What Happens To Bones When We Age?
Aging happens to everyone, but how we age is, to some degree, something we can influence. Our body is always in flux, changing in one way or another. During some periods of growth, like puberty, that span of change is fairly rapid and dramatic. The process of what and how things change throughout our body in our later years can be more gradual, depending on the individual and their context. But whether we experience more abrupt changes or slow shifts over time, every system in our body changes as we get older. In this article, part two in our series on tissues and aging, we’ll look specifically at how aging affects our bones.
What’s a bone?
Remember that bone is a living tissue. It’s not a dry, desiccated, or unchangeable substance. It’s actually a type of connective tissue. Bone has a particular extracellular organic matrix, which its components, cells, lipids, and inorganic minerals, exist within. That matrix is collagen, as well as some additional proteins. Within that matrix, we find bone cells. Bone cells include osteoblasts, osteocytes, and osteoclasts. Osteoblasts synthesize bone. Osteocytes help regulate the mineral concentration in bone. Osteoclasts participate in the process of breaking down bone, called bone resorption.
Our bones have multiple functions. They have the obvious structural and mechanical function of providing support for our structure. The particular way that bones come together at a joint allows for movement and helps distribute load and force. Bones also serve as a production site for blood cell production and a reservoir for certain minerals.
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How do bones develop?
Our bones begin to form in the embryo, initially as cartilage. The cells present in that tissue differentiate into bone cells, and eventually, that cartilaginous tissue ossifies into what we recognize as bone. Our bones continue growing and changing in character as we move through stages of childhood and early adolescence. Our long bones, like our femur, tibia, and humerus, for example, stop getting longer around puberty. However, our bones continue a different growth process, called appositional growth, well into our adulthood. That process allows them to get wider and change shape as a response to mechanical loading.
How do bones change as we age?
There are many normal changes that happen to our bones as we move into our middle and later years. One of the biggest changes is that our overall bone mass commonly begins to decline as we move from early adulthood into our middle years. This happens because the balance between new bone creation by osteoblasts and the breaking down of bone by osteoclasts shifts in favor of more resorption compared to new bone creation. In addition to the loss of bone mass, we typically experience an increase in bone marrow fat.
Exactly why this shift in favor of more resorption and less bone creation happens is uncertain. There appear to be some unavoidable age-related reasons for this change. But, some amount of bone loss may also be correlated with a decrease in physical activity that is common in middle age through our older years.
Bone mass decline
The loss of bone mass potentially starts earlier than you might expect. Peak bone mass typically occurs between ages 15 and 30, depending on individual factors, especially gender. Women typically develop peak bone mass earlier, between ages 15 and 20. Men typically reach peak bone mass later. Declining bone mass commonly starts somewhere around age 40, if not before. Men generally have a higher peak bone mass than women. Women typically experience an accelerated decline in bone mass at menopause due to the somewhat abrupt decline in estrogen.
Other changes
In addition to the change in the balance of bone formation with breaking down, there are other distinct age-related changes in bone that occur in both men and women. Changes in protein content, mineral content, and bone architecture, as well as an increase in microfractures, usually occur. Decreased calcium and vitamin D availability due to the overlapping effects of nutrition and age-related hormonal changes reduces the availability of those minerals. That can accelerate bone loss by decreasing bone formation. The cumulative effect of these age-related changes is that bones become more fragile in our later years.
Why do age-related changes to bone matter?
Age-related changes to our skeletal system and bones can have wide-ranging effects. These changes, perhaps surprisingly, can affect other body systems because they are intricately interwoven. Recent research shows an interesting two-way relationship between bone growth and maintenance and immune cell development and maintenance. Changes in bone mass can therefore affect our immune system, and vice versa. Similarly, our vascular system is interrelated with our skeletal system. Age-related decline in our vascular health can affect our bone health.
Osteoporosis and fractures
More specifically related to our skeletal system, a decrease in bone mass in our later years can lead to osteoporosis. This is the term used to describe a particularly significant decrease in bone mass, usually age-related, along with structural changes to our bones, which make them more fragile.
The loss of bone mass, generally, and osteoporosis specifically, can increase the risk of bone fractures during a fall as well as increase the risk of spinal deformities and other bony dysfunction. As you might remember from my last article on aging and muscles, our balance and proprioception tend to decline as we age, making falls more likely. The increasing fragility of our bones makes the potential consequences of those falls more serious.
What can you do to slow bone decline?
Age-related changes in bone are not consistent across the population. Some people experience more dramatic bone-related changes than others, and some people experience them much sooner than others. There are multiple factors that affect the individual trajectory of this change. Some part of what we experience is influenced by our genetics. However, healthy lifestyle choices also significantly affect how we experience aging.
Healthy lifestyle choices to maximize our peak bone mass delay the effects of age-related bone mass decline. Those healthy lifestyle choices that increase our likelihood of maximizing peak bone mass include things like maintaining optimal exercise routines. That means incorporating weight-bearing exercise in multiple planes by doing strength-training as well as activities like Pilates or yoga. A healthy lifestyle for maximizing bone mass also includes not smoking, maintaining optimal nutrition, including sufficient calcium and vitamin D, and avoiding excessive alcohol.
References
Boskey, A. L. and R. Coleman. 2010. Aging and bone. J Dent Res. 89(12):1333-48.
Kiebzak, G.M. 1991. Age-related bone changes. Experimental Gerontology. 26(2–3): 171-187.