The 3 Components of Body Composition Analysis

What is body composition?

Body composition is the breakdown of what your body is made of — muscle, fat, bone, and water. Rather than just tracking your weight on a scale, body composition analysis gives you a deeper look at your health. As we age, naturally our body composition begins to change. We start to slowly lose muscle and it becomes much easier to gain fat. This combination leads to inflammation, insulin resistance and numerous other health consequences. Additionally, our bone mineral density begins to decline as we age, putting us at risk of developing osteopenia or osteoporosis. By performing body composition analysis routinely, we gain deeper insights into our current health and are able to track metrics over time and evaluate which lifestyle modalities are helping or hurting our body composition.

The 3 components of body composition analysis

Body composition can be broken down into 3 components: fat mass, lean mass & bone mineral density.

Fat Mass

Fat mass is the absolute weight of all the fat in your body, usually measured in pounds. We can then divide fat mass into two categories: subcutaneous fat and visceral fat.

Subcutaneous fat is the fat found underneath the skin. It is the fat found around the hips, thighs, belly and buttocks. Visceral fat, or non-subcutaneous fat, is the fat that is around our internal organs.

When an individual begins to accumulate fat, fat is first stored subcutaneously. But as the individual accumulates more and more fat, eventually the fat spills over and begins to get stored as visceral fat. This storage as visceral fat is important because it comes with detrimental effects on metabolic health. Visceral fat is a major source of inflammation and with its proximity to internal organs, it causes negative effects on those organs.

Unfortunately, we cannot control when fat will begin to get stored as visceral fat. When the fat begins to "spill over" into the viscera is different for every individual and is genetically driven. But what we do know is that if you can decrease your total body fat then the amount of visceral fat you have will also decrease.

Lean Mass

Lean mass is the absolute weight of your body besides fat. This includes muscle, water, connective tissue, bone and internal organs. Skeletal muscle makes up the majority of lean mass and is beneficial for numerous reasons.

As we age, muscle mass begins to decline and maintaining muscle is important for preventing frailty and preventing falls. Muscle mass is inversely associated with mortality, meaning the more muscle you have the less likely you are to die. Muscle mass is also important for metabolic health as skeletal muscle is the largest site of glucose disposal in the body. After a meal, glucose is taken out of the bloodstream by skeletal muscle for either storage or utilization, preventing blood glucose spikes. This serves as evidence that it is clearly in our best interest to maintain as much muscle as possible as we age. I don't think I have ever heard anyone over the age of 60 complain about "having too much muscle".

When evaluating lean mass via body composition analysis, we are able to break it down into two indices: fat free mass index (FFMI) and appendicular lean mass index (ALMI).

Fat free mass index

Fat free mass index is a measure of all mass that isn't fat (muscle, water, organs, bone, connective tissue). The FFMI value is relative to height as a very tall individual and a very short individual with the same fat free mass would have very different muscularity perceptions.

We calculate the fat free mass index (FFMI) with the following equation:

FFMI = total lean mass (kg) / [height in meters²] + (6.1 x [1.8 – height in meters])

For men, an FFMI of 18-20 is considered normal. An FFMI greater than 25 is considered extremely muscular.

For women, an FFMI of 15-17 is normal, an FFMI of 20 is athletic and an FFMI of 22 is extremely muscular.

Appendicular lean mass index (ALMI)

The appendicular lean mass index is a measurement of the amount of mass in the arms and legs that is not fat (muscle, bones, connective tissue, skin). This value is also relative to height for the same reasons as noted above. The ALMI is a purer estimation of muscle mass compared to FFMI because it doesn't include internal organs given that there are no organs in the extremities. The ALMI is our preferred metric for informing lifestyle interventions as it gives a more accurate estimation of lean mass in the form of skeletal muscle which is what we are interested in when it comes to longevity and health.

The calculation for ALMI is as follows:

ALMI = lean mass in arms (kg) + lean mass in legs (kg) / height in meters²

The NHANES (National Health and Nutrition Examination Survey), a large ongoing research program run by the CDC, provides the following info regarding ALMI values for healthy individuals.

• Men ages 20-39: ALMI 8.5-9.5 kg/meter²
• Women ages 20-39: ALMI 6.5-7.5 kg/meter²
• Men with an ALMI less than 7 and women with an ALMI less than 5.5 would be considered to have sarcopenia

Bone Mineral Density

The final component of body composition analysis is bone mineral density (BMD). Bone mineral density is a measurement of how much mineral content (mainly calcium and phosphorus) is packed into a section of bone. It reflects bone strength and is the standard way to assess risk for osteoporosis and fractures. BMD declines with age, increasing our risk of fractures and mortality.

When you perform body composition analysis, bone mineral density gets reported with different scores: a Z-score and a T-score.

A Z-score compares your bone mineral density to the average bone mineral density of a healthy person of your exact same age and gender. It indicates the number of standard deviations your score is above or below this age matched average.

A T-score compares a patient's bone mineral density to that of a healthy young adult of the same sex. The T-score is used to diagnose osteopenia and osteoporosis. A T-score of 0 represents normal bone mineral density. Slightly above or below 0 is also normal. A score between -1 and -2.5 indicates low bone mass (osteopenia). A score of -2.5 or lower indicates significantly low bone density and high fracture risk (osteoporosis).

The bottom line on body composition analysis

Fat mass, lean mass, and bone mineral density provide a complete picture of body composition. By understanding these components, we can move beyond the number on the scale and gain insight into true health risks and strengths. Whether the goal is disease prevention, performance, or healthy aging, body composition analysis is one of the most powerful tools to track meaningful progress.

All materials contained in this blog post are for informational purposes only and should not be construed as medical advice. Please consult with your physician concerning any and all medical questions and/or problems.