Effects of aging on the digestive system
Ronni Chernoff
Introduction
There is little change that can be attributed to age in gastrointestinal (GI) function in the absence of disease due to the large reserve capacity of this multiorgan system. The GI tract serves two major functions in the body, the first is the ingestion, digestion, absorption and excretion of nutrients and their byproducts; the functional reserve is greatest in the midgut, pancreas and liver. Intestinal segments may adapt, and functional reserve tends to buffer change so that only long-term observation may uncover abnormalities. This is less true for the proximal (esophagus) and distal (large colon) portions of the gut. The second major function of the gut is as an organ that filters and defends against pathogens.
The ingestion, digestion and absorption of nutrients are essential processes that are part of the maintenance of nutritional status. The function of the GI tract is, therefore, intricately involved in nutrition and a factor in an individual’s nutritional status. Because of the physiological changes that occur with advancing age, older people may have difficulties in meeting their nutritional requirements and fighting microorganisms. Common GI symptoms are often nonspecific and do not indicate the exact nature or severity of disease (Ravindran et al., 2014).
The physiological changes associated with advancing age include the loss of lean body mass and body protein compartments, a decrease in total body water, reduction in bone density and a proportional gain in total body fat. Loss of lean body mass and muscle integrity, as well as a decrease in vagal sensitivity, may impact peristalsis and the ability to move a bolus through the GI tract. In an older individual, adequate nutrition is an important factor for the maintenance of health and recovery from disease. It is important to note that many studies that describe changes in the GI tract with aging are conducted on subjects who have chronic conditions that may affect GI physiology or function (Ravindran et al., 2014).
Nutritional requirements in aging
Energy
The maintenance of health status and the provision of adequate nutrition in elderly people requires an understanding of the impact of age on nutritional requirements. The most well-documented change that occurs over time is the decrease in energy metabolism. This reduction in energy requirements is related to a decrease in total protein mass rather than a reduction in the metabolic activity of aging tissue.
Basal energy requirements reflect the energy needed for all of the metabolic processes that are involved in maintaining cell function and keeping the brain and vital organs functioning; the reduction of active metabolic mass will result in lowered energy needs.
Protein
Protein requirements in elderly individuals might be expected to decrease to accommodate a lower total lean body mass. However, studies appear to indicate that protein requirements may be slightly higher in older subjects. One explanation is that a lower calorie intake contributes to reduced retention of dietary nitrogen, therefore requiring more dietary protein to achieve nitrogen balance (Campbell et al., 2014). It has been reported that ingestion of 25–30 g of high-quality protein at each meal may help older adults maintain muscle mass, although longitudinal research is needed to assess this notion (Paddon-Jones & Rasmussen, 2009).
Protein needs are also affected by immobility, which contributes to negative nitrogen balance. Elderly people who are bed-bound, wheelchair-bound or otherwise immobilized will require higher levels of dietary protein to achieve nitrogen equilibrium. Surgery, sepsis, long-bone fractures and unusual losses, such as those that occur with burns or GI disease, increase the need for dietary protein.
Some clinicians have been wary of providing high levels of protein for fear of precipitating renal disease in elderly individuals. Research has shown that there is no evidence that dietary protein induces deterioration of renal function in individuals who have no pre-existing evidence of renal disease. For elderly patients who have a measurable decline in renal function, therapeutic regimens should be followed.
Fat
The major contribution of fat in the diet is energy, essential fatty acids and fat-soluble vitamins. Because only small amounts of fat are needed to provide essential fatty acids, and fat-soluble vitamins are available from other dietary sources, the primary contribution from dietary fat is the provision of calories. For older people, restricting dietary fat, thereby reducing caloric intake, is a reasonable strategy to maintain caloric balance without restricting intake of other nutrients; however, in some individuals, too rigid restrictions on dietary fat may contribute to energy deficits.
