Nutrition (3 page)
• peptidases act on any remaining peptides to release their constitutive amino acids
• lipase continues the digestion of triglycerides into free fatty acids and glycerol.
So far in its journey through your intestines, food has been mechanically chewed and churned, then attacked with chemicals and enzymes to form a porridge-like slurry. This slurry is teeming with valuable sources of energy (especially sugars and fatty acids), protein building blocks (amino acids) and micronutrients ready for absorption.
The inner lining of the jejunum and ileum is ideally suited for this task as it is covered in finger-like projections (villi), which are between 0.5 mm and 2 mm in length.
Compared with a flat surface, these villi increase the surface area over which absorption can occur by a factor of 30. The villi within the lining of the jejunum are longer than those within the ileum, as this is where most nutrients are absorbed.
Water-soluble nutrients, such as amino acids and sugars, are taken up by tiny blood capillaries within the villi and sent, via the hepatic portal vein, straight to the liver for processing: amino acids are reassembled to form proteins such as clotting factors, while excess glucose is converted into fats or glycogen for storage.
In contrast, fatty substances such as fatty acids and cholesterol have to be repackaged to make them soluble before they can enter the bloodstream. This repackaging occurs in the intestinal lining cells on the surface of the villi. Known as enterocytes, these cells wrap fatty acids and cholesterol with special proteins to form soluble particles (chylomicrons) that are released into small lymph vessels (lacteals) within the villi itself. Then, rather than going to the liver, they travel via the lymphatic system to be released into the subclavian vein under the collarbone. As these chylomicrons travel around the body, some are broken down by enzymes (lipase) found in the circulation to release fatty acids that can be used as a fuel by body cells (especially in your heart and skeletal muscle), or stored in fat cells (adipocytes). Those that reach the liver are further processed and repackaged – for example, excess dietary saturated fats are converted into cholesterol.
THE COMPLEXITY OF VITAMIN B12 UPTAKE
Before it can be absorbed in the last part of the ileum, vitamin B12 must be bound to a substance called gastric intrinsic factor, which is secreted by specialized cells in the stomach lining. As you get older, you tend to produce less intrinsic factor, so that less vitamin B12 is absorbed. This can lead to a B12 deficiency that is associated with a pernicious anaemia and a number of neurological symptoms, such as muscle weakness. Severe lack of B12 can also lead to irreversible damage to the spinal cord, and dementia.
Between them, the jejunum and ileum process 9 to 10 litres of fluid per day; 2 to 3 litres from your diet, and 7 to 8 litres of digestive juices. Absorption is so efficient, however, that only 1 to 2 litres of fluid are left to pass through the ileocaecal valve that marks the end of the small intestines and the start of the large bowel.
Large intestines
The large bowel acts as a waste-packaging unit for the body. Around 1 metre long, it mainly consists of the colon and rectum. At the start is a blind-ending pouch, into which the ileum protrudes to act like a one-way valve. An increase in pressure in the small intestine allows the valve to open, while increases in pressure in the large bowel squeezes it closed. The appendix, which branches off from the first part of the large bowel (caecum), was long considered an evolutionary remnant (vestigial organ), but new research suggests it does play a useful yet non-essential role. It acts as a reservoir of ‘friendly’ gut bacteria that can readily repopulate the bowel after a bout of food poisoning, or after a course of antibiotics (which can kill beneficial bacteria as well as those associated with infection).
The colon absorbs fluid, salts and minerals from bowel contents. It also contains a trillion digestive bacteria – more than the number of human cells in your body. These bacteria ferment dietary fibre and synthesize nutrients such as vitamin K, biotin and folic acid, which you can absorb and use. Of around 2 litres of bowel contents received into the colon each day, on average only 10 per cent is voided as semi-solid waste.
Intestinal contractions
Once you swallow a mouthful of food, you have no further voluntary control on how it progresses through your intestines.
Bowel contents are propelled forward by waves of muscular contractions known as peristalsis and, while involuntary (in that they occur without you thinking about them), they are beautifully coordinated by the action of special receptors and hormone signals that detect when food is present. These peristaltic waves pass through your duodenum and jejunum at a frequency of around twelve contractions per minute – about as often as you breathe in and out at rest. In the ileum, the wave slows to around nine times per minute as fluid is absorbed and the volume of intestinal contents decreases. Peristalsis within the colon occurs at between two and six contractions per minute.
Ring-like segmentation contractions also occur in the gut at regular intervals, which move intestinal contents (chyme) to and fro so that as many nutrients as possible come into contact with the bowel wall for absorption.
A third type of contraction only occurs in the colon – the so-called mass action contraction. This constriction causes a large area of the colon to push the relatively dry waste material that remains after absorption of nutrients and fluid into the rectum. Rectal filling and distension then trigger the desire to open your bowels (defaecation reflex).
Normally, your absorptive processes are so efficient that most of the ‘waste’ reaching your colon consists of undigested dietary fibre. This is fermented by bowel bacteria to produce some useful nutrients, such as short-chain fatty acids (which you can absorb and use as a fuel). In fact, over half the weight of your stools consists of these bacteria!
CHOLECYSTOKININ (CCK)
As food distends the stomach, a hormone called cholecystokinin (CCK) is secreted into the duodenum. This causes the colon to contract and often brings on a strong desire to open the bowels (gastrocolic reflex). This is why babies often fill their nappies as they feed. CCK also plays a role in appetite control, as discussed later.
