A second family of ideas points to causes of aging at the cellular level. One notion focuses on the number of times cells can divide, which presumably limits the life span of a complex organism. Cells grown in laboratory culture dishes undergo only a fixed number of divisions before dying, with the number of possible divisions dropping depending on the age of the donor organism; this
Physical Changes 67
Why Do Most People Think We Age?
What does the average person believe about how and why we age physiologically. To find out, list the various organ and body systems discussed in this chapter. Ask some people of different ages two sets of questions.
First, ask them what they think
happens to each system as people grow older. Then ask them what they think causes these changes. Compile the results from your interviews and compare them with what you discover in this chapter. To what extent were your interviewees
correct in their descriptions. Where were they off base. Does any of the misinformation match up with the stereotypes of aging we considered in Chapter 1.
Why do you think this might be the case. How accurate are people in describing aging.
phenomenon is called the Hayflick limit, after its discoverer, Leonard Hayflick (Hayflick, 1996). For example, cells from human fetal tissue are capable of 40 to 60 divisions; cells from a human adult are capable of only about 20. What causes cells to limit their number of divisions? Evidence suggests that the tips of the chromosomes, called telomeres, play a major role in aging by adjusting the cell’s response to stress and growth stimulation based on cell divisions and DNA damage (Aubert & Lansdorp, 2008). Healthy, normal telomeres help regulate the cell division and reproduction process. An enzyme called telomerase is needed in DNA replication to fully reproduce the telomeres when cells divide. But telomerase normally is not present in somatic cells, so with each replication the telomeres become shorter. Eventually, the chromosomes become unstable and cannot replicate because the telomeres become too short (Saretzki & Zglinicki, 2002). Some researchers believe that in some cases cancer cells proliferate so quickly because telomeres are not able to regulate cell growth and reproduction (Londono-Vallejo, 2008). Current thinking is that one effective cancer treatment may involve targeting telomerase (Harley, 2008). Research also shows that exercise may slow the rate at which telomeres shorten, which may help slow the aging process itself (Cherkas et al., 2008).
68 CHAPTER 3
A second cellular theory is based on a process called cross-linking, in which certain proteins in human cells interact randomly and produce molecules that are linked in such a way as to make the body stiffer (Cavanaugh, 1999c). The proteins in question, which make up roughly one third of the protein in the body, are called collagen. Collagen in soft body tissue acts much like reinforcing rods in concrete. The more cross-links there are, the stiffer the tissue. For example, leather tanning involves using chemicals that create many cross-links to make the leather stiff enough for use in shoes and other products. As we age, the number of cross-links increases. This process may explain why muscles, such as the heart, and arteries become stiffer with age. However, few scientific data demonstrate that cross-linking impedes metabolic processes or causes the formation of faulty molecules that would constitute a fundamental cause of aging (Hayflick, 1998). Thus, even though cross-linking occurs, it probably is not an adequate explanation of aging.
A third type of cellular theory proposes that aging is caused by unstable molecules called free radicals, which are highly reactive chemicals produced randomly in normal metabolism (Cristofalo et al., 1999). When these free radicals interact with nearby molecules, problems may result. For example, free radicals may
cause cell damage by changing the oxygen levels in cells (Lu & Finkel, 2008). The most important evidence that free radicals may be involved in aging comes from research with substances that prevent the development of free radicals in the first place. These substances, called antioxidants, prevent oxygen from combining with susceptible molecules to form free radicals. Common antioxidants include vitamins A, C, and E, and coenzyme Q. A growing body of evidence shows that ingesting antioxidants postpones the appearance of age-related diseases such as cancer, cardiovascular disease, and immune system dysfunction (Lu & Finkel, 2008), but there is no direct evidence yet that taking antioxidants actually increases the life span, although research is ongoing (Miwa, Beckman, & Muller, 2008). Nevertheless, it can’t hurt to include foods high in antioxidants, such as apples, berries, and red beans (Jacka & Berk, 2007).