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Physiology: What is Normal Aging?

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Most people would agree that in normal aging one would expect an increase in blood pressure, blood sugar and weight, and a decrease in bone density. While these changes may be associated with aging (age-related) they are not age-determined. Neither can they be considered harmless. Each of the conditions above promotes disease. Each can be modified with proper lifestyle intervention such as diet, exercise and smoking cessation. The pitfalls to the theory of normal aging are two-fold: Age-related changes carry no risk to well being, and age-related changes are inevitable and unresponsive to intervention.

For this reason, we will now consider usual aging.

Usual aging refers to the physiologic changes that occur in all individuals due to age. The sequence of the changes is consistent among all people, the rate, however, can differ greatly. Both genetics and environment impact age-related changes. Siblings with the same genetic background can have marked differences in longevity due to environmental factors or secondary aging. The causes of secondary aging may be preventable: arteriosclerosis, type 2 diabetes, essential hypertension and osteoporosis.

The research team of Rowe and Kahn promote the theory of “successful” aging. They start with the premise that positive lifestyle factors can mitigate or delay the onset of disease. Positive lifestyle changes are diet and exercise patterns that impact body composition (fat to lean ratio), blood glucose levels, and cardiovascular function. By affecting disease onset and severity, life expectancy and years of enjoying good health can be prolonged.

In terms of the physiology of the aging body, tissue function generally reduces with age. This leads to a diminished capacity to resist disease and use nutrients efficiently. After age 30, most organs and systems experience a decrease in efficiency at a nearly linear rate with age. All organs and systems, however, do not experience aging in a uniform fashion. Some systems change little with age, the gastrointestinal (GI) system being one of them. What declines significantly is the ability of an organ or system to respond to an illness or stress situation.

Before we look at the effect of aging on the different systems, we should first note that many studies, were done using institutionalized or hospitalized older people as their subjects. These researchers, therefore, were not recording the effects of usual aging, but more probable, disease or chronic conditions that these elderly were experiencing. Often disease situations have a greater impact on the nutritional status of the aged than the effects of usual aging.

The Gastrointestinal (GI) System

The GI tract in general maintains its functional level well into old age. There are three areas that are the exceptions: altered taste, decreased stomach acid and decreased blood flow to the liver. In the mouth, there is a decrease in saliva which affects chewing and swallowing. Also, altered taste makes food less palatable. 

The stomach is where we begin to see real differences between the young and the old. Changes occurring in the stomach lining can lead to decreased secretion of acid. Another condition, atrophic gastritis, which is a chronic inflammation of the stomach leading results in a lack of acid secretion. Calcium from food and supplements are better absorbed in an acidic environment as are iron and folate. Vitamin B12 is protein-bound and needs acid to split this bond so it can be absorbed. Another outcome of reduced gastric secretions is bloating and gas produced when foods are not completely broken down. Bacteria normally kept in check by stomach acid, can escape from the stomach and colonize the small intestine, competing with the body for available B vitamins and macronutrients. Antibiotics are the treatment of choice for this condition.

The colon, the holder and processor of fecal waste, can be a site for major problems in the aged. Collagen fibers can replace smooth muscle cells weakening the colon wall and setting up a structural change called diverticulosis. Those in this group need to avoid small, indigestible foods, such as seeds and nuts. The major functional change during old age is constipation. Lifestyle changes appear to mitigate the incidence of constipation in the elderly. Exercise, proper hydration, regular meals, high fiber intake, a quick response to the body’s signal to defecate,  and nonuse of laxatives can help prevent constipation.

The Skeletal System

Osteopenia and osteoporosis both deal with bone loss; the difference is the degree of loss. Between the ages of 30 and 40, men and women start to lose bone mass and enter a condition called osteopenia, a decreased or lower mineral content of the bone. Low bone density is defined as bone mineral content between 1 and 2.5 standard deviations below the mean value of young adults. Osteoporosis, then is a greater degree of osteopenia, being a bone mineral content that is more than 2.5 standard deviations below this same mean. Osteoporosis involves both a decrease in bone minerals and a decrease in bone matrix.

Osteoporosis leads to brittle bones that often fracture, either at the hip, pelvis, leg, arm, hand or ankle region. It is rarely a clean break as, for example, snapping a piece of chalk in half. Rather the bone explodes into many small fragments that cannot be pieced together. Many elderly suffering hip fracture do not walk again and about a fifth die from complications. If bones do not break, at the very least, the body’s weight on the fragile vertebrae of the spine can crush down on them and pinch the major nerves, a very painful condition. They also may compress into wedges called kyphosis or “dowager’s hump” resulting in a loss of height of up to 6 inches and alterations in posture.

