A strong skeleton is just as important as a healthy heart.

Bones form the frame that keeps our bodies from collapsing and serve as a bank for minerals essential to multiple bodily functions. In fact, 99% of the body’s calcium is found in the bones and teeth (NIH n.d.). The skeleton anchors everything fitness professionals deal with every day: muscles, joints, tendons, the whole kinetic chain.

But how much do you really know about bones? Although bones are vital to life itself, knowledge and interventions geared at preventing unnecessary bone loss and subsequent osteopenia and osteoporosis are still lacking. According to the Surgeon General, “The biggest problem is a lack of awareness of bone disease among both the public and health care professionals” (OSG 2004).

Bone loss and sarcopenia (muscle loss) are a normal part of aging, and being aware of the pathophysiology of these processes can help fitness professionals to develop effective preventive strategies and to better inform clients. Consider that half of women over 50 and a quarter of their male counterparts suffer broken bones because of osteoporosis (NOF 2016). Health authorities project about 17.2 million new cases of osteoporosis or osteopenia between 2010 and 2030, a 32% growth rate compared with 2005–2010 (Wright et al. 2014). No doubt, you’re aware of the ample opportunity—and great earnings potential—in this population.

To excel at informing these clients and helping them develop effective preventive strategies, you need insight on the basic pathophysiology of aging bone. We’ll start by reviewing how bones grow, why they shrink with age, and what the difference is between osteoporosis and osteopenia.

Bone Modeling and Remodeling

The skeleton is composed of two types of bone: cortical and trabecular. Cortical (compact) bone comprises 80% of the volume in the adult skeleton and forms the outer layer of bone (Lerner 2012). Trabecular (cancellous) bone makes up the inner layer; has a spongy, honeycomb structure; and is mostly found in the skull, pelvis, sacrum and vertebrae. Although peak bone mass is reached in late adolescence, bones never stop changing. An adult skeleton replaces its bone mass every 10 years (OSG 2004).

Bones adapt via processes called “modeling” and “remodeling”:

Modeling—formation of new bone in one site and removal of old bone in another—occurs during childhood and adolescence. This process allows bone to grow in size and shift in space so the skeleton can adapt on the way to adulthood. Modeling is crucial to bone because it’s in the modeling years that peak bone mass develops—a significant indicator of fracture risk in later life (Rizzoli 2014). A 10% higher peak bone mass may delay the development of osteoporosis by 13 years (Hernandez, Beaupre & Carter 2003).

Remodeling happens in the fully-formed adult skeleton. Remodeling does not change the size or shape of bones; it gradually replaces them.

Osteoblasts, Osteoclasts and Osteocytes

Three types of cells make modeling and remodeling possible: osteoblasts, osteoclasts and osteocytes. Osteoblasts form bone and cause it to mineralize. Osteoclasts do the opposite, degrading bone tissue. An optimum balance between osteoclasts and osteoblasts keeps bone mass constant. Too many osteoclasts cause too much bone to be dissolved, shrinking bone mass, but if osteoclasts are too few, bones will not be hollowed out enough for the marrow. Both of these imbalances can cause osteoporosis (Gilbert 2000).

Osteocytes, the third kind of cell, originate from osteoblasts and make up 90% of all bone cells (Lerner 2012). Osteocytes are deeply embedded in cortical and trabecular bone tissue and have extensive dendritic (branching) processes through which they communicate with osteoblasts and other osteocytes. These signaling pathways play a crucial role in the skeleton’s ability to continually adapt in response to mechanical loading, because osteocytes can sense total mechanical load and trigger biomechanical responses. These responses can either encourage formation of new bone to handle heavier loads or remove bone in the absence of load (Bonewald 2006).

The Bone-Muscle Unit

Mechanical loading is the most important determinant of bone strength, influencing muscle size and force, which in turn correlate with bone mineral density (BMD) (Tagliaferri et al. 2015). We call this concept the “bone-muscle unit” because the development of one directly influences the other. This unit has extra importance during childhood and adolescence because accruing lean tissue mass during the formative years affects adult bone strength (Tagliaferri et al. 2015). If you work with children and adolescents, the bone-muscle unit represents a prime opportunity to influence present and future bone health.

Bone Loss

Bone loss and aging are inseparable: “The skeleton is a systemically regulated mass of mineralized material that is born, grows, reaches a more or less high peak, and then declines faster or slower as to develop a correspondingly high or low fracture risk”(Ferretti et al. 2003). Musculoskeletal aging—declining bone and muscle mass, increasing joint pain and stiffness, and decreasing physical mobility—is a normal part of aging. However, how rapidly or slowly bone mass declines depends on different factors. While genetic abnormalities account for 70% of the variance in skeletal strength (Rizzoli 2014), other factors—like hormones, nutrition, physical activity and toxins—play crucial roles in developing bone and losing it.

Understanding bone loss starts with distinguishing between the two basic diagnoses: osteopenia (low bone mass) and osteoporosis (advanced loss of bone tissue). Then, the smart thing to do is learn about risk factors and preventive measures.

Osteopenia vs. Osteoporosis

More clients may come to you with a diagnosis of osteopenia than with the better-known osteoporosis. Osteopenia points to low bone mass when overall bone mineral density hasn’t fallen far enough to cause serious concern. It’s critical to note that an osteopenia diagnosis does not mean osteoporosis is inevitable. How­ever, if a client mentions this diagnosis, you have an excellent opportunity to explain that physical activity and nutrition are important modifiable factors that can prevent further decline.

We distinguish between osteopenia and osteoporosis via the T-score system, which quantifies BMD (Karaguzel & Holick 2010). Because the T-score measures bone loss, it is expressed as a negative number. A T-score between -1
and -2.5 is defined as osteopenia, while a T-score below -2.5 is defined as osteoporosis.

