Understanding the Impact of Aging on the Body

The Decline in Muscle Mass and Bone Density

Getting old is not for the weak! As we transition to middle age (40-64), we start noticing prominent body changes. These changes include loss of muscle mass, bone density, muscular strength, cardiorespiratory fitness, and gains in body fat. Collectively, we could describe these changes as “hey – what happened? This sucks!”

And I didn’t even mention the lower libido and disrupted sexual function that can affect both men and women. Such is the burden of aging. Why?   

Hormonal Changes and Their Effect on Metabolism

Decreased hormone production explains much of our age-related transformation (dare I say-deterioration?) For men, testosterone levels increase during puberty, peaking at around 18 years of age. During adulthood, testosterone levels remain stable until middle age. After 44 years or so, andropause kicks in, following which testosterone levels drop by ~1-2% per year.

For women, the hormone estrogen follows a pattern like that of testosterone in men. Estrogen increases during puberty and remains stable through young adulthood. And then, sometime around age 50, menopause occurs. Following menopause, estrogen levels decrease drastically. Although it is considered “the male hormone,” testosterone is present in women as well, though in much (1/10 to 1/20) smaller quantities. Further, testosterone levels increase during puberty but start to decline by early adulthood (age 25) in women. 

Growth hormone increases throughout childhood and peaks during late adolescence for both sexes. After age 30, it decreases by about 15% per decade. This progressive loss is termed somatopause.      

_{Sources: Bruce & Collier, Wisner, Garcia}

Collectively, losses in testosterone, estrogen, and growth hormone contribute to negative changes in body composition and losses in physical function. Namely changes in body composition such as decreases in muscle protein synthesis, decreases in bone mineral density, and gains in body fat. These structural changes lead to functional losses in maximum aerobic power and muscular power, increased susceptibility to bone fractures, and decreases in muscular strength, endurance, and flexibility. And then you die.

It's grim, I know. I try to be a positive person, so let's focus on what we can do to reduce the burden of aging: exercise, eat a balanced diet, and (potentially) seek medical therapy. Stay tuned for Part 2 and Part 3 of Fitness Over 40!

1. _{Bruce, D. F. (2024, July 23). Testosterone & Estrogen in women: High vs. low vs. normal levels. WebMD. https://www.webmd.com/women/normal-testosterone-and-estrogen-levels-in-women}

2. _{Choi, Y., Kim, D., & Kim, S. K. (2024). Effects of Physical Activity on Body Composition, Muscle Strength, and Physical Function in Old Age: Bibliometric and Meta-Analyses. Healthcare (Basel, Switzerland), 12(2), 197. https://doi.org/10.3390/healthcare12020197}

3. _{Corona, G., Rastrelli, G., Maseroli, E., Forti, G., & Maggi, M. (2013). Sexual function of the ageing male. Best practice & research. Clinical endocrinology & metabolism, 27(4), 581–601. https://doi.org/10.1016/j.beem.2013.05.007}

4. _{Garcia JM, Merriam GR, Kargi AY. Growth Hormone in Aging. [Updated 2019 Oct 7]. In: Feingold KR, Anawalt B, Blackman MR, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279163/}

5. _{Pearce, M. J., & Hawton, K. (1996). Psychological and sexual aspects of the menopause and HRT. Bailliere's clinical obstetrics and gynaecology, 10(3), 385–399. https://doi.org/10.1016/s0950-3552(96)80021-6}

6. _{Wisner, W. (2024, November 29). How do testosterone levels change throughout your lifetime?. Health. https://www.health.com/chart-of-testosterone-levels-by-age-8743223}