One Avocado a Day Keeps the Doctor Away?

Written by Zachary Wenger

November 26, 2021

Wang, Li et al. “Effect of a moderate fat diet with and without avocados on lipoprotein particle number, size and subclasses in overweight and obese adults: a randomized, controlled trial.” Journal of the American Heart Association vol. 4,1 e001355. 7 Jan. 2015, doi:10.1161/JAHA.114.001355

Introduction:

The monounsaturated fatty acid (MUFA) powerhouse, avocados, are generally recognized as a dietary constituent that can beneficially impact heart health, which includes improvements in blood lipids (1). As such, the public health guidelines suggest limiting saturated fat (SFA) in the diet to <10% of total calories, and to replace those calories with MUFA, along with polyunsaturated fats (PUFA), another unsaturated fatty acid (2).

The dietary sources of MUFA utilized in the mediterranean dietary pattern, may explain the cardioprotective benefits that are consistently observed. One of the better illustrations of this is the PREDIMED (Prevención con Dieta Mediterránea) trial (3). Notably, the participants were supplemented with 30g of mixed nuts per day, 4 tbsp of extra virgin olive oil per day, or simply told to reduce total fat (control). At 5-years of follow-up, these interventions reduced the incidence of major CVD events by ~30%, relative to the control, in individuals at high cardiovascular risk.

One Hass avocado contains ~13 g of the MUFA, oleic acid, which is similar to the amount of oleic acid found in 1.5 oz of almonds or 2 tablespoons of olive oil (4). As a function of the fatty acid composition, avocados would be expected to elicit similar effects on reducing cardiovascular disease risk, via improvements in blood lipids. However, what if we were able to control for MUFA in the diet, in order to examine if avocados have any unique lipid lowering properties beyond the MUFA content? While studies have been conducted investigating avocados in relation to blood lipids in the past, the methodological rigor in this more recent publication is unrivaled (5).

The Study:

The study was a randomized, 3‐period crossover study design with 45 study participants. Firstly, the participants were assigned a 2‐week “run‐in” average American diet (AAD), which was considered baseline. Then, the participants were allocated to all 3 of the different study arms for 5 weeks each, separated by 2 week compliance breaks between diet periods, with the goal of maximizing adherence.

The inclusion criteria targeted overweight and obese men and women, with LDL‐C in the range of 105 to 194 mg/dL for males; 98 to 190 mg/dL for females. All participants were nonsmokers, had normal/well-controlled blood pressure, free of CVD, diabetes, liver, or renal disease, and not taking lipid‐lowering medications or supplements.

The three interventions utilized in the study were a low fat (LF), moderate fat (MF), and an avocado diet (AV). The LF diet was designed by replacing 6-7% of calories from SFA (from the run-in AAD) with carbohydrates. Similarly, the AV and MF diets were designed to replace 6-7% of calories from SFA with MUFA. This was achieved from either one Hass avocado per day (AV) or sunflower oil and canola oil (MF). Table 2 illustrates one day menus for each intervention. Evidently, the MF and AV diets were nearly identical, except for the substitution of the sunflower/canola oil in the MF arm for the whole avocado in the AV arm. Primary outcome measures included lipids and lipoproteins, as well as lipoprotein particle size.

Results:

Adherence to the interventions was excellent at about 90%, the participants maintained their body weight during each diet period (± 2 kg), and the dropout rate was low. 

Relative to the AAD, the LW, MF, and AV diets resulted in significant reductions in total cholesterol (TC) and other blood lipids. Below, I’ll present some of the key results surrounding blood lipids:

  • LDL-C, Non-HDL-C, and TC: The reduction in LDL-cholesterol (LDL-C), non-HDL cholesterol (non-HDL-C), and TC by the AV diet, was significantly greater than the LF and MF diets.
    • LDL-C: 120.5 mg/dL (LF), 119.0 mg/dL (MF), 113.7 mg/dL (AV)
    • Non-HDL-C: 145.9 mg/dL (LF), 141.5 mg/dL (MF), 135.3 mg/dL (AV)
    • TC: 190.7 mg/dL (LF), 188.7 mg/dL (MF) 182.2 mg/dL (AV)
  • TG and VLDL-C: The LF diet significantly increased triglycerides (TG) and very-low-density lipoprotein cholesterol (VLDL-C), while the MF and AV diets did not.
    • TG: 134.3 mg/dL (LF), 111.1 mg/dL (MF), 108.6 mg/dL (AV)
    • VLDL-C: 25.6 mg/dL (LF), 22.6 mg/dL (MF), 21.7 mg/dL (AV)
  • ApoB: The AV and MF diets significantly decreased ApoB from BL and relative to the LF diet.
    • ApoB: 99.0 mg/L (LF), 95.3 mg/L (MF), 92.4 mg/L (AV)
  • HDL-C: Relative to the AAD run-in, HDL-cholesterol (HDL‐C) significantly decreased in all groups, though less on the MF diet and AV diet than the LF diet.
    • HDL-C: 44.8 mg/dL (LF), 47.2 mg/dL (MF), 46.9 mg/dL (AV)
  • LDL-P: Relative to the AAD run-in, the AV diet was the only arm that significantly reduced LDL-P.
    • LDL-P: 1168.1 nmol/L (LF), 1099.7 nmol/L (MF), 1054.6 nmol/L (AV)

Analyzing:

Strengths: The study possessed incredibly well-matched diets, in which two, the MF and AV diets, were nearly identical to one another. In addition, including canola oil and sunflower oil as comparators in the MF arm, which are also recognized for their cholesterol-lowering properties relative to SFA (6,7). Next, by maintaining a high degree of adherence among the study participants, the maintenance of body weight throughout the trial, and experiencing a low dropout rate. Subsequently, a glaring strength was the utilization of the 3-period crossover study design, which if the dropout rate is low (which it was), eliminates between-subject variability. Lastly, to successfully investigate avocados beyond their MUFA content, in order to potentially detect additional compounds that may have cardioprotective properties.

