Eating lean beef as part of MED diet could improve gut diversity

By Dr. Sanchari Sinha Dutta, Ph.D.Reviewed by Lauren HardakerNov 21 2025

New clinical trial evidence suggests that replacing an American-style diet with a Mediterranean eating pattern, while still consuming lean beef, can significantly reduce TMAO levels and enhance gut microbial diversity, highlighting the importance of overall diet quality over meat quantity.

Study: Effect of Varying Quantities of Lean Beef as Part of a Mediterranean‐Style Dietary Pattern on Gut Microbiota and Plasma, Fecal, and Urinary Metabolites: A Randomized Crossover Controlled Feeding Trial. Image credit: Sea Wave/Shutterstock.com

A study by researchers at Pennsylvania State University suggests that Mediterranean-style diets with moderate amounts of lean beef offer better metabolic and gut health benefits than an average American diet with similar amounts of beef. The study is published in the Journal of the American Heart Association.

How diet shapes TMAO

Excessive intake of red meat is known to increase the risk of cardiovascular disease, probably by elevating blood levels of trimethylamine N‐oxide (TMAO). This metabolite is produced during gut microbiota-mediated metabolism of animal foods and is associated with atherosclerotic plaque formation.

Although several studies have linked beef intake to increased TMAO production, emerging evidence from clinical trials suggests that the inclusion of lean, unprocessed meat in heart-healthy dietary patterns does not increase levels of conventional cardiovascular risk factors.

The Dietary Guidelines for Americans recommend intake of 737 grams of meats, poultry, and eggs per week for a 2000‐kcal diet. Clinical evidence on heart-healthy dietary patterns reveals that a Mediterranean-style (MED) diet with a higher intake of red meat (500 grams per week) increases blood levels of TMAO compared to a lower intake (200 grams per week).

Given the potential impact of red meat on cardiovascular health, researchers at the Pennsylvania State University, USA, conducted a randomized, controlled-feeding study to investigate the metabolic and gut health effects of MED diets with different amounts of lean beef, and compared the outcomes with an average American diet with a similar amount of beef.

Four-period crossover trial

The study included three MED diets with 14, 71, and 156 grams of lean meat consumed per day per 2000 kcal. The impact of these diets on gut microbiota composition and blood, urinary, and fecal levels of TMAO was compared with that of an average American diet, which included 71 grams of beef per day, equivalent to 2000 kcal.

A total of 30 healthy individuals participated in the study, who consumed each diet for four weeks with at least a one-week washout period. Analytical samples were collected from participants at baseline and at the end of each 4‐week diet period. 

The authors noted that Greek yogurt was included in the MED diets, while conventional yogurt was used in the American diet, contributing to some observed microbial differences.

How diets changed outcomes

The study analysis revealed that consumption of a MED diet with 14 g or 71 g of lean beef per day per 2000 kcal is associated with higher gut microbiota diversity compared to a typical American diet with 71 g of beef per day per 2000 kcal. The MED diet containing 156 g/day did not significantly increase diversity compared with the American diet.

The consumption of MED diets with 14 and 71 grams of lean meat, respectively, resulted in 1.7-fold and 2-fold reductions in blood TMAO levels compared to the American diet containing an equal amount of beef.

Similarly, MED diets with 14, 71, and 156 grams of lean beef, respectively, caused 1.8-fold, 2-fold, and 1.7-fold reductions in urinary TMAO levels compared to the American diet.

Notably, the study found that the relationship between gut microbiota diversity and TMAO levels was only observed when participants were classified as high or low TMAO producers, suggesting a complex relationship between diet, microbiota, and TMAO.

Implications for heart health

The study demonstrates that incorporating moderate amounts of lean beef into a healthy Mediterranean-style diet can improve certain aspects of gut microbiota composition in healthy adults, without increasing TMAO levels associated with cardiovascular disease risk.

The three Mediterranean-style diets, even with progressively higher amounts of lean beef, did not raise TMAO levels in a dose-dependent way among healthy participants. This suggests that a Mediterranean diet rich in plant-based foods and fiber may help limit TMAO production by shaping gut microbiota, even when lean beef intake is higher.

The study reports that Mediterranean diets lead to higher blood levels of carnitine, a TMAO precursor, than the American diet. However, the authors do not interpret this as evidence of increased conversion of carnitine to TMAO, and it should not be taken to mean that the American diet drives greater TMAO production from carnitine.

Participants who consistently showed higher TMAO levels across all four diet periods also had higher gut microbiota diversity when grouped as high or low TMAO producers, despite this being an unfavorable metabolic profile. This suggests that the microbiota’s functional activity, rather than its overall diversity, is more important in driving TMAO production.

However, the study did not observe diet‐related shifts in predicted microbial functional pathways, suggesting that predictive analysis may have limitations in capturing true functional changes in the microbiota, which are best captured by direct measures of microbial activity, such as metatranscriptomics or proteomics.

The study included metabolically healthy individuals, which may restrict the generalizability of its findings to individuals with metabolic disorders. Furthermore, the American diet analyzed in the study has higher fiber content than the average American fiber intake. This may restrict an accurate comparison between healthy MED diets and typical American diets.  

Overall, the study findings highlight the role of healthy dietary patterns in influencing gut microbiota‐mediated metabolic pathways and suggest that the inclusion of up to 156 grams per day of lean beef in an MED diet does not affect the novel cardiovascular disease risk factor TMAO in healthy populations.

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