By Ronenn Roubenoff, MD
A normal part of ageing is the loss of muscle mass, and with it of strength, that has been termed sarcopenia, from the Greek for "poverty of flesh". This process is a universal one, as befits a feature of normal ageing, and is not a product of a disease. Many illnesses, however, can cause accelerated loss of muscle, which older persons are less able to tolerate than the young because they are already experiencing age-related sarcopenia. In this case, the combination of the two processes can be devastating. Although little is known about the determinants of sarcopenia, there is a growing body of evidence regarding the detrimental effects of muscle loss and weakness. Fortunately, much is known today about how to minimize and treat sarcopenia in older persons, using strengthening exercises that can be done at home with simple, low-cost techniques.
What is Sarcopenia? Sarcopenia involves loss of muscle, the largest component of the lean body mass compartment of the human body (the other components of lean mass are the visceral organs, immune cell mass, and extracellular water and connective tissue). There is reasonable evidence that there is a limit on how much lean body mass can be lost before death supervenes. The available data, based on starvation, AIDS patients, and critical illness, suggest that loss of more than about 40% of baseline lean mass is fatal.1 "Baseline" is a slippery concept here, because again absolute mass is not explanatory - basketball players do not necessarily outlive jockeys - but rather the amount of loss as a function of the baseline mass with which the individual started. Kehayias et al.2 defined baseline as the mean for adults aged 20-30 years; no healthy subjects were found below approximately 70% of that standard, and there was a steady decline in body cell mass for both men and women across age groups between 30 and 100 years (Figure 1). Most of this decline has been shown to occur in the muscle, as visceral organ size, and connective tissue mass do not change appreciably with age.
Figure 1. Decline in body cell mass (primarily muscle) across age groups in healthy, ambulatory, successfully ageing adults aged 20 -100. TBK = total body potassium, the reference method for cell mass. Data are expressed as a % of the 20-30 year old group for men and women separately. (From Kehayias et al. 1997 with permission.)
The decline in muscle mass has two important effects on the body. First, strength is directly proportional to the amount of muscle, so that people with less muscle mass are necessarily weaker than people with more muscle. In fact, estimates of muscle mass explain the vast majority of the difference in strength between men and women, and between young and old. Although there is much research currently focusing on the question of whether there is a decline in muscle quality with age, it is clear that loss of muscle quantity is a critical determinant of weakness in the aged. The loss of strength has direct functional implications: weaker persons have more difficulty with tasks that are needed for independent living, and thus are more likely to require assistance or be institutionalized. This functional effect of sarcopenia is shown in Figure 2, which compares the strength needed to rise from a chair for two persons of the same weight. Young persons can do this with only a fraction of their maximal voluntary contraction (MVC, or strength), and thus have little difficulty rising from a chair. Older persons, however, who have lost a significant amount of muscle will require nearly all of their strength to achieve the same work, so they can only do it once or twice before the muscle is too fatigued to accomplish another repetition. Since roughly two-thirds of human muscle is below the waist, it is basic motions such as rising from a chair or bed, walking, and standing that are affected by sarcopenia, and these activities in turn are the fundamental units of independence - the ability to stand up, cross a room, prepare a meal, shop for food and other necessities, etc. Sarcopenia affects all of these in profound ways.
Figure 2. Effect of loss of strength on the ability to perform an action such as rising from a chair, for a young, healthy adult (left) and for a sarcopenic older persons adults (right). MVC = maximal voluntary contraction. Actions that exceed MVC cannot be performed. (Adapted: Frontera and Meredith 1995).
The second major impact of sarcopenia is a metabolic one, as an indicator of reduced protein stores for times of physiologic stress such as an acute illness. It is well accepted that during illness, gluconeogenesis increases in importance, while ketogenesis is relatively suppressed, so that protein is burned for energy in excess of the levels seen in starvation adaptation. Given the anorexia caused by acute illness, and by the iatrogenic limitation on dietary intake that often occurs in hospitals, endogenous protein stores are crucial in determining the availability of metabolic substrate to cope with the illness, and thus the ability to survive it. Therefore, it is no wonder that older, sarcopenic patients fare worse than young, healthy adults for almost all diseases. Tellado et al.3 have shown that measurement of body cell mass was the only independent determinant of survival in intensive care unit patients in multivariate analysis, removing the significance of univariate predictors such as albumin, age, and even diagnosis. Thus, the metabolic significance of sarcopenia in illness should be considered independently of its functional impact during times of better health, as both are important to the survival and well being of older persons.
