Effects of Protein Type and Mineral Form on Postprandial Amino Acid Availability in Healthy Young Adults: a Randomized Cross-over Study

All living tissues are in a constant state of protein turnover, with synthesis and breakdown processes ensuring structural integrity, metabolic flexibility and the capacity for repair. Amino acids are the building blocks of proteins and must be supplied through the diet to maintain a positive protein balance. Ingestion of high-quality protein sources (e.g. meat, dairy) leads to a postprandial increase in circulating amino acids, and as such can facilitate tissue protein synthesis. The magnitude and the kinetics of the amino acid response largely depend on the amino acid composition combined with protein digestion and amino acid absorption rate.

Beyond their structural role, amino acids also function as signalling molecules involved in gastrointestinal hormone secretion and appetite control. One such key hormone is glucagon-like peptide 1 (GLP-1), a gut-derived hormone that plays an essential role in metabolic regulation such as insulin secretion and appetite suppression. Given its role in metabolic health, particularly in the context of obesity and type 2 diabetes, increasing GLP-1 availability has become a target for support in metabolic care. However, many pharmacological approaches to elevate GLP-1 are costly and carry risks of harmful side-effects.

Nutritional strategies that modulate GLP-1 secretion and availability therefore offer an attractive alternative. Emerging evidence suggests that not only protein ingestion itself, but also the matrix in which the protein is delivered can modulate GLP-1 secretion. For example, co- ingestion of specific minerals, especially calcium, has been shown to enhance GLP-1 secretion beyond the effects of protein ingestion alone. These data indicate that mineral-protein interactions may play a meaningful role in appetite regulation and metabolic signalling, warranting further investigation.

Bovine milk contains two primary high-quality protein fractions: whey (~20%) and casein (~80 %). Whey protein is rapidly digested, resulting in fast but transient increases in circulating amino acids, whereas casein protein is digested more slowly, leading to more prolonged but sustained amino acid availability (13-15). To optimize the functional properties of casein, various processing techniques can be applied, including the formation of caseinate salts through binding casein with different minerals such as calcium, sodium, magnesium, and potassium. These caseinate forms differ in mineral composition, which may influence not only digestion and absorption kinetics, but also GLP-1 secretion capacity. Although the differences in digestion kinetics between whey and casein are well-established, a direct comparison of postprandial amino acid and GLP-1 responses following ingestion of whey versus various mineral-bound caseinates has not yet been performed.