Reference
First-Pass Splanchnic Extraction
**First-pass splanchnic extraction** is the fraction of dietary amino acids retained and used by the gut and liver during their first pass through the body, before the remainder reaches systemic circulation and becomes available to skeletal muscle.
How it works
When protein is digested, amino acids are absorbed across the intestinal wall and travel through the portal vein directly to the liver. The tissues along this route — collectively the splanchnic bed — are metabolically active and consume a meaningful share of those amino acids for their own purposes: maintaining the rapidly renewing gut lining, synthesizing digestive enzymes, producing plasma proteins, and supporting hepatic metabolism. Whatever the splanchnic tissues extract never reaches the bloodstream, so it cannot contribute to muscle protein synthesis.
This is why the amount of protein that appears in peripheral circulation after a meal is always lower than the amount swallowed. The difference is not waste so much as a competing claim: the gut and liver are served first.
Why it matters with age
Splanchnic extraction tends to rise with age. Older adults retain a larger proportion of ingested amino acids in the gut and liver, leaving less to reach muscle from the same meal. This is one of the mechanisms thought to contribute to anabolic resistance — the blunted muscle-building response to protein seen in older adults — and it is part of the rationale for recommending higher per-meal protein doses later in life.
Because a smaller fraction of each meal arrives at the muscle, the practical response is to raise both the dose and the quality of protein consumed, so that the amount surviving first-pass extraction still crosses the leucine threshold needed to trigger synthesis.
Relevance to potato protein
The amino acids that survive first-pass extraction matter more than the total ingested, which makes protein quality and digestibility central. Potato protein isolate is a single-ingredient protein with a high digestibility profile; the Digestible Indispensable Amino Acid Score for potato protein isolates has been reported as high as 100 (Food Science & Nutrition, Herreman et al., 2020, PMID:33133540). A 2020 study found potato protein isolate stimulated muscle protein synthesis at rest and with resistance exercise in young women (Nutrients, 2020, PMID:32349353).
Leucine content is the other lever. When a plant protein blend supplied roughly half the leucine of an equivalent whey dose, adding free leucine to match brought its muscle protein synthesis response in line with whey (J Nutr, 2024). A higher absolute leucine intake helps offset what the splanchnic bed retains.
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