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Bone Density Assessment

Posted Oct 16 2009 10:03pm

Lex Rooker, a reader of PaNu and a blogger in his own right, has asked me to comment on the results of his recent DEXA bone density scan.

I believe Lex had this done for two reasons. Firstly, he was a vegan for 20 years, which to the degree he was relying on “healthy whole grains” and was likely deficient in Vitamin K2 from animal sources, he might legitimately be concerned about his bone density. Secondly, Lex is in the avant-garde of VLC carnivory, following a raw meat/no dairy regimen. He needs to assuage those (not me) who think his diet is deficient relative to the SAD, or those who think (like Loren Cordain, for instance) that one must use a pH meter on food to avoid dissolving your bones due to “acid base imbalance”, as if our kidneys are not smart enough to do that for us. Unless Lex eats butter, there might still be some concern about K2, but a priori I am generally not concerned that a Steffanson- like diet that otherwise has enough D3 and K2 is bad for your bones.

He has posted his results here.

Here are Lex’s comments and the abstracted results:

“I finally got the results of my DEXA scan.  Considering that I was a devout vegan for 20 years, and dental x-rays from 6 or 7 years ago showed that my deteriorating dental health was due in large part to loss of bone density, I’m pretty happy with the results.”  

Age: 58   Gender: M   Height: 73 inches   Weight: 165 pounds

 

Region              BMD    Young Adult%   T Score     Age Matched%     Z Score

--------------------------------------------------------------------------------

L1-L4              1.212             99%              -0.1              104%            +0.4

Neck                0.976             91%              -0.7              103%            +0.2

Ward               0.805             84%              -1.2              103%            +0.2

Trochanter     0.858             92%              -0.7                98%            -0.2

Total               1.009             93%              -0.3              101%              0.0

 

Studies were performed using a Lunar DPX IQ.  Technical quality of the scans were excellent with no artifacts. According to the World Health Organization guidelines, the patient is classified as NORMAL.  Based on these results, a followup exam is recommended in two to three years.

“This is my first DEXA scan so this becomes the base line.  At this point I really don’t know much about them and the best my doctor could do was read the report which indicates that everything is normal – whatever that means.  If anyone can provide a more comprehensive analysis it would be greatly appreciated.”

Lex had a dual-photon x-ray absorptiometry scan or DEXA scan. This study uses low dose x-rays at two different energies to acquire data used to calculate body density. A thin beam is passed through the body part with two energies. How much the body attenuates (diminishes) the x-ray flux and the difference in attenuation between the two energies allows calculation of the density of bone in the scanned area.

Geeks can go here.

The first thing to understand is the reason we might be interested in bone density. We are interested in it to the degree it predicts bone strength, and we care about bone strength to the degree it predicts fractures. However, in the same way that the density of a pile of bricks and a mortared brick wall might be the same, but they obviously differ in strength, we use density of bones as a proxy for bone strength with the critical assumption that the structure of the bones is normal. This may or may not be the case in the pathologic state, but with healthy bone of normal structure, it’s a fair assumption that is clinically accurate.

The density of your bones, properly considered, is in units of mass per unit volume, for example grams per cubic centimeter or g/cm3. If you have bone density measured with quantitative CT (an accurate technique not used today as it has a fairly high radiation dose) the results are reported in g/cm3. Confusingly, DEXA scans give a measure that is clinically accurate but sounds like it comes from “ flatland ” – the units of density are in grams per centimeter squared or g/cm2. This is because the data are acquired from a planar (two dimensional) projection of a three-dimensional volume – your body. A density reported in g/cm2 is disturbing to those of us with training in the physical sciences, but it correlates with true density measurements closely, and is the basis for all the clinical data acquired on populations.

So what do the numbers mean?

In interpreting these numbers, it’s good to know what we know as well as what we don’t know. We know from clinical studies that there is a certain risk of fracture for white women that can be estimated from their age and the bone density measured at the proximal femur (hip) or the spine.

Note that age and bone density are independent predictors of fracture risk. A 65 year old with t-score (explained below) of -3.5 has the same fracture risk as a 80 year old with a better t-score of -2.5. Not all fracture risk relates to bone fragility. Weakness due to age-related sarcopenia (muscle loss) and neurological degenerative diseases also affect fracture risk.

We know also fairly well how some drugs can mitigate this risk of fracture in white women. In those with a history of previous fracture or with t-scores worse than -2.5, bisphosphonate drugs can reduce fracture risk by as much as half.

We don’t really know as well how data from other ethnic groups or from white men predict fractures.

From Lex’s study:

Region              BMD    Young Adult%   T Score     Age Matched%     Z Score

--------------------------------------------------------------------------------

L1-L4              1.212             99%              -0.1              104%            +0.4

 

L1-L4 means the first through fourth lumbar vertebrae in the spine (low back) were measured. BMD of 1.212 means the areal (flatland) bone density at these four levels averaged together is 1.212 g/cm2.

Young adult % means Lex’s BMD at L1-4 is 99% of the mean for a male of age 30.

