CHAPTER 3 The Physiology of Age-Related and Lifestyle-Related Decline Marybeth Brown,PT,PbD,FAPTA INTRODUCTION in each system may differ person to person,but total any age.Indeed,there is a growing body of evidence in- our ntir litean e spend about 7 even for sarcopenia,th age-relat vast hundreds of theories on why we age there is no one unifying theory that satisfactorily ac 一e83aao2 counts for all the changes the body undergoes.Indeed, the study in its infancy.Althoughenor By and nd fo mental activities of daily living for 90 to 100 vears the recognition that whole-body inflammation is an im- Sarcopenia is distinct from another muscle wasting con ant con三butor to aging是 dition,cachexia.Cachexia is rapid and as wear and tea frequeal occurs oximately half of therapy is highly successful for the modification of sarco penia;however,physical therapist intervention cannot consequence of re iate cachexi I be dis assed below t can ac Aging The cellul ar and ubcellula describe what occurs in selected systems for the purpose (which puts stresses on tissues,increases inflammation. of understanding the functional consequences of aging gneceseaikcohoelandy predisposes towar ally.Fo ne in bon ven decline may result in the loss of lung capacity.renal clearance.or aerobic endurance. changes that affect balance,diminish lung capacity,and shorten step and stride length.Once cellular changes are ther inactivity-and obviously results in the loss of half of the tissue function dressed Thus physical therapists must consider all the still have a normal life span.As examples of normal ag sequelae of health disorders ing there are 90-year-c tissue or system that does However,only those ,1993 by Mosb 27
Copyright © 2012, 2000, 1993 by Mosby, Inc., an affiliate of Elsevier Inc. 27 Because so much of the decline with aging is lifestyle related, physical therapists have ample opportunity to intervene along the way, with successful results likely at any age. Indeed, there is a growing body of evidence indicating that exercise is a powerful modifier of inactivityrelated decline, even for sarcopenia, the age-related wasting of muscle.9-13 Loss of skeletal muscle mass and force is inevitable with aging and can be further exacerbated by a host of variables, such as nutrition and disease. However, sedentary lifestyle is likely to take the greatest toll.2,3,7,14-16 By and large, men and women who include physical activity in their daily routine should have sufficient muscle mass and force to achieve all of the fundamental activities of daily living for 90 to 100 years. Sarcopenia is distinct from another muscle wasting condition, cachexia. Cachexia is rapid and relentless muscle wasting that frequently occurs before death. Cachexia occurs with terminal disease such as cancer. Physical therapy is highly successful for the modification of sarcopenia; however, physical therapist intervention cannot remediate cachexia, as will be discussed below. Aging is manifested by cellular and subcellular changes within all tissues. The intent of this chapter is to describe what occurs in selected systems for the purpose of understanding the functional consequences of aging as they present to the physical therapist clinically. For example, the natural decline in bone mineral content may predispose patients to osteoporosis. It is not uncommon for those with osteoporosis to manifest postural changes that affect balance, diminish lung capacity, and shorten step and stride length. Once cellular changes are described, other inactivity- and lifestyle-related events that further contribute to systemic decline will be addressed. Thus, physical therapists must consider all the sequelae of health disorders. There is not a single tissue or system that does not undergo age-related changes. However, only those INTRODUCTION Aging is a fundamental process that affects all of our systems and tissues. The rate and magnitude of change in each system may differ person to person, but total body decline is an inevitable part of life for everyone. Ironically, we spend about 75% of our entire life span undergoing the process of decline. Although there are hundreds of theories on why we age there is no one unifying theory that satisfactorily accounts for all the changes the body undergoes. Indeed, the study of aging is still in its infancy. Although enormous strides have been made in our understanding of the aging process, there is still much to discover about the science of age-related decline. A recent advancement is the recognition that whole-body inflammation is an important contributor to aging-related decline: a significant shift from concepts such as wear and tear and the biological clock based on genetic programming. Also it has only recently been realized that approximately half of the decline with age has a genetic basis.1-5 The remainder of age-related change is the consequence of lifestyle, primarily physical inactivity that can account for the