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High Cholesterol, Blood Pressure Targeted in AHA, ACC Guideline to Reduce Cardiovascular Risk

By Jennifer Fleming MS, RD and Penny M. Kris-Etherton, PhD, RD

Quick Taste: Current dietary guidance recommends replacing SFA with unsaturated fat, including both PUFA and MUFA, to reduce LDL-C to decrease CVD risk.  The evidence is especially strong for the cardiovascular benefits of n-6 and n-3 PUFA. Thus, incorporating food sources that are high in n-6 fatty acids and also n-3 fatty acids, such as soybean oil, into a low SFA DASH dietary pattern is a simple and effective strategy for decreasing LDL-C and the risk of CVD.

Two major risk factors for cardiovascular disease (CVD)– an elevated LDL-cholesterol (LDL-C) and high blood pressure (BP)—are targeted in the 2013 American Heart Association (AHA) and American College of Cardiology (ACC) Guideline on Lifestyle Management to Reduce Cardiovascular Disease Risk. 1 The guideline advises adults who would benefit from LDL-C and/or BP lowering to consume a DASH (Dietary Approaches to Stop Hypertension)-type dietary pattern that emphasizes vegetables, fruits, and whole grains; includes low-fat dairy products, poultry, fish, legumes, non-tropical vegetable oils, and nuts; and limits intake of sweets, sugar-sweetened beverages and red meats.  In addition, reducing saturated fat (SFA) and trans fatty acids to lower LDL-C, and decreasing sodium to lower BP, are recommended.

To lower LDL-C, a dietary pattern that provides 5%-6% of calories from saturated fatty acids (SFA) is recommended.1  This amount is about one-half the current consumption in the United States (U.S.).  Major sources of SFA in the U.S. diet are full-fat dairy products, fatty meats and grain-based desserts.  To lower dietary SFA to meet the new guideline, skim milk and low-fat dairy products, plant proteins and lean meats are recommended.  Solid fats like butter should be replaced with liquid vegetable oils and soft margarines.

The evidence for the SFA recommendation comes from many randomized controlled trials (RCTs). A recent Cochrane Database Systematic Review conducted to assess the effect of reduction and/or modification of dietary fats on total mortality, cardiovascular mortality, and cardiovascular morbidity reported that reducing SFA lowered the risk of cardiovascular events by 14%.2  While replacing SFA with unsaturated fat (both monounsaturated and polyunsaturated fatty acids, MUFA and PUFA, respectively) decreases cardiovascular events, evidence is needed about the specific amounts of MUFA and PUFA that should be recommended.2 In a systematic review and meta-analysis of eight randomized controlled trials (with 13,614 participants), Mozaffarian et al.3 reported that replacing SFA with PUFA decreased risk of coronary heart disease (CHD).  PUFA consumption was approximately 15% and SFA was less than 10% of energy.  The overall risk reduction was 19%, which corresponded to a 10% reduced risk of CHD for each 5% energy increase in PUFA.  The authors concluded, “A shift toward greater population PUFA consumption in place of SFA would significantly reduce rates of CHD.” In 2009, an AHA Science Advisory recommended 5%-10% of energy from n-6 PUFA for CHD risk reduction.  The Advisory also noted that if n-6 PUFA were reduced from the current intake (≈7% of energy), that CHD risk would likely increase.  This conclusion was based on the evidence from RCTs of morbidity/mortality outcomes, case-control and cohort observational studies of coronary disease outcomes, short-term RCTs of CHD risk factors, and long-term animal experiments.  In support of the current recommendations to replace calories from SFA with unsaturated fat, in 2009, in a pooled analysis of 11 cohort studies from the U.S. and Europe of 344,696 subjects followed for 4 to 10 years, Jakobsen et al.4 reported that replacing 5% of energy from SFA with PUFA was associated with a decreased risk of coronary events by 13% and coronary deaths by 26%.  In an accompanying editorial, Katan et al.5 reaffirmed the recommendation of the AHA Science Advisory for omega-6 PUFA on the basis of the analysis published by Jakobsen et al.4 and the existing RCT evidence.

Most recently, two studies demonstrated benefits of the n-6 PUFA linoleic acid (LA) on CHD risk.   In a systematic review and meta-analysis of 13 prospective cohort studies, Favrid et al.6 showed that dietary LA intake was inversely associated with CHD risk in a dose-dependent manner.  The highest LA intake group had a 15% lower risk of CHD events compared to the lowest intake group.  In the Cardiovascular Health Study, Wu et al.7 reported that high circulating LA levels were inversely associated with both total and CHD mortality in adults (n=2,792) 65 years and older.

In contrast, the evidence supporting the replacement of SFA with MUFA is not as clear. In the analysis conducted by Jakobsen et al.,4 MUFA or carbohydrate (CHO) replacement of SFA was not associated with a decreased risk of CHD death or events.   Numerous reasons could account for the lack of benefits reported for CHO and MUFA substitution for SFA in the observational studies conducted to date.  As noted by Katan et al.,8 the clinical studies show that replacing SFA with CHO decreases LDL-C, as well as HDL-cholesterol (HDL-C), thereby often resulting in an unchanged total cholesterol (TC):HDL-C ratio.  However, there is a need to evaluate the type of CHO substituted for SFA since refined grains and whole grains have been shown to elicit different lipid/lipoprotein responses.  It is important to appreciate that in these meta-analyses, a mixture of CHO-containing foods was studied as a substitute for SFA, and most were derived from simple CHO and refined grains.  The lack of a benefit associated with replacing SFA with MUFA was surprising given the benefits of a Mediterranean diet.  However, this observation may reflect the food sources of MUFA (dairy products, meat and partially hydrogenated oils) and/or the presence of other “confounders” associated with animal food sources of MUFA, including smoking, high BMI and decreased physical activity.

