Understanding Body Composition: Why It Matters

Team Rx Performance
Feb 5
3 min read

Updated: Mar 24

What BMI Actually Measures (and What It Misses)

BMI (Body Mass Index) is calculated as:

Weight (kg) ÷ Height² (m²)

It was designed as a population-level screening tool, not a diagnostic health marker for individuals.

Limitations of BMI

BMI:

Does not distinguish between fat mass and muscle mass.
Does not account for bone density.
Does not show fat distribution.
Does not reflect metabolic health.

That’s how you can have:

Two people with the same BMI.
One lean and muscular.
One carrying high body fat and low muscle.

Yet both are classified the same.

BMI can’t tell the difference between fat loss and muscle loss.

Why Body Fat % Tells a Better Story

Body fat percentage breaks weight down into:

Fat mass
Lean mass (muscle, organs, bone, water)

This matters because:

Losing muscle while losing weight worsens metabolic health.
Maintaining or gaining muscle improves insulin sensitivity.
Fat location matters more than fat quantity alone.

A person whose weight stays the same—but replaces fat with muscle—is almost always healthier, even if the scale doesn’t move.

Measuring Body Fat: What’s Best vs What’s Practical

Gold-standard methods:

MRI – most accurate, but expensive and inaccessible.
DEXA scans – very good accuracy for fat, lean mass, and bone density.

At-home bioimpedance scales:

Less accurate for absolute numbers.
Useful only if used consistently to track trends over time.

Not All Fat Is Equal

Subcutaneous Fat

Located between the skin and fascia.
What you can pinch.
Often what “hides the abs.”

Visceral Fat

Located beneath the fascia.
Surrounds organs like the liver and intestines.
Strongly associated with metabolic disease, insulin resistance, cardiovascular risk, and inflammation.

Reducing visceral fat has a far greater impact on health than simply lowering scale weight.

What Is an “Optimal” Body Fat Percentage?

Let's go beyond the talk about abs and biceps.

We are not aiming for:

<18% body fat for women.
~5% body fat for men.

Those levels are often unsustainable and can harm hormones, immunity, and mental health.

What Does Matter

Mortality risk rises sharply when body fat exceeds ~35–40%.
Reducing visceral fat lowers long-term disease risk.
Genetics play a role in fat storage and loss (yes, some people lose weight more easily).

Health is about risk reduction, not chasing the lowest number.

Muscle Mass: The Underrated Health Marker

Building muscle is not just for aesthetics. Yes, it helps to build strength, but having more muscle mass is associated with:

Lower mortality risk.
Better insulin sensitivity.
Higher metabolic flexibility.
Improved bone density.
Greater independence with age.

If you’re sitting around 25–30% body fat, the smartest strategy is often:

Build lean muscle first—not just lose weight.

Muscle improves body composition even before fat loss accelerates.

Insulin Resistance: The Hidden Driver

Insulin resistance occurs when muscle and liver cells become less responsive to insulin. The truth is that genetics play a role. But there are changes and steps we can take to improve insulin sensitivity:

Strength training.
Zone 2 aerobic work.
Sleep quality.
Stress and cortisol management.
Nutrition quality and protein intake.

Weight loss becomes easier after insulin sensitivity improves.

The Real Goal: Metabolic Health

The body most people want isn’t simply a lighter body. It’s a body with:

More lean muscle.
Less visceral fat.
Better insulin sensitivity.
Strong bones.
Sustainable energy.

The scale can change without your health improving. Weight is what you see. Metabolic health is what runs the system. And body composition, not just BMI, is the signal worth paying attention to.

Conclusion: Focus on What Matters

Train for strength.

Protect muscle.

Reduce visceral fat.

Play the long game.

By focusing on these aspects, I can achieve lasting health and performance improvements. Remember, it’s not just about the number on the scale; it’s about how I feel and function every day.

For more insights on achieving your fitness goals, check out Rx Performance.