Altering the type and amount of dietary fat in the diet of older adults is somewhat controversial. As a controllable variable in the reduction of the risk of heart disease, there are major differences in opinion regarding the need for dietary fat alteration in adults over 65. One approach to lowering risk in older adults is to reduce the intake of saturated fat and simultaneously increase the intake of polyunsaturated and monounsaturated fat, keeping the total fat intake about the same. In one study cited by deLorgeril and Salen (2011) in their review, subjects who had previously suffered a myocardial infarction were put on a Mediterranean-style diet with more complex carbohydrate, fruit, green vegetables and fish with less beef, lamb, or pork, and monounsaturated cooking oils. The group on this type of diet had fewer cardiac events and deaths at 2-year follow-up than did a control group who made no dietary modifications. Even on this modified diet, there was no change in total cholesterol or low-density lipoprotein cholesterol levels. The Mediterranean diet is recommended routinely as secondary prevention of coronary heart disease.
Carbohydrates
Carbohydrate (CHO) intake in the diets of elderly people should be approximately 55–60% of the total caloric intake, with an emphasis on complex CHOs. The ability to metabolize CHO appears to decline with advancing age. Nevertheless, glucose is an efficient energy substrate that can be used by all body tissues but is necessary for energy production in brain and red blood cells (Kohlmeier, 2003; Keim et al., 2014).
It is important to encourage complex CHO intake in elderly people because foods in this group provide fiber, a constituent of the diet that enhances bowel motility, which tends to decrease over time. Bowel disorders that can be managed by a diet high in fiber include constipation and diverticular disease. Dietary fiber intake also decreases total cholesterol, LDL-cholesterol and triglycerides levels (Clemens et al., 2012). Fresh fruits and vegetables are difficult to chew if oral health status is not optimal or dentures do not fit properly, and these foods are expensive when they are out of season. Cereal fibers should be encouraged as an alternative; however, it is difficult to obtain adequate fiber from cereal foods alone.
Vitamins
Vitamin requirements for adults over 65 are mostly speculative, although there is much ongoing research. Vitamin deficiencies may exist subclinically in the elderly, particularly for some of the water-soluble vitamins. In times of stress, after illness or injury, a depleted reserve capacity may not be able to compensate for rapid depletion of tissue stores and the individual may become overtly deficient. Subclinical deficiencies may exist in people who have adequate but not excess dietary intake, because the absorption and utilization of these vitamins may be compromised by the use of multiple medications or single nutrient supplements or by the declining efficiency of the small bowel to absorb micronutrients.
The water-soluble vitamins that are often the focus of attention are vitamin C and vitamin B12. Although there appears to be no age-related alteration in vitamin C (ascorbic acid) absorption, deficiency in this vitamin is often linked with wound-healing problems or a tendency to bruise easily. Vitamin C is an essential factor needed to make collagen, the protein matrix that holds cells together, and is therefore required when new tissue is being made. The recommended daily allowance (RDA) for vitamin C is 60 mg/day, a level that is far exceeded in most American diets. With large doses of supplemental vitamin C, tissue saturation is reached rapidly and the excess vitamin is excreted in urine. Very large doses (greater than 1 g/day) may contribute to some serious side-effects, such as the formation of kidney stones or chronic diarrhea in sensitive individuals. There is little evidence that massive doses of vitamin C aid in wound healing, ward off the common cold or cure cancer.
Vitamin B12 is a vitamin for which many older adults may be at risk for deficiency. The major dietary source of vitamin B12 is red meat and organ meats, which many elderly people have eliminated from their diets because of the fat and cholesterol content. In addition to dietary inadequacy, some older adults have a condition called atrophic gastritis, in which gastric acid production is decreased. Gastric acid is necessary for the release of vitamin B12 from a series of protein carriers; it is then linked to an intrinsic factor that forms a complex with the vitamin, allowing it to be absorbed. Production of intrinsic factor is also decreased with atrophic gastritis. Symptoms of vitamin B12 deficiency are generally nonspecific but include irritability, lethargy and mild dementia.
