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The interpretation of DEXA-derived body composition parameters and its implication

By Huiling Estelle Shao and Dr Vince Vardhanabhuti

Over the last century, with better health care, healthier lifestyles, sufficient food and reduced child mortality, human lifespan has seen a significant increase. As we live longer we hope to add years to the middle of our years when we are more vibrant rather than to the ends of our lives. Therefore in recent years, we see a transition in medical research from reactive healthcare to proactive healthcare. Proactive healthcare, in other words, preventive healthcare, differs from reactive in the sense that actions are taken before symptoms manifest. The efficiency of preventive healthcare relies on powerful biomarkers which can predict disease risk and mortality and body composition (BC) parameters are among these predictive biomarkers. While computed tomography (CT) and magnetic resonance imaging (MRI) have conventionally been gold standard in measuring body composition parameters, there is increasing demand for more accessible and affordable methods to measure BC, e.g., dual-energy X-ray absorptiometry (DEXA) scans. In our previous two blog posts, we gave an overview of DEXA scans and reported clinical evidence supporting the correlation between BC parameters and mortality risk. We have known that DEXA scan measures bone mineral density (BMD), fat mass and lean mass, which can be applied to evaluate disease and mortality risk through evidence-supported correlation with these measurements. This article serves as an extension of our previous posts.

The purpose of this article is to:

1. Provide ways to interpret DEXA results

2. Help to understand the alert messages DEXA results convey

3. Highlight healthy lifestyles to avoid such alert messages


Bone mineral density (BMD)


Bone mineral density measures how much calcium and other types of minerals are in an area of your bone. This test helps clinicians to detect osteoporosis and predict the risk for bone fractures. The output of a DEXA test concerning bone mineral density is a number called T score. T scores represent numbers that compare the condition of your bones with those of an average young person with healthy bones. The diagnosis of osteoporosis is generally based on BMD T score at the hip (1). The BMD threshold used to diagnose osteoporosis differs between sexes. For women, three general diagnostic categories have been proposed by WHO (2) and modified by the International Osteoporosis Foundation (3):

1. Normal

2. Low bone mass (osteopenia)

3. Osteoporosis


Meanwhile, suitable diagnostic cutoff values for men are less well-defined than for women. Some studies suggest that a similar cutoff value for hip BMD that is used in women can be used in the diagnosis of osteoporosis in men (3).


The risk of fracture is very high when osteoporosis is present, but the risk of fracture is by no means negligible when BMD is normal (1). Besides, several clinical factors are associated with a fracture risk that is greater than what can be accounted for by bone mineral density alone (4). Fracture risk assessment, therefore, should employ specific risk factors in addition to bone mineral density. For example, age is a powerful independent risk factor that has largely been ignored in previous clinical guidelines. In women with a T score of -2.5, the probability of hip fracture is five times greater at the age of eighty years than it is at the age of fifty years (5). Thus, fracture risk can be assessed more accurately by considering both age and bone mineral density than it can by considering bone mineral density alone. This can be achieved by a Z score, which compares your bone density to the average bone density of people sharing your age and gender. Similarly, other clinical risk factors, including but not limited to family history, glucocorticoids and prior fracture, contribute independently to fracture risk6.


You need sufficient calcium to keep your bones healthy and vitamin D to help your body absorb calcium. According to the National Health Service (NHS) of the United Kingdom, adults need 700 milligrams of calcium and 10 micrograms of vitamin D per day. Common sources of calcium include dairy foods, nuts and plant-based drinks (such as soya drink). With vitamin D, it’s difficult to get from our diet. We get most of our vitamin D from the action of the sun on our skin. Although after age 65, bone breakdown outpaces bone formation and bone loss often accelerates, staying active can help slow down this natural process and prevent osteoporosis. People over 65 are recommended to get 150 minutes of moderate-intensity exercise every week and muscle strength exercise at least twice a week. It’s also a good idea to do activities to improve balance and flexibility, such as yoga, twice a week as this can reduce your risk of falling.


Fat mass and lean mass


Fat body mass provides information regarding the amount of fat tissue you have in your body and is a significant risk factor for increased morbidity and mortality, most importantly from cardiovascular disease and diabetes, but also from cancer and chronic diseases, including osteoarthritis, liver and kidney disease, sleep apnea and depression (7). Body mass index (BMI) is typically used to define overweight and obesity in epidemiological studies. However, BMI has low sensitivity and there is a large inter-individual variability in the percent body fat for any given BMI value, partly attributed to age, sex, and ethnicity. For instance, Asians have a greater percentage of body fat compared to Caucasian counterparts with the same BMI. Greater cardiometabolic risk has also been associated with increased body fat percentage, as well as the localization of excess fat in the visceral adipose (8). A point to be noted is that below normal body fat percentage is also regarded as a warning sign of poor health. Men who have less than 6 percent body fat and women with less than 16 percent body fat are considered too low. According to a 12-month case study conducted by the International Journal Sports Physiological Performance, low body fat percentage is linked to a higher risk of diseases like heart disease, gastrointestinal problems, damage to the nervous system as well as the risk of organ shrinkage and an adverse effect on your immune system. Also if you don’t have enough fat and a calorie deficit, your body will start using muscles for energy, which leads to an overall weakness in body strength. Having too less body fat as a woman can also affect the ability to maintain pregnancy. Besides, too little body fat can cause deficiencies of fat-soluble vitamins, which your body can only absorb with fat (9).