other half of the decline with age. Coupling sedentary lifestyle with inadequate nutrient intake, excess body weight (which puts stresses on tissues, increases inflammation, predisposes toward disease), and variables such as smoking and excessive alcohol intake, the biological decline is more precipitous and greater in magnitude.6-8 Even though age-related decline may result in the loss of lung capacity, renal clearance, or aerobic endurance, we have enough tissue reserve in each of our systems to get through 80 to 90 years without infirmity. Indeed, those who surgically donate a kidney or lung, which obviously results in the loss of half of the tissue function, still have a normal life span. As examples of normal aging there are 90-year-olds who can run marathons, do finger-tip push-ups, and dance vigorously. C H A P T E R 3 The Physiology of Age-Related and Lifestyle-Related Decline Marybeth Brown, PT, PhD, FAPTA
28 CHAPTER3 The Physiology of Age-Related and Lifestyle-Related Decin ity to render opuim. and decrease ne a sease a spect to the issue of drug clearance through the kidney. to illness and has low capacity to combat the effects of the illness:a bout of infl enza may kill. n a separate When a person is in h and balanee obis and endurance.When the inactive older adult with stable constipation. AGING:A DECLINE IN HOMEOSTASIS sets of circumstances, Homeostasis is a critical concept that summarizes all of ameophsioloeicailstcgs,thsPeauce5eposiilir agingfrom a functionl standpoint.Homeostasis refers to the physiological at maintain a s hich th win nd of homeostasis will influence susceptibility to illness and the body to draw on a“well" Creasy As we age the c modifiable with lifestyle of immune function, trength, endu ofa0ateresourcsinorder by Muce and Male stress to meet the of day. Tho e who maintain homeostasis will continue to thrive. various levels of physical stress and the adaptive re- whereas men and women unable to maintain homeosta- sponses of tis sis against even small stress the le hallenge homeosta The successfully aging older adult maintains a high maintenance of a large physiological reserve that main- capacity to hereas he person whe tains homeostasis even in the pres logical stress and,thus,provide a wider homeostasis be arrested nutritionally. Cachexia is associated with window is possible using principles incorporated in the tha adults with more reserve”may rece ver from state,most people do not,and rarely do older adults Effect of Physical Stress on Tissue Adaptation Death Loss of adaptatior Injury ed tole (hypertrophy Th Maintenance egp7e The other term that must be defined is sarcopenid, which is the muscle wasting of old age.Sarcopenia is Death Loss of adaptatior FIGURE 3-1 Effect of varying levels of physical stress (inadequately e 70 years have sarcopenia;for those older than age 80 ap
28 CHAPTER 3 The Physiology of Age-Related and Lifestyle-Related Decline systems that physical therapists treat directly or affect the ability to render optimal care will be discussed in this chapter. Gastrointestinal or genitourinary systems, for example, will not be discussed in detail, except with respect to the issue of drug clearance through the kidney. Skeletal muscle is also excluded, as it will be covered in a separate chapter. Finally, some attention will be given to age-related issues that are amenable to change with exercise: sleep, sexual function, depression, gastroesophageal reflux disease (GERD), gastric motility, and constipation. AGING: A DECLINE IN HOMEOSTASIS Homeostasis is a critical concept that summarizes all of aging from a functional standpoint. Homeostasis refers to the physiological processes that maintain a stable internal environment of the body. The extent to which the body can adapt to physiological stressors and maintain homeostasis will influence susceptibility to illness and injury. As we age the capacity to tolerate stressors decreases but remains partially modifiable with lifestyle adaptations. The physical stress theory (PST) proposed by Mueller and Maluf17 captures the essence of homeostasis. Figure 3-1 illustrates the relationship between various levels of physical stress and the adaptive responses of tissue. Figure 3-2 provides a conceptual picture of the relationship of successful and unsuccessful aging to a tolerance for challenges to homeostasis and the effect of varying levels of challenge on homeostasis. The successfully aging older adult maintains a high capacity to tolerate physiological stress, whereas the person who is aging unsuccessfully generally has a low tolerance to physiological stressors that challenge homeostasis. The ability to improve tolerance for physiological stress and, thus, provide a wider homeostasis window is possible using principles incorporated in the PST. Tolerance range increases in response to exercise, and decreases with the addition of chronic disease and greater inactivity. The older individual with very