The liver
Your liver deserves a special mention as, like a major warehouse, it plays a vital role in processing and distributing the nutrients you absorb from your food. The liver is located in the upper part of your abdomen, just below the diaphragm, with the main bulk on the right-hand side.
The liver is unique in that it receives two separate blood supplies. Water-soluble nutrients absorbed from your intestines into your circulation pass directly to the liver within the hepatic portal vein. Dietary fats take a more convoluted route. First, they are made soluble within the lining cells of your small intestine, then absorbed via the lymphatic system as described earlier. Those that are not immediately taken up by muscle or fat cells then reach the liver within the hepatic artery. Once in the liver, blood from the hepatic portal vein and hepatic artery mingle within relatively large spaces (sinusoids) from which the liver cells extract the oxygen and nutrients.
The highly specialized cells where the main work of the liver is carried out are called hepatocytes. As well as secreting bile (which aids the digestion of fat in the duodenum), liver cells have many other important nutritional roles, such as:
• making proteins from amino acids
• converting ammonia, a waste product of amino-acid metabolism, into urea
• making glucose from glycerol, lactic acid and certain amino acids (e.g. alanine)
• storing excess glucose as glycogen – a starchy emergency fuel
• processing fatty acids to make triglycerides and cholesterol
• storing fat-soluble vitamins (A, D, E and K plus vitamin B12) and some minerals (e.g. iron and copper)
• filtering out foreign proteins carried from the intestines to reduce their impact on the immune system and allergies
• removing and detoxifying dietary poisons (e.g. alcohol)
• generating heat to warm passing blood and help regulate your metabolic rate.
So far, we have considered the perfect scenario in which foods are digested and absorbed when the gut is in perfect working order. However, there are times when food processing doesn’t go so smoothly. If bowel contents pass through the intestinal tract too rapidly (for example, due to food poisoning, or excessive stress), nutrient and water absorption is incomplete resulting in diarrhoea. If this is prolonged, it can lead to dehydration, lack of energy and salt imbalances that can be life-threatening. Worldwide, diarrhoeal disease is the second leading cause of death in children under the age of five (after pneumonia). At the other end of the scale (for example, due to lack of dietary fibre, or the effects of morphine-related painkillers), bowel contents may pass through too slowly, so that excessive water is absorbed and wastes become dry and difficult to pass. Constipation can have a profound effect on your quality of life, and is a contributory cause of volvulus in which the bowel twists round on itself, provoking a surgical emergency.
Malabsorption can also develop, in which certain nutrients are not absorbed properly. Examples include lactose intolerance, due to insufficient production of the enzyme lactase, needed to digest milk sugar (lactose); coeliac disease, in which a type of gluten protein found in wheat causes an immune reaction that damages the lower jejunum; and an inability to digest and absorb dietary fats (which can have a number of causes such as lack of bile or pancreatic enzymes, to inflammatory bowel disease). Similarly, an inability to absorb vitamin B12 in the ileum can be due to lack of intrinsic factor (produced in the stomach to assist B12 absorption), or disease of the ileum itself.
Assuming digestion and absorption go as planned, however, the body receives a plentiful supply of both macronutrients (protein, carbohydrates, fat) and micronutrients (vitamins, minerals and trace elements) to meet its needs. The next chapter looks at macronutrients and what we do with them.
Just as you have to fill your car with petrol, diesel or, increasingly, plug it into an electricity supply, you need food to fuel your energy requirements. Energy is the dynamic force that underpins all your biological processes from growth, metabolism and reproduction to physical and mental exertion. Even digestion and absorption themselves require energy for chewing, secreting digestive enzymes, propelling food through the gut and transporting nutrients around the body. The extra workload of the liver also generates heat, and it is estimated that at least 10 per cent of the total energy yield of a meal is used up during its processing.
The energy in food is provided by three broad categories of chemicals – proteins, carbohydrates and fats. They have other roles too, of course. Proteins also act as building blocks for making enzymes and body tissues, while fats are an important component of cell membranes and are used to make hormones as well as regulating inflammation in the body. Even carbohydrates, which used to be thought of in terms of pure energy, are now known to be involved in the biological signalling that tells cells what to do.
Dietary proteins
Proteins form the basic structural units of your body. But as we have seen, the human body cannot absorb them in their complete form. First they must be digested down into their basic building blocks before absorption can take place. These building blocks are then used by your cells to make all the different human proteins in your body including antibodies, blood-clotting factors, cell receptors, transport proteins, lean muscle tissue and the collagen and elastin in your skin, cartilage and ligaments.
The building blocks within dietary proteins are called amino acids. These are linked together to form different chain lengths: chains containing between two and ten amino acids are known as peptides; those containing ten to 100 amino acids are called polypeptides; chains of over 100 amino acids, which fold into complex three-dimensional shapes, are known as proteins.
Protein digestion starts in your stomach when the enzyme pepsin cleaves the links in proteins and polypeptides to produce short peptide chains. Once in your small intestine, these are further attacked by enzymes released from the intestinal wall and pancreas. Some chains of double and triple amino acids (dipeptides and tripeptides) are absorbed into your gut lining cells (enterocytes), where they are broken apart to release single amino acids into the bloodstream.
Twenty-one amino acids are important for human health. Twelve of these can be synthesized from other building blocks within your own cells, but the remaining nine must come from your diet and are known as the nutritionally essential amino acids. These are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
On average, you need to obtain around 1 gram of dietary protein per day for each kilogram of your body weight. Someone weighing 70 kg, therefore, needs to obtain roughly 70 g of protein per day from their diet. This represents about 15 per cent of daily energy intake.
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