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AHA: Polyunsaturated fats as substitute for saturated fats lower risk for CVD
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The American Heart Association recommended substituting saturated fat with monounsaturated and polyunsaturated fats to lower cholesterol and reduce the risk for CVD, according to a presidential advisory published in Circulation.
The benefit was specifically noted with polyunsaturated fat, often found in oils such as peanut, corn and soybean.
“We want to set the record straight on why well-conducted scientific research overwhelmingly supported limiting saturated fat in the diet to prevent diseases of the heart and blood vessels,” Frank M. Sacks, MD, FAHA, professor of cardiovascular disease prevention at the Harvard T. H. Chan School of Public Health, said in a press release. “Saturated fat increases LDL … which is a major cause of artery-clogging plaque and [CVD].”
Sacks and colleagues reviewed various studies that provide evidence on the effects of a diet that replaces saturated fat with unsaturated fat or carbohydrates on the risk for CVD. Saturated fat has been shown to increase LDL. Current AHA/American College of Cardiology guidelines suggest lowering saturated fat intake to 5% to 6% of total daily calories for patients with elevated LDL, although the average intake for adults in the United States is 11%.
Early studies on saturated fat
Studies from the 1950s and 1960s reviewed the benefits of replacing saturated fats with polyunsaturated fats. Patients experienced reduced serum cholesterol, fewer primary events or MI, and decreased risk for events relating to CVD and ischemic stroke.
“However, these trials were conducted in the 1960s, before widespread use of statins, when serum cholesterol levels were higher than now, as was the saturated fat content of the diet,” Sacks and colleagues wrote.
Researchers of the randomized controlled trials wrote that polyunsaturated fats reduced CVD events by an estimated 30%, which is similar to what can be attained by statin therapy.
Other trials have studied the effects of replacing saturated fat with carbohydrates. Those with a diet low in saturated fat and high in carbohydrates had minimal reductions in serum cholesterol, CVD events and stroke.
“In contrast to trials of polyunsaturated fat, adherence to the low-fat regimen fell short of the intention, impairing the ability of the trials to test a biologically based or efficacy hypothesis,” Sacks and colleagues wrote.
Saturated fats and CHD
Researchers performed multivariable regression analysis, which included prospective observational studies from 2009 to 2015.
“It’s important to look very carefully at the quality of the studies that are being cited,” Steven E. Nissen, MD, MACC, chairman of the Robert and Suzanne Tomsich Department of Cardiovascular Medicine at the Cleveland Clinic’s Sydell and Arnold Miller Family Heart and Vascular Institute and Cardiology Today Editorial Board Member, said in an interview. “They typically are small. They’re observational. They’re not randomized controlled trials. There’s a huge potential problem with publication bias, where studies that show what people expect are published and studies that are less likely to be submitted for publication.”
The risk for CHD was reduced when patients replaced saturated fats with monounsaturated fats (15%), polyunsaturated fats (25%) or carbohydrates from whole grains (9%). Prevalence of CVD death, all-cause death, lung disease, cancer and neurodegenerative disease declined due to polyunsaturated and monounsaturated fats, Sacks and colleagues wrote.
Meta-regression analysis and various other studies, including DASH, DELTA and DASH-Sodium showed that polyunsaturated fats decreased LDL more than monounsaturated fats and contributed to decreased risk for CVD, according to the authors.
The size of LDL may also play a factor in the amount of its effect on CVD, but the findings vary. Some studies showed that large LDL is linked to a higher risk for CVD, others showed that large and small LDL particles predict CVD, and one study determined that there is no association between the two. In one study, patients who replaced carbohydrates with monounsaturated fat had a reduction in small and medium LDL particles and a shifted distribution of larger LDL.
“The effects of replacing carbohydrates with various kinds of fats qualitatively at least may be similar by increasing larger and decreasing smaller LDL sizes,” Sacks and colleagues wrote.
Effects on HDL, triglycerides
HDL and triglycerides are other blood lipid biomarkers that are affected by replacing saturated fats with polyunsaturated fat, monounsaturated fats or carbohydrates. Polyunsaturated and monounsaturated fats slightly decreased HDL by 0.2 mg/dL and 0.1 mg/dL respectively, whereas carbohydrates lowered HDL by 0.4 mg/dL. Atherogenic properties have been found in triglyceride-rich lipoproteins, and monounsaturated and polyunsaturated fats can lessen their effect on the prevalence of CVD. Carbohydrates increased triglycerides by an estimated 1 mg/dL, and monounsaturated fat (0.4 mg/dL) and polyunsaturated fat (0.9 mg/dL) decreased triglycerides.
Researchers also analyzed data regarding the effects of replacing carbohydrates with individual saturated fatty acids. A meta-regression analysis found that individual saturated fatty acids such as myristic, lauric and palmitic acids increased both HDL and LDL as it decreased triglycerides. Stearic acid, which is found in pork fat, beef, lamb fat and chocolate, did not have the same effects, although replacing it with unsaturated fat lowered LDL. Patients are advised to not use coconut oil, as it has been shown to increase LDL.
Dairy fat, which contains 51% saturated fatty acids, has had mixed results in studies. Some clinicians said they believe that cheese may protect patients against CVD because it is produced by fermentation, but data do not consistently show the benefit. In one clinical trial, diets rich in beef and cheese did not increase LDL, and increased HDL. Other studies have shown that cheese decreased the risk for CHD or stroke. A diet in which carbohydrates from whole grains was substituted for dairy fat resulted in a 34% lower prevalence of CHD and 16% lower risk for stroke.
“This analysis demonstrates again that it is essential to analyze the effects of unsaturated fats, refined carbohydrates and whole grains separately to reach an informed and useful result for dietary advice,” Sacks and colleagues wrote.
