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Another nail in the coffin for the saturated fat—heart disease hypothesis?

The idea that total and saturated fat intake is a risk factor for heart disease still persists. In this review, Cliff examines a recent meta-analysis that casts further doubt on this.

Dietary total fat, fatty acids intake, and risk of cardiovascular disease: a dose-response meta-analysis of cohort studies

Yongjian Zhu, Yacong Bo & Yanhua Liu

Lipids in Health and Disease | Volume 18, Article number: 91 (2019)

https://lipidworld.biomedcentral.com/articles/10.1186/s12944-019-1035-2?fbclid=IwAR0mFp2qUNCQjpDCtw8EgfIlUTRv5vs1kfuCuUbmzJOqtVYf4-W20OYHVoA

Abstract

Background

Several epidemiological studies have investigated the association between dietary fat intake and cardiovascular disease. However, dietary recommendations based on systematic review and meta-analysis might be more credible.

Methods and results

Pubmed, Embase and Cochrane Library were searched up to July 1st, 2018 for cohort studies reporting associations of dietary fat intake and risk of CVDs. By comparing the highest vs. the lowest categories of fat or fatty acids intake, we found that higher dietary trans fatty acids (TFA) intake was associated with increased risk of CVDs [RR:1.14(1.08–1.21)]. However, no association was observed between total fat, monounsaturated fatty acids (MUFA), saturated fatty acids (SFA), and polyunsaturated fatty acids (PUFA), and risk of CVDs. Subgroup analysis found a cardio-protective effect of PUFA in the studies that has been followed up more than 10 years [0.95(0.91–0.99), I2 = 62.4%]. Dose-response analysis suggested that the risk of CVDs increased 16% [1.16 (1.07–1.25), Plinearity = 0.033] for an increment of 2% energy/day of TFA intake.

Conclusions

This current meta-analysis of cohort studies suggested that total fat, SFA, MUFA, and PUFA intake were not associated with the risk of cardiovascular disease. However, we found that higher TFA intake is associated with greater risk of CVDs in a dose-response fashion. Furthermore, the subgroup analysis found a cardio-protective effect of PUFA in studies followed up for more than 10 years.

This current meta-analysis of cohort studies suggested that total fat, SFA, MUFA, and PUFA intake were not associated with the risk of cardiovascular disease

Comment

Cardiovascular disease is still the leading cause of death globally. Diet is seen as being the leading contributor to this, along with other lifestyle factors such as exercise, and we are increasingly aware of the roles of stress and mental health, and negative changes to sleeping patterns. Critical to the debate around cardiovascular disease and diet is the topic of saturated fat and whether or not it has a major role to play in the incidence and severity of heart disease and to all-cause mortality. As previously mentioned in this issue of The Carb-Appropriate Research Review, the evidence overall does not suggest any strong link between saturated fat and either all-cause or cardiovascular disease mortality and yet, the reduction of saturated fat is still universally recommended. Much of this recommendation relies on substitution data, especially from the Hooper analysis1 which has also recently fallen under some debate due to a reanalysis of the data using different (and probably more appropriate) statistical methods which showed no effect of limiting saturated fat for the prevention of cardiovascular disease.2

In this paper, Zhu and colleagues appraised over 100,000 papers for inclusion and finally analysed 43 publications to determine the effect of saturated fat on cardiovascular mortality outcomes. Some of the key findings included:

  • No effect on cardiovascular risk from total fat intake (RR 0.97; 95% CI, 0.93–1.01)
  • No effect of highest vs lowest intakes of saturated fat on cardiovascular risk (RR 0.97; 95% CI, 0.93–1.02)
  • No influence of monounsaturated fat on cardiovascular disease risk (RR 0.97; 95% CI, 0.93–1.01)
  • No effect of highest vs lowest intakes of polyunsaturated fats on cardiovascular disease risk (RR 0.97; 95% CI, 0.93–1.004)
  • Highest vs lowest intakes of trans-fatty acids were associated with increased risk of cardiovascular disease (RR 1.14; 95% CI, 1.08–1.21)

Dose-response

Dose-response analysis didn’t yield significant results and the results were also conflicting. Trends were observed for reduced risk with increased fat intake overall, increased saturated fat (to around 30% of calories), monounsaturated fat, with reduced risk from baseline for increased % of calories from polyunsaturated fats, and worsened risk with increased intakes of trans-fats.

Dose-response analyses of the linear association between dietary total fat (a), trans-fatty acids (b), saturated fatty acids (c), monounsaturated fatty acids (d), and polyunsaturated fatty acids intake (e) and the risk of cardiovascular disease (% energy)

However, when absolute intakes (i.e. grams per day) were analysed the results are even less clear as you can see in the graph below.

Dose-response analyses of the linear association between dietary total fat (a), trans-fatty acids (b), saturated fatty acids (c), monounsaturated fatty acids (d), and polyunsaturated fatty acids intake (e) and the risk of cardiovascular disease(g/d)

What does this tell us overall?

This study is consistent with the existing research. While some studies show a very small potential risk from saturated fats, others show no association or even benefit. But the overall effect size is very small and when we consider all the other factors at play, is so small as to be ‘statistical noise’.

In this particular study this is shown, with small effect sizes that cross over the ‘risk line’ of 1.0 (meaning that we can’t be sure whether they benefit, harm) leaving us to conclude that they probably have no effect whatsoever. This is true not just for saturated fats, but for all fat types!… And indeed, for fat intake overall.

The only fat to show harm is trans-fat and even that has a fairly modest effect. Not that I suggest people eat trans-fats! BUT the harm from inadvertently ingesting some trans-fats is likely to be very small indeed and of course, some naturally occurring trans-fats are likely to be health-promoting (like vaccenic acid from meat and dairy for example).

Overall, the take-home message is that overall fat intake, or the type of fat that you consume, is not likely to be a significant modifier of your future health. The key is that the fats, and the foods overall that you consume, are mostly natural, whole, and unprocessed wherever possible.

Overall, the take-home message is that overall fat intake, or the type of fat that you consume, is not likely to be a significant modifier of your future health

References

1.            Hooper L, Martin N, Abdelhamid A, Davey Smith G. Reduction in saturated fat intake for cardiovascular disease. The Cochrane database of systematic reviews. 2015(6):Cd011737.

2.            Thornley S, Schofield G, Zinn C, Henderson G. How reliable is the statistical evidence for limiting saturated fat intake? A fresh look at the influential Hooper meta-analysis. Internal Medicine Journal. 2019;0(ja).

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