Measurement of protein digestibility in humans : Revisiting basic nutrition science
24 Jul 2018
By Prof. Anura Kurpad, Margdarshi Fellow
St. John's Research Institute, Bengaluru
Proteins are important dietary components that contribute energy as well as amino acid building blocks and nitrogen for synthesis of biomolecules like DNA. Dietary proteins can be categorized based on their amino acid content. Of the 20 amino acids that we need, our body cannot synthesize nine; therefore, it is necessary that our diet include them. Dietary proteins containing all the nine ‘indispensable amino acids’ are called ‘complete proteins’; proteins lacking one or more of them are called ‘incomplete proteins’. Dietary proteins also differ based on how easily the body digests and absorbs them.
Characterization of dietary protein quality and digestibility is important for nutrition scientists to make protein intake recommendations tailored to age, health, and availability of various food items. A bottleneck is the inability to accurately measure the amino acid content of a food item that is actually utilized by the body. Our team devised a new method to assess protein utilization using stable (nonradioactive) isotopes.
We used spirulina, a well-studied blue-green algae and dietary supplement, labelled with the stable isotope of carbon (13C). It was fed in combination with amino acids labelled with the stable isotope of hydrogen (2H) to six healthy individuals. Additionally, we grew chickpea and mung bean using water labelled with 2H, and compared the digestibility and absorption of their proteins (in a curry) with that of labelled spirulina. On measuring the blood levels of the labelled amino acids that were absorbed from the digested proteins, we found that 85% of the spirulina protein was absorbed into the blood compared to approximately 57% for both chickpea and mung beans. We also examined the impact of food processing on protein absorption and found that dehulling the mung beans increased the relative absorption by approximately 10%. In further experiments, we fed laying hens with amino acids that were labelled with 2H to obtain labelled eggs and meat. The measurement of digestibility of the protein in these foods showed that about 89–92% of the protein was absorbed, making them high quality proteins.
Better characterization of amino acid availability of various food types can be achieved using the dual-tracer method. Our finding will be very relevant in regions of the world where high-quality protein foods are not commonly consumed. More specifically, the dual-tracer method could be instrumental in optimizing protein nutrition in growing young children and individuals suffering from chronic infections and illness.
Measurement of protein digestibility in humans by a dual-tracer method. Devi S, Varkey A, Sheshshayee MS, Preston T, Kurpad AV. American Journal of Clinical Nutrition. 2018; 107:984-991.
Ileal digestibility of intrinsically labeled hen's egg and meat protein determined with the dual stable isotope tracer method in Indian adults. Kashyap S, Shivakumar N, Varkey A, Duraisamy R, Thomas T, Preston T, Devi S, Kurpad AV. American Journal of Clinical Nutrition, In Press