This study analyzed food frequency questionnaire and stool sample data from older adult participants (ages 50–64) in the Swedish Cardiopulmonary Bioimage Study (SCAPIS).

The intake of processed red meat was correlated with lower alpha-diversity, even after adjusting for age, sex, diet, physical activity, smoking, and education, but not after also adjusting for BMI.

Meat intake was also associated with differences in the relative abundance of species, including the following:

White meat: more Streptococcus australis (oral bacteria),^[1] less B. longum subsp. longum (a probiotic).
Unprocessed red meat: more Amedibacillus dolichus (associated with visceral fat mass),^[2] less Streptococcus mutans (tooth decay bacteria).^[3]
Processed meat: more Latilactobacillus sakei (found in sausage)^[4] and fewer Clostridium species.

How might gut microbiota reflect dietary meat consumption? Original paper