Subsequent to the study by Jakobsen et al.,4 other epidemiologic studies also have reported no association between SFA intake and CVD risk.4,9-11  In a meta-analysis of several large cohort studies, Skeaff and Miller reported that SFA intake was not significantly associated with CHD death.10 Similarly, in a meta-analysis of prospective cohort studies of 347,747 subjects, Siri-Tarino et al. reported no significant association between SFA and CHD, stroke and CVD.9  In these studies, SFA was replaced with dietary CHO and specifically refined and simple CHO since it is the predominant CHO source in Western diets.

There are methodological limitations that have been raised about the epidemiologic studies that did not show adverse associations between SFA and risk of coronary diseases/events.  One major problem relates to limitations of the dietary assessment methodology used.  For example, Skeaff and Miller10 concluded that their findings were problematic due to inconsistent dietary assessment methods and the potential for bias.  Specifically, they identified the possible underestimation of an association due to variability and/or measurement error related both to the dietary data (for SFA) as well as the CVD event data.10  Large within-individual variability in SFA intake is another limitation noted by Kromhout et al.12 of the recent epidemiologic studies,9,10 and therefore, the use of 24-hour recalls to evaluate long term dietary habits is problematic.6 In addition, there are limitations with food frequency questionnaires to assess dietary data that are commonly used in epidemiologic studies; many have a limited number of foods listed and some are not updated periodically.  Furthermore, over-adjusting for serum lipids and dietary lipids also could flaw these meta-analyses, considering that the effect of SFA on CVD is partly dependent upon serum lipids.

More recently, a meta-analysis published in the Annals of Internal Medicine, evaluated the relationship between dietary, circulating and supplemental fatty acids and coronary risk.13   This study evaluated 32 observational studies of dietary fatty acid intake, 17 observational studies of fatty acid biomarkers, and 27 randomized, controlled trials of fatty acid supplementation.  The authors concluded that the relative risks for coronary disease for dietary saturated, monounsaturated, long-chain n-3 PUFA, n-6 PUFA and trans fatty acids were not significant for the top third and bottom third of intakes.  Likewise, the corresponding circulating fatty acids were not significantly associated with coronary disease.  As noted by Willett et al. in an archived letter to the Editor, the article contained multiple errors and omissions.14 Specifically, there were gross errors in data abstraction from the original papers, and important studies were omitted, especially those on PUFA.  Moreover, the PUFA results reported by Chowdhury et al. are not supported by the preponderance of evidence from epidemiologic studies.  Some scientists have recommended that the article be withdrawn.

Of note is that a recent systematic review and meta-analysis of observational studies15 with 251,049 individuals reported benefits of alpha linolenic acid (ALA) on fatal-CHD, non-fatal CHD and total CHD. The authors observed an overall pooled RR of 0.86 (95% CI: 0.77, 0.97).  The results were consistent for dietary and biomarker studies, however only the dietary studies reached significance.  Thus, more research is needed to establish the cardiovascular benefits of ALA.  Nonetheless, the evidence to date demonstrates cardiovascular benefits of PUFA, which affirms current recommendations to replace SFA with unsaturated fat especially n-6 and n-3 PUFA.

Both LA, an n-6 PUFA, and ALA, an n-3 PUFA, are essential fatty acids and must be provided in the diet. Acceptable macronutrient distribution ranges established by the Institute of Medicine16 for LA and ALA are 5%-10% and 0.6%-1.2% of total energy, respectively.  Total PUFA accounts for approximately 7.3% of total calories, the majority of which (6.5%) comes from LA, which is derived from vegetable oils such as corn, sunflower, and soybean oils.  Omega-3 PUFA accounts for approximately 0.7% of total calories, the majority of which (0.6%) comes from ALA, which is derived from certain vegetable oils such as soybean, canola, flaxseed and walnuts.

Soybean oil is the most commonly consumed vegetable oil in the U.S. and its popularity continues to grow as consumers begin to heed the recommendations to replace SFA with unsaturated fats. For example, replacing 1 Tbsp. of butter with 1 Tbsp. of soybean oil would result in a reduction of 5g of SFA and an increase of 7.5g PUFA (6.6 g LA, 0.9 g ALA). Similarly, replacing high fat meats with soy products (i.e. edamame, tofu) also will reduce SFA and increase dietary fiber which is found in many plant protein foods. Also, replacing 3 oz. of a high fat meat (9g SFA) with 3 oz. firm tofu or ½ cup edamame (<0.5g SFA) will decrease SFA intake by 8.5g and increase PUFA intake by 1 to 1.5g. Moreover, total caloric intake will decrease by 150 to 200 kcal and fiber intake will increase by 2 to 4g. Another alternative for reducing SFA while increasing PUFA is to replace a high fat protein food (high fat meat, bacon, cheese) in salad with a lean protein (i.e., lean beef, chicken, tuna).  Walnuts or a soybean oil based salad dressing can also be added to make up some or all of the calories lost from the fatty protein food substitution.

The benefits of soybean oil and walnuts go beyond SFA reduction as both are sources of ALA.  However, increasing consumption to 2 to 3g/day, as has been suggested,17 has proven to be challenging as many vegetable oils are being reformulated to contain higher amounts of oleic acid.  Current intake of ALA in the U.S. is 1.8g/day for men and 1.4g/day for women, which is below suggested recommendations. To attain a daily intake of 2 to 3g, ALA consumption would have to be increased by almost 2 grams.  This recommendation could be met with a 1 oz. serving of walnuts (2.6g of ALA) or 2 Tbsp. soybean oil (1.8g ALA); both of which can be incorporated in the diet (e.g., in mixed dishes or in salads) as a replacement strategy for high fat meats or cheeses to reduce dietary SFA.

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