When we talk about body composition, we’re primarily concerned with two key elements: fat mass and lean mass. Each of these has distinct effects on our health and longevity.

Fat Mass

Adipose tissue, commonly referred to as “fat mass,” is linked to a higher risk of all-cause mortality when present in large amounts throughout the body. However, not all fat mass is created equal. Fat exists in two main forms based on its location:

• Subcutaneous adipose tissue: Found underneath the skin, typically around the hips, thighs, belly, and butt. This represents the “normal” form of fat storage.
• Non-subcutaneous adipose tissue: Found around the viscera (soft internal organs), and incorporated into the pancreas, liver, and within muscle. This form of fat, particularly visceral adipose tissue (VAT), poses the most significant threat to metabolic health.

Non-subcutaneous fat is particularly harmful because it’s a major source of inflammation. It can secrete pro-inflammatory cytokines and attract immune cells, worsening inflammation. Its proximity to major organs can contribute to the development of atherosclerosis and metabolic syndrome.

A large study using UK Biobank data showed that those in the highest quartile of visceral adiposity index had a:

• 20% higher risk of all-cause mortality
• 22% higher risk of cancer mortality
• 46% higher risk of cardiovascular disease mortality compared to those in the lowest quartile.

Lean Mass

Lean mass includes skeletal muscle mass, which makes up the majority of lean mass, as well as other non-fat and non-bone body parts like internal organs and skin. Increased muscle mass prevents frailty as we age, improves quality of life, and decreases fall risk in old age. Muscle mass also has positive effects on glucose tolerance, improving overall metabolism.

An abundance of data suggests that muscle mass is strongly and inversely associated with mortality. For example, one prospective cohort study found that in men and women aged 70-79, greater loss of thigh muscle over a five-year period was associated with a higher mortality risk over the subsequent 12 years.

Changes in Body Composition with Age

Body composition changes naturally with age, making it critical to monitor these metrics over time. From birth to our early/mid-twenties, we rapidly gain muscle mass, but after about age 40, muscle mass begins to decline, a process that accelerates with age. Fat mass tends to increase throughout life.

Determining Body Composition

Several methods exist for estimating body composition, ranging from imprecise to extremely accurate. These include:

• Anthropometric Estimates: Includes BMI, waist circumference, and bioelectrical impedance analysis (BIA).
• MRI and CT: Highly accurate but impractical for regular use due to cost and radiation exposure.
• Dual-energy X-ray absorptiometry (DEXA): Offers the best balance of costs and benefits.

DEXA-Derived Measures of Body Composition

DEXA scans provide several key metrics:

• Fat Free Mass Index (FFMI): Measures all mass that isn’t fat relative to height.
• Appendicular Lean Mass Index (ALMI): Measures the amount of mass that isn’t fat in the arms and legs relative to height.
• Total Body Fat: Measured as a percentage of total body mass.
• Visceral Adipose Tissue (VAT): Estimated as the fat mass that lies within the L4-L5 region of the abdomen.

Improving Your Body Composition

Optimal interventions for improving body composition depend on which areas are of greatest concern at baseline. Individuals can be categorized into four groups:

• Under Muscled and Overnourished
• Adequately Muscled and Overnourished
• Under Muscled and Adequately Nourished
• Adequately Muscled and Adequately Nourished

Each category requires different interventions, ranging from calorie restriction and increased protein intake to specific exercise regimens.

Conclusion

Body composition is a vital indicator of health and a predictor of chronic disease and mortality. By understanding and monitoring your body composition, you can make targeted changes to your lifestyle to optimize health and longevity. Remember, it’s not just about weight—it’s about the quality of that weight in terms of lean mass versus fat mass.

As a physician, I encourage you to look beyond the scale and consider getting a DEXA scan to truly understand your body composition. Armed with this knowledge, you can make informed decisions about your diet and exercise regimen to support a longer, healthier life.