The Effects Of Monounsaturated Fatty Acid-enriched Diets With And Without Avocados On Cardio-metabolic Risk Factors

Open Access
Author:
Wang, Li
Graduate Program:
Nutritional Sciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 13, 2014
Committee Members:
  • Penny Margaret Kris Etherton, Dissertation Advisor
  • Penny Margaret Kris Etherton, Committee Chair
  • Joshua D Lambert, Committee Member
  • Jeffrey Maurice Peters, Committee Member
  • Michael Thomas Green, Committee Member
  • Mosuk Chow, Committee Member
Keywords:
  • avocado
  • antioxidant
  • CVD
  • LDL-C
  • lipid
  • lipoprotein
  • metabolic syndrome
  • MUFA
  • nutrition
  • oxidation
  • small dense LDL.
Abstract:
Dietary guidance recommends a healthy dietary pattern that is low in saturated fat (SFA) to lower low-density lipoprotein cholesterol (LDL-C), a primary target for cardiovascular disease (CVD) risk reduction. Using either carbohydrates (CHO) or unsaturated fat to replace saturated fat in an average American diet may achieve a similar reduction in LDL-C, but a high intake of dietary CHO may increase triglycerides (TG), lower high density lipoprotein cholesterol (HDL-C), and increase small, dense LDL, along with promoting insulin resistance thereby increasing the risk of metabolic syndrome and type 2 diabetes. These cardio-metabolic risk factors also are targets for dietary intervention to achieve a greater CVD risk reduction. When SFA is replaced with unsaturated fat, the predominant fatty acid class is monounsaturated fatty acids (MUFA) because the recommended intake of polyunsaturated fatty acids (PUFA) is less than 10% of energy. Overall, the substitution of MUFA instead of CHO (especially refine grains and added sugar) for SFA calories may favorably affect other lipid risk factors of CVD. The oxidative modification of LDL particles is a key factor in the initial process of atherosclerosis. Dietary antioxidant vitamins, polyphenols, and other bioactive compounds from foods (e.g., fruits and vegetables) have been a focus of nutrition because of their role in improving antioxidant status and lowering LDL oxidation. Identifying foods that can beneficially improve multiple cardio-metabolic risk factors is needed for the primary prevention of CVD. Avocados are a nutrient-dense source of MUFA, rich in vitamins, minerals, fiber, phytosterols and polyphenols. It is unclear whether avocados affect cardio-metabolic risk factors beyond their fatty acid profile. This dissertation investigated the effects of a MUFA enriched moderate fat diets including one avocado per day on established and novel cardio-metabolic risk factors. A randomized, cross-over, controlled feeding trial was conducted with 45 healthy overweight/obese participants with baseline LDL-C levels in the 25-90th percentile. After a 2 week run-in average American diet (34% fat, 13% SFA, 11% MUFA, 51% carbohydrate, 16% protein) at baseline, three cholesterol-lowering diets (6-7% SFA) were fed (5 weeks each) with a random sequence: a lower-fat diet (LF: 24% fat, 7% SFA, 11% MUFA, 9% PUFA, 59% carbohydrate, 16-17% protein); and two moderate fat diets matched for macronutrients and fatty acids (34% fat, 6% SFA, 17% MUFA, 9% PUFA, 49% carbohydrate, 16-17% protein): the avocado diet included one fresh Hass avocado (136 g) per day, and the moderate fat diet provided mostly the same foods (as the avocado diet) but used high oleic acid oils and low fat dairy products to match the fatty acid profile of one avocado. All three cholesterol-lowering diets met current dietary recommendations on macronutrient percentage range and the serving amount for each food group. Specifically, the grain product in the lower fat diet contained more than half whole grains, which meets current dietary guidelines. Compared to baseline, all three diets decreased total cholesterol and LDL-C (p<0.001 for all). The LF diet significantly increased TG (20.8mg/dL, p<0.0001) and very-low-density lipoprotein cholesterol (VLDL-C, 2.6mg/dL, p=0.0003), while the AV and MF diets did not. There was a greater HDL-C decrease on the LF diet versus the AV and MF diets (p=0.03 and 0.04, respectively). The reduction in LDL-C and non-HDL cholesterol on the AV diet (-13.5mg/dL, -14.6 mg/dL) was greater (p=0.04 and 0.01) than the MF (-8.3 mg/dL, -8.7 mg/dL) and LF (-7.4 mg/dL, -4.8 mg/dL) diets. Furthermore, only the AV diet significantly decreased LDL particle number (LDL-P, -80.1nmol/L, p=0.0001), small dense LDL cholesterol (LDL3+4, -4.1 mg/dL, p=0.04), and the ratio of LDL/HDL (-6.6%, p<0.001). In addition, compared to baseline, only the AV diet decreased oxidized-LDL (-7.0 U/L, -8.8%, p=0.0004) while the LF diet (-1.6 U/L p=0.1) and the MF diet (-3.2 U/L, p=0.2) did not. Oxidized-LDL after consumption of the AV diet was significantly lower (p=0.05 and 0.03) than the MF diet and LF diet. HPLC analysis showed that only the AV diet increased plasma lutein by 68.7% from baseline (p<0.0001). The increase in lutein in response to the AV diet was significantly greater than the increase by the MF (21.1%, p=0.7) and LF (37.6%, p=0.1) diets. Both MF and AV diets significantly increased plasma α-carotene (72.8% and 68.4%, p<0.01 for both) and β-carotene (15.4% and 12%, p<0.05 for both) compared to baseline. The LF diet did not elicit changes in plasma antioxidant vitamins, except for a decrease in γ-tocopherol (-7.8%, p=0.03). Interestingly, the change of oxidized-LDL was significantly correlated with the change in small LDL-P (r=0.32, p=0.0002) and small, dense LDL-C (r=0.47, p<0.0001) by not large LDL-P (r=0.15, p=0.09) or large, buoyant LDL-C (r=-0.03, p=0.8). Overall, our results showed that a high MUFA, moderate fat diet elicits a more favorable blood lipid profile compared to a lower fat, high carbohydrate diet. Furthermore, the inclusion of one avocado per day as part of a moderate fat, cholesterol-lowering diet has additional benefits on lowering LDL-C, LDL-P, non-HDL-C, small, dense LDL, and oxidized LDL, and increasing plasma lutein. The change in oxidized LDL by the avocado diet may be due to its effect on lowering small, dense LDL. Our results demonstrate that avocados have beneficial effects on cardio-metabolic risk factors that extend beyond their cardio-protective fatty acid profile. Avocados are a source of many cardio-protective nutrients and further larger, long-term clinical studies are warranted to evaluate their role in primary and secondary CVD prevention.