It is less likely that elderly people will be deficient in fat-soluble vitamins (A, D, E, K) because of the ability to store these vitamins in liver tissue. The greatest risk is for deficiency of vitamin D, particularly for homebound or institutionalized elderly people. Limited exposure to sunlight, the use of sunscreens and an inadequate intake of dairy products contribute to this risk. It is also known that the amount of vitamin D precursor in skin, which is stimulated by sunlight, particularly ultraviolet rays, decreases with age. Dietary vitamin D goes through several conversions in the liver and kidney, resulting in production of the active form of the vitamin; the kidney becomes less efficient at the final step of conversion with advanced age. Because vitamin D is an important nutrient in bone mineralization and immune function, it is wise to encourage the inclusion of foods rich in vitamin D in the diets of elderly individuals who may be at risk of deficiency.
For vitamin A, the risk of vitamin toxicity is greater than the risk of deficiency. This is especially true of older people who are taking over-the-counter vitamin supplements, many of which have very high levels of vitamin A. Beta-carotene, a vitamin A precursor, has received a great deal of attention in recent years because of its apparent protective effect against various types of neoplasms. The long-term effects of high doses of beta-carotene have not been adequately explored.
Minerals
The requirements for most minerals do not change with age. An exception is iron, for which there is a decreased requirement because of a tendency to increase tissue iron stores with advancing age and a cessation of menstrual blood loss in women. Calcium requirements have attracted much attention in recent years. Investigators have suggested that the recommendations for dietary calcium intake increase from 800 mg/day to 1200 or 1500 mg/day to reduce the risk of osteoporosis. However, the controversy surrounding calcium requirements in older people has not yet been settled, with many investigators believing that the recommendations should not be changed.
For most other major minerals, such as sodium and potassium, requirements are not changed by the aging process but are affected by the presence of acute or chronic diseases and their treatment (medications).
Water
Water is an important nutrient for older people. Inadequate fluid intake may lead to rapid dehydration and precipitate associated problems: hypotension, elevated body temperature, constipation, nausea, vomiting, mucosal dryness, decreased urine output and mental confusion. It is particularly noteworthy that these problems are rarely attributed to fluid imbalances, which can be easily corrected.
Fluid intake should be adequate to compensate for normal losses (through kidneys, bowel, lungs and skin) and for unusual losses associated with increased body temperature, vomiting, diarrhea or hemorrhage. A reasonable estimate of fluid needs is approximately 1 ml of fluid/kcal ingested or 30 ml/kg actual body weight. The minimum intake for all older adults regardless of their size or caloric intake should be approximately 1500 ml/day. Fluid needs can be met with water, juices, beverages such as tea or coffee, gelatin desserts and other foods that are liquid at room temperature. Tube-feeding formulas contain approximately 750 ml water per liter of solution; it is wise to compensate for the solid displacement by adding 25% of the volume of the tube feeding as additional free water.
Meeting all of these changes is often challenging. Encouraging older adults to consume an adequate diet may be linked to a functional and healthy GI tract. Age does have an impact on GI structure and function and it is worth assessing GI function in older adults.
Age and the GI tract
The aging oral cavity
The changes associated with the aging process affect the structures of the mouth. Bone loss is a common problem and, in the oral cavity, where the alveolar bone is more prone to brittleness and fragility, there is an increased likelihood of tissue damage occurring because of oral trauma, periodontal disease and loss of teeth. Nutritional deficiencies are also manifested in periodontal and perioral tissue, which can impair chewing and normal ingestion of food.
As lean body mass decreases, gum tissue may be lost because of disease and atrophy. This process, along with bone resorption, leads to an increased risk of root caries, periodontal disease and loss of structure to support dentures. These changes, along with others in oral musculature and the mucous membranes, contribute to difficulty in chewing food adequately. Many individuals alter their dietary intake to compensate for their diminished efficiency in chewing, thereby putting themselves at risk for malnutrition. Malnutrition is associated with negative outcomes and adds an additional burden to the challenge of rehabilitation.
Other changes that may occur in the mouth and affect nutritional status include decreased taste and smell sensitivity, loss of taste and smell, and decreased salivary flow, which may be associated with disease conditions or the effects of medications. In chronically ill patients, the possibility that this condition may be present should be investigated. It is important to assess the ability of an individual to consume adequate nutrients to restore or maintain nutritional status through a period of rehabilitation.
