Synthesis and absorption of intestinal microbial lysine in humans and non-ruminant animals and impact on human estimated average requirement of dietary lysine.

PURPOSE OF REVIEW: While there are reports on the nature of synthesis and absorption of intestinal microbial lysine in humans and non-ruminant animals, there are few efforts to quantify microbial amino acid absorption in human subjects. We review the available information on the synthesis of microbial lysine and the quantification of its absorption and utilization by the human host and monogastric model animals. In addition, we explore the impact of microbial lysine on the current estimated average requirement of dietary lysine. RECENT
FINDINGS: It is still uncertain whether microbial amino acids are absorbed primarily from the small or the large intestine in humans. In the pig, the majority of microbial lysine is absorbed in the small intestine. It appears that microbial lysine contribution is responsive to the nutritional status of the host. Estimates for microbial lysine contribution in adult humans on adequate or low protein diets range from 12 to 68 mg/kg per day. It is unlikely that these estimates represent net values because of methodological concerns related to the 15N tracer methodology used.
SUMMARY: We conclude that microbial lysine contributes to the lysine homeostasis in humans and other non-ruminant mammals. Microbial lysine utilization by the host is a continuous process and occurs both with low, adequate, and high protein intakes, and under protein-free and low lysine dietary conditions in growing and adult individuals. We also conclude that the estimated average lysine requirement for humans already considers lysine contributed by the intestinal microbiota.