Unsaturated fatty acids
Trans unsaturated fatty acids occur in meat and milk of animals such as sheep and cattle. Numerous studies have illustrated its adverse effects on risk factors for CVD, including LDL, triglycerides, apolipoprotein B and HDL. An increased intake of trans fatty acid has been associated with a higher risk for CHD.
“The concordance between the adverse effects of trans fatty acids on lipid risk factors for CVD and the robust association of higher trans fatty acid intake with elevated CHD risk in observational studies provides the impetus for current policy actions of many local and national jurisdictions to reduce industrial trans fatty acids in the food supply,” Sacks and colleagues wrote.
The advisory also addresses the effects of omega-3 fatty acids. There is no sufficient evidence that backs the benefit of alpha-linolenic acid, although some studies show that it may reduce fatal CHD.
Mediterranean diets have been reviewed in studies, including Seven Countries Study, Lyon Heart Study and PREDIMED, which have shown that they decrease the risk for CVD along with all-cause death, CV death, MI and stroke.
“I recommend the Mediterranean diet, but I don’t tell patients that they’re going to drop dead if they have an occasional pat of butter or if they eat full-fat yogurt because I just don’t know,” Nissen told Cardiology Today.
Benefits in children
Children may also benefit from a diet of reduced saturated fats, according to the statement. Intake of animal fat, dairy and starches has been associated with increased BMI, total serum cholesterol and LDL. In studies such as Cardiovascular Risk in Young Finns, NHLBI National Growth and Health Study and STRIP trial, children whose diets were low in saturated fats and high in fruits and vegetables had lower BMI, waist circumference, LDL and body fat.
“Reduction in total dietary fat or a goal for total fat intake is not recommended. This shift from saturated to unsaturated fats should occur simultaneously in an overall healthful dietary pattern such as the DASH or Mediterranean diet, as emphasized by the 2013 AHA/[ACC] lifestyle guidelines and the 2015 to 2020 Dietary Guidelines for Americans,” Sacks and colleagues wrote.
Nissen said the evidence remains too weak to justify definitive recommendations.
“We can’t tell people we have an answer based upon limited evidence,” Nissen told Cardiology Today. “We can’t say as a society we need to strictly limit saturated fat. We’ve made that mistake before. People looked at observational data where women who took estrogen after menopause seemed to have a lower incidence of heart disease, and tens of millions of American women were treated with estrogen. When we finally did the randomized controlled trials, estrogen increased the risk for heart disease, not decreased it. When you make these kinds of large societal decisions based upon poor-quality evidence, you make mistakes.” – by Darlene Dobkowski
Disclosures: Nissen and Sacks report no relevant financial disclosures. Please see the full advisory for a list of the other authors’ relevant financial disclosures.
Perspective
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Kim Allan Williams, Sr., MD, MACC, FAHA, MASNC, FESC
This advisory is designed to put evidence on saturated fat into scientific perspective, and to decrease some of the confusion on this topic. It is an excellent review of existing literature, clearly demonstrating that saturated fat substitution, particularly with polyunsaturated fat, can reduce risk factors, cardiac events and mortality.
Every day in clinical medicine, we deal with the common diseases and complications of risk factors, most of which can be drastically improved or eliminated by careful attention to diet. These include hypertension, diabetes, obesity, MI, stroke and CV death. Thus, the implications of dietary knowledge are huge for family practitioners, primary care and CV specialists, including our patients, our families and ourselves.
Dietary fats, particularly trans fat and saturated fat, increase serum cholesterol and caloric density, resulting in more obesity and atherosclerotic CVDs.
At this point, the data are quite clear — saturated fats come primarily from animal sources and are associated with higher morbidity and mortality, while fats from vegetable sources are less likely to have these outcomes. However, unanswered questions revolve around plaque regression and cardiac events of a very low or no-fat plant-based diet in comparison with more liberal plant-based diet that contains polyunsaturated and monounsaturated fat on a regular basis. We know that, as an analogy, recent studies indicate that fish consumption increases CV and all-cause mortality, but it does so far less than consuming processed red meat. Therefore, studies that include fish appear to show benefit when actually, it is a relative or substitutionary benefit. The same may be true for polyunsaturated fat vs. saturated fat.
Also, there are fewer data comparing isolated nutrients such as polyunsaturated vs. saturated fat separated from their sources. This confounds conclusions, since vegetable sources are not only generally have less saturated fat, but are devoid of dietary cholesterol, heme iron, animal protein and insulin-like growth factor-I, and are lower in creatine, choline and phosphatidylcholine, which increase production of trimethylamine-N-oxide by gut bacteria. Each of these compounds have been associated with CVD and events.
Lastly, vegetable sources of saturated fat, such as coconut oil, need further study of their role in increasing cardiac events. While clearly raising serum LDL, there is not clarity on whether this worsens cardiac outcomes. The authors conclude that coconut oil is best avoided, but clearly state that this is expert opinion.
In keeping with the principle, "the fat you eat is the fat you wear," I instruct my overweight and obese patients to reduce all source of fat. Given the prevalence of heart disease in my practice at Rush University Cardiology, I recommend plant-based nutrition routinely, but I titrate the recommendation for intake of polyunsaturated and monounsaturated fat based on patient's individual needs.
I am encouraged to see more focus on nutrition throughout the CV community and medical literature. Even at our recent American Medical Association meeting, resolutions were introduced and approved that seek to reduce unhealthy food in hospitals and our government SNAP program. There is now a much greater focus on prevention through nutrition than even just 2 or 3 years ago. If successful, this will have a dramatic impact on our population health and future expenditures by Medicare.
I like to tell people that it's their patriotic duty to practice the very best dietary habits and exercise routinely.
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