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Scientific Updates /

The saturated fat debate

9 February 2022

Type:

Review

Fat is an important part of our diet as a source of energy and essential fatty acids (omega-3 and omega-6). Adequate fat in the diet also supports absorption of essential nutrients including vitamins A, D, E and K. Dietary fats are categorised according to the type of fatty acids they contain:

  • Saturated fatty acids (SFA)

  • Mono-unsaturated fatty acids (MUFA)

  • Poly-unsaturated fatty acids (PUFA)

  • Trans-fatty acids (TFA)

All foods contain different ratios of these fatty acids. Notably, red meat and dairy contain a higher ratio of saturated fat, whereas plant-based foods such as nuts, soya and vegetable oils tend to contain higher levels of unsaturated fats (with the exception of coconut, palm and shea). Saturated fats tend to be solid at room temperature and unsaturated fats are typically liquid.

Dietary fats, cholesterol and CVD

Both SFA and TFA have been shown to increase low-density lipoprotein cholesterol (LDL-C) which increases risk of cardiovascular disease (CVD) including coronary heart disease (CHD).() Conversely, unsaturated fat, particularly PUFA have a cardioprotective effect.() Worldwide, non-optimal intakes of PUFA, SFA and TFA have been estimated to result in 81,500, 33,800 and 38,700 deaths respectively from CHD per year, accounting for 10.9%, 4.5%, and 5.2% of CHD mortality.()

Read more about plant-based eating and cholesterol

The relationship between SFA, LDL-C and CHD has been confirmed through a wide variety of studies, including longitudinal observational studies such as the Nurses’ Health Study and the Health Professionals Follow-up Study (total >43,000 participants).()

A systematic review including 15 randomised control trials, the highest quality of evidence, concluded that reducing dietary saturated fat reduces the risk of CVD events by 17% and CVD mortality by 5%.() Meta-analyses of cohort studies have also demonstrated a 9-13% reduction in CVD events and 21-26% reduction in CVD mortality when the equivalent of 5% of total caloric intake from SFA is replaced by PUFA.()

The variation in outcomes may be explained by differences between study methodologies in terms of the source of PUFA and demographics of participants. However, overall, replacing SFA with PUFA is consistently associated with lower risk of heart disease.

Why is saturated fat still being debated?

Since the 1960s, dietary recommendations have been consistently advising the reduction in total dietary fat and substituting saturated for “heart-healthy" unsaturated fats. However, in recent years there have been several controversial studies which have questioned these recommendations and concluding that there is no association between SFA and heart disease incidence and mortality. This contradicts the widely accepted research and international recommendations.

Many of the studies questioning the current recommendations used data from two cohort studies from the 1960-70s: the Minnesota Coronary Experiment and the Sydney Diet Heart Study.() This has understandably raised concern and confusion amongst both health professionals and consumers. However, these controversial findings can be explained by the significant limitations in the study methodologies:

  1. Lack of differentiation with what nutrient replaced SFA
    This is a critical factor as the effect of SFA reduction on CHD risk is dependent on what takes its place in the diet. Various studies have shown that replacing 5% total energy intake from SFA with equivalent from PUFA, MUFA or wholegrains reduces the risk of CHD by 26-30%, 20% and 11% respectively, whereas replacing saturated fats with protein, TFA or refined carbohydrates is associated with no benefit or an increased risk of CHD.()

  2. Not accounting for TFA in the PUFA replacements
    The main source of unsaturated fat in both the Sydney Diet Heart Study and the Minnesota Coronary Experiment was industrial margarine. These margarines, due to the production processes at the time, would have been high in TFA. As the association between TFA, LDL-C and CVD risk had not been identified at the time of the original studies, the researchers did not control for TFA intakes in their participants. It is therefore likely that TFA intakes confounded the data and led to the finding that PUFA increased CVD risk.() Additionally, these studies’ findings do not reflect the impact of replacing SFA with any other food sources of PUFA such as oils, nuts and seeds.()

  3. Exceeding recommended PUFA and total fat intake
    In the Sydney Heart Study, the intake of dietary PUFA in the intervention group increased from 6.1% at baseline to 15.4% of energy at follow up. The total fat consumption in both control and intervention groups was around 40% of energy intake. These levels of fat intake exceed recommendations for good health which may have impacted the study outcomes.() In the Minnesota Coronary Experiment, PUFA intakes were also increased dramatically to more than double the recommendations.()

  4. Not accounting for changes in the control group diet
    In the Sydney Heart Study, the dietary intakes of the control group were not adequately controlled. The SFA and total energy intake of the control group decreased notably during the intervention which would have impacted their CVD outcomes. The researchers in both studies did not control for other dietary factors which influence CVD risk such as n-3 PUFA, fibre and refined carbohydrates.

  5. Results not generalisable to other populations
    Both studies were carried out in very specific populations and therefore the results are not generalisable to the general population. The Sydney Heart Study included only men between 30 and 59 years with existing CVD. The Minnesota Coronary Experiment included participants with normal cholesterol levels who were therefore not considered at risk for CVD.

