I bet most of the Better Bones community have heard of the infamous bone density T score. But I suspect that few understand what the T score means, and even fewer know about the hidden value of the Z score. Contrary to popular (mis)understanding, the T score does not measure fracture risk or indicate that you need bone drugs. So what then does the T score indicate?
Putting the T score in perspective
The T score measures how closely your bone density compares to that of an average 30-year-old of the same sex. This comparison is expressed in terms of the “standard deviation,” or SD, which you may recognize from a statistics class as being the amount that represents the typical distance above or below the mean for individual measurements. So, if your bone density differs from the average 30-year-old’s (which it probably will!), that difference will be characterized as “–1.5” if your bones are 1.5 SD below the mean, or “+1.2” if they’re 1.2 SD above the mean.
As the chart above shows, a T score of –1.0 is described as osteopenia, and –2.5 is described as osteoporosis — even though bone density bears little relationship to fracture risk (but that’s another story).
In this second chart (below), 68%, or around two-thirds, of the population falls within 1 SD of the mean, either lower or higher than the average, and another 32% (the remaining third) of the population falls more than 1 SD beyond the mean. Keep in mind that, starting in her mid-30s, the average woman will lose 35% of her cortical bone mass and 50% of her trabecular bone mass over her lifetime (assuming she doesn’t take steps to limit bone loss). So it is totally reasonable to expect that an older woman is not going to have the bone density of a 30-year-old woman, and that her T score will be a negative rather than a positive number.
The overlooked Z score
Elsewhere in your testing results, you will find out your Z score. The Z score compares an individual to others their own age and sex — which makes it a much more realistic assessment of how your bones are faring in their lifelong journey. Where an active, healthy 80-year-old woman, compared to a 30-year-old, might have a T score of –2.0, when compared to other 80-year-old women, she could very well have a Z score of +1.0 or even +2.0!
It’s a pity that the Z score, which “compares apples to apples,” gets so much less attention than the T score. It hardly seems realistic to be worried about having lower bone density that a 30-year-old — but there’s good reason to pay attention when your bone density is much lower than people your own age. For instance, if your Z score is –2.0 or more, it means that very few people your age have a bone density that low, and it signals the need for a complete medical workup looking for all possible cause of excessive bone loss.
Unraveling the mysteries of bone density test results
Now that you know the story of the infamous T score and the neglected Z score, hopefully it will encourage you to learn more from your bone density test. If you want to do just that, look into our new online class: A worried woman’s micro-course on understanding bone density tests. This class includes a live group Q&A with Dr. Brown where you can ask your questions about the course material. Details coming soon!
Hunter, D, and Sambrook, PN. Bone loss: Epidemiology of bone loss. Arthritis Res. 2000:2(6):441-445.
The saying, “You can never be too rich or too thin” is definitely not true when it comes to bone health! It’s well established that women with low body weight have lower bone density and are at increased risk for many types of fracture.
Let’s better understand this link between body weight and fracture risk and then look at the many steps thinner women can take to start strengthening their bones.
- Experience twice the rate of hip fracture as do “healthy weight” women. (See chart below.)
- Have lower hip bone density, lower cross-sectional bone area, and less bone bending strength than “normal” weight women.
- Experience more vertebral and wrist fractures (but fewer lower leg fractures).
- Tend to lose more bone during the menopause transition than do heavier women.
So just who is included in this “underweight” category?
The “underweight” designation is determined by body mass index (BMI) using weight and height. A BMI of 18.5 or less (which would translate into a 5’3” women weighing 105 lbs or less, or a 5’5” person weighing 115 lbs or less) is considered underweight.
While the “underweight” category includes only very thin folks, there exists a weight–bone gradient link whereby lighter weight individuals have both decreased bone density and increased risk of various fractures. Those falling into the “underweight” category are at highest risk, but slender individuals (that is, those whose BMI is 18.6–21.5) are often reported to be at increased fracture risk, too, particularly if they also have low muscle mass.
How can underweight or slender women reduce their fracture risk?
This is indeed a complicated topic, which I can only begin to address here. As always, at the Center for Better Bones we look for the root cause of the problem and seek a solution with this in mind. Being significantly underweight likely indicates a serious imbalance within the system, such as digestive, emotional, or disease-related issues:
- Low body weight is often associated with weak digestion, food allergies, or food intolerances. Addressing these issues is central to improving your metabolism. Check out our 10 steps to stronger digestion and try a simple elimination diet.
