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Z-Score For Bone Mineral Density: What It Is & Ranges

Written by Biljana Novkovic, PhD | Last updated:
Puya Yazdi
Medically reviewed by
Puya Yazdi, MD | Written by Biljana Novkovic, PhD | Last updated:

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Knowing your Z-score can help you compare your own bone mineral density (BMD) to those of a healthy person of the same age and body size as you, and help predict your risk of bone fractures or osteoporosis. Keep reading to learn about low and high BMD Z-scores, as well as ways to increase it.

What is a Z-Score?

Your Z-score is a comparison of your individual bone mineral density (BMD) to what is expected for a person of the same age and sex. The Z-score represents how far off your score is (measured in the number of standard deviations) from the average score of healthy people of similar age, ethnicity, and gender [1, 2].

Z-score differs from T-score, which compares your BDM to that of a healthy young adult [1, 2].

Normal Range of Z-Score

A normal BMD Z-score ranges from -2.5 to 2.5 [3, 4].

A normal Z-score means that you have a similar BMD to other healthy people in your age group. A lower Z-score means your BMD is lower and a higher Z-score means it’s higher.

Low Z-Score Causes/Associations

A low Z-score is associated with secondary osteoporosis. Osteoporosis is when bones become weak and brittle, resulting in a higher risk of bone fractures [5, 6].

Unlike primary osteoporosis which is age-related and has unknown causes, secondary osteoporosis is not age-related and results from a specific health disorder or disorders [6].

Causes shown here are associated with lower BMD and Z-scores. Work with your doctor or another health care professional to get an accurate diagnosis.

Remember, a Z-score can’t be used to diagnose osteoporosis. Your doctor will interpret your Z-score, taking into account your medical history, symptoms, and other test results (such as the T-score).

1) Alcoholism

Alcohol abuse can decrease bone density.

In a study of 105 people, alcoholics had lower Bone Mineral Density (BMD) Z-scores than non-alcoholics. Increased alcohol consumption can also lead to osteoporosis [7].

2) Malnutrition

If you don’t get enough vitamins and minerals in your diet, your bones can become weak. Protein and vitamin D deficiency are two common causes of osteoporosis. People with malnutrition generally have lower bone mineral density than people who get enough nutrients from their diet [8, 9, 10].

Scientists have found that women with anorexia have lower BMD, impaired bone metabolism, and low BMD Z-scores. Even after patients gain weight, they can still have bone density deficits [11].

3) Celiac Disease

People who have celiac disease have an increased risk of low BMD. In one study, newly diagnosed celiac disease patients had a significantly low Z-score. The study suggests that BMD improves with a gluten-free diet [12].

4) Inflammatory Bowel Disease (IBD)

People with IBD, especially Crohn’s disease, can have low BMD and bone loss. Both those with ulcerative colitis and Crohn’s disease may have a lower BMD Z-score [13, 14].

5) Diabetes

Researchers found that type 2 diabetics tend to have lower bone mineral density compared to healthy people. Diabetic medications can also reduce BMD. In one study, 67 diabetics had lower BMD Z-scores compared to healthy people [15].

6) Liver Disease

According to research, people who have chronic liver disease and liver scarring (cirrhosis) patients have lower BMD and Z-scores [16].

7) Thyroid Conditions

Both hyperthyroidism (overactive thyroid glands) and hypothyroidism (underactive thyroid glands) have been associated with osteoporosis and increased bone fracture risk. People with toxic goiters (which eventually leads to hyperthyroidism) have low BMD Z-scores [17, 18].

8) Hyperparathyroidism

Overproduction of parathyroid hormone from the parathyroid glands (hyperparathyroidism) increases bone fracture risk [19].

9) Premature Ovarian Failure

Premature ovarian failure, or primary ovarian insufficiency (POI), is the loss of ovarian function before the age of 40. Women with POI have significantly lower BMD and Z-scores than healthy women [20].

