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SYNOPSIS: The authors of this review of key nutritional studies compiled evidence-based information on foods and dietary patterns that support cardiovascular health and demystified those that have incorrectly been identified as beneficial to cardiovascular health.
SOURCE: Freeman AM, Morris PB, Barnard N, et al. Trending cardiovascular nutrition controversies. J Am Coll Cardiol 2017;69:1172-1187.
The foundation for prevention of atherosclerotic cardiovascular disease is a heart-healthy diet. Freeman et al examined data from more than 30 countries and included a variety of studies — meta-analyses, systematic reviews, randomized, controlled trials (RCTs), case control studies, and cohort studies — to shed light on the evidence for recommending particular foods and the reasons for the hype surrounding certain diets.1 (See Table 1.)
Eggs and Dietary Cholesterol. Consumption of cholesterol increases the blood serum cholesterol, with each 100 mg increment in dietary cholesterol increasing total cholesterol by 2.2 mg/dL. After reviewing a meta-analysis of 17 studies examining the effects of eggs and cholesterol, the authors recommended that clinicians should continue to advise patients to reduce dietary cholesterol intake.
Vegetable Oils: Not All Oils Are Created Equal. It is generally assumed that plant-based oils are healthier than animal-based oils. However, not all plant oils are created equally, as they contain varying amounts of fatty acids, including saturated fatty acids (SFAs), polyunsaturated fatty acids (PUFAs), and monounsaturated fatty acids (MUFAs) even within the same family. When oils high in MUFAs and PUFAs are used as a substitute to other saturated fats, there is a reduction in low-density lipoprotein cholesterol (LDL-C) levels, although there are no data on cardiovascular disease (CVD) outcomes. Although healthier substitutes of virgin coconut oil retain the bioactive polyphenols lost in refinement and have been promoted for cardiovascular benefits, no well-designed studies have shown evidence of CVD benefit from virgin coconut oil consumption. In addition, this oil contains high amounts of SFAs and, therefore, should be avoided. Ecological data of Asians who consume coconut as a staple show low incidence of atherosclerotic CVD. However, these populations also are consuming lower quantities of SFAs in general. Palm oil is high in SFAs and, thus, is not a heart-healthy oil and should be avoided. Plant and animal oils high in saturated fatty acids should be replaced with oils high in PUFAs or MUFAs to help lower LDL-C.
In general, the issue of plant vs. animal is a result of the varying percentage of PUFAs and MUFAs. Freeman et al only discussed the differences between the plant-based SFAs and reminded people that just because the oil is derived from a plant does not mean it is healthy.
The cardioprotective effect of olive oil has been assessed in RCTs examining both outcomes and CVD biomarkers. Freeman et al reported that olive oil’s effect for cardioprotection may be due to its polyphenol content. In addition, they reported that participants in the PREDIMED study had mean 30% reduction in the primary endpoint of composite of myocardial infarction, stroke, and CVD death, hence providing first level evidence of the health benefits of extra virgin olive oil (~4 tablespoons per day).
Antioxidants: Whole Foods or Supplements. Anthocyanins are highly concentrated in purple eggplant, red cabbage, red radishes, strawberries, and blueberries. Data from the Nurses’ Health Study among postmenopausal women showed a trend toward decreased myocardial infarction risk and statistically significantly lower blood pressure in those who consumed about 1 cup of blueberries a day. However, excess antioxidant supplementation can lead to neutral or negative health effects because of a process known as hormesis, whereby substances are effective at low doses but are ineffective or harmful at higher doses. Therefore, the authors noted that fruits and vegetables provide the healthiest, most beneficial source of antioxidants.
Nuts. Freeman et al reported that large, prospective studies demonstrated the consistent cardioprotective effect of nut consumption (four servings of nuts/week) on CVD outcomes. However, since nuts are calorie dense, portion control is key to prevent excess caloric intake.
Green Leafy Vegetables. Dark green leafy vegetables contain inorganic nitrate, which is converted to nitrite by salivary amylase, followed by acidification in the stomach to nitric oxide. Significant decreases in mean systolic blood pressure (-7.5 mmHg), which peaked at two hours postprandial, were observed in people who consumed 8.8 ounces of dark green leafy vegetables. Celery contains a vasodilator (3-n-butylphthalide) and has been found to decrease blood pressure in people with hypertension. Lutein, another component of dark green leafy vegetables, is inversely associated with incident hypertension. A diet rich in a variety of green leafy vegetables has a significant beneficial effect on atherosclerotic CVD risk and can be recommended to patients on warfarin, provided intake is consistent.
