high saturated fat content, which affects serum lipid profiles [6], increasing LDL-cholesterol and risk of atherosclerosis. However, there is evidence to suggest that its heme iron content may be important. Free iron can be considered toxic [4] as it catalyses the formation of hydroxyl radicals by the Fenton reaction [7], causing cellular oxidative stress [8]. Consequently, there can be inhibition of the translocation of GLUT4 transporters to the plasma membrane and insulin binding [8], and damage to pancreatic β-cells, affecting insulin synthesis and secretion [7]. This may result in insulin resistance and T2D. Moreover, free radicals damage DNA, and heme iron promotes the formation of N-nitroso compounds and stimulates epithelial proliferation [9], suggesting it also has a role in the pathogenesis of cancer.

Further to this effect, processed meat contains preservatives for flavour enhancement, colour retention and shelf-life extension. Endogenous reactions with amines can convert nitrates to nitrosamines [10], and high temperature cooking such as frying or barbecueing [11] forms N-nitroso compounds, polycyclic aromatic hydrocarbons and heterocyclic amines 9. These compounds are carcinogenic, causing DNA mutations or adduct formation 8. Moreover, the 4-fold higher sodium content of processed compared to fresh red meat [12] may also increase intravascular volume and blood pressure [13], causing vascular dysfunction [12] and increasing risk of CVD.

To begin by considering total mortality risk, a positive association between total red meat consumption all-cause mortality has been reported [14][15], with intakes of >100g/day reducing survival time by 1 year, and >300g/day reducing it by 2 years [16]. When separating this into processed and unprocessed meat, it is more common to observe no association with unprocessed meat [14] but a greater risk of mortality for higher processed meat consumption [15]. However, this data does not elucidate the cause of death, and therefore it is likely that overall dietary pattern and other lifestyle factors may contribute.

Similar to the results by Lenighan et al., a meta-analysis observed no association between red meat consumption and chronic heart disease (CHD) [12], yet Pan et al. suggested that 8.6% of CVD deaths in men and 12.2% in women could be prevented by reducing consumption of red meat to below 0.5 servings/day [9]. To reinforce the mechanisms by which red meat may increase risk of chronic disease, it was observed that there was a moderate attenuation of the results by adjustment for SFA and cholesterol, yet a positive association still remained [9]. This suggests that iron may have a significant role in increasing CVD risk. Moreover, it has been reported that each serving of processed meat per day increases risk of CHD by 42% [12] and risk of CVD mortality by 15% [14], which may be accounted for by the elevation of blood pressure resulting from high sodium intake [13]. Despite this, total meat consumption is more commonly associated with other criteria for metabolic syndrome, with an increase in risk of central adiposity [17], hypertriglyceridemia and low HDL cholesterol [18]. It is important to note that these observations may be limited in their generalisability due to differences in meat consumption habits between populations [19].

The association between red meat consumption and stroke is more uncertain, with one study showing no association between red meat consumption and stroke [20], yet another observing a 13% greater risk of stroke per 100g/day increase in red meat consumption [21]. Moreover, it has been suggested that there is a significant positive association above 70g/day [6], with an 8% higher risk per each 50g/day increase [20]. The inconsistencies may be explained by the frequent lack of dietary measurements beyond baseline, with inaccurate classification of participants resulting from dietary changes being made throughout a study period [22].

Lenighan et al. observed no association between processed meat and markers of T2D [2], yet a positive association between total meat consumption and T2D has been reported, with a 2-fold higher risk for those consuming ≥3 servings per day compared to those consuming <2 servings per day. However, no significant association was found when this was separated into processed and non-processed meat [23]. In contrast, Fretts et al. observed an increased risk of developing T2D for processed meat [24], and Micha et al. a 19% increased risk of incident T2D per daily perving, most notably with a 2-fold higher risk from each additional serving of bacon per day [12]. Consequently, it seems that there is high likelihood for an association between T2D and processed meat, particularly those with high quantities of added nitrates.

Although it was not investigated in the study by Lenighan et al., the World Cancer Research Fund recommends eating very little, if any processed meat [25] due to its association with increased risk of cancer, most notably colorectal cancer. This has been discussed to be biologically plausible due to exogenously and endogenously produced carcinogens damaging DNA in the GI tract. Research frequently supports an association, with a meta-analysis by Chan et al. observing an increased risk of colorectal, colon and rectal cancer of 14%, 25% and 31% respectively with higher processed meat consumption. It has, however, been proposed that the risk increase is linear until 140g/day, beyond which the increase in risk is less pronounced [26], suggesting that there is great benefit to be obtained from reducing intake of processed meat, even for those with low habitual levels. Moreover, a positive association between gastric cancer and processed meat such as bacon, ham and sausages, has been observed [27], and a 19% increased risk of pancreatic cancer per 50g/day serving due to N-nitroso compounds being transported via the blood stream [28]. However, the association between meat consumption and cancer risk may not be limited to processed meat as a 36% increased risk of colorectal adenomas per 100g/day additional intake of red meat has been shown [29], although this effect may be attributed to beef more so than pork [27]. Nonetheless, it is likely that smoking status may modify the results relating to cancer incidence. This is of particular importance due to the generally poorer dietary habits and greater processed meat consumption within smokers and the presence of carcinogens within cigarette smoke.

Impacts

The null findings with regards to the effect of processed meat consumption on CVD and T2D from the study by Lenighan et al. [2] have been compared with wider research, within which there are inconsistencies in observations. These may be accounted for by the heterogeneous nature of the ‘red and processed meat’ category [12], as well as dietary measurement errors such as underreporting, attenuating results towards the null [9]. Nonetheless, the biological plausibility of the role of red meat in the pathogenesis of the main chronic diseases means it may be justified to conclude that there is likely to be a positive association between its consumption and their incidence, particularly if the overall dietary pattern is poor. However, as it is an important source of essential nutrients, including heme iron and high-quality protein, it could be said that red meat should still remain within a balanced diet. In contrast, when solely considering processed meat, the research suggests intake should be minimal due to the increased risk of CVD, T2D and colorectal cancer observed as a result of the high nitrate and sodium content. Consequently, the current SACN recommendations of limiting red and processed meat to 70g/day, with low amounts from processed meat, could be thought appropriate advice.

It is important to ensure that cooking method is considered as use of high temperatures and direct heat exposure in frying and barbecuing can enhance the formation of carcinogenic N-nitroso compounds and polycyclic amines. It would therefore be suggested that cooking meat in the oven or at lower temperatures may be one way in which red meat and processed meat can be enjoyed but with fewer adverse health effects.

Finally, when removing foods from the diet there must be replacement with alternative items. Reductions in mortality of 10% for fish, 17% for poultry, 13% for legumes, 12% for low-fat dairy and 16% for wholegrains have been shown when one serving of processed meat per day is replaced with these items [9]. It could therefore be recommended that individuals consuming in excess of 50-70g/day of red and processed meat, which is the equivalent of approximately two to three slices of ham, two sausages or a quarter pounder burger [30] should reduce the amount in their diet and instead opt for a portion of poultry, fish or legumes. This substitution should facilitate intake of sufficient iron and protein, yet reduce risk of chronic disease to improve long term health.

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