Nutritional epidemiology provides evidence about the diet-disease relationship to inform Public Health Nutrition (PHN) and enable the development of prevention methods [Boeing, 2013].

Almost 60% of global colorectal cancer (CRC) cases are seen in developed countries [Ferlay et al., 2010]. In the UK, CRC is the fourth most common cancer; accounting for an incidence rate of 47 per 100,000 in 2011 [CRUK, 2014a].

The WCRF/AICR [2007] recognise the complexities of determining associations between diet and cancer, and conclude that red and processed meat (RAPM) are ‘convincing’ causes of CRC. This is reinforced by subsequent meta-analyses by Chan et al. [2011] and Xu et al. [2013]. In contrast, Alexander and Cushing [2011] suggest there is inadequate evidence to support this relationship .

The aim of this essay is to critically appraise two cohort studies by Norat et al. [2005] and Cross et al. [2007], observing the relationship between RAPM consumption and CRC risk. These studies have been selected due to their large sample size and robust study design. Since CRC is largely influenced by diet [Willett, 2005], understanding these influences will enable development and implementation of PHN strategies at a national and local level [Webster-Gandy, Madden and Holdsworth, 2012].

Critical Discussion

Study Design

Cohort studies are the strongest observational design for diet and cancer [Gibney et al., 2004]. Although randomised controlled trials (RCTs) are considered the ‘gold standard’ of quantitative research methods [Aveyard and Sharp, 2009], they are not always appropriate [Morrow, 2010]. Since cancer develops slowly, applying intervention trials can be difficult [Gibney et al., 2004]. It may also be unethical to use a RCT, as some participants would not receive the potentially beneficial treatment [Sanson-Fisher et al., 2007]. Cohort studies however are considered to be ethical, and are useful for establishing a sequence of events [Heneghan and Badenoch, 2006]. In contrast, they do involve enrolling numerous participants which is costly [Willett, 2013]. There is also potential for selection bias, information bias and confounding bias which can reduce the reliability and validity of the findings [Healey and Devane, 2011].


Both studies used Cox regression to adjust for confounders. CRUK [2014b] outline the dietary and lifestyle factors believed to be associated with CRC risk. Both studies have failed to account for some of these factors, which could lead to an inappropriate assumption that the exposure increases disease risk [Healy and Devane, 2011].

In the XXXX study, cooking methods were not considered, even though certain methods have been linked with carcinogen production [WCRF /AICR, 2007]. Genetic susceptibility was also not accounted for, despite the evidence of an association between family history and CRC [Johns and Houlston, 2001]. Additionally, socioeconomic factors were not taken into account, despite the positive association between deprivation and CRC incidence in males [PHE, 2014].

In the YYYY study, the highest consumers of red meat also had other CRC risk factors. Therefore, residual confounding may have contributed to the positive associations observed. Additionally, 42.8% of women reported use of hormone replacement therapy (HRT), which wasn’t adjusted for, even though HRT is believed to reduce CRC risk [CRUK, 2014b].


Alexander, D.D. and Cushing, C.A. (2011) ‘Red meat and colorectal cancer: a critical summary of prospective epidemiological studies’. International Association for the Study of Obesity. 12(5), pp. e472-e493.

Aveyard, H. and Sharp, P. (2009) A beginners guide to evidence based practice in health and social care. Berkshire: Open University Press

Boeing, H. (2013) ‘Nutritional epidemiology: New perspectives for understanding the diet-disease relationship?’ European Journal of Clinical Nutrition. 67, pp. 424-429.

Cancer Research UK (CRUK) (2014a) Bowel Cancer Incidence Statistics. Available at:

Cancer Research UK (CRUK) (2014b) Bowel Cancer Risk Factors. Available at:

Chan, S.M., Lau, R., Aune, D., Vieira, R., Greenwood, D.C., Kampman, E. and Norat, T. (2011) ‘Red and Processed meat and Colorectal Cancer Incidence: meta-Analysis of Prospective Studies’. PloS ONE. 6(6), pp. 1-11.

Cross, A.J., Leitzmann, M.F., Gail, M.H., Hollenbeck, A.R., Schatzkin, A. and Sinha, R. (2007) ‘A Prospective Study of Red and Processed Meat Intake In Relation to Cancer Risk’. PLoS Medicine. 4(12), pp. 1973-1984.

Ferlay, J., Shin, H.R., Bray, F., Forman, D., Mathers, C. and Parkin, D.M. (2010) ‘Estimates of worldwide burden of cancer in 2008: GLOBOCAN 2008’. International Journal of Cancer. 127(12), pp. 2893-2917.

