DAX body composition analysis | Sports Medicine | UC DAvis Health

DXA body composition analysis 

What does this scan show?

The DXA, or "Dual X-ray Absorptiometry", is a quick and pain free scan that can tell you a lot about your body. It provides you with an in-depth analysis of your fat tissue, lean mass and bone density.

The DXA is a sleek, inconspicuous machine with an “arm” that branches out over the table that you lay on. Due to its open design patients can comfortably enjoy the test without feeling claustrophobic. It works by sending dual low power x-ray beams that can accurately and precisely differentiate between bone mineral, lean mass and fat mass.

Example analysis from a DXA scan (PDF)

The main goal of the DXA is to provide you with an in-depth analysis of the main components of your body; fat, muscle and bone. After the scan, you will be given a multi-paged print out where you will see percentages, mass, and images accounting for the various data obtained.

The great thing about the DXA scan is that it requires very minimal preparation. For more accurate results you should make sure you are well hydrated and not have any food in your stomach (at least 3 hours since your last meal). It is also important to not take calcium supplements 24 hours prior to your test to ensure accurate bone density readings.

Upon arriving at our medical office you will be greeted and taken back to meet with the licensed technologist who will perform your scan for you. After measuring your height and weight, you will be asked to lie down and get comfortable and the scan will begin. The scan takes 6 minutes.

Once the scan is over you will be able to sit down with the exercise specialist to go over your results. Your results will be explained to you and suggestions will be given according to goals that you have (i.e., lose fat, gain muscle, increase bone density). You will be able to keep your packet of results as a reference in the case that a follow up is desired in the future. Note: it is beneficial to do this scan every 3-6 months for body composition and every year if you are looking to modify something specific such as bone density.  

Because this test gives so much detailed information regarding various components in your body, it is a scan that can be used for anyone. Athletes can get this scan done if they are curious to track their muscle mass as well as overall fat percentage. Due to its broad uses, the average person who is simply curious about their health could get this scan in order to gain insight regarding their body composition.

Total Body Fat Percentage (%BF): The percent of the body that is composed of fat. This will change based on the amount of fat there is as well as the amount of lean mass there is.

Fat Mass Index (FMI): The total amount of fat you have (in kilograms) relative to your height (in meters2).  It is a measure of how much total fat you have, relative to your size and independent of lean mass.

Visceral Adipose Tissue (VAT): VAT is a hormonally active component of total body fat. The measurement reflects the amount of internal abdominal fat around the organs.  This is different than subcutaneous fat, which lies beneath the skin.  Increased VAT has a high correlation to cardiovascular and metabolic disease risk. Current research shows and elevated risk at around 100-160 cm2 and

Android to Gynoid Ratio: The “apple” to “pear” shape ratio. It describes where the fat is stored. Android (apple shape) refers to having most of the fat around the stomach and mid-section. Gynoid (pear shape) refers to having the fat stored around the hips. A bigger number means more android and a smaller number means more gynoid. From a health risk standpoint, ideal values are believed to be less than 0.8 for women and 1.0 for men.

Fat Free Mass Index (FFMI): The amount of mass that is not fat, relative to your height. This includes muscle, bone, organs and connective tissue. It can be used to gauge relative muscle mass in lean individuals.

Skeletal Muscle Mass (SMM): An estimate of the total amount of skeletal muscle you have.  Because muscle has approximately the same density as other organs (liver, skin, etc…) and other types of muscle (heart, smooth muscle, etc…) we are not able to directly the amount of skeletal muscle you have. This is true of any commercially available body composition measurement (bioelectrical impedance, underwater weighing). However, several scientific studies have been performed that demonstrate good accuracy between our estimated SMM and that measured by MRI or CT scanning.

Skeletal Muscle Mass Percentage (%SMM): The percent of the body that is composed of skeletal muscle. Like %BF, this number is relative to your total mass.

Appendicular Lean Mass to Height Ratio (ALM/Height2): The amount of lean mass in the arms and legs relative to height. A low ALM/Ht2 is a risk factor for sarcopenia (low muscle mass), primarily in lean individuals. Cut points in research are generally around 5.5 for women and 7.0 for men.

Appendicular Lean Mass to BMI Ratio (ALM/BMI): The amount of lean mass in the arms and legs relative to Body Mass Index. This can also be used as a risk factor for sarcopenia and is more relevant than ALM/ht2 in overweight individuals. Cut points in research are generally around 0.51 for women and 0.79 for men.

Resting Metabolic Rate (RMR): The number of calories the body needs to maintain its current mass under resting conditions. The value provided by the DXA scan is estimated from the amounts of different tissues, and tissue specific metabolic rates. It can be thought of as the “ideal” RMR, given an individual’s body size and composition.

Bone Density: Shows how dense the bones are and can be used to assess the risk of osteopenia and osteoporosis. The z-score compares your bone density to what is normal for people similar in age and body size. The t-score compares your bone density to that of a 30-year-old. Anything higher than -1.0 is representative of normal bone density. Between -1.0 and -2.5 is a risk for osteopenia. Anything under -2.5 is a risk for osteoporosis.