low tolerance to physiological stressors is highly susceptible to illness and has low capacity to combat the effects of the illness: a bout of influenza may kill. When a person is in homeostasis, exercise results in robust positive change with systemic adaptation. Strength and balance can increase as can aerobic and muscle endurance. When the inactive older adult with stable chronic diseases engages in exercise, positive change also occurs, albeit more slowly and of smaller magnitude. Under both sets of circumstances, a widening of the window of homeostasis occurs, providing greater tolerance to physiological stress, thus reducing the possibility of moving out of homeostasis into cachexia and death. The wider the window of homeostasis, the greater the chance of survival and of maintaining independence in physical function. Furthermore, the wider the window, the greater the physical reserve as well as the capacity of the body to draw on a “well” of immune function, strength, and endurance among other resources in order to meet the demands of another day. The natural corollary of homeostasis is survival. Those who maintain homeostasis will continue to thrive, whereas men and women unable to maintain homeostasis against even small stressors may become cachexic, or succumb to a devastating illness such as pneumonia. One of the biggest challenges of current practice is to promote wellness and enhance survival through the maintenance of a large physiological reserve that maintains homeostasis even in the presence of large stressors. It is necessary to define several terms that characterize many older adults. Cachexia typically refers to an inexorable decline in muscle (and body) wasting that cannot be arrested nutritionally.18-20 Cachexia is associated with end-stage cancer, AIDS, tuberculosis, and certain infectious diseases and is a response to one or more pathologies that overwhelm the body. Although some young adults with more “reserve” may recover from a cachectic state, most people do not, and rarely do older adults recover from cachexia. The cachexia of old age typically precedes death and is the final stage of chronic obstructive pulmonary disease (COPD), chronic heart failure (CHF), and other terminal pathologies. Although the cause of cachexia is not well defined, it is believed to be the consequence of a massive increase in inflammatory cytokines, which will be discussed later in this chapter.18-21 The other term that must be defined is sarcopenia, which is the muscle wasting of old age.19 Sarcopenia is present if muscle mass as determined by dual-energy x-ray absorptiometry is two or more standard deviations below values obtained for young adults.22-26 Approximately 22% of all men and women older than age 70 years have sarcopenia; for those older than age 80 years the number of sarcopenic individuals approaches 50%, with a higher percentage for men than women.27 Effect of Physical Stress on Tissue Adaptation Physical stress level Death Injury Increased tolerance (e.g., hypertrophy) Maintenance Decreased tolerance (e.g., atrophy) Death Loss of adaptation Loss of adaptation Thresholds for adaptations FIGURE 3-1 Effect of varying levels of physical stress (inadequately low to excessively high) on tissue’s ability to adapt and to maintain homeostasis. (Reprinted with permission, from Muellér MJ, Maluf KS: Tissue adaptation to physical stress: a proposed “Physical Stress Theory” to guide physical therapist practice, education, and research. Phys Ther 82(4):383–403, 2002.)
CHAPTER 3 The Physiology of Age-Related and Lifestyle-Related Dedine 29 Death Death FIGURE3-2 A depiction of in in who have age dotted lines repres the cal stress thatm Maintenance range of ho Maintenance range of homeostasis ct of in ase stre tolera cha 9 ain tiss tdeongnonlhga at hnabyogaFpgeoeen ability to maintain homeostasis Death Nonsuccessful aging Successful aging The major distinctio heemthetosc wasting of ble to shange.Indeed.sar letel known that women have a faster rate of bone mas capable of responding to strength-training exercis this phas of oda Peak bone mass rally In the United States and around the world.the fastest growing segment of th ongevity continue age group are physically dependent on others for basic essentials such as shopping,cooking,housekeeping,medi- ng,and athing.Some d en often the consequence of inactivity- have re- 1020304050 6070 80 sulted in frailty.Sarcopenia is frequently the hallmark of women with frailty is FIGURE 3-3 B The t e( tive impact through physical therapy.Further discussion of me sarcopenia is included in the chapter on impaired muscle Physic occurs in all systems The e age-reth enhanced tissue and organ function through physical therapy is also discussed. a Skeletal Tissue ing ado Skeletal tissue is remarkably susceptible to change in compromise (e.g car accident with pro on to the ongoing