The esophagus
The esophagus is the conduit that serves to transport food from the mouth to the stomach. Although it may not seem to be a very important part of the GI tract, esophageal dysfunction may have a profound impact on nutritional status and, therefore, on the recovery from an illness or other physiological problem.
The most common dysfunction of the esophagus is swallowing disorders (Robbins & Banaszynski, 2014). Swallowing problems may be characterized by pain, choking, spitting or vomiting. These symptoms are usually associated with an obstruction, cerebrovascular accident, neurological disease or degenerative muscular disease. Gastroesophageal reflux may be a secondary problem resulting from weakness in the lower esophageal sphincter, failure of peristalsis, or an injury or illness in the stomach (Katz et al., 2013).
Diagnosis and correction of esophageal problems are key to safe ingestion of food and liquids. Depending on the etiology and severity of the dysfunction, dietary modification may be the appropriate treatment. More severe problems require medical, pharmacological or surgical interventions. In either case, ensuring adequate nutritional intake is important to maintaining nutritional status.
The stomach
The stomach serves several functions in the digestive process: its mechanical action breaks up food; it digests food through chemical and enzymatic actions; and it serves as a reservoir to hold partially digested food until it can be released into the small intestine. There is no evidence that age has a significant effect on gastric function; however, age-related conditions and diseases may result in altered gastric function.
The gastric conditions most commonly seen in elderly individuals are atrophic gastritis, peptic ulcer disease and gastroesophageal reflux disease. Atrophic gastritis may contribute to a perception of food intolerance but, more importantly, it may be a major factor in vitamin B12 deficiency because gastric acid is required for the digestion process that allows this vitamin to be absorbed. Folic acid may also be malabsorbed with this condition (Johnson et al., 2010).
Peptic ulcer disease is increasing among the elderly, although the incidence in the general population appears to be declining (Wang & Peura, 2011). Medications, such as H2 (histamine) antagonists and antacids, may have multiple side-effects, which could lead to other problems, including constipation, obstruction, osteomalacia, diarrhea, dehydration and electrolyte disturbances.
Gastroesophageal reflux disease is usually associated with the incompetence of the lower esophageal sphincter. There is no evidence that this is an age-related condition but some older individuals do experience this condition.
The pancreas
There is no strong evidence that age affects the pancreas in any significant way; however, glucose intolerance seems to increase and insulin secretion tends to decrease with advanced age and there appears to be a reduction in secretory output (Scheen, 2005). This reduction is not considered clinically significant until pancreatic output is less than 10% of normal or these changes become symptomatic.
Diseases of the pancreas do commonly occur in older people. Acute pancreatitis occurs in older patients and may have severe consequences, resulting in sepsis and shock. An uncomplicated course may have a brief period of pain, nausea and vomiting, and tends to occur in individuals who have biliary tract disease. A more severe occurrence may result in abscesses, other septic symptoms or shock, and may require surgery and stress metabolic management.
Chronic primary inflammatory pancreatitis is a disease of older people. Symptoms include steatorrhea, diabetes, pancreatic calcification and weight loss. This is often a pain-free condition with an unpredictable response to therapy.
The aging liver
The liver tends to get smaller in mass with advancing age, which can lead to changes in structure and function. This may be important because many of the functions of the liver (synthesis, excretion and metabolism) are crucial for the maintenance of health. These functions are more affected by systemic disease and liver disease, both of which are common in elderly people.
The changes that occur which are important considerations in elderly people include alterations in drug metabolism and a decrease in the rate of protein synthesis. Both of these factors contribute to a diminished ability to respond appropriately to drug therapy, to adequately clear drugs through the liver or to tolerate the physiological burden associated with disease. In elderly people, who often are receiving multiple prescription medications, this clearance ability may be a major factor in drug-related symptoms (McLachlan & Pont, 2012)
The small bowel
The GI tract, beginning at the mouth and ending at the anus, is a large muscle that propels food and its digested products through the body. Food is ingested and almost immediately acted upon by digestive enzymes, chemicals and mechanical actions. Many of the critical digestion and absorption functions occur in the small bowel. Age and disease can have an impact on the normal function of the small bowel.