Later meta-analyses have also concluded that reduction in dietary saturated fat is not associated with reduced risk of CVD.() However, these analyses were heavily skewed by the inclusion of the above flawed studies, so much so that re-analyses of the data excluding the above studies demonstrated a 19% reduction in CVD risk when SFA is replaced with PUFA.()

Current dietary recommendations

Following extensive analysis of the current research literature, international organizations including the World Health Organization (WHO)(), American Heart Association (AHA)() and Scientific Advisory Committee on Nutrition (SACN)() continue to recommend limiting saturated fats to <10% total energy intake and increasing consumption of unsaturated plant-based fats for optimal cardiometabolic health.()

Low SFA plant-based diets, such as the Mediterranean diet, and the Dietary Approaches to Stop Hypertension (DASH) diet are advised by ESC/EAS to improve blood lipid profile, lower blood pressure and reduce CVD risk.()

Plant-based diets and dietary fat

Plant-based diets are typically low in saturated fat and high in unsaturated fat, including the essential n-3 and n-6 PUFA. In addition to their beneficial fat profile, plant foods tend to also provide other nutrients (e.g., fibre) and bioactive compounds associated with improved lipid profiles and CVD outcomes. Eating smaller amounts of animal foods, which are the main sources of saturated fat, and replacing these with soya, legumes, oils, nuts, and seeds is an excellent way to improve the fat quality of the diet.()

Conclusion

Despite much debate and confusion, dietary fat recommendations have remained consistent for several decades. The scientific evidence is stronger than ever that reducing saturated fat and replacing with unsaturated fat improves cardiovascular health outcomes.

References

  1. Sacks FM, Lichtenstein AH, Wu JHY, et al. Dietary Fats and Cardiovascular Disease: A Presidential Advisory from the American Heart Association. Circulation. 2017;136:e1–e23. https://doi.org/10.1161/CIR.0000000000000510
  2. Catapano AL, Graham I, De Backer G, et al. ESC Scientific Document Group, 2016 ESC/EAS Guidelines for the Management of Dyslipidaemias. Eur Heart J. 2016;2999–3058. https://doi.org/10.1093/eurheartj/ehw272
  3. Wang Q, Afshin A, Yakoob MY et al. Impact of Nonoptimal Intakes of Saturated, Polyunsaturated, and Trans Fat on Global Burdens of Coronary Heart Disease. J Am Heart Assoc. 2016;5(1): e002891. https://doi.org/10.1161/JAHA.115.002891
  4. Li Y, Hruby A, Bernstein AM, et al. Saturated Fats Compared with Unsaturated Fats and Sources of Carbohydrates in Relation to Risk of Coronary Heart Disease: A Prospective Cohort Study. J Am Coll Cardiol. 2015;66(14):1538-1548. https://doi:10.1016/j.jacc.2015.07.055
  5. Hooper L, Martin N, Abdelhamid A, et al. Reduction in saturated fat intake for cardiovascular disease. Cochrane Database of Systematic Reviews. 2015;6. https://doi.org/10.1002/14651858.CD011737
  6. Farvid MS, Ding M, Pan A, et al. Dietary linoleic acid and risk of coronary heart disease: a systematic review and meta-analysis of prospective cohort studies. Circulation. 2014;130(18):1568-1578. doi:10.1161/CIRCULATIONAHA.114.010236. https://dx.doi.org/10.1161%2FCIRCULATIONAHA.114.010236
  7. Jakobsen MU, O'Reilly EJ, Heitmann BL, et al. 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. doi: 10.3945/ajcn.2008.27124. https://dx.doi.org/10.3945%2Fajcn.2008.27124
  8. Ramsden CE, Zamora D, Majchrzak-Hong S, et al. Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73). BMJ. 2016;353:i1246. https://doi.org/10.1136/bmj.i1246
  9. Ramsden C E, Zamora D, Leelarthaepin B, Majchrzak-Hong S F, Faurot K R, Suchindran C M et al. Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis. BMJ 2013;346:e8707. https://doi.org/10.1136/bmj.e8707
  10. Siri-Tarino PW, Chiu S, Bergeron N, Krauss RM. Saturated Fats Versus Polyunsaturated Fats Versus Carbohydrates for Cardiovascular Disease Prevention and Treatment. Annu Rev Nutr. 2015;35:517-43. https://doi.org/10.1146/annurev-nutr-071714-034449
  11. de Souza RJ, Mente A, Maroleanu A, et al. Intake of saturated and trans unsaturated fatty acids and risk of all-cause mortality, cardiovascular disease, and type 2 diabetes: systematic review and meta-analysis of observational studies. BMJ. 2015;351:h3978. https://doi.org/10.1136/bmj.h3978
  12. Hamley S. The effect of replacing saturated fat with mostly n-6 polyunsaturated fat on coronary heart disease: a meta-analysis of randomised controlled trials. Nutr J. 2017;16(1):30. https://doi.org/10.1186/s12937-017-0254-5
  13. WHO. Healthy diet. WHO 2020. Accessed Feb 2022. https://www.who.int/news-room/fact-sheets/detail/healthy-diet#:~:text=Intake%20of%20saturated%20fats%20should,4%2C%205%2C%206
  14. Lichtenstein AH, Appel LJ, Vadiveloo M, et al. 2021 Dietary Guidance to Improve Cardiovascular Health: A Scientific Statement From the American Heart Association. Circulation 2021;144(23): e472–e487. doi.org/10.1161/CIR.0000000000001031
  15. Public Health England. Saturated fats and health: SACN report. Gov.UK 2019. Accessed Feb 2022. https://www.gov.uk/government/publications/saturated-fats-and-health-sacn-report

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