Saturated Fat Consumption Not Associated With Mortality

Lara C. Pullen, PhD

August 11, 2015

Consumption of saturated fats is not associated with all-cause mortality, cardiovascular disease, coronary heart disease (CHD), ischemic stroke, or diabetes. In contrast, however, consumption of trans fats is associated with all-cause mortality, total CHD, and CHD mortality, according to a systematic review and meta-analysis of observational studies. However, the authors caution that the results are confounded by heterogeneous evidence and methodological limitations.

Russell J. De Souza, ScD, RD, from McMaster University, Hamilton, Ontario, Canada, and colleagues published their synthesis of observational evidence online August 11 in the BMJ. They focused their review on apparently healthy adults. The underlying studies tended to rely on food frequency questionnaires, 24-hour recalls, or 7-day food records.

The investigators note that the findings from prospective cohorts were consistent with the findings from case-controlled studies.

The researchers attempted to both synthesize and quantify the literature in the field. To that end, they focused their attention on studies of similar design that measured comparable outcomes. They then used the grading of recommendations assessment, development, and evaluation (GRADE) approach to evaluate the quality of the body of evidence in a way that was able to quantify the presence, strength, and direction of the effect.

“Very Low” Certainty for Saturated Fats

Based on the GRADE approach, the investigators labeled the certainty of association between saturated fat and all outcomes "very low." Overall, however, the investigators noted that they were unable to explain the tremendous heterogeneity present in most of the analyses of saturated fats.

When they synthesized the results from the studies, they found no association between saturated fat intake and all-cause mortality, despite inclusion of positive data from the Seven Countries' Study. They note, however, that they were unable to effectively combine the positive association found in the Seven Countries' Study with the associations from other studies because of differences in methodology.

Although saturated fats were not associated with total CHD, there was a trend for an association between saturated fats and CHD mortality. Specifically, when the investigators pooled prospective cohorts with nested case-control studies, they found a borderline significant association between saturated fats and CHD mortality.

There was no association, however, between saturated fat and ischemic stroke. That said, studies in Asian countries revealed that the relative risk for stroke in the highest quartile of saturated fat consumption was 18% less (0.82; 95% confidence interval, 0.69 - 0.98) than the risk for stroke in the lowest quartile of saturated fat consumption.

Although some researchers believe saturated fats compromise insulin sensitivity, the small randomized trials that tested this hypothesis yielded inconclusive results. Dr de Souza and colleagues confirmed a documented inverse association between dairy products and type 2 diabetes. In particular, the odd-chain saturated fats associated with diary intake were inversely associated with incident type 2 diabetes.

Stronger Associations for Trans Fats

The investigators labelled the certainty of association of trans fats with CHD outcomes as "moderate."

Dr de Souza and colleagues found "reliable and strong positive associations" between intake of trans fats and CHD and CHD mortality. In contrast, the association between trans fats and ischemic stroke was less clear, and the researchers found that the two prospective studies on the subject yielded inconsistent results.

Although there was no association between trans fats and type 2 diabetes, the data suggest trans-palmitoleic acid (found in dairy fat) may be able to protect against type 2 diabetes.

The systematic review revealed striking differences between industrially produced trans fats and ruminant trans fats. The investigators note, however, that it was difficult to distinguish between consumption of specific trans fatty acids (industrial vs ruminant), especially when individuals eat only a small amount of ruminant trans fats relative to high volumes of industrial trans fats.

The authors propose, then, that the association between consumption of trans fats and mortality may thus reflect a higher intake of industrial trans fats, as opposed to ruminant trans fats. Future studies will be able to address this issue better as industrially produced partially hydrogenated oils are phased out of several countries.

Until the phase out of industrially produced trans fats is a reality, however, the researchers suggest that currently available data support the hypothesis that industrially produced trans fats, but not ruminant trans fats, are associated with risk for CHD. In contrast, both industrial trans fats and ruminant trans fats appear to have similar effects on low- and high-density lipoprotein cholesterol levels.

Limitations of the Studies

The investigators note that observational studies can only document associations; they cannot demonstrate causation. Moreover, the measurement error in epidemiologic studies of diet and disease is often significant. These measurement errors often bias the associations toward the null, making it possible that health effects are being missed.

The authors also call attention to the implications of their findings for nutritional recommendations. They suggest that dietary guidelines that call for the removal of saturated fats and trans fats consider the health implications of the macronutrients (carbohydrates and/or protein) that will be used to replace the fat. Such a consideration is especially important in light of the current systematic review, as well as new data on the health effects of carbohydrates.

The authors have disclosed no relevant financial relationships.

BMJ. Published online August 11, 2015.


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