Lean body mass (LBS) is the total weight of your body minus all the weight due to your fat mass. LBM includes the weight of organs, skin, bones, body water and muscle mass. Lean body mass help to avoid calorie imbalances, boost the immune system and protect against bones becoming weaker.


A DEXA scan measures lean body mass and fat body mass, based on which we can calculate body fat percentage. We here present an Ideal Body Fat Percentage Chart (Table 1), proposed by Jackson and Pollock (10, 11). Find your age on the left-hand column, then see the corresponding body fat percentage to the right. So if you are a 40-year-old woman, a body fat percentage of around 22.2% is considered ideal.



A DEXA scan also provides an estimation for visceral adipose. Body fat tissue is traditionally distributed into two main compartments with different metabolic characteristics: The adipose tissue beneath the skin is called subcutaneous adipose tissue (SAT), whereas those around the internal organs are termed visceral adipose tissue (VAT)(12). VAT accumulation also determines a comprehensive cardiovascular risk profile and increases the susceptibility to ischemic heart disease and arterial hypertension (13). It is essential to develop quantitative criteria for defining visceral obesity. However, to date, these criteria have not been clearly defined in any modality.


According to the Obesity Prevention Strategies released by the School of Public Health, Harvard University, promising strategies for obesity prevention include:

· Choosing healthier foods (whole grains, fruits and vegetables, healthy fats and protein sources) and beverages

· Increasing physical activity

· Limiting “sit time”, such as television time

· Improving sleep

· Reducing stress




Summary


In this article, we presented the ways to interpret DEXA test results applied to evaluate disease and mortality risk and understand the clinical significance underlying these DEXA-derived body composition parameters. BMD values allow us to predict the risk of fracture while fat body mass and lean body mass provide information regarding obesity and muscle-related parameters. We also summarized prevention strategies to maintain BC parameters mentioned above in the ideal range. A lifestyle of a healthy diet and regular exercise helps us stay in the ideal BC parameter interval mitigating the development of chronic disease that may ensue if we don’t look after our bodies well.



References


1. Kanis JA. Diagnosis of osteoporosis and assessment of fracture risk. Lancet 2002; 359: 1929-1936. 2002/06/12. DOI: 10.1016/S0140-6736(02)08761-5.

2. Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. Report of a WHO Study Group. World Health Organ Tech Rep Ser 1994; 843: 1-129. 1994/01/01.

3. Kanis JA and Gluer CC. An update on the diagnosis and assessment of osteoporosis with densitometry. Committee of Scientific Advisors, International Osteoporosis Foundation. Osteoporos Int 2000; 11: 192-202. 2000/05/29. DOI: 10.1007/s001980050281.

4. Unnanuntana A, Gladnick BP, Donnelly E, et al. The assessment of fracture risk. J Bone Joint Surg Am 2010; 92: 743-753. 2010/03/03. DOI: 10.2106/JBJS.I.00919.

5. Kanis JA, McCloskey EV, Johansson H, et al. Case finding for the management of osteoporosis with FRAX®—assessment and intervention thresholds for the UK. Osteoporosis International 2008; 19: 1395-1408. DOI: 10.1007/s00198-008-0712-1.

6. Kanis JA, Oden A, Johansson H, et al. FRAX® and its applications to clinical practice. Bone 2009; 44: 734-743. DOI: https://doi.org/10.1016/j.bone.2009.01.373.

7. Pi-Sunyer X. The medical risks of obesity. Postgrad Med 2009; 121: 21-33. 2009/11/27. DOI: 10.3810/pgm.2009.11.2074.

8. Chooi YC, Ding C and Magkos F. The epidemiology of obesity. Metabolism 2019; 92: 6-10. 2018/09/27. DOI: 10.1016/j.metabol.2018.09.005.

9. Jeanes YM, Hall WL, Ellard S, et al. The absorption of vitamin E is influenced by the amount of fat in a meal and the food matrix. Br J Nutr 2004; 92: 575-579. 2004/11/04. DOI: 10.1079/bjn20041249.

10. Jackson AS, Pollock ML and Ward A. Generalized equations for predicting body density of women. Med Sci Sports Exerc 1980; 12: 175-181. 1980/01/01.

11. Jackson AS and Pollock ML. Generalized equations for predicting body density of men. 1978. Br J Nutr 2004; 91: 161-168. 2004/01/30.

12. Mittal B. Subcutaneous adipose tissue & visceral adipose tissue. Indian J Med Res 2019; 149: 571-573. 2019/08/17. DOI: 10.4103/ijmr.IJMR_1910_18.

13. Shuster A, Patlas M, Pinthus JH, et al. The clinical importance of visceral adiposity: a critical review of methods for visceral adipose tissue analysis. Br J Radiol 2012; 85: 1-10. 2011/09/23. DOI: 10.1259/bjr/38447238.


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