Plant-based Diet. Freeman et al noted that RCTs and epidemiological studies support that plant-based diets are associated with improvement in atherosclerotic CVD risk factors. For example, in the European Prospective Investigation into Cancer and Nutrition, vegetarians (avoid animal products, but some dairy products and eggs) had lower systolic blood pressure and a 32% lower risk of developing coronary heart disease. Populations, such as the Seventh-day Adventists, Okinawans, rural Chinese, central Africans, Papua New Guinea highlanders, and Tsimané, who predominantly consume a plant-based diet, rarely develop CVD. In a European study, vegetarians had a lower HDL-C.
Gluten. For those without a true gluten sensitivity or allergy, Freeman et al found the claims of health benefits for avoiding gluten are unsubstantiated.
The compilation of evidence-based information from various studies around the world supports that a diet rich in whole plant-based foods with limited quantities of animal products and oils decreased the risk of atherosclerotic CVD. The main strength of this study is that the authors examined data from studies worldwide, including Stabilization of Atherosclerotic Plaque by Initiation of Darapladib Therapy (39 countries), the U.S. Nurses’ Health Study, and the Prevención con Dieta Mediterránea (Spain). There are challenges in studies of dietary patterns because of various factors: 1) Most use self-reporting, which may lead to under- or over-reporting certain foods; 2) There are confounders: People who eat certain foods also tend to exhibit certain behaviors that may affect the outcome; and 3) Researchers may not be measuring the factors that directly influence the outcome, but rather may be measuring the influencers.
A key challenge of epidemiological data is the risk of ecologic fallacy; conclusions obtained from populations may not be true for individuals. Because of genetic differences, an emphasis on particular lifestyle recommendations may be even more important in some individuals than others. For example, certain people genetically remove cholesterol from serum more quickly than others, while other people with APOA1 single nucleotide polymorphism have higher levels of HDL-C4. This ability for cholesterol efflux is dependent on the functional capability of HDL and not just the quantity of HDL. Individuals with the same HDL levels may have different functional capacities of HDL. Other investigators have studied the serum cholesterol efflux capacity of HDL.2,3 Khera et al showed that the greater the efflux capacity of HDL, the lower the prevalence of coronary artery disease.4 Therefore, two individuals with the same quantitative HDL but differing functional capabilities of HDL most likely would need different nutritional advice. This may be the future of medicine: targeted and individualized nutritional recommendations based on evidence-based medicine.
This study very briefly touched on genetic differences in intestinal cholesterol absorptive capabilities that led to varied increase of total cholesterol with the same amount of dietary cholesterol. Some individuals had a greater increase in the measured cholesterol than others even with the same increase of dietary cholesterol. It is important for providers to retain a suspicion for a varied genetic response to dietary changes.
Part of the challenge of offering nutritional recommendations is that often providers do not know the key genetic differences of their patients. It will be interesting to see how the nutritional recommendations change as our understanding of cholesterol mechanisms improves. It is important for providers to expect that some of these recommendations will change as our understanding continues to evolve and to keep up to date with the changes. Current evidence shows that diets rich in whole foods from vegetables, fruits, and plant protein are naturally low in saturated fatty acids and are associated with a decreased atherosclerotic CVD risk. There are many fad diets and myths about healthy foods; therefore, it is important for providers to stay up to date on the latest evidence. In summary, this review of the current fad diets provides clear evidence for clinicians to engage in discussions with their patients, emphasizing that the certain foods — whole foods, vegetables, fruits, and plant protein — likely will have a positive effect on atherosclerotic CVD risk factors.
Financial Disclosure: Integrative Medicine Alert’s Executive Editor David Kiefer, MD; Peer Reviewer Suhani Bora, MD; AHC Media Executive Editor Leslie Coplin; Editor Jonathan Springston; and Editorial Group Manager Terrey L. Hatcher report no financial relationships relevant to this field of study.
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