As mentioned in the previous two sections of this article, the evidence is straightforward when it comes to the impact of both plyometric and strength training: these two types of training, when applied in a safe and logical manner, tend to lead to favorable outcomes related to bone density. The evidence is murkier when it comes to the impact of cardiovascular training on bone health.
There are two important considerations that must be taken into account when we look at cardio and bone health: 1) the impact of the activity itself as well as 2) the impact on energy availability. Let’s examine the impact of the activity itself first:
While cardiovascular training can be helpful for many aspects of your health including heart health (duh), respiratory health and even mental health, it is clear that all forms of cardio are not created equally when it comes to the promotion of bone health. For example, it’s been well-established that individuals who use modalities such as swimming, biking and rowing as their primary source of endurance training tend to have lower bone mineral density (BMD) scores compared to people who choose activities such as running or field sports for their cardio. Why would one cardio tool be superior to another when it comes to bone health? What do swimmers, bikers, and rowers all have in common, compared to runners or field sport athletes? Swimming, biking and rowing are all what we would consider non or low weight-bearing activities, while things like walking, running, hiking and lifting weights are all considered weight-bearing activities.
If you reflect back to part 2 of this article, the mechanism we’re trying to manipulate here is the rate of bone formation relative to the rate of bone breakdown within your skeleton. The degree to which an activity requires you to bear weight, the more stress is placed on your skeleton as a whole. This in turn leads to an increased rate of bone formation, as your body will attempt to “heal itself” stronger in response to the stress placed on it. While there is nothing wrong with using tools such as swimming, biking, and rowing to help enhance all of those other qualities, it’s important to note that in order to keep your skeleton healthy, you’ll need to be doing some weight-bearing activities on a regular basis. Running is a great example, but if it’s not your thing or if it’s caused you pain or injury in the past, there are plenty of options you can choose from! More on those options later.
This idea of weight-bearing as a necessity for bone health also goes hand in hand with our second important consideration mentioned above, energy availability. Simply put, energy availability refers to the amount of energy your body has its disposal to keep you functioning at a healthy level. We can get an idea of your energy availability if we know how many calories your body requires, compared to the amount of calories your body has to meet those requirements. A decrease in overall energy availability has been shown to lead to a decrease in BMD and bone health in general. Without getting too far into the nutritional weeds here- cardio tends to be much more costly when it comes to caloric expenditure when compared to lifting weights or performing plyometric exercise as described in earlier sections. Therefore, we must be careful not to over-do the cardio component of our training, and if we’re adding a significant amount of cardio to your weekly regimen, you must also add a comparable amount of caloric intake in order to maintain an energy balance.
In summary, our two major considerations for cardio related to bone health are: 1) the weight-bearing which occurs during the activity itself, and 2) the impact of the activity on energy availability. These two points are underlined by some of the research that has been done on weight loss programs: As an in the Journal of Nutrition pointed out, “Importantly, weight loss and weight-cycling throughout adulthood and older age were shown to increase hip fracture risk. Losing as little as 5% of body weight increases the fracture risk in postmenopausal women, especially in those who are relatively thin in middle age.” The findings above are significant: 5% reduction in weight isn’t much, just 7.5 pounds for a 150-pound individual. The fact that this small of a weight reduction leads to an increased fracture risk in women helps to underline both of the considerations laid out in this article: 1) By losing weight, you are in fact bearing less weight during any activity you perform and therefore the stimulus for bone formation, all else being equal, decreases. As a compounding variable, in order to lose weight, you’ll also find yourself in a degree of caloric deficit, which is the very definition of a decrease in energy availability.
If you have any reason for concern regarding your bone health, these factors should give you reason to be thoughtful if you’re attempting to lose weight, and would be a great reason to seek out the advice of a coach.