In formulating his views regarding conventional wisdom relating to saturated fats Dr Smith appears to have been strongly influenced by a number of popular books most written by lay authors, and examination of several original publications1. Lay authors may not understand the inherent complexities, imprecisions and inaccuracies associated with individual studies and, whether by design or simply because they have not carried out a formal search of data bases, may not be in possession of the totality of evidence. “Cherry picking” of evidence whether by design or default can lead to serious misinterpretation of existing knowledge
Two examples taken from Dr Smith’s feature serve to illustrate how individual studies may be misinterpreted. Dr Smith describes the Women’s Health Initiative (WHI) as “the biggest test of the saturated fat hypothesis”. This is incorrect. The saturated fat hypothesis assumes that the effect of this nutrient on coronary heart disease (CHD) is to a considerable extent mediated via cholesterol. In the WHI the low fat intervention only achieved a 0.09 mmol/l reduction in cholesterol which would not have been expected to achieve a reduction in CHD2. Furthermore the lower fat higher carbohydrate was associated with a small weight loss compared with the usual diet, a finding consistent with a systematic review of all intervention studies of lower fat diets. The fallacious ecological argument that Smith invokes to infer that lower fat intakes in some countries have led to increasing rates of obesity could be applied equally to the opposite phenomenon in China (18% of the world’s population) where the increasing fat intakes correspond closely with rising obesity rates3.
The Hooper et al. Cochrane Collaboration systematic review has also been misinterpreted4. The 14% reduction in cardiovascular events associated with a reduction in saturated fat is what might have been expected on the basis of the expected reduction in cholesterol5. This would translate into appreciable public health benefit. No benefit was observed in terms of mortality but nor would this have been expected given the relatively short duration of the intervention studies4.
Systematic reviews also differ in their findings based on how the research question as well as the qualifying criteria for inclusion of studies is defined. This opens the door for cherry picking of results, as is often seen in the popular press and, unfortunately, some academics who are not trained in nutritional science or who may have vested interests. Systematic reviews have shown the importance of limiting saturated fat in favour of MUFAs and PUFAs 4, 6, 7 but approaches and interpretations differ8.
Dietary guidelines developed by authoritative bodies around the world (e.g. by World Health Organisation and national governments) are based on careful and systematic reviews of the literature, the type recommended by the Cochrane library9-11, and are derived through processes such as those outlined by GRADE12. The guidelines lead to the conclusion that a wide range of dietary patterns (and macronutrient compositions) are consistent with a healthy diet. However, they are consistent in their advice to limit intake of saturated fat in favour of mono and poly-unsaturated fats, and limit sugar and highly refined carbohydrates in favour of wholegrains, fresh fruit and vegetables. Or, in the words of another popular author “eat [whole] food, not too much, mostly plants”13.
Rachael McLean, Murray Skeaff, Jim Mann and Lisa Te Morenga
1. Smith R. Are some diets "mass murder"? BMJ. 2014;349:g7654.
2. Howard BV, Manson JE, Stefanick ML, Beresford SA, Frank G, Jones B, Rodabough RJ, Snetselaar L, Thomson C, Tinker L, Vitolins M, Prentice R. Low-fat dietary pattern and weight change over 7 years: the Women's Health Initiative Dietary Modification Trial. JAMA. 2006;295(1):39-49.
3. Zhai F, Wang H, Du S, He Y, Wang Z, Ge K, Popkin BM. Prospective study on nutrition transition in China. Nutr Rev. 2009;67 Suppl 1:S56-61.
4. Hooper L, Summerbell CD, Thompson R, Sills D, Roberts FG, Moore HJ, Davey Smith G. Reduced or modified dietary fat for preventing cardiovascular disease. Cochrane Database Syst Rev. 2012;5:CD002137.
5. Baigent C, Keech A, Kearney PM, Blackwell L, Buck G, Pollicino C, Kirby A, Sourjina T, Peto R, Collins R, Simes R. Efficacy and safety of cholesterol-lowering treatment: prospective meta-analysis of data from 90,056 participants in 14 randomised trials of statins. Lancet. 2005;366(9493):1267-78.
6. Mozaffarian D, Micha R, Wallace S. Effects on coronary heart disease of increasing polyunsaturated fat in place of saturated fat: a systematic review and meta-analysis of randomized controlled trials. PLoS Med. 2010;7(3):e1000252.
7. Jakobsen MU, O'Reilly EJ, Heitmann BL, Pereira MA, Bälter K, Fraser GE, Goldbourt U, Hallmans G, Knekt P, Liu S. Major types of dietary fat and risk of coronary heart disease: a pooled analysis of 11 cohort studies. Am J Clin Nutr. 2009;89(5):1425-32.
8. Chowdhury R, Warnakula S, Kunutsor S, Crowe F, Ward HA, Johnson L, Franco OH, Butterworth AS, Forouhi NG, Thompson SG, Khaw K-T, Mozaffarian D, Danesh J, Di Angelantonio E. Association of Dietary, Circulating, and Supplement Fatty Acids With Coronary RiskA Systematic Review and Meta-analysis. Ann Intern Med. 2014;160(6):398-406.
9. Scientific Advisory Committee on Nutrition. Draft Carbohydrates and Health report. Online: http://www.sacn.gov.uk/reports_position_statements/reports/scientific_co.... Scientific Advisory Committee on Nutrition, 2014.
10. Nordic Council of Ministers. Nordic Nutrition Recommendations 2012. Part 1: Summary, principles and use. Online http://www.norden.org/en/publications/publikationer/nord-2013-009. Copenhagen: 2013.
11. Brazil MoHo. Dietary Guidelines for the Brazilian Population. Brazilia: Ministry of Health of Brazil, 2014.
12. Guyatt GH, Oxman AD, Vist GE, Kunz R, Falck-Ytter Y, Alonso-Coello P, Schunemann HJ. GRADE: an emerging consensus on rating quality of evidence and strength of recommendations. BMJ. 2008;336(7650):924-6.
13. Pollan M. Unhappy Meals. New York: New York Times; 2007 [cited 2014 19 December]; Available from: http://www.nytimes.com/2007/01/28/magazine/28nutritionism.t.html?pagewan....
Competing interests: No competing interests