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The anatomy and physiology of ageing 3 - The digestive system

This third article in our series on ageing explores the major anatomical and physiological changes that occur within the digestive system.

Authors
Yamni Nigam, PhD, MSc, BSc; John Knight, PhD, BSc
; both lecturers in biomedical science, School of Health Science, Swansea University.
Nigam, Y., Knight, J. (2008) Exploring the anatomy and physiology of ageing. Part 3 - The digestive system. Nursing Times; 104: 33: 22-23.

The digestive system is complex, performing a range of functions, each of which is differentially affected by the ageing process (Fig 1). Its major role is to mechanically and chemically break down food into simple components that can be absorbed and assimilated. Subsequently the gut and accessory organs play an important role in the elimination of waste, not just indigestible food components but also bile pigments, toxins and excess salts. For further details on the physiology of the gastrointestinal tract and its accessory organs, see Montague et al (2005).

It is well established that food intake diminishes with age (Redfern and Ross, 1999). For each decade after the age of 50, the calorific requirement drops by around 10% (reflecting a general slowing of the metabolism and a gradual loss of lean muscle mass). However, basic nutritional requirements do not change with age.

Effects on smell and taste

The number of fibres in the olfactory bulb and olfactory receptors decrease noticeably with age (Boyce and Shone, 2006), reducing the sense of smell. Olfactory loss may also be caused by age-related bone growth in the skull pinching off sensory nerve fibres. Such changes may account for age-related loss in ability to discriminate between smells.

In the mouth, regional deficits in taste occur widely in older people. However, what is perceived as a taste defect (gustatory dysfunction) is often a primary defect in olfaction, although some studies suggest that normal ageing produces taste loss due to changes in the taste cell membranes.

Since food is chosen on the basis of smell and taste, the reduction in these senses can lead to appetite suppression, resulting in weight loss and malnutrition. Increased consumption of salt and sugar-rich foods to compensate for their lack of taste may aggravate health conditions such as hypertension and diabetes.

The mouth and oesophagus

The lips, tongue, salivary glands and teeth play a major role in the ingestion and mastication (chewing) of food. With age, shrinkage of the maxillary and mandibular bones, and reduced calcium content of the skeleton, causes a slow erosion of tooth sockets, leading to gum recession and eventual tooth loss. Lack of teeth or poorly fitting dentures can make eating and chewing extremely difficult, which may lead to malnutrition or to eating more highly refined, easy to chew foods that are lacking in dietary fibre. Reduced fibre intake (along with other factors mentioned below) will affect bowel function, causing problems such as constipation.

Dry mouth (xerostomia) is common among older people but while the number of acinar (saliva-producing) cells on the tongue decreases with age, there is contradictory evidence regarding whether saliva production actually decreases. Most researchers agree there is no age-associated fluid diminution from the main salivary glands (parotid), although the amount produced by the sub-mandibular salivary gland may reduce (Heft and Baum, 1984).

Having formed a bolus of food, the mouth prepares to swallow. This is a complex, coordinated, neurological and muscular interaction. The bolus reaches the posterior pharyngeal wall where the musculature contracts around it, passing the food through the upper oesophageal sphincter into the oesophagus.

The muscular contractions that initiate swallowing slow with age, increasing pharyngeal transit time, which may lead to dysphagia (swallowing difficulty). This increases the likelihood of choking and the sensation that food (or medication) is stuck in the throat.

The strength of oesophageal peristalsis also decreases and may no longer be triggered with each swallow. Both upper and lower oesophageal sphincters lose tension. Relaxation of the lower sphincter leads to ‘heartburn’ or gastro-oesophageal reflux.

The stomach

The stomach acts as a reservoir for food allowing the ingestion of large meals. With increasing age, it cannot accommodate as much, primarily due to decreased elasticity of the stomach wall. The stomach secretes gastric juice as a part of the digestive process; its lytic secretions, primarily pepsin and hydrochloric acid, remain constant through the ageing process. There is no recorded reduction in acid production in the healthy stomach, although disorders involving hyposecretion, such as chronic atrophic gastritis, are more common in older adults.

The capacity of the stomach lining to resist damage declines with age due to a weakened mucosal barrier. Gastric bicarbonate (HCO3-) and mucus normally provide an alkaline gel layer that defends against lumen acid and pepsin. The secretion of HCO3- declines significantly with age. In addition, the protective prostaglandin content in mucus declines, rendering older people more prone to gastro-mucosal injury (such as lesions and ulcers), especially following ingestion of NSAIDs (Lee and Feldman, 1997).

The small intestine

The main function of the small intestine is food digestion and absorption. It produces a range of enzymes and also uses secretions from the pancreas and liver. Following digestion, absorption of nutrients occurs in the terminal jejunum and ileum. To enable this, the lining of the small intestine is made up of microscopic folds (villi), increasing its surface area for maximal absorption. The villi shrink and broaden with age, significantly reducing the surface area.

Although amino acid absorption is not impaired, lipid absorption is reduced (Redfern and Ross, 1999). There is also evidence that lactase levels decrease, often leading to intolerance to dairy products. Certain bacteria residing in the small intestine overpopulate with age leading to bloating, pain and decreased absorption of nutrients such as calcium, folic acid and iron, which may affect health status.

The large intestine

With increasing age, peristalsis slows, increasing the transit time of waste in the large intestine, which can lead to constipation. It is believed that the mucosa and muscle layers of the colon atrophy leading to reduced and weaker peristalsis. The colon walls sag, prompting the formation of pouches (diverticuli). Straining to eliminate faeces may put additional pressure on weakening blood vessel walls, leading to haemorrhoids. There is also a drastic rise in the incidence of several gut pathologies including colon cancer (Montague et al, 2005), which may be partly due to the declining rate of cell division of the digestive epithelium which cannot repair and replace itself as needed.

Accessory organs

The pancreas generates four major digestive enzymes. With age, its weight decreases and some tissue undergoes fibrosis. There is evidence that its exocrine function is impaired with age, reducing the secretion of chymotrypsin and pancreatic lipase (Vellas et al, 1988) and adversely affecting the digestive ability of the small intestine.

The liver, which undertakes over 114 functions, shrinks with age, and blood flow to it decreases. This reduces its functional capacity. For example, the rate of protein synthesis and metabolism decreases, as does the liver’s ability to detoxify many substances.

Drugs are no longer inactivated quickly, and since they are more likely to cause dose-related side-effects, drug dosages must be carefully checked for older people.

The production and flow of bile (involved in fat emulsification) decreases with ageing. In addition, bile becomes thicker with higher cholesterol content. Gallstones are therefore more likely to form.

References

Boyce, J,M., Shone, G.R. (2006) Effects of ageing on smell and taste. Postgraduate Medical Journal; 82: 239-241.

Heft, M.W., Baum, B.J. (1984) Unstimulated and stimulated parotid salivary flow rate in individuals of different ages. Journal of Dental Research; 63: 10, 1182-1185.

Lee, M., Feldman, M. (1997) The ageing stomach: implications for NSAID gastropathy. Gut; 41: 425-426.

Montague, S.E. et al (2005) Physiology for Nursing Practice (3rd ed). Oxford: Bailli貥 Tindall.

Redfern, S.J., Ross, F.M. (1999) Nursing Older People (3rd ed). Edinburgh: Churchill Livingstone.

Vellas, B. et al (1988) Exocrine pancreatic secretion in the elderly. International Journal of Gastrointestinal Cancer; 3: 6, 497-502.

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