Very low ileal nitrogen and amino acid digestibility of zein compared to whey protein isolate in healthy volunteers

Abstract

Background Whey protein and zein are of nutritional interest due to their high leucine content, but little data are available on their amino acid (AA) ileal digestibility. Objective This study aimed to determine ileal digestibility of whey protein isolate (WPI) and zein in healthy volunteers by use of the naso-ileal intubation method, which allows continuous collection of postprandial ileal digesta. Methods Twenty-two healthy volunteers were intubated with a naso-ileal sampling device positioned at the terminal ileum level. They received a single meal of protein-free biscuits and a drink containing zein (n = 8), WPI (n = 7), or no protein (protein free, n = 7). Ileal effluents and plasma samples were collected over a 9-h postprandial period. Total nitrogen and AA contents were quantified in effluents. True ileal digestibility was calculated after correction for endogenous losses evaluated in the protein-free group. Results True ileal nitrogen digestibility of zein was markedly lower than WPI (60.2 ± 4.5% and 91.2 ± 2.6%, respectively, P = 0.0003). True ileal digestibility of AAs ranged from 87.4 ± 2.7% for threonine to 98.4 ± 1.0% for methionine in the WPI group, and from 59.3 ± 5.6% for methionine to 69.0 ± 5.8% for arginine in the zein group. The digestible indispensable AA (IAA) score was 1.03 (histidine) for WPI and close to 0 for zein, owing to its negligible lysine content. Plasma IAA concentration significantly increased after WPI intake (P = 0.0319), whereas no effect of zein on aminoacidemia was observed, including plasma leucine, despite its high leucine content. Conclusions Our findings provide data on ileal digestibility of WPI and zein AAs in healthy humans and, in contrast to WPI, zein is poorly digestible. This study was registered at clinicaltrials.gov as NCT03279211.

Determinants of amino acid bioavailability from ingested protein in relation to gut health

“…It appears that for spirulina, which has a digestibility 12–15% lower than that of the other two proteins, the data dispersion is dramatically expanded. We observed the same in humans, for instance, concerning zein digestibility, which was the lowest we observed among our intestinal tube studies [ 8 ▪ ]. The SD was 13% (for a mean of 60%), with the lowest individual value being 39% and the highest 82%.…”

“…Measuring ileal digestibility is complex and requires invasive procedures, depending on the model. In humans, intestinal tubing allows for the continuous collection of effluents and adequate measurements of ileal amino acid losses [ 8 ▪ ]. However, this method is very invasive and ileostomized patients have therefore been used as convenient alternative due to the direct access to digesta.…”

“…However, the amino acid digestibility of whole eggs can be severely decreased, down to 75%, when ingested with black tea [ 18 ], a result that can be ascribed to the polyphenols. Using ileal tubes to measure the true ileal digestibility (endogenous protein being evaluated in a protein-free group), mean amino acid bioavailability from whey was close to that of egg and meat, whereas that of zein was very low [ 8 ▪ ], a phenomenon that can be partly ascribed to the isolate's low solubility. Using the IAAO method, Rafii et al [ 19 ] determined the metabolic availability of the limiting amino acids in corn meal, lysine and tryptophan, and found higher values for these individual amino acids than those found for zein by Calvez et al [ 8 ▪ ].…”

“…Using ileal tubes to measure the true ileal digestibility (endogenous protein being evaluated in a protein-free group), mean amino acid bioavailability from whey was close to that of egg and meat, whereas that of zein was very low [ 8 ▪ ], a phenomenon that can be partly ascribed to the isolate's low solubility. Using the IAAO method, Rafii et al [ 19 ] determined the metabolic availability of the limiting amino acids in corn meal, lysine and tryptophan, and found higher values for these individual amino acids than those found for zein by Calvez et al [ 8 ▪ ]. It must be pointed out that in the latter study, the digestibility of lysine and tryptophan could not be determined because zein was almost completely deficient in both amino acids.…”

Combining Plant Proteins to Achieve Amino Acid Profiles Adapted to Various Nutritional Objectives—An Exploratory Analysis Using Linear Programming

“…or have a protein structure resistant to hydrolysis. However, with some exceptions ( 35 ), real ileal digestibility studies in humans have reported high digestibility estimates for plant protein isolates (89–92%) that are comparable to those of meat or egg proteins (90–94%) ( 3 ). Indeed, isolate extraction processes can reduce or suppress the activity of antinutritional factors ( 36 – 38 ) and preparation processes (such as heating) can often markedly improve protein resistance to hydrolysis via protein denaturation ( 29 ).…”