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Trans fats, but not saturated fats, were associated with a greater risk of death and cardiovascular disease in a new meta-analysis.

Pooled data from studies involving hundreds of thousands of participants indicated that those who consumed the most trans fats faced a greater risk of all cause mortality (relative risk 1.34, 95% CI 1.16 to 1.56; P<0.001), coronary heart disease mortality (RR 1.28, 95% CI 1.09 to 1.50; P=0.003), and total coronary heart disease (1.21, 95% CI 1.10 to 1.33; P<0.001), compared with those who consumed low amounts of trans fats.

There was no such association with ischemic stroke risk (1.07, 95% CI 0.88 to 1.28; P=0.50) or with type 2 diabetes (1.10, 95% CI 0.95 to 1.27; P=0.21).

Saturated fat was not associated with increased risk for all-cause mortality, cardiovascular mortality, ischemic stroke, or diabetes, according to the authors of the study, who were led by , at McMaster University in Canada. But there was no convincing lack of association between saturated fat and some outcomes, and the data for saturated fats were limited, found the study, which appeared this week in .

, at SUNY Downstate Medical Center in New York, said in an interview with 番茄社区 that the findings on saturated fats fit in with what was already known.

"You can't keep doing these same studies over and over again and expect to get different results," he said. He suggested that the science was "settled a long time ago" when findings emerged finding a lack of evidence for an association between saturated fat and heightened risk of heart disease.

National recommendations have long recommended cutting saturated fats, but recent years have seen a shift with the realization that the evidence for the recommendations was lacking. A 2013 commentary in BMJ argued that, "It is time to bust the myth of the role of saturated fat in heart disease and wind back the harms of dietary advice that has contributed to obesity."

According to , a member of the American Board of Obesity Medicine, the latest study doesn't indicate that saturated fats present any kind of a harm. "This result will be surprising to some since recommendations to lower saturated fat are still widely circulated, but there seems to be little basis for that -- at least this study didn't find any," he wrote in an email.

De Souza and colleagues found one to six studies for each association between trans fat and each health outcome they looked at; in total, 20 studies were included. To be eligible, the studies had to be observational and report a measure of association between trans or saturated fats and specific cardiovascular outcomes. Retrospective studies had to include an odds ratio to be included. Researchers rated the trans fat studies based on how reliably they measured exposure to trans fat.

They found that individuals in the highest quintiles or quartiles (depending on how the individual studies were designed) of industrially produced trans fats were associated with a higher risk of coronary heart disease, compared with those in the lowest strata. But no association was seen with consumption of ruminant-derived trans fats (i.e., from animals). This is consistent with other research, according to the authors.

There were three to 12 cohort studies for each association between saturated fat intake and all cause mortality, cardiovascular mortality, total coronary heart disease, ischemic stroke, and type 2 diabetes. Researchers looked at 41 studies on saturated fat overall. None of the associations linked saturated fats to a heightened risks of these outcomes, but the evidence against an association between saturated fat and coronary heart disease mortality was not convincing (RR 1.15, 95% CI 0.97 to 1.36; P=0.10), the authors argued.

"The certainty of associations between saturated fats and all outcomes was 'very low'," de Souza and colleagues concluded.

, at Washington University in St. Louis, wrote in an email to 番茄社区 that more research is needed to understand how fat consumption impacts health when consumed as part of a whole diet.

"Continuing to focus on individual fatty acids does not provide research that speaks to how people eat," she wrote. "The role of diet and health is about the whole, not the individual parts."

She added that an important part of the discussion around fats is what patients eat in place of them. "What nutrients replace a reduction in saturated fat intake is key to reducing disease risk and clearly simple carbohydrates are not the choice," she wrote.

A similar sentiment was echoed by , at the Albert Einstein College of Medicine in New York. "It depends on what you replace fat intake with," he wrote in an email. He added that as a registered dietitian he feels that his field sometimes gets blamed for giving bad dietary advice when they tell patients to cut some saturated fats. "We never said to replace them with white bread and jam," he said.

It's important to consider that trans fat consumption is only part of the larger picture of what can increase risk, according to , at St. Joseph Mercy Oakland Hospital in Michigan. He added that the study had significant limitations and only confirmed what was already known. "I am impressed with the authors' own recognition of the weakness of their analysis design and flaws of their paper," he wrote in an email to 番茄社区.

Limitations of the meta-analysis included the possibility of the included studies being different enough to compromise the findings. There was substantial heterogeneity in the examined studies and a small number of cohorts for some associations. In addition, the studies evaluated were observational and could not provide causal evidence.

The authors added that there are several questions that remained unanswered, including whether different kinds of saturated fats have different effects on health.

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