Here's a brief review for those who haven't been following along here more than two years ago. In May 2010, I broke my ankle in a seemingly benign mountain biking mishap. I dabbed while going slowly and rolled my ankle. The resultant triple fracture was anything but benign. Several hours later, a plate, six screws and two pins put my tibia and fibula back together again.
Later that fall, after a complete recovery, my doctor and I agreed it was worthwhile to get a bone density scan due to the freakiness of my fracture coupled with the fact I seemed to have lost some stature recently, around a half-inch in height. Sure enough, I had very low bone density for my age. My doc gave me options of going on prescription meds or bringing weight bearing, impact exercise into my routine. I chose the latter.
So here I am, two years after the initial diagnosis of osteopenia (not a disease, just term for low bone density), with another DEXA bone scan result. I had high hopes of making some improvements the natural way without taking huge risks with the bisphophonate drugs that come with serious side effects. My bone density has changed very little in two years.
This first plot gives my raw bone density in grams/square centimeter over the last three measurements. Ideally, my spine should be around 1.2g/cm^2 and my hips should be around 1.1g/cm^2. I'm well be low these levels. This doesn't say anything about how I compare to the rest of the population or how this increases my relative risk of fracture. We'll get to that in a moment. The point here is I didn't get worse, and my left hip improved from two years ago, probably because it atrophied more than my right while I was in a cast for nine weeks of 2010.
This next chart gives my young adult T-score for the three measurements spanning two years. Because my BMD hasn't changed much and we lose bone density slowly at age 50, these lines look similar to the absolute bone density. But the T-score tells us a little more. The units are in standard deviations, so it takes my population distribution (Caucasian male) into account. Average is zero. Negative numbers are below average. A T-score of -2.0 says about 97% of my population group has higher bone density than me. In other words, I'm at the very bottom end of the distribution. This already sounds scary, but it still doesn't say what my risk is.
One more chart. The age-matched Z-score goes one step further than the T-score, in that it ranks me with other 50 year old white males. Finally, we start to see a positive trend here. There is a slight upward improvement in Z-score, which says on an age adjusted basis, I'm improving my bone density. These improvements are very small, pretty much within the error measurement bounds. A Z-score of -1.5 says I'm in the bottom 7% of 50 year olds. Still wicked scary when you consider the risk I take every time I hop on a bicycle.
Ok, so 93% of men my age have stronger bones than I do. What is my relative risk of fracture? This gets into some really messy population studies and number crunching. It can be summarized pretty easily though. For hips, every standard deviation (Z-score number) below zero increases relative risk of fracture 2.6x. My left hip is -1.4 SD. 2.6^1.4 = 3.8x relative increase in hip fracture risk. Another way to put this, if I crash with a bunch of guys in a master's 50+ road race, I'm about four times more likely to break my hip than the other guys (assuming they're normal). As you well know, even young guys with strong bones break hips in bike crashes. My risk is four times greater than other old guys that have lost a little bone density since their youth. My risk is more than six times greater than young guys when using the T-score.
My right hip fares a little better. With a Z-score of -1.0, I'm at a 2.6x greater chance of fracturing my right hip than other 50 year old guys. Hardly a consolation.
So what about my spine? The gradient of spine fracture risk is different, only 1.6. So with a Z-score of -1.4 for my spine, we have 1.6^1.4 = 1.9x relative increase in spine fracture. Although this seems much better than 4x for my left hip, it still scares the willies out of me.
For these reasons, I become quite pragmatic in managing riding risk over the past year. It is sad I didn't compete in a single road race, but I deem some of the events I used to compete in too risky, even though I have never crashed in a road race. Likewise, riding off-road, there's stuff I just won't try or charge into anymore. I can still derive full satisfaction from cycling with what amounts to rather minor changes in riding.
So where do I go from here? I've run only about 130 hours over the span of these three BMD measurements. From what I've read, ideal running distance for bone density is 15-20 miles per week. I run half this amount. Running more than 20hrs/wk can actually have an adverse impact on BMD, in fact. It is also interesting to note that those who only cycle are seven times more likely to have osteopenia than runners. That used to be me. No weight bearing impact.
I wish I knew how I got here. Some people, due to lifestyle and poor diet, never reach normal peak bone density. Then in early adulthood, you start the long, downward slope from an already low density. I was pretty active as a kid (weight bearing), got lots of sunlight (vitamin D) and was always a big dairy eater (calcium). I'd like to think I reached normal bone density. I didn't become sedentary until my mid-20's.
It could be that my sedentary period from roughly 25-35 years old saw me lose BMD more rapidly than average. Even when I began cycling, lost weight and became quite fit into my 40's, I was still essentially sedentary from my bone's perspective. All-day desk job and no impact when I exercised. Couple this with the fact that athletes can consume acidifying carbohydrates and lose calcium in their sweat, you have perfect conditions to mount an attack on your bones.
I will probably continue running for now. I derive satisfaction from it and running can get you a "fix" in a pinch. I may even try more running events in 2013. BMD changes very slowly, up or down, maybe only a percent or two per year. I do not plan to get another scan for two years. If running can at least maintain my bone density, my Z-score will eventually catch back up to the normal range, but it may take 20 years.