According to the findings of a large-scale clinical trial in Israel, the green-Mediterranean diet – with added portions of duckweed and green tea – reduced visceral adipose tissue (VAT) by 14% compared to 7% in the Mediterranean diet and 4.5% in a healthy diet.
“In this 18-month dietary intervention study, the green-MED diet, richer in dietary polyphenols and green plant-based proteins and lower in red meat, might be a more effective strategy for VAT loss than the traditional healthy MED diet achieving more than twice the degree of VAT reduction, despite similar weight loss,” the team of researchers wrote.
Published in the BMC Medicine journal, the study was funded through grants from the German Research Foundation, the Israel Ministries of Health, the Israel Ministry of Science and Technology, and the California Walnut Commission. It is part of the wider DIRECT-PLUS (Dietary Intervention Randomized Controlled Trial Polyphenols Unprocessed Study) led by researchers from Ben-Gurion University of the Negev in collaboration with the Harvard School of Public Health and the University of Leipzig.
Modifying the Mediterranean diet
The concept of the green-Mediterranean diet emerged in 2020 from a first study conducted by the DIRECT PLUS research team on the effects the green-MED on cardiometabolic risk. After identifying improvements in LDL cholesterol, diastolic blood pressure and insulin resistance, subsequent research explored the diet’s beneficial outcomes on age-related brain atrophy and non-alcoholic fatty liver disease.
“We learned from the results of our experiment that the quality of food is no less important than the number of calories consumed and the goal today is to understand the mechanisms of various nutrients, for example, positive ones such as the polyphenols, and negative ones such as empty carbohydrates and processed red meat, on the pace of fat cell differentiation and their aggregation in the viscera,” Professor Iris Shai of Ben-Gurion University of the Negev and a lead author on the study, said in a press release.
The modified Mediterranean diet removes red meat completely in favor of plant-based proteins and incorporates polyphenol-rich green tea, walnuts and Wolffia globose, a green plant also known as Mankai duckweed.
“Wolffia globosa, which had the highest magnitude of VAT reduction, is an aquatic plant rich in polyphenols and high-quality protein with beneficial effects on postprandial and fasting glycemic control, known to provide bioavailable essential amino acids, iron and available B12 vitamin,” the researchers noted.
Independent lab tests have also confirmed that Mankai is rich in phenolic metabolites including ellagic acid, benzoic acid, naringenin, luteolin, quercetin, p-coumaric acid and caffeic acid.
The 18-month intervention evaluated the effects of three different diets on 294 adults with abdominal obesity or dyslipidemia recruited from the workforce of the Shimon Peres Negev Nuclear Research Center in Dimona, Israel. Participants were randomly assigned to one of three groups: a healthy-dietary guidelines (HDG) control group, a traditional Mediterranean diet group (MED) or a green-Mediterranean (green-MED) diet group. Dietary interventions were combined with physical activity.
The MED and green-MED diets were similarly calorie-restricted (1500–1800 kcal/d for men and 1200–1400 kcal/d for women) with identical components including 28g of walnuts a day. Participants in the green-MED group, however, consumed three to four cups of green tea a day, as well as 100 grams frozen duckweed plant cubes as a green shake dinner substitute for meat protein – doubling the polyphenol content in their diet.
Abdominal fat depots were assessed at baseline and at 18 months, while weight, waist circumference and blood and urine biomarkers were evaluated at baseline, six months and 18 months. Polyphenol compounds were measured in both plasma and urine samples.
Polyphenols as prebiotic
The DIRECT-PLUS team noted a significant association between total plasma polyphenols and VAT loss, hypothesizing that benefits may be found along a gut-fat axis. Polyphenols, widely studied for their potent anti-inflammatory and antioxidant effects, have also shown a ‘prebiotic’ effect by modulating microbiota in the gut.
“The beneficial effects of the green-MED diet on VAT loss might be explained by polyphenols,” the researchers put forth. “While exploring specific polyphenols, we found that urolithin A, a derived gut microbiota metabolite of ellagitannin, was significantly correlated with VAT loss.”
The trial correlated elevated urolithin A with walnut and Mankai consumption, since its precursor, ellagic acid, was found in both. In addition, plasma hippuric acid – a combination of glycine and benzoic acid metabolites present in Mankai – was significantly associated with VAT reduction even after controlling for waist circumference change.
“Hippuric acid is an end product of microbial metabolism of different classes of dietary polyphenols, and elevated fasting plasma levels indicate an upregulation of microbiome total polyphenol metabolism,” the study noted. “Hippurate appeared to be the single most important metabolite linking diet and visceral fat.”
The researchers called for future studies to explore the exact mechanisms of specific polyphenol-rich foods on visceral adiposity.
Source: BMC Med 20, 327 (2022)
“The effect of high-polyphenol Mediterranean diet on visceral adiposity: the DIRECT PLUS randomized controlled trial”
Authors: Zelicha, H., Kloting, N., Kaplan, A. et al.