Limitations: In light of what we know about the role of dietary fiber on blood lipids, it would have been serviceable to account for total fiber. Between the MF and AV arms, the researchers eloquently accounted for nearly the entirety of the nutrient profiles within the diets, except for fiber, which were 26g/d and 35g/d, respectively. This could have been achieved with a fiber supplement, such as psyllium husk, which wouldn’t have altered the MUFA content. We will discuss this limitation in greater detail in the following section.

Biological Plausibility: 

Conventional proposed mechanisms for the lipid lowering effects of avocados generally begin with the MUFA content. However, as this study outlined, the MUFA content doesn’t completely explain the effect. The AV diet lowered LDL‐C, TC, LDL‐P, non‐HDL‐C, TC/HDL‐C, and LDL/HDL‐C significantly more than the MF diet, which was MUFA equalized. 

The researchers suggest that the β‐sitosterol, a plant sterol, may possibly explain the findings. Plant sterols do indeed lower LDL-C by inhibiting cholesterol absorption (10). However, there’s ~103mg of β‐sitosterol in one avocado and ~359.7mg in 100g of canola oil (4,8). That’s ~100.7mg of β‐sitosterol in 2 tbsp of canola oil. While this may contribute towards the overall reduction in blood lipids, this doesn’t appear to explain the differences observed between the AV and MF arm.

The more persuasive mechanism would be the difference in fiber content between the MF and AV arm, which was 26g/d and 35g/d, respectively. That’s a difference of 315g of fiber within the 5 weeks. The total fiber content of avocados are composed of 70% insoluble and 30% soluble fiber (4). While soluble fiber has a more pronounced effect on lowering blood lipids than insoluble fiber, due to the binding affinity of soluble fiber to bile acids, both types of fiber have lipid lowering properties, and ultimately, avocados contain both soluble and insoluble fiber, just not in equal proportions (9). 

While the differences in fiber was a limitation that I mentioned earlier, and while I personally  would have liked it accounted for, similarly to the accounting of MUFA, that variance appears to be a deliberate part of the methodology. The researchers state: “The effects of the AV diet on sdLDL may be due to the combined effects of MUFA and other bioactives, especially phytosterols and fiber.” In other words, the researchers are well aware of the imbalance in dietary fiber between the MF and AV arm and suspect that it may be a plausible mechanism to explain the lipid lowering effects.

Take Home Message: 

In overweight and obese men and women, with elevated LDL-C, consuming one avocado per day, as part of a moderate-fat, cholesterol-lowering diet, has favorable effects, beyond the MUFA content, on decreasing CVD risk factors. Further research is warranted to examine avocado consumption on CVD outcomes, preferably well conducted prospective cohorts.

References:

  1.  “The Skinny on Fats.” Www.Heart.Org, https://www.heart.org/en/health-topics/cholesterol/prevention-and-treatment-of-high-cholesterol-hyperlipidemia/the-skinny-on-fats.
  2. Office of Disease Prevention and Health Promotion. Cut Down on Saturated Fats. 2016. Available online: https://health.gov/sites/default/files/2019-10/DGA_Cut-Down-On-Saturated-Fats.pdf
  3. Estruch, Ramón et al. “Primary Prevention of Cardiovascular Disease with a Mediterranean Diet Supplemented with Extra-Virgin Olive Oil or Nuts.” The New England journal of medicine vol. 378,25 (2018): e34. doi:10.1056/NEJMoa1800389
  4. Dreher, Mark L, and Adrienne J Davenport. “Hass avocado composition and potential health effects.” Critical reviews in food science and nutrition vol. 53,7 (2013): 738-50. doi:10.1080/10408398.2011.556759
  5. Wang, Li et al. “Effect of a moderate fat diet with and without avocados on lipoprotein particle number, size and subclasses in overweight and obese adults: a randomized, controlled trial.” Journal of the American Heart Association vol. 4,1 e001355. 7 Jan. 2015, doi:10.1161/JAHA.114.001355
  6. Schwingshackl, Lukas et al. “Effects of oils and solid fats on blood lipids: a systematic review and network meta-analysis.” Journal of lipid research vol. 59,9 (2018): 1771-1782. doi:10.1194/jlr.P085522
  7. Ghobadi, Saeed et al. “Effects of Canola Oil Consumption on Lipid Profile: A Systematic Review and Meta-Analysis of Randomized Controlled Clinical Trials.” Journal of the American College of Nutrition vol. 38,2 (2019): 185-196. doi:10.1080/07315724.2018.1475270
  8. Mo, Shunyan et al. “Quantitative analysis of phytosterols in edible oils using APCI liquid chromatography-tandem mass spectrometry.” Lipids vol. 48,9 (2013): 949-56. doi:10.1007/s11745-013-3813-3
  9. Jenkins, D J et al. “Effect on blood lipids of very high intakes of fiber in diets low in saturated fat and cholesterol.” The New England journal of medicine vol. 329,1 (1993): 21-6. doi:10.1056/NEJM199307013290104
  10. de Jong, Ariënne et al. “Metabolic effects of plant sterols and stanols (Review).” The Journal of nutritional biochemistry vol. 14,7 (2003): 362-9. doi:10.1016/s0955-2863(03)00002-0