Stomach
The muscular pouch of the digestive tract that stores, churns and acidifies food, and whose acidity governs protein digestion and the absorption of several micronutrients.
The stomach has three functions: it stores the meal and releases it gradually, it churns the food mechanically, and it acidifies it with hydrochloric acid secreted by its parietal cells.
Its pH falls to between 1.5 and 3.5, an acidity comparable to concentrated vinegar. It plays three roles: activating pepsin, which breaks down protein, destroying a large proportion of ingested micro-organisms, and releasing iron and vitamin B12 from their food-bound forms.
This is why insufficient acidity has nutritional consequences. The point is counter-intuitive: stomach problems are spontaneously associated with “too much acid”, whereas too little acid (due to age, gastritis or prolonged treatment with proton pump inhibitors) interferes with the absorption of iron, B12, calcium and magnesium.
Gastric emptying varies greatly with the composition of the meal: quick for liquids and simple carbohydrate, markedly slower in the presence of fat and protein. This is one of the mechanisms of satiety, and it also explains why a fatty meal “sits” on the stomach for longer.