CHAPTER 3 The Physiology of Age-Related and Lifestyle-Related Decline 29 The major distinction between the muscle wasting of sarcopenia versus cachexia is that sarcopenia is amenable to change. Indeed, sarcopenic muscle is completely capable of responding to strength-training exercise, with significant increases in muscle mass and strength.10,12,28 In contrast, cachexic muscle will not respond to exercise, and physical therapy treatment to improve strength at this phase of old age is generally unwarranted. In the United States and around the world, the fastest growing segment of the population are those adults who are age 85 years and older. Although longevity continues to increase, quality of life frequently does not. Indeed, approximately half of all individuals in the 851 years age group are physically dependent on others for basic essentials such as shopping, cooking, housekeeping, medication management, walking, and bathing. Some of the decline in functional ability is secondary to sarcopenia; for others, accumulated declines in strength, balance, and endurance—often the consequence of inactivity—have resulted in frailty. Sarcopenia is frequently the hallmark of frailty and the number of men and women with frailty is growing exponentially.19 The increasing incidence of sarcopenia and frailty provides limitless opportunities for positive impact through physical therapy. Further discussion of sarcopenia is included in the chapter on impaired muscle performance. Physical decline occurs in all systems. The age-related changes in the systems most applicable to physical therapy are presented in the following sections. The potential for enhanced tissue and organ function through physical therapy is also discussed. Skeletal Tissue Skeletal tissue is remarkably susceptible to change in response to day-to-day nutrient intake, inactivity, weight bearing, hormones, and medications.29-34 These day-to-day changes occur in addition to the ongoing Physical stress Death Nonsuccessful aging Death Successful aging A B C Death Death Decreased tolerance (deconditioning, frailty) Maintenance range of homeostasis Increased tolerance: tissue adaptation Excessive stress overwhelming homeostasis: disease, trauma, illness Decreased tolerance (deconditioning, frailty) Maintenance range of homeostasis Increased tolerance: tissue adaptation Excessive stress overwhelming homeostasis: disease, trauma, illness Inability to adapt to even low levels of stress Inability to adapt to even low levels of stress FIGURE 3-2 A depiction of the differences in range of homeostasis tolerance and ability to adapt to stress in individuals who have aged nonsuccessfully and those who have aged successfully. The dotted lines represent the limits of homeostasis centered around the range of physical stress that maintains tissue at physiological equilibrium and effect of increased or decreased stress on tolerance to challenges to homeostasis. A, Inadequate ability to adapt (maintain tissue homeostasis) against even small stresses. B, Level of stress that maintains homeostasis tolerance at the same level. C, Level of stress that overwhelms the tissue’s ability to maintain homeostasis x x x x x x x x Peak bone mass Age in years Bone mass 0 10 20 30 40 50 60 70 80 Car accident Alcoholism Osteoporosis FIGURE 3-3 Bone mass profiles of three women throughout the course of a lifetime. The top line (n) represents usual lifestyle, including adequate nutrition including calcium, occasional or no weightbearing exercise, some outdoor time (vitamin D exposure), minimal inactivity-related diseases, including obesity, modest alcohol intake, no drugs that diminish bone. The middle line (X) reflects optimal bone mass in a woman who embraced a healthy lifestyle over the course of her lifetime. Healthy lifestyle includes adequate nutrition including protein and calcium intake, a regular weight-bearing exercise program, routine exposure to sunshine, minimal disease burden, modest alcohol consumption, no drugs that diminish bone. The bottom line () reflects one of several possibilities: inadequate calcium during the teenage years and/or amenorrhea as a teen or early adult stage of life, or anorexia as a teenager with inadequate calcium and protein intake. Anorexia often results in low estrogen values as well. Major points: Calcium intake during adolescence is critical; loss of normal serum estrogen results in accelerated bone loss with age or failure to maximize bone stock in youth; poor lifestyle choices (e.g., alcoholism, sedentary lifestyle, poor nutrition) diminishes bone at all ages; and serious physical compromise (e.g., car accident with prolonged bed rest) has lifelong consequences. decline in bone mineral that begins in the 3rd decade and continues on through life (Figure 3-3). It is well known that women have a faster rate of bone mass loss during the menopause, where the typical yearly decrease of 0.5% to 1% doubles to about 2% per year