10) Cushing Syndrome

Cushing syndrome is a disorder that occurs when the body is exposed to too much cortisol. High cortisol levels can trigger bone loss, resulting in a lower Z-score [21].

11) Multiple Myeloma

Multiple myeloma is a cause of secondary osteoporosis; patients have lower BMD and BMD Z-scores compared to healthy people. Scientists have found that after treatment for myeloma, their BMD dramatically increases [22].

12) Drugs

The following drugs can cause secondary osteoporosis and are associated with low BMD Z-scores [5, 17, 23]:

  • Steroids (glucocorticoids – they have an effect similar to cortisol)
  • Antiepileptic drugs
  • Antidepressants
  • Proton pump inhibitors
  • Anticoagulants (blood thinners)

Ways to Increase Your Z-Score

The most important thing is to work with your doctor to find out what’s causing your low Z-score and to treat any underlying conditions. The additional lifestyle changes listed below are other things you may want to discuss with your doctor. None of these strategies should ever be done in place of what your doctor recommends or prescribes!

Diet

Eating more fruit and vegetables can help improve your bone health. In two studies (n=926, n=3089), fruit and vegetable intake was associated with increased BMD and Z-score. Elderly people who ate veggies also had a reduced risk of osteoporosis [24, 25].

Get more calcium, which is essential for healthy bones. In a study of 4,797 adults, milk consumption was associated with higher BMD and Z-score [26].

Get enough protein, but not too much. Protein is required for healthy bones, but as your body digests protein, it releases acids into the bloodstream, which the body neutralizes by drawing calcium from the bones. Over time, this can result in weaker bones [27].

Limit your caffeine intake. Drinking a lot of coffee (about four or more cups per day) can increase the risk of bone fracture. Caffeine promotes calcium excretion in the urine, which negatively impacts your calcium balance [28, 29].

Lifestyle Changes

Don’t go overboard with alcohol. In a study of 105 people, alcohol consumption was associated with bone loss, bone breakdown, and low BMD Z-scores. When these people stopped drinking alcohol, vitamin D and osteocalcin levels increased, indicating an increase in bone mass and BMD [7].

Supplements

Discuss the following supplements with your doctor. Studies suggest they can be beneficial for bone health:

High Z-Scores

A high BMD Z-score indicates that you have a high bone mineral density. A high BMD is not necessarily healthy. BMD may be elevated in certain medical conditions [4].

Causes of High Z-Scores

Causes listed below are associated with higher BMD and Z-scores. Work with your doctor or another health care professional to get an accurate diagnosis.

The following conditions result in a high BMD [4]:

  • Fractured vertebrae
  • Spondylosis (spinal disc degeneration)
  • Ankylosing spondylitis (an inflammatory disease that affects the spine and large joints)
  • Paget’s disease (a bone disorder that disrupts the replacement of old bone tissue with new bone tissue)
  • Myelofibrosis (bone marrow disorder)
  • Acromegaly (overproduction of growth hormones)
  • Renal osteodystrophy (a disease with abnormal levels of calcium and phosphorus
  • in the kidneys)
  • Fluorosis (white lines or streaks on the teeth caused by overexposure to fluoride)
  • Sclerosteosis (a genetic disorder that causes bone overgrowth)
  • Diffuse idiopathic skeletal hyperostosis (hardening of ligaments in areas that attach to the spine)
  • Tuberous sclerosis (the growth of many non-cancerous tumors in the skin, brain, lungs, heart, and other organs)
  • Tumors

It’s important to work with your doctor to find out what’s causing your high Z-score and to treat any underlying conditions!

About the Author

Biljana Novkovic

Biljana Novkovic

PhD
Biljana received her PhD from Hokkaido University.
Before joining SelfHacked, she was a research scientist with extensive field and laboratory experience. She spent 4 years reviewing the scientific literature on supplements, lab tests and other areas of health sciences. She is passionate about releasing the most accurate science and health information available on topics, and she's meticulous when writing and reviewing articles to make sure the science is sound. She believes that SelfHacked has the best science that is also layperson-friendly on the web.

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