- Clinically, I also see low body weight associated with a tendency towards anxiety, nervousness, and at times even straight out emotional distress and fear. While uprooting these negative emotional responses can take some time and good guidance, a good place to begin is with our free e-book, 7 secrets to reducing stress.
- If you’re an “eat-and-run” person or you regularly skip meals, discipline yourself to sit down, relax, and consume three tasty, wholesome meals every day.
- Try not to get extra calories from simple carbohydrates like sugars, pastas, and flours. Instead, increase your intake of root crops and wholesome fats like avocado, nuts, seeds, and olive oil.
- Remember, bone and muscle are built together — so include some weight-bearing, muscle-building exercise in your weekly routine.
As you can see, there’s a lot that a thin, small-framed person can do to strengthen bone and reduce their fracture risk. Being aware of your risk is the important first step!
Johansson, H. et al. A meta‐analysis of the association of fracture risk and body mass index in women. J Bone Miner Res., 2014;29: 223-233. doi:10.1002/jbmr.2017
Søgaard AJ, et al. Abdominal obesity increases the risk of hip fracture. A population‐based study of 43 000 women and men aged 60–79 years followed for 8 years. Cohort of Norway. J Intern Med. 2015;277: 306–317.
Compston, J. E., et al. Relationship of weight, height, and body mass index with fracture risk at different sites in postmenopausal women: The Global Longitudinal Study of Osteoporosis in Women (GLOW). J Bone Miner Res. 2014;29: 487-493. doi:10.1002/jbmr.2051
Our skeletons were meant to last a lifetime, so when they show signs of not living up to this promise, we need to stop and ask ourselves, “What’s causing bones to weaken?”
Seems only logical, right?
While this is how thoughtful osteoporosis treatment begins, having spoken with thousands of individuals diagnosed with osteoporosis, I can’t help but notice a glaring difference between how women and men are treated when a bone health concern arises.
The double standard of conventional osteoporosis treatment
As a rule, women are told to take bone drugs when their T scores get to -2.5 or greater — without any investigation into the possible causes of their bone weakening. Often, they’re not even carefully questioned about their history, or even given a simple test for vitamin D adequacy. For a man with similar bone density or fracture history, however, a medical workup is usually ordered; he’s not simply handed a bone drug script.
Let me illustrate these statements with the cases of Sally and Robert.
Sally, a 61-year-old woman, experienced a spinal compression fracture and was found to have low bone density. Her hip and spine T-scores were -2.5 and -2.7, so she was diagnosed with osteoporosis. The doctor felt Sally’s osteoporosis was potentially dangerous, so he recommended bone drugs, despite all their limitations and unwanted side effects, as treatment. His recommendation was accompanied by remarks like, “You’ll fracture if you don’t.” When Sally expressed doubts, she heard, “You’ll end up disabled.”
At the Center for Better Bones, we take the position that if the doctor thinks the problem is serious enough to warrant bone drug use, then it is surely serious enough to warrant a search for the causes of this problem. Yet Sally wasn’t given even one of the medical tests commonly used to identify causes of bone weakening. She was simply and forcefully told to take bone drugs.
Richard, a slender, 55 year-old man with T-scores of of -3 in the hip and -2.5 in the spine, had no fractures. Like Sally, Richard was diagnosed with osteoporosis and offered bone drugs — but beforehand, he was subjected to one of the most comprehensive osteoporosis workups I’ve ever seen. He was tested for vitamin D, parathyroid hormone, loss of calcium in the urine, blood calcium, bone formation markers, celiac disease, autoimmune factors, a hormone panel, comprehensive nutrition testing — even tests looking for unusual bone marrow disorders. The irony is, men in his family have a tendency to have low bone mass, yet none ever had a significant osteoporotic fracture — so he was given this battery of tests (and offered drugs) despite personal and familial history suggesting his fracture risk was low.
While these are just two cases, this is something I typically see in my clients: women are rarely given the same sort of medical workup for osteoporosis as men, even when (as in Sally’s case) they have already had an osteoporotic fracture.
Finding hidden causes of osteoporosis
Of course, we should all review and correct any shortcomings in diet, lifestyle, exercise, behavior, and even thought patterns that contribute to less-than-ideal bone health. But if you have excessive bone loss or low-trauma fracture, women especially should seek out a physician who will look for underlying medical causes of bone weakness. As a guide, you might print out my list of osteoporosis workup medical tests to give your doctor. Follow this up by politely asking to be treated… like a man.