One caveat about BMD in the spine. Measuring BMD at the spine can be “false negative”. If there is enough arthritis, bone spurs can falsely elevate the bone mineral density measured, and give a reading that seems normal when the central structure of the bone is actually weak. The interpreting radiologist has the responsibility to look out for this situation by looking at a low-resolution image obtained at the same time as the DEXA scan.

The t-score is the same statistic normalized mathematically, and expressed as standard deviations above or below the mean for a 30 year old, presumed to be the peak of bone mass. A t-score of zero means you have the same BMD as a 30 year old of your sex that anatomic site, regardless of your age.

T-score is the clinically used measure and the one on which fracture risk estimates and treatment decisions are based. A good rule of thumb is that each decrement of t-score of -1.0 doubles your lifetime risk of fracture. So a t-score of -2.0 is about 4 times the risk as 0.0. For hip and spine BMD, a t-score of -1.0 equates to having BMD about 15% lower than those with t-score of 0.0 (normal 30 year old). A woman will typically lose about 1% of bone density per year after age 60, so if starting at -1.0 it would take about 10 years to get to -2.0.

The WHO (world health organization) arbitrarily defines t-scores of 0 to -1.0 as normal, -1.0 to -2.5 as osteopenia. (More than half of post-menopausal women are “osteopenic” by this definition.) The WHO defines t-scores lower than -2.5 to be osteoporosis. Scientifically, the risk of fracture is a continuum. These WHO definitions are more about enabling comparative epidemiology and big pharma marketing of drugs than they are about science.

Lex’s t-score at L1-4 is -0.1 meaning his BMD is just 1/10 of a standard deviation below the mean for 30 year old men. 

The z-score is your bone density relative to others of the same age. In Lex’s case, his L1-L4 score is 4% higher than the average 58-year-old.

The z score is not predictive of fractures, it just tells you how bad or good your numbers are relative to your peers. Being the winner of the nursing home bone density derby among your 90-year old peers does not give you any protection. It’s the t-score that counts.

Here are some charts from a good review article in JAMA. You can look at these along with my short narrative to get a feel for fracture risk and bone density.

 

 

Continuing with Lex’s results:

 

Neck                0.976             91%              -0.7              103%            +0.2

Ward               0.805             84%              -1.2              103%            +0.2

Trochanter     0.858             92%              -0.7                98%            -0.2

Total               1.009             93%              -0.3              101%              0.0

 

For the rest of Lex’s study, the numbers mean the same, they are just numbers from different body sites. Neck means femoral neck of the proximal femur or thigh bone. Ward means ward’s triangle, a particular small anatomic area. Trochanter is another specific area of the proximal thighbone. Total refers to the whole hip area.

Femoral Neck and Total are the most predictive of future fracture risk. Incidentally, it is hip fractures that are most significant clinically. Spontaneous insufficiency spine fractures are common and painful, but are not as life-limiting and life threatening as hip fractures.

Finally, what can we say about Lex’s bone mineral density?

At the least, we can say that we see no evidence of any significant abnormally at any of the areas surveyed, at least based on the stated numbers and assuming the spine numbers are not falsely elevated.

There is absolutely no way to know if his bone density might be higher now if he had never had a vegan diet, but there is also no real reason to suspect that it would be.

From what we know of the way those with severe nutritional deficiencies are able to correct their bone density once they get proper nutrition, it is reasonable to think that if Lex did have osteopenia from his veganism that it may well be fully corrected by now.

In the context of the concern that his present raw paleo diet might be “leaching the calcium from his bones” due to acid-base imbalance from lack of lettuce or some other nonsense, we also see no evidence for that.

Finally, we should be aware that all the reference data, the "normal" values, may be representing pathology which we have made normative by our perverse definition of normal. It seems quite likely that the epidemic of hip fractures in old white women, heretofore blamed on northern european genetics and long lifespans, may be yet another disease of civilization caused by deficiency of Vits D3 and K2 and grain consumption.

So should you have your bone density assessed?

If you have a history of long term steroid use (prednisone, etc.), if you are a woman over age 50 on the SAD, or if you have some concern that prior or current dietary indiscretions (lots of healthy whole grains and lack of D3 and K2, for instance) have affected your bone health, then DEXA is a safe and reasonably effective test to get reassurance.

I have a DEXA machine at my imaging center. We are not necessarily the cheapest, but if you mention the PaNu weblog and want to come in for a scan you will get a small discount and also something you never get anywhere else - a sit down consultation with the radiologist to review your scan. We also can do coronary calcium scoring on our 64-slice CT. I don’t think everyone needs calcium scoring (blog posts on that are coming up eventually), but if you decide you want it we can review those results with you, too.

Finally, the Vitamin D post discusses osteoporosis (the medical definition, not the WHO one) and my recommendation from that post remains:

"Vitamin D, grain avoidance and eating grass-fed butter and hard cheeses (for the K2) are my strong recommendations for avoiding osteoporosis."

References:

Clinical Use of Bone Densitometry - Cummings, Bates and Black

Clinical Challenges in Management of Osteoporosis - Vondracek, Minne, McDermott

 

 

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