30 CHAPTER3 The Physiology of Age-Related and Lifestyle-Related Decin B0X3-1 Factors for Bone Loss nd Modifiable Risk one size of wor to men,women ible use.Theep ated risk Nonmodifiable Risk Factors for Bone Loss porosis in the postmenopausal woman is a staggering Caucasian race eoporosis Foundation). 15 ee cel the the whic nily history o nd the maintains bone.These three cell types form the basi re at menonause Childhood n labsorption disease ce activ of musde mass difiable Risk Fact shift.believed to have a hormonal basis,that causes ei- ther a higher bone ac at ause age one de than ing age, index(5) course,is what occurs during growth. Factors other than aging may affect the health and eing o and hout the life span equate vitamin D many factors affecting bone mass are modifiable with lifestyle.Factors that thos zed in fo Recently.it has beco me evident that restosterone and Exercise is critical to the health and well-being estroger are independent mediators of bone health in of skeletal tissue.The natural pull of contracting men Thus, any condition affecting sex hormones muscles one mineral density;inac cancer automati The fact that tomo w's oste otic women are in a cast or goes into space.The loss of bone in space Centers for Dise Control ntion drink has been estimated at 0.5%to 1.0%per day because the muscl coactions are not prod cing any demane and are Several studies have indicated that exercise without adequate ho and one u system or more IDHEA]tcosterone.fropon nbi don,ca endous nd it is du of 12 to 18t tha nd we men.Dalsky.40 for sed loadin exercise as the stimulus in 60-to 70-vear-old women dur teenager no milk, eats pizza and burgers da an no ex outs 0 ne mineral col leagues foun their teens with a skeletal profle ear-old.Ar ment therap (HRT)gained additional bone mineral the other end of the age spectrum is the older women density (BMD)in the spine and hip with loading exercise. who spends 23.5 hours per day ight training and wearinga ese womer wh becoming and frail.and m 75 ars)on HrT also had signifcant increases o predispo falling and bone breakage with each gPoihsof7oitanceandaerobceaercminie proximately 3.5%in lumbar spine BMD with
30 CHAPTER 3 The Physiology of Age-Related and Lifestyle-Related Decline for the 5-year peri- and menopausal era. Given the smaller bone size of women compared to men, women are much more susceptible to developing osteopenia with menopause. The current estimated risk for osteoporosis in the postmenopausal woman is a staggering 50% (International Osteoporosis Foundation). Bone is composed of three cell types: the osteoclast, which breaks down bone; the osteoblast, which produces and increases bone mineral; and the osteocyte, which maintains bone. These three cell types form the basic metabolic unit (BMU) of bone as suggested by Frost.35 Under normal circumstances, there is a balance between osteoblastic and osteoclastic activity such that the loss/ gain ratio each day is one-to-one. With aging, there is a shift, believed to have a hormonal basis, that causes either a higher bone breakdown rate or reduced bone accretion rate, which is what causes age-related bone decline.29,31 Thus, with advancing age, the BMU favors bone catabolism rather than bone anabolism which, of course, is what occurs during growth. Factors other than aging may affect the health and well-being of men and women throughout the life span and account for more decline in bone mass than aging alone. Some of these factors are nonmodifiable, but many factors affecting bone mass are modifiable with lifestyle. Factors that are modifiable with lifestyle and those that are not modifiable are summarized in Box 3-1. It is important to realize that estrogen is critical for the maintenance of bone mass in both men and women. Recently, it has become evident that testosterone and estrogen are independent mediators of bone health in men.26 Thus, any condition affecting sex hormones (e.g., prostate cancer, breast cancer) automatically affects skeletal health in both sexes. The fact that tomorrow’s osteoporotic women are being created among the youth of today gravely concerns the Centers for Disease Control and Prevention (CDC).36 Young women are not drinking milk, are highly sedentary, are not using their muscles, are not going outside routinely for sun exposure, and are eating nutritionally poor foods without adequate calcium, protein, and vitamin D. Each day spent without the building blocks of bone robs the skeletal system of more mineral. During the teenage years, bone mass increases tremendously and it is during the ages of 12 to 18 that the ultimate skeletal profile is determined. Thus, if a teenager drinks no milk, eats pizza and burgers most days of the week, and gets no exercise outside, chances increase that these adolescents will emerge from their teens with a skeletal profile of a 60-year-old. At the other end of the age spectrum is the older women in a nursing home who spends 23.5 hours per day lying in bed or sitting inside.33,34 These women, who already are at