Gibney, M.J., Margetts, B.M., Kearney, J.M. and Arab, L. (2004) Public Health Nutrition. Oxford: Blackwell Science.

Healy, P. and Devane, D. (2011) ‘Methodological considerations in cohort study designs’. Nurse Researcher. 18(3), pp. 32-36.

Heneghan, C. and Badenoch, D. (2006) Evidence-based Medicine Toolkit (pdf). 2nd edn. Massachusetts: Blackwell Publishing. Available at:

Johns, L.E., Houlston, R.S. (2001) ‘A systematic review and meta-analysis of familial colorectal cancer risk’. Am J Gastroenterol. 96(10), pp. 2992-3003.

Morrow, B. (2010) ‘An overview of cohort study designs and their advantages and disadvantages’. International Journal of Therapy and Rehabilitation. 17(10), pp. 518-523.

Norat, T., Bingham, S., Ferrari, P., et al (2005) ‘Meat, fish, and colorectal cancer risk: the European Prospective Investigation into cancer and nutrition’. J Natl Cancer Inst. 97(12), pp. 906–916.

Public Health England (PHE) (2014) National Cancer Intelligence Network: Cancer by Deprivation in England (pdf). Available at: file:///C:/Users/Megan/Downloads/Cancer_by_Deprivation_in_England_1996_2011.pdf

Sanson-Fisher, R.W., Bonevski, B., Green, L.W. and D’Este, C. (2007) ‘Limitations of the Randomised Controlled Trial in Evaluating Population-Based Health Interventions’. American Journal of Preventive Medicine. 33(2), pp. 155-161.

Willett, W.C. (2005) ‘Diet and cancer: an evolving picture’. JAMA. 293(2), pp.233-234.

Willett, W.C. (2013) Nutritional Epidemiology. 3rd edn. New York: Oxford University Press.

Webster-Gandy, J., Madden, A. and Holdsworth, M. (2012) Oxford Handbook of Nutrition and Dietetics. 2nd edn. Oxford: Oxford University Press.

World Cancer Research Fund (WCRF)/American Institute for Cancer Research (AICR) (2007) Food, nutrition, physical activity and the prevention of cancer: a global perspective (pdf). Available at:

Xu, X. Yu, E., Gao, X., Song, N., Liu, L., Wei, X. Zhang, W. and Fu, C. (2013) ‘Red and processed meat intake and risk of colorectal adenomas: A meta-analysis of observational studies’. International Journal of Cancer. 132(2), pp.437-448.

I am module lead for Nutritional Epidemiology module. This assessment is a challenge for the students as it tests their critical writing skills (and nutrition as a subject area that provides many complexities with this skill). This essay excerpt is taken from a student who achieved a first class mark for this assessment. This essay (of which excerpts are shown) was well structured, with good use of a range of referenced sources, which were well integrated to demonstrate good understanding of epidemiology concepts as well as excellent skills in critical appraisal of the evidence.

Function: Describe

This sentence leaves the reader asking the question, what are the strategies? It would have been better to provide further details.

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Good that it relates to the 'gold standard', and is supported by a reference.

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Good introductory sentence, helping the reader understand the intended direction of the writing.

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Quality: Authority

Please note that 'et al.' is a Latin term and normally shown in italics.

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Ensure that all abbreviations are explained within the text initially. It is then acceptable to use them from that point forward. This rule also applies to citations.

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An excellent critical review on the use of cohorts, with good use of evidence and structure for the discussions.

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Bold statement and supported well with ref

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A very good range of references have been used for this extract of the essay.

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This is a bold statement and supported well with a reference.

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Quality: Structure

This is a very good mini critique to decipher evidence.

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Once again good here, you have split your studies here, which makes sense to get your various confounding related points over with clarity

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Once again this is a good approach. Splitting the studies has helped put forward various confounding related points across with clarity.

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Function: Analyse

You are going to appraise if this is a robust design

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This is a very good introduction, you set scene well and also in context of public health nutrition - good use of refs and also integrated critical skills - essay introduced very well

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This is not a very scientific term, it is much better to be more specific.

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This is very thorough critique here, you have gone outside simple critique, drawing on additional concepts from lecture material and showing very sound understanding of confounding as a concept

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Although this is a good use of a reference, it does not give the reader any suggestions as to why it is an important.

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This sentence clearly indicates that the author will critically appraise robust designs.

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This is a very thorough critique. It draws on additional concepts from lecture material, and shows a very sound understanding of confounding as a concept.

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This is a very good introduction, it sets the scene well within the context of public health nutrition. Good use of references and integrated critical skills.

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The discussion begins.

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