There are two types of osteoporosis, both characterized by weakened bone and increased risk for fracture: primary (bone loss through aging) and secondary (caused by a variety of diseases, medications and toxic agents). We will focus only on primary osteoporosis since it is the more common type and also the type that fitness professionals can help clients with through physical activity and nutrition.

Osteoporosis is often called the “silent disease” because it’s usually not diagnosed until after a fracture (Porter et al. 2016). Indeed, nearly 80% of older adults who suffer bone breaks have never been tested or treated for osteoporosis (NOF 2016). Following a fracture, the risks of mortality and disability accelerate: 1 in 5 hip fracture patients end up in a nursing home (OSG 2004), and 24% of hip fracture patients over 50 die in the year after the fracture (NOF 2016).

Thus, fitness professionals who help older exercisers confront the risk factors of osteoporosis can be lifesavers.

Diet, Exercise and Bone Health

As a fitness pro, you can’t fix the genetic and environmental contributors to bone loss, but you can encourage physical activity and proper nutrition, both of which improve bone health. More than 70% of Americans don’t get the recommended amount of physical activity (Laskowski 2012), and 50% are considered deficient in vitamin D (Karaguzel & Holick 2010). One systematic review rated calcium, vitamin D, dairy and physical activity/exercise as the most important modifiable lifestyle factors that can influence the development of peak bone mass (NOF 2016).


Proper diet pulls double duty: developing skeletal strength and maintaining bone’s role as a mineral storehouse. Minerals such as calcium and phosphorus, which the body must have in order to perform every day, are stored in bone. If the body can’t get these minerals from our diet, it takes them from our bones, reducing bone mass and strength (OSG 2004).

After age 30, calcium deficiencies can lead to a gradual bone loss as high as 0.5% per year (Karaguzel & Holick 2010). Calcium is a crucial part of young people’s diets, as half of the calcium in the adult skeleton is deposited during the ages of 13–17 (Karaguzel & Holick 2010).

Consuming calcium on its own, however, is not enough. Proper absorption of calcium depends on sufficient vitamin D intake, and some research has even suggested that calcium supplements without vitamin D supplementation may increase the risk of myocardial infarction (Rizzoli 2014). Interestingly, this increased risk is not evident when calcium is consumed from food, which is the intake method that is most highly recommended. Calcium can be obtained from a variety of food sources and, even for people with dietary restrictions such as dairy intolerance, there are many food sources available.

Dairy products, however, are the most efficient way to get enough calcium. One study evaluated the importance of dairy in calcium delivery and found that although calcium supplements can influence bone remodeling, dairy products have an additional benefit for bone growth. Dairy products provide more calcium, protein, magnesium, potassium, zinc and phosphorus per calorie than any other food (Rizzoli 2014). It would take 48 servings of whole grains or 24 servings of green vegetables to provide as much calcium as there is in a 200-millileter (6.76-ounce) glass of milk.

You may also have seen milk advertised as the perfect postworkout drink. Research into the effectiveness of fat-free milk versus an isoenergetic carbohydrate drink after resistance training found that milk promoted fat loss, lean-mass gains and BMD (Rizzoli 2014). Because calcium and vitamin D go hand in hand, many milks and yogurts are now fortified with vitamin D. Note that exposure to sunlight is still the most effective way to get enough vitamin D (Karaguzel & Holick 2010).

Although vitamin D is the main factor in calcium absorption, research suggests that absorption increases with higher dietary protein consumption in postmenopausal women (Tagliaferri et al. 2015) and that higher protein intake correlates to a lower rate of age-related bone loss (IOF 2017).

Note: Be aware of your scope of practice. You can inform clients about nutrition, but you cannot prescribe specific nutrients. Always refer your clients to their physician. In fact, suggesting they talk to their physician about bone health could be a motivating factor. Studies show that patients who are aware of their BMD numbers and fracture risk are more likely to adjust their calcium intake (Rizzoli 2014).

Physical Activity

According to the Surgeon General, physical activity “is one of the most important controllable lifestyle changes to help prevent (or reduce the risk of) a number of chronic diseases” (OSG 2004), and although it is a leading health indicator, more than 80% of adults do not meet the guidelines for both aerobic and muscle-strengthening activities (ODPHP 2018).

McMillan et al. (2017) state that physical inactivity, or sedentary behavior, has been described as “the major public health problem of our time.” Physical activity is known to influence both bone and muscle metabolism; therefore, inactivity—or a decline in activity—can affect bone through those two pathways (Tagliaferri et al. 2015). Osteogenesis occurs in response to mechanical loading. Inactivity, with its lack of loading, prevents bones from receiving the signal to adapt, which causes bone loss. It’s a basic use-it-or-lose-it scenario.

While all exercises affect muscle and bone, recent research indicates that some activities help bones more than others. In program development, it is important to consider that the skeletal sites closest to the engaged muscle will have the biggest BMD increases (McMillan et al. 2017). For example, in sprinters and shot putters, BMD is highest in the legs and dominant arm, respectively. This is helpful to know when working with clients who have compromised BMD in certain sites.

Muscles are the key factor in exerting mechanical force on the skeleton, so increasing muscle strength is important—especially in the bones most likely to fracture (hips, wrists, vertebrae, etc.). See the sidebar “Activities and Bone Health” for bone-building exercises.

Making Bone Health a Priority

You probably don’t have many clients coming in and saying, “I want to grow a strong skeleton.” But you know that skeletal health is an essential foundation for all the work fitness professionals do. Therefore, prioritize knowing the basics of bone development and loss and figuring out how to help your clients grow and retain the strongest bones possible.