unusually high risk for fracture, are becoming more osteoporotic and frail, and more predisposed to falling and bone breakage with each passing day.37,38 Exercise is critical to the health and well-being of skeletal tissue. The natural pull of contracting muscles is what maintains bone mineral density; inactivity robs bone of a critical stimulus for osteoblastic activity. A classic example is the remarkable amount of bone loss that occurs when someone is immobilized in a cast or goes into space. The loss of bone in space has been estimated at 0.5% to 1.0% per day because muscle contractions are not producing any demand on bone.39 Several studies have indicated that exercise or hormone replacement therapy (dehydroepiandrosterone [DHEA], testosterone, estrogen, or estrogen/progesterone combined), either alone or in combination, can add bone mineral density to the osteopenic framework of older men and women. Dalsky,40 for example, used loading exercise as the stimulus in 60- to 70-year-old women during a 1-year study and observed a 3% to 6% increase in bone mineral content.38,41 Kohrt and colleagues found that older women who were already on hormone replacement therapy (HRT) gained additional bone mineral density (BMD) in the spine and hip with loading exercise. Activities consisted of weight training and wearing a weighted vest while ascending stairs.42 Furthermore, Villareal demonstrated that frail older women (older than age 75 years) on HRT also had significant increases of approximately 3.5% in lumbar spine BMD with 9 months of resistance and aerobic exercise training.43,44 BOX 3-1 Nonmodifiable and Modifiable Risk Factors for Bone Loss Nonmodifiable Risk Factors for Bone Loss Genetics: women with small frames Caucasian race Hispanic women Age: female older than age 50 years Family history of osteoporosis Premature at birth Low estrogen: menopause Childhood malabsorption disease Seizure disorder—using Dilantin Age-associated loss of muscle mass Modifiable Risk Factors Calcium intake: 1200 mg/day or more is required Excessive alcohol intake: maximum allowable is not defined Smoking cigarettes Low body mass index (,18.5) Low estrogen: amenorrhea, anorexia Low estrogen: ovariohysterectomy Inactivity, immobilization Substituting soda for milk, especially among children Insufficient protein at all ages Inadequate vitamin D Hyperthyroidism Prednisone and cortisone use, hyperparathyroidism
CHAPTER 3 The Physiology of Age-Related and Lifestyle-Related Dedine 31 Thus, that b Age-related shift in body mas of the few studies that incuded men 100 DHEA was given for 2 years to subjects of both sexes 0 Women on DHEA increased spin year and 60 men.4s Given the ent trend of increasing oste in men,successful therapies are needed.Natural alterna- 40 20 ar whet tors (SERMs)such a ifen or raloxifene affect bone 0 in a synergistic and additive fashion. 20 40 60 80 100 Age in years -■-Fat mass*Lean mass Body Composition Throughout the decades there is a gradual shift in FIGU body composition such that lean mass decreases and 3-4 0 body ears that are repre mmon of 85/15.Even if this same man maintains body weight for the next 50 years he is likely to have a lean/fat ratio movement in all directions.Over the decades,subtle nass of 509 at age 80 ye ars ever of the hange occurs in an wate changes wil Of co f the which Collagenous tissues are composed of collagen,which is now believed to be a significant contributor provides substantial tensile strength,and a surrounding increa nati occurs with age othe the e binds water a crease lde past oneh th decreases considerably.The most obvious consequence active metabolic tissue,and its contribution to age of the water loss is body shrinking or height loss pri decline and disease is just beginning to be marily because of water from the inte ertebra bdominal fat the the risk for s water and s).Clne cally.the loss of water manifests itself in two ways Women are particularly vulnerable to these diseases after reduced range of motion and loss of "bounce,"that is, menopause as the protec tive effectso absorb shock.From an exercise stand tha n men at a 2 point,work ng towar rang mes more an walked is about 10o calories burned.When the losses from limiting function.Exercises too should be heart rate goes up in response to exercise and muscles are shifted away from activities that are jarring such as ngaged,metabolic ate increa ned as fue jumping from high surfaces.Although plyometric exer men of a are con active a1 are sedentary.2 Cons que less whole-body inflammation and less disease. stressful on an older body less able to absorb the impact. Collagenous Tissues use the number also Collagen is probably the most ubiquitous tissue type in come manifest?a decreased ranee of motion andan the body,comprising the skin,tendons,ligaments,fascia, increase in stiffness.Even though end range is dimin and a I activities of daily living,including
