The absorption of food is dependent upon its chemical structure, the rate of its transit along the gastrointestinal tract, the intestinal absorption surface and receptors and the interaction between the nutrient components of the meal.
• Cooking may modify the rate of nutrient absorption.
The uptake of food is dependent upon the total intake and bioavailability, i.e. absorption. No food is of value to the individual until it has been absorbed.
An important function of the intestine is to regulate the intake of minerals which are essential though poisonous in excess. Some essential nutrients are very labile, and consequently there is the potential for nutritional deficiency.
The availability of foods for absorption from the lumen of the intestine (Figure 10.9) is dependent upon a number of contributing factors, mucosal absorption, pathological factors altering intestinal absorption and alterations in luminal availability, e.g. interaction between accompanying nutrients within a meal. Other factors which may affect absorption include deficiency of necessary intestinal secretions and enteric bacteria, medicinal drugs and surgical procedures. Partial gastrectomies or gastroenterostomies will decrease gastric emptying time and accelerate absorption, endocrine response and cause hypoglycaemia, i.e. dumping.
The uptake by the mucosa of various nutrients may depend on the individual’s nutritional needs whereby intestinal absorption may be either partially or totally controlled.
1. In the gastric mucosa and small intestine the epithelial cells of the small intestine have three major functions: digestion, absorption and secretion. This process is under the control of hormones intestinal nervous network and the stomach and intestinal contents.
2. The small intestinal enterocytes are constantly being lost into the lumen from the tips of the villi and are replaced every 48 hours.
3. Absorption from the lumen involves movement from the bulk phase across the ‘unstirred layer’ to the enterocyte surface, movement across the brush border membrane, movement across the cytoplasm, and removal to the blood stream through the basolateral membrane.
4. Water-soluble nutrients of low molecular weight pass through the intestinal epithelium by two major routes, paracellular and transcellular. Paracellular transport is through highly permeable tight junctions joining the brush border membranes, the rate of transport depends upon concentration gradient and solvent drag. The transcellular route is across the brush border membrane of the absorptive cells, followed by crossing the cytosol of the cell interior by diffusion and exit through basolateral membranes by facilitated diffusion.
5. There are two main methods of transport across the cell membrane: (i) simple and facilitated diffusion which are passive transport systems; and (ii) mediated transport, which is specific for each structurally related substance, e.g. amino acid across the membrane.
6. The availability of foods for absorption from the intestinal lumen depends on a number of contributing factors, mucosal absorption, pathological factors altering intestinal absorption and alterations in luminal availability, e.g. interaction between accompanying nutrients within a meal. Deficiency of necessary intestinal secretions and enteric bacteria, medicinal drugs and surgical procedures may also affect absorption.
7. The colon is a conserving organ filled by a large bacterial mass.
8. Movement along and absorption from the gastrointestinal tract is dictated in part by local hormones.