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A meal presents the digestive system with proteins held within food structures and surrounded by other nutrients.
A nutrition label can tell us the grams of protein in a serving. To understand the journey toward muscle, we need to look further. What amino acids does that protein supply? How accessible is it to digestion? What accompanies it? And how does the body respond to the resulting supply?
These questions place the whole meal at the beginning of our investigation.
Protein is a collection of amino acids
Dietary proteins consist of amino acids joined in particular sequences. Digestion breaks these proteins into smaller components, allowing the body to absorb and use their amino acids.
The body does not transfer a bean protein or an egg protein intact into a muscle fiber. It uses the available amino acids to assemble its own proteins according to cellular instructions.
Nine amino acids are considered essential for adults because the body cannot manufacture enough of them to meet its needs. They must come from the diet. Other amino acids can be synthesized by the body, although requirements and production capacity can change under particular conditions.
For muscle protein synthesis, the supply must provide the amino acids needed to complete the proteins being assembled. An abundance of one amino acid cannot replace a shortage of another.
Leucine will receive particular attention later because of its role in nutrient signaling. It is also a building material. Both roles matter, and neither makes the other essential amino acids dispensable.
Protein sources differ
Proteins differ in amino acid composition. Their physical surroundings also differ: protein held within an intact plant cell and protein extracted into a powder present different starting conditions for digestion.
Here we establish what a meal contains and how its components are organized. Chapter 12 will use these ideas to compare protein choices and portions.
Protein arrives within a food structure
The term food matrix describes the physical and chemical setting in which nutrients occur.
In an intact plant food, protein exists within a structure that also includes cell walls and other components. In yogurt, protein is part of a fermented dairy food. In an egg, proteins occur alongside—or separately from—the fats and other nutrients of the yolk, depending on what is eaten.
Preparation can change this setting. Cooking, grinding, fermenting, and extracting protein alter the food presented to the digestive system.
The surrounding meal adds another level. A protein source may be consumed alone in a laboratory drink or eaten with vegetables, grains, fats, and other foods at the table. Those are distinct conditions that deserve direct investigation.
This is a central question for EpiNutrition: how does the composition and structure of a meal influence the handling and use of its nutrients?
Following food structure through the body
Food structure gives us a biological question to investigate: does changing the setting around a protein alter its digestion or its use?
Human studies have examined casein in different matrices and compared whole eggs with egg whites. [1,2] Chapters 4 and 5 explore these experiments in detail as we follow absorption and the influence of the surrounding meal.
From the plate to the next stage
The protein content of a meal is valuable information. Its biological meaning becomes clearer when we also understand the amino acids supplied, the food structure, the preparation, and the context in which it is eaten.
Our task is to follow those features through the journey. We will ask which differences alter digestion, which change amino acid availability, and which influence incorporation into muscle protein.
The next stage begins when the meal enters the digestive tract.
Before its amino acids can be used, the proteins must be taken apart.
References
1. Churchward-Venne TA, Snijders T, Linkens AMA, et al. Ingestion of casein in a milk matrix modulates dietary protein digestion and absorption kinetics but does not modulate postprandial muscle protein synthesis in older men. Journal of Nutrition. 2015;145:1438–1445.
2. van Vliet S, Shy EL, Abou Sawan S, et al. Consumption of whole eggs promotes greater stimulation of postexercise muscle protein synthesis than consumption of isonitrogenous amounts of egg whites in young men. American Journal of Clinical Nutrition. 2017;106:1401–1412.