Cutting-Edge Application | Relaxation Rates for Protein Digestion Research: A New Perspective on Personalized Nutrition in Functional Foods

Background

With the rise of functional foods, food scientists and nutritionists have developed a range of products with specific health benefits, such as low‑GI foods for blood sugar control, or products for weight management and skin improvement. Among these, personalized nutrition—developing foods that optimize nutrient absorption according to individual needs—has become particularly important. This requires a deep understanding of protein behavior in the human digestive system, including its digestion in the stomach.

Simulated gastric fluid (SGF) is commonly used to mimic gastric digestion in vitro. Pharmacopoeia recommends preparing SGF by mixing 16.4 mL of dilute hydrochloric acid with approximately 800 mL of water and 10 g of pepsin, then diluting to 1000 mL with water [1]. In food and nutrition research, however, SGF formulations are required to be more physiologically relevant—for example, using biomimetic digestive systems or more complex SGF compositions that include gastric mucus or food matrices, in addition to HCl and pepsin, to more accurately simulate the gastric environment [2].

Meanwhile, in some digestion studies—such as the addition of exogenous substances to proteins [3] or the construction of composite hydrogel encapsulation systems [4]—low‑field NMR has been used to interpret the different relaxation characteristics of various water types (e.g., free water, weakly bound water, and bound water) in food matrices, which is a classic approach for studying water distribution and migration in foods.

Novel Application of Relaxation Rates in Protein Digestion

Similar to the principle of relaxation rates in contrast agents, the relaxation rate of SGF is influenced by the chemical environment and physical dynamics of water protons in the sample, primarily due to protein‑water interactions and the effect of pH on water proton dynamics. By measuring relaxation rates (R₁ and R₂) and establishing correlations with protein concentration and hydrogen ion concentration (reflected by pH), a mathematical model can be developed to predict the extent of protein digestion under different pH conditions (gastric acid environments).

Experimental Procedure

1. Sample preparation: Two concentrations of whey protein isolate (WPI) gels—15 wt% and 20 wt%—were prepared and placed in standard SGF (pH = 5, 37 °C).
2. Data acquisition: Measurements were taken at selected time points (t = 0 min before digestion, every 5 min within the first 30 min, and every 30 min from 30 min to 120 min). The pH of each sample was measured using a pH meter, and protein concentration was determined using a BCA protein assay kit. For relaxation rate measurements, the CPMG sequence was used to measure T₂ relaxation time, and the CWFP sequence [5] was used to measure T₁ relaxation time, yielding R₂ = 1/T₂ and R₁ = 1/T₁.

Experimental Results

Linear regression analysis was performed using statistical software to establish empirical equations relating R₁ and R₂ to protein concentration and pH (hydrogen ion concentration [H⁺]):

R₂ = 0.46 + 0.05 · c_protein − 1.31 · c_protein · [H⁺] (R² = 0.99)

R₁ = 0.41 + 0.006 · c_protein − 0.02 · c_protein · [H⁺] (R² = 0.96)

Summary

This article describes a novel approach for developing functional foods targeted at individuals with digestive disorders (e.g., hypo‑ or hyperchlorhydria). By using empirical equations relating relaxation rates, protein concentration, and pH, personalized dosage or formulation customization can be achieved, enabling precision nutrition.

References

[1] Pharmacopoeia of the People‘s Republic of China, 2020 Edition, Part IV, General Chapter 0921, “Disintegration Test,” Notes.

[2] Deng, R., Seimys, A., Mars, M., Janssen, A. E. M., & Smeets, P. A. M. (2022). Monitoring pH and whey protein digestion by TD‑NMR and MRI in a novel semi‑dynamic in vitro gastric simulator (MR‑GAS). Food Hydrocolloids, 125, 107393.

[3] Liu, X. Y., Ye, Y. H., Bai, W. D., et al. (2024). Effects of exogenous substances on the conformation and in vitro digestibility of low‑salt tilapia surimi proteins. Science and Technology of Food Industry, 45(1), 80‑87.

[4] Liu, Z. Q., Chen, J. L., Ren, G. Y., et al. (2023). Construction and mechanism of gelatin/sodium hexametaphosphate/transglutaminase composite hydrogel encapsulation system. Food Science, 44(16), 81‑90.

[5] Pereira, F. M. V., Pflanzer, S. B., Gomig, T., Gomes, C. L., de Felício, P. E., & Colnago, L. A. (2013). Fast determination of beef quality parameters with time‑domain nuclear magnetic resonance spectroscopy and chemometrics. Talanta, 108, 88‑91.