Risk factors for osteoporosis are: age; female gender; Northern European descent; small, lean build; poor calcium and vitamin D intake throughout life, but especially during the teenage and young adult years; lack of physical activity; use of alcohol and tobacco; and possibly, excess intake of animal protein, sodium, and inadequate vitamin K. Side-effects from steroids, anticonvulsants and others drugs may cause secondary osteoporosis.

Because bone resorption outpaces bone development during the middle adult years, age is the strongest predictor of osteoporosis. Next to age comes gender. Women account for 80% of the cases of osteoporosis mainly due to the precipitous drop in estrogen after menopause. (Data from the Women’s Health Initiative published in the Fall of 2002 shows a link between estrogen replacement therapy and an increased risk of breast cancer, heart disease and stroke.)

The Nervous System

Loss of mental function and aging do NOT go hand in hand. Mental capacity does show a decrease in middle age (45) and a steeper drop after 65, but individual variation is great and appears somewhat dependent on how much mental stimulation one receives. The same applies to memory. Memory losses more often affect short-term memory rather than long-term memory.

Although brain weight decreases by 10% by age 90, its size or number of cells does not correlate to mental function. A loss of brain cells can be compensated for by an increase in dendrites, the cell “extensions” that transmit nerve impulses, thus preserving brain function. The exception to this would be in the case of senile dementia of the Alzheimer type (SDAT) in which there is a loss of brain cells, the appearance of senile plaques, and strikingly lower levels of neurotransmitters. SDAT is not reversible. It is more common in women and has a genetic link.

The warning signs of Alzheimer's Disease are the following: repeatedly retelling a story, repeatedly asking the same question, relying on other to ask and answer questions, getting lost in familiar locations, and forgetting how to do routine tasks.

Usual aging is accompanied by a lower production of neurotransmitters, but only when the drop approaches 50% will dementia ensue. About 15% of the elderly have severe dementia. If the dementia is the result of acute electrolyte imbalances of sodium or potassium, thyroid dysfunction, drug toxicity or illness, it can be reversed upon treatment of the causal factor.

The incidence of Parkinson’s disease is linked to age, peaking at age 75. As in SDAT, there is a loss of brain cells, but in this case, the loss is in those cells at the base of the brain that produce dopamine, a neurotransmitter.

Pain sensation is also decreased in the elderly.

The Cardiovascular System

The heart of an elderly person could be smaller than normal due to malnutrition, larger than normal due to severe high blood pressure, or unchanged in size. Usual aging causes the left ventricular wall to thicken and the diameter and length of the aorta, a major artery leaving the  heart, to increase. Fat will accumulate in the heart muscle as a response an increase in total body fat.

Other body organs such as the brain and the kidneys rely on a well-functioning heart. Diminished blood flow (and therefore oxygen) to the brain can affect mental function. The kidneys depend on a good pumping heart to sustain an appropriate filtration rate for efficient removal of waste products.

Renal Function

Unlike others systems that only show a deterioration when under physical or mental stress, the older kidney will have a decreased ability to handle normal and stressful (disease and illness) situations. This is because the older kidney has a decreased capacity to conserve water during low fluid intake and to excrete excess fluid. The kidneys are less able to concentrate urine. It is harder for the older kidney to regulate sodium and potassium. Excess sodium and potassium are more difficult to excrete.  And yet when the pair are available at normal levels, they are harder to conserve.

Pulmonary Function

The function of the lungs is to take in oxygen and release carbon dioxide. This is done in the air sacs. With age these sacs enlarge and lose elasticity. The surface area and number of small blood vessels in the air sacs decrease which leads to a concomitant decrease in oxygen uptake and carbon dioxide output. Decreases begin at age 20 and are progressive. A 20 year old takes in 4.1 liters of oxygen per minute compared to 1.5 for a 75 year old who doesn’t smoke or have emphysema. Oxygen utilization is lower in women than men because women have less muscle mass than men and less hemoglobin in the red blood cells to carry the oxygen. As lung volume decreases so to exercise capability.

Immune Function

Infections occur more frequently in the elderly and are more often severe. Whether this is due to the aging process itself or from poor nutrition and chronic disease is in question.

More information on this topic can be accessed through the Texas Gerontology Curriculum Consortium at http://www.eastfieldcollege.edu/ger/future.html

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