The most common disorder of CHO metabolism is the disaccharidase deficiency of lactase. Lactase deficiency occurs with age and with common GI diseases such as viral gastroenteritis, Crohn’s disease, bacterial infections and ulcerative colitis. Symptoms are associated with the ingestion of milk and milk products, and occur when the ingestion of lactose exceeds the production of lactase in the small bowel.
Another disorder with vague symptoms is celiac disease; this involves sensitivity to gluten, a protein commonly found in wheat products. It frequently results from an injury to the small bowel from exposure to gluten, which contributes to malabsorption and steatorrhea. The treatment is to eliminate gluten, found in wheat, rye or barley-based products, from the diet and replace it with products made from corn, rice or potato. Replacement of malabsorbed nutrients (iron, folic acid, calcium, vitamin D) should be part of the therapy.
Another source of malabsorption in older individuals is bacterial overgrowth. This may be associated with the decrease in gastric acid production by the stomach and the age-related decrease in bowel motility. Generalized malabsorption may result from this condition; vitamin B12 is a nutrient that is at risk of being malabsorbed.
Other conditions that may damage the small bowel and impair its ability to digest and absorb essential nutrients include radiation enteritis and inflammatory bowel diseases. Radiation enteritis is often a consequence of treatment for cancer of the cervix, uterus, prostate, bladder or colon. Because of their rapidly dividing characteristics, the cells in the small intestine are vulnerable to damage from radiation. Symptoms of diarrhea, nausea, cramping and distension often occur years after the period of therapy and may go unreported. Malabsorption and dehydration are potential nutritional consequences. Inflammatory bowel disease may occur, with its symptoms attributed to other conditions because it is less common in older adults.
Along with the digestive and absorptive bowel functions is a mucosal immune system that exists independently of the peripheral immune system and functions separately from the nutritional functions of the bowel mucosa. Age-related deterioration of immune function has been well recognized in older adults; the incidence of infection, autoimmune diseases and cancer is higher among older adults. Although immunosenescence in both host and cell-mediated systems is well described, mucosal immunity is less well understood (Spencer & Belkaid, 2012).
The large intestine
The primary function of the large intestine is the absorption of water, electrolytes, bile salts and short-chain fatty acids. The major conditions related to the large intestine that are experienced by older people are colon cancer, diverticulosis and constipation. If diagnosed early enough, colon cancer is treatable with surgery and radiation therapy. Diverticular disease may be asymptomatic in elderly patients until an infection occurs and the individual becomes symptomatic. Dietary treatment is the same for older patients as it is for younger patients.
Constipation is a common complaint among older adults. It may occur as a result of many conditions: neurological disease, drug effects, systemic disease, inadequate fluid intake, lack of dietary bulk and physical inactivity. However, the primary issue may be aging smooth muscle; there has been very little exploration of this physiological process and extensive research is needed. Treatment should be based on the etiology of the condition and include adequate hydration, dietary fiber and physical activity (Bitar & Patil, 2004; Toner & Claros, 2012; Cherniak, 2013).
Dietary management of malnutrition
As with other nutritional problems, the patient who is in rehabilitation should be encouraged to eat as much as possible. Underlying disease conditions should be treated first with nutritional adequacy encouraged as appropriate. Smaller, frequent meals may be accepted more readily by elderly patients with smaller appetites and early satiety. Oral liquid supplements can be added to solid food if fluid overload is not a contraindication. The goal of refeeding should be to provide 35 kcal/kg of the patient’s actual weight and at least 1 g of protein/kg. Experience has demonstrated that only 10% of elderly people who have protein energy malnutrition can consume adequate calories orally to correct their nutritional deficiencies; most patients therefore require more aggressive nutritional intervention, such as enteral or parenteral feeding.
Conclusion
The impact of aging on GI tract function happens slowly over time but will often contribute to nutritional challenges that may affect the ingestion, digestion and absorption of nutrients. In older individuals, the ability to maintain nutritional status will also be affected by chronic conditions and episodes of acute illness that require adequate nutritional reserve. For most of the changes encountered, nutritional solutions can be devised; the greatest challenge is to recognize that there is a problem and to start interventions as soon as possible.
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