Incorporation of urea nitrogen into fecal protein and plasma protein aminoacids in elderly human volunteers after ingestion of lactic acid bacteria

Health effects of fermented milks have been associated with the metabolic a ctivity of lactic acid bacteria in the gastrointestinal tract. It has been proposed that an increased excretion of urea nitrogen via microbial protein may decrease the workload on kidneys and liver. Therefore, a study was car ried out in healthy elderly human subjects to investigate the incorporation of [N-15(2)]urea nitrogen into plasma and fecal proteins and amino acids. Over a period of 10 d 13 healthy elderly subjects ingested daily a freeze-d ried microbial preparation which contained different genera of lactic acid bacteria and is used to produce fermented milk products. One of the strains was originally isolated from stool samples of elderly people from the Cauc asus region (Lactobacillus plantarum). No stimulation of fecal protein-nitr ogen excretion and no increase in N-15-abundances in fecal protein was meas ured following the administration of the viable microbial preparation and a [N-15(2)]urea bolus. Tentatively, it was concluded that this may have been caused by the inability of the microbial culture to survive the gastro-int estinal passage and (or) by the absence of additional fermentable carbohydr ates in the diet as energy source for bacterial protein synthesis in the la rge intestine. However, using a highly sensitive GC-C-IRMS method we observ ed a significant incorporation of N-15 into plasma protein amino acids. N-1 5-Enrichments in single amino acids were found according to their participa tion in transamination reactions. The slight enrichment of lysine which is not transaminated in mammalian tissues may indicate a microbial synthesis a nd absorption of bacterial lysine.