American Society for Nutrition (ASN), a professional nutrition association recently shared a collection of some of the most discussed clinical research published in the nutrition journals in 2021. These clinical papers have received special attention from researchers and practitioners in the field either on Twitter, Facebook, in the news, in policy documents, or elsewhere during the past year. 

   
    Among the 20 papers listed, there were two about avocado and overweight/obesity, both reported on different aspects of the same avocado supplementation study which I happen to have referred to one of them in a previous blog One Avocado a Day May Lower Cholesterol.

   
    In the USA, ∼70% of the adult population is either overweight or obese, which has been referred to as a major public health challenge and epidemic. These papers have drawn a lot of interest is because, with the obesity epidemic, any knowledge that can lower abdominal obesity would be helpful.

   
    In the paper that I referred to by Khan et al. (1), they tried to evaluate the effects of one avocado per day consumption on abdominal adiposity, insulin resistance, oral-glucose-tolerance test, and estimated β-cell function. They recruited 105 adults aged 25–45 y (61% female) with BMI ≥25 kg/m2. The study participants were randomly assigned to an intervention (n = 53) that received a daily meal with 1 fresh Hass avocado or control (n = 52) that received an isocaloric meal with similar ingredients without avocado for 12 weeks.

   
    They measured abdominal adiposity parameters such as visceral adipose tissue (VAT), subcutaneous abdominal adipose tissue (SAAT), and the ratio of VAT to SAAT (VS Ratio). The abdominal fat can be divided into the visceral adipose tissue (VAT, the fat surrounding the internal organs) and subcutaneous abdominal adipose tissue (SAAT, the fat under the skin). Relative to other adipose tissue depots, VAT accumulation, surrounding internal organs such as the liver, is associated with type 2 diabetes, dyslipidemia, inflammation, increased risk of thrombosis, and nonalcoholic fatty liver disease. The less visceral adipose tissue is better for health.

   
    The study showed that daily consumption of 1 fresh Hass avocado changed abdominal adiposity distribution among females but did not facilitate improvements in peripheral insulin sensitivity or β-cell function among adults with overweight and obesity. Among females, the treatment group exhibited a greater reduction in VAT [1.6 ± 89.8 g (control) compared with –32.9 ± 81.6 g (treatment), p = 0.021] and VS Ratio [0.01 ± 0.05 (control) compared with –0.01 ± 0.03 (treatment), p = 0.001]. Among males, there was no significant difference between groups in changes in abdominal adiposity or glycemic outcomes. The decrease in VS Ratio among treatment group participants suggests that avocado intake imparts a beneficial abdominal adiposity profile, thus more likely to reduce the risks of developing obesity-related metabolic disorders (1).

   
    The average –32.9 ± 81.6 g of visceral fat reduction in the women who received avocado may not seem to be much over 12 week time period. However, if considering what is the typical amount of visceral fat a woman may have, for example, one paper reported an average of 258 ± 226 g of visceral fat in 20-30-year-old healthy European women (n=217 with ~20% of these women were overweight/obese)(2), then  33 g would be more than 10% for that population. All the women in the avocado study were with BMI ≥25 kg/m2 either overweight or obese (potential a larger visceral fat accumulation to start with compared to those in the European women), thus the change in VAT tissue among females in the treatment group in this study was ∼5%.

   
    The change of 5% would be considered modest.  However, this study was relatively short (3 months) and did not include an exercise component, the modest change in visceral fat is not surprising. It is possible that maintaining the treatment regimen for a longer period could have provided the necessary cumulative reduction in visceral fat to be clinically meaningful. Previously, a large prospective study had observed annual increases of 3%, 4%, and 3% of VAT, SAAT, and VS Ratio, respectively, were related to an increased risk of a diagnosis of diabetes among adults with higher BMI. Also, a 26% reduction in VAT was observed in a study over 12 months, using a healthy eating and physical activity/exercise program, which corresponded to significant improvement in cardiorespiratory fitness, plasma inflammatory markers, lipid profiles, and oral glucose tolerance test (1).

   
    The reasons why avocados may contribute to visceral fat changes are possible because of the higher fiber and monounsaturated fatty acids content. A whole avocado contains ∼10 g of total fiber, contributing to ≤40% of the dietary recommendations among females or 30% for males. Greater dietary fiber intake is associated with lower visceral fat and is related to lower gains in visceral fat. Supplemental fiber intake can reduce BMI and waist circumference independent of caloric restriction. Dietary fiber, specifically insoluble fiber (contributing to 70% of the fiber in an avocado), could affect visceral fat by increasing fecal bulk and shortening transit time in the gut and lowering the absorption of nutrients and energy. Meals with higher dietary fiber can result in a moderate postprandial blood glucose response, stimulating a greater sensation of satiety in healthy adults (1).

   
    The other paper in the 20 most discussed list that mentioned avocado reported the microbiota aspect of avocado's effect in the same group of study participants (3).  Daily avocado consumption resulted in lower fecal bile acid concentrations, greater fecal fatty acid, and short-chain fatty acids, and greater relative abundances of bacteria capable of fiber fermentation, providing evidence that this nutrient-dense food affects digestive physiology, as well as the composition and metabolic functions of the intestinal microbiota.

   
    Avocados are rich in fiber, including soluble hemicelluloses and pectins that can be metabolized by intestinal microorganisms to produce short-chain fatty acids, similar to what had been observed with greater adherence to a Mediterranean diet rich in fiber and monounsaturated fatty acids. High-fiber diets increase fecal weight, which could have contributed to the decreased fecal bile acid concentrations observed in the avocado treatment group (3).

   
    These findings provided valuable insight regarding the impact of avocado intake on abdominal fat and the intestinal microbiota, which have important implications for dietary interventions conducted among the growing at-risk population of adults with overweight or obesity. That is why avocados were among the most discussed in 2021.

 

References:

1. Naiman A Khan, et al.  (2021). Avocado Consumption, Abdominal Adiposity, and Oral Glucose Tolerance Among Persons with Overweight and Obesity. J Nutr. 2021 Sep; 151(9): 2513–2521.

2. Miazgowski, T., et al. (2017). Visceral fat reference values derived from healthy European men and women aged 20-30 years using GE Healthcare dual-energy x-ray absorptiometry. PLOS ONE, 12(7), e0180614. doi:10.1371/journal.pone.0180614

3. Thompson, S. V., et al. (2020). Avocado Consumption Alters Gastrointestinal Bacteria Abundance and Microbial Metabolite Concentrations among Adults with Overweight or Obesity: A Randomized Controlled Trial. The Journal of Nutrition. doi:10.1093/jn/nxaa219