When you think of how your body is aging, it's probably the most visible changes that come to mind. Maybe you've noticed more gray hair, or your skin doesn't seem as smooth as it used to be. These are just external signs of a series of processes going on within your cells and bodily systems that together constitute normal aging.
While this is one of the few experiences all humans have in common, the National Institutes of Health say aging is actually one of nature's least understood processes. Here are some of the leading theories about how time affects your personal biology:
Aging is pre-programmed:
- Cellular clock theory: Since many human cells do not continue to reproduce indefinitely, this theory suggests that aging is a result of cells hitting their programmed reproductive limit.
- Neuroendrocrine hypothesis: Shifting hormone levels determine rate of aging. Menopause is an example of an age-related hormone change, which in turn results in aging (less estrogen, greater susceptibility to conditions like osteoporosis).
- Immune system decline: Specific immunity cells decline in number and how well they function with age. This leaves the body more vulnerable to life-threatening diseases like cancer.
Aging is a result of cell damage:
- Parts of cells just wear out: An example of this is the shortening of telomeres, the caps on cell chromosomes. Each time a cell reproduces, the chromosome or genetic material loses a bit of the protective cap until the cell can no longer divide.
- Oxidative (free radical) stress: The theory that free radicals -- the toxic byproducts of cell metabolism or oxidation -- are responsible for damaging DNA and therefore a cause of cell death. Though cleanup systems within cells exist, the damage is cumulative over time. Mitochondria, the engines of cell metabolism, are thought to play a central role because of the disproportionate free radicals they produce. First proposed in the 1950s, this theory has led to the belief that consuming antioxidants in food would counteract this process.
- Cross-linking (glycation): Over time, cells' proteins and DNA or genetic material develop unnecessary attachments, or cross-linkages, which can interfere with cell function.
- Genome error: Caused by environmental toxins or spontaneous mutations, damage to our DNA that isn't repaired on a cellular level can lead to cell death and organ malfunction, accelerating the aging process.
Over time, gerontologists will determine which of the above theories, or combination of theories, is the greatest contributor to aging.
Age-Related Changes in Your Body
While people age at different rates depending on genetics, diet, culture, activity levels and environmental exposure, certain long-term population studies give us clues about what changes we can expect. Here's what researchers in the Baltimore Longitudinal Study of Aging, in which more than 1,400 people from age 20 to 90 have been tracked since its inception in 1958, have observed:
Cardiovascular System: Heart muscle thickens and blood vessels stiffen with age, reducing the amount of oxygen available to the brain and body. Breathing capacity declines by as much as 40 percent between 20 and 70 years of age.
What you can do: Regular, sustained exercise can improve heart and lung function at any age.
Brain and Nervous System: Some nerve cell structure is lost, along with some function of individual nerve cells. Adult nerve cells may be reproduced, but the extent of this regeneration is not fully understood. Most severe decline in mental function is a result of diseases like Alzheimer's and dementia, not normal aging.
What you can do: Scientists are just beginning to learn how plastic, or adaptable, the brain is. You can improve memory and other brain functions by trying brain exercises and learning new skills (like dancing or playing a musical instrument).
Kidneys, Bladder and Urinary Systems: Kidneys lose efficiency at clearing waste from the blood. Bladder capacity declines and urinary incontinence can occur, especially in women.
What you can do: Medication and behavioral techniques can improve bladder capacity.
Bones and joints: Bone density diminishes on average beginning at age 35, with an accelerated rate of loss in menopausal women.
What you can do: Weight-bearing exercise, like strength training, in addition to walking and running, has been shown to help maintain bone density.
Muscles: Between the ages of 30 and 70, muscle mass declines more than 20 percent in men and women, in the absence of regular exercise.
What you can do: The same regular exercise that slows bone loss will help maintain muscle mass.
Eyes: By age 40, the lens begins to stiffen, resulting in difficulty focusing on near objects. Age-related eye conditions, such as macular degeneration and glaucoma, can develop. Cloudy areas on the lens will become cataracts in more than half of people by the age of 80, according to the National Eye Institute.
What you can do: It's believed that limiting tobacco use and ultraviolet light exposure will help prevent cataracts. Cataract surgery is a safe and effective remedy.
Ears: Two types of hearing loss that commonly occur with advancing age are presbycusis, which tends to run in families, and tinnitus, a ringing or hissing in the ears that can also be a sign of other problems, like high blood pressure. Hearing declines with age more rapidly in men than in women.
What you can do: An audiologist or hearing specialist may recommend a hearing aid. Other devices, like phone or television amplifying devices, may make declining hearing ability easier to manage.
Aging Under the Microscope. Public Information Booklet. U.S. National Institutes of Health, National Institute on Aging. Accessed October 17, 2011. http://www.nia.nih.gov/health/publication/biology-aging/aging-under-microscope
Florian L. Mullera, Michael S. Lustgartenb, Youngmok Janga, Arlan Richardson, Holly Van Remmen. Free Radical Biology; Medicine 43 (2007) 477-503. http://www.rand.org/content/dam/rand/www/external/labor/aging/rsi/rsi_papers/2009/benz5.pdf
Hearing Loss and Older Adults. Public Information Sheet. U.S.National Institute on Deafness and Other Communications Disorders. Accessed October 17, 2011. http://www.nidcd.nih.gov/health/hearing/Pages/older.aspx
The Beginning of Something New-The Baltimore Longitudinal Study of Aging. Public Information Sheet. U.S. National Institutes of Health, National Institute on Aging. Accessed October 14, 2011.http://www.grc.nia.nih.gov/branches/blsa/blsafindings.pdf
Why and How do we Age? Public Information Sheet. American Federation for Aging Research. Accessed October 17, 2011. http://www.afar.org/pdfs/AFAR-Guide-to-Theories-of-Agingsm.pdf