CHAPTER 3 The Physiology of Age-Related and Lifestyle-Related Decline 31 Thus, the evidence suggests that bone in women of all ages is able to respond to HRT and to exercise with additive effects. In one of the few studies that included men, DHEA was given for 2 years to subjects of both sexes aged 65 to 75 years. Women on DHEA increased spine BMD 1.7% the first year and by 3.6% after 2 years of supplementation. No increases in bone were observed for men.45 Given the current trend of increasing osteoporosis in men, successful therapies are needed. Natural alternatives such as genistein and other food additives are being investigated in both sexes. It is also still unclear whether exercise coupled with selective estrogen receptor modulators (SERMs) such as tamoxifen or raloxifene affect bone in a synergistic and additive fashion. Body Composition Throughout the decades there is a gradual shift in body composition such that lean mass decreases and fat mass proportionately increases (Figure 3-4). To provide a typical example, it is not uncommon for a man in his 20s to have a lean body mass/fat mass ratio of 85/15. Even if this same man maintains body weight for the next 50 years he is likely to have a lean/fat ratio of 70/30. For women, it is not uncommon to observe a fat mass of 50% at age 80 years even though the individual appears to be no more than “pleasingly plump.” Of considerable significance is the fact that most of the fat increase occurs inside the peritoneum,7,46-51 which is now believed to be a significant contributor to the increased inflammation that occurs with age. The increase in intra-abdominal fat is also believed to predispose older individuals, particularly women, to elevated lipids and prediabetes.51,52 Fat is an extraordinarily active metabolic tissue, and its contribution to agerelated decline and disease is just beginning to be understood. The more intra-abdominal fat the greater the risk for heart disease, metabolic syndrome, diabetes, and cancer. Women are particularly vulnerable to these diseases after menopause as the protective effects of estrogen are gone and women have more fat than men at all ages.50 Exercise plays an important role in controlling intra-abdominal fat.49,52,53 Every mile walked is about 100 calories burned. When the heart rate goes up in response to exercise and muscles are engaged, metabolic rate increases and fat is burned as fuel. Men and women of all ages who are consistently active do not add intra-abdominal fat to the same extent as those who are sedentary.52 Consequently, active men and women have less whole-body inflammation and less disease.43 Collagenous Tissues Collagen is probably the most ubiquitous tissue type in the body, comprising the skin, tendons, ligaments, fascia, and a host of lesser entities. Essentially, collagenous tissues hold us together while still permitting freedom of movement in all directions. Over the decades, subtle change occurs in all collagenous tissues, but only three of these changes will be discussed here: loss of water from matrix, increase in crosslinks, and loss of elastic fibers.54-57 Collagenous tissues are composed of collagen, which provides substantial tensile strength, and a surrounding semiliquid matrix that binds water and permits collagen fibers to easily glide past one another. Matrix composition changes over the years such that water content decreases considerably. The most obvious consequence of the water loss is body shrinking or height loss primarily because of water loss from the intervertebral discs. Articular cartilage also loses water and becomes more susceptible to breakdown (osteoarthritis). Clinically, the loss of water manifests itself in two ways: reduced range of motion and loss of “bounce,” that is, the ability to absorb shock. From an exercise standpoint, working toward end range becomes more and more important with advancing age to prevent range losses from limiting function. Exercises too should be shifted away from activities that are jarring such as jumping from high surfaces. Although plyometric exercises are recommended as an excellent stimulus to increase bone mass, care should be taken to choose exercises that act as a stimulus to bone without being too stressful on an older body less able to absorb the impact. Because the number of collagen crosslinks also increases with age, two observable clinical changes become manifest: a decreased range of motion and an increase in stiffness. Even though end range is diminished with advancing years, range should still be sufficient to accomplish all activities of daily living, including * * * * * * * * Age in years Percent 0 20 40 60 80 100 100 80 60 40 20 0 Age-related shift in body mass Fat mass Lean mass FIGURE 3-4 Typical shift in fat and lean mass in an aging male. Lean mass, which is mostly muscle, declines continuously after the 3rd decade. Fat mass increases concomitantly. In this individual, body weight has not changed over the 60 years that are represented