How Common is Sarcopenia? There is not much information about the prevalence of sarcopenia. There is one population-based study of the prevalence of sarcopenia with advancing age. Data are available from the New Mexico Elder Health Survey by Baumgartner et al.4 who measured appendicular muscle mass by dual-energy x-ray absorptiometry (DXA) in 883 older Hispanic and non-Hispanic white men and women. The subjects were selected randomly from the Health Care Financing Administration (HCFA) Medicare listing for Bernalillo County, New Mexico. A total of 2,200 subjects were sampled; 534 had died, moved, could not be contacted, or were ineligible. Of the 1666 eligible subjects contacted, 1,130 (67.8%) completed the home interview and 883 (53%) underwent DXA. Sarcopenia was defined as a muscle mass 2 or more standard deviations below the mean for young healthy participants in the Rosetta Study, a large cross-sectional study of body composition in New York. The prevalence of sarcopenia by this definition increased from 13-24% of persons under age 70 to over 50% of those over age 80 years (Figure 3).
What Can Be Done About Sarcopenia? Muscle is an amazing organ because it can adapt to new demands to a remarkable degree. The difference in body habits between Olympic weightlifters and marathon runners shows the vast range of muscle mass and content that humans can achieve. Muscle will grow if it is repeatedly stressed to the point of failure. In other words, lifting weights that are heavy enough to make the last one or two repetitions of the motion extremely difficult or impossible to achieve will lead to hypertrophy of the muscle and a gain in strength to the point that the exercise can now be accomplished. Even more remarkable in many ways is the finding that this adaptability is maintained throughout life, even into ages 90 and above.5 Data from our laboratory and others' have shown clearly that muscle strength and mass can be regained even in very frail elders, including those suffering from comorbid conditions such as heart failure, kidney disease, arthritis, diabetes, and obesity. In addition, there is now good evidence for improvement in these chronic conditions with strength training, as the exercise reverses many of the peripheral effects of the disease on muscle, generally without worsening the disease, and often with improvement in that as well.6
Baumgartner et al., Am J Epidem 1998; 147:755-63In order for strength gain and muscle hypertrophy to occur, it is crucial that the exercise be intensive and progressive. Intensive in that only 6 to 8 repetitions can be done in one set before failure occurs, and the weight can no longer be lifted in good form. Progressive in that as soon as strength has increased to the point that the exertion required to perform the exercise is not intensive, the weight or resistance is increased. Thus, it is not adequate to lift soup cans or milk bottles - it is necessary to increase weights as people become stronger. Usually, 2-3 sets of exercises are performed two to three times a week. The large muscles should be exercised, since that is where the bulk of body protein is. Conversely, it is not necessary to use elaborate or expensive equipment in order to achieve the desired results. Training can be done in people's homes, using second-hand or home-made dumbbells and ankle weights, or two or more people can share equipment and exercise together. Such a "buddy system" is often a useful way to continue training after the novelty has worn off. The most important issue, however, is to learn how to do the exercises properly, with good form, so that gains occur as rapidly as possible while injury is avoided. This is best done by following training principles learned over the past four or five decades. An excellent book by Miriam Nelson, PhD, from Tufts University, called Strong Women Stay Young (Bantam, 1997), outlines in great detail and with many illustrations the right way to strength train for older persons. Despite the title, it is equally applicable to men, and it has been translated into 11 languages and is available worldwide.
In summary, sarcopenia is physiologic part of ageing. We still do not know much about what causes it, but we do know how to treat it. Such treatment - and its application at the public health level - will be a major determinant of whether the burgeoning older population we expect in the 21st century will be made up of millions of incapacitated, frail, fractured, and unhappy persons, or whether we can in fact extend not only chronologic age, but also functional status, so that the "golden years" are in fact valuable to the majority of elders. The research is clear. The challenge now is translating it into practice,
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Life expectancy at birth for men and women by UN Region in the year 2000 Men & women age differently. It varies from about 2 yrs difference between men & women in Africa to 8 yrs in Europe - in very old age women outnumber men by about 2:1. Research shows this may be partly due to biological factors (e.g., endogenous hormones protect women from ischaemic heart disease until menopause). Other factors are lifestyle & socioeconomic issues (i.e., men are exposed to work hazards, more often, have more accidents, drink alcohol more excessively, and tend to smoke more (although women are now smoking more, especially in developing countries. Source: UN, the Population Prospectus, database update 1998 |
