Why BMI Doesn't Tell The Whole Story

TTG Staff
4 days ago
5 min read

Part 2 of The Training Ground Guide to GLP-1 Weight Loss Medications

Balance scale with yellow fat and red muscle, black weights, and two human silhouettes in a gray studio background.

If you've ever been told you're overweight, obese, or a healthy weight, there's a good chance that conclusion came from a single calculation:

BMI.

Body Mass Index.

For over 50 years, BMI has been one of the most commonly used tools for assessing body weight.

Doctors use it.

Health insurers use it.

Researchers use it.

Government health organisations use it.

And yet, despite how widely it is used, BMI is one of the most misunderstood measurements in health and fitness.

At The Training Ground, we don't believe BMI is useless.

But we do believe it only tells part of the story.

And sometimes, that missing part is the most important.

What Is BMI?

BMI is a simple calculation that compares your body weight to your height.

The formula is:

BMI = Weight (kg) ÷ Height² (m)

The result places people into categories such as:

Underweight
Healthy weight
Overweight
Obese

The appeal of BMI is obvious.

It's quick.

It's free.

It's easy to calculate.

And when you're looking at large populations, it can provide useful information.

The problem begins when we use it to make assumptions about individuals.

Because individuals are not averages.

What BMI Gets Right

Before we criticise BMI, it's important to acknowledge something.

BMI exists for a reason.

Across large populations, higher BMI values are generally associated with increased risks of:

Type 2 diabetes
Cardiovascular disease
Hypertension
Certain cancers
Premature mortality

This is why BMI became widely adopted.

If you are looking at thousands or millions of people, BMI can help identify broad health trends.

That's valuable.

The mistake is assuming it tells us everything we need to know about a specific person sitting in front of us.

What BMI Doesn't Measure

This is where things become important.

BMI knows:

Your height
Your weight

That's it.

BMI does not know:

Your body fat percentage
Your muscle mass
Your bone density
Your training history
Your fitness level
Your physical capacity
Your metabolic health

In other words, BMI can tell us how heavy you are for your height.

It cannot tell us why.

And that distinction matters.

Two People. Same BMI. Completely Different Story.

Imagine two people.

Both are:

178cm tall
95kg

Both have exactly the same BMI.

According to the calculation, they are identical.

But what if:

Person A has:

High body fat
Low muscle mass
Minimal physical activity

While Person B has:

Significant muscle mass
Regular strength training
Excellent cardiovascular fitness

BMI sees them as the same person.

Reality does not.

This is one of the biggest limitations of relying on BMI in isolation.

The Problem We See Most Often

One of the most common conversations we have at The Training Ground goes something like this:

"My doctor told me my BMI is too high."

Sometimes that's a useful observation.

Sometimes it's an incomplete one.

Because the next question should be:

Why is the BMI high?

Is it:

Excess body fat?
High muscle mass?
A combination of both?

BMI cannot answer that question.

And yet that answer may completely change the conversation.

The Weight-Loss Trap

This becomes particularly important when someone starts a weight-loss journey.

Imagine two people both lose 10kg.

The scales celebrate both outcomes equally.

BMI improves equally.

But what if:

Person A loses:

8kg fat
2kg muscle

While Person B loses:

4kg fat
6kg muscle

Those are very different outcomes.

Yet BMI treats them the same.

This is one reason we become more interested in body composition than body weight alone.

Why Body Composition Matters

At The Training Ground, we're usually less interested in how much someone weighs and more interested in what makes up that weight.

Body composition helps us understand:

Fat mass
Muscle mass
Bone mass
Water balance
Segmental muscle distribution

These measurements provide context.

Because health isn't simply about becoming lighter.

It's about improving the quality of the weight you're carrying.

A Real Example

Not long ago, I sat down with a client who was taking a GLP-1 medication.

He was excited.

The scales showed he had lost weight.

On the surface, everything looked positive.

But when we reviewed his body composition, we found that a significant proportion of that weight loss had come from active tissue rather than predominantly body fat.

The scales called it a win.

The body composition data told us we needed to make adjustments.

That doesn't mean the medication wasn't working.

It means the scales weren't telling the whole story.

Why We Use BIA Scans

This is one of the reasons we invested in body composition analysis at The Training Ground.

A scale can tell you:

How much weight changed.

A body composition assessment helps answer:

What changed?

We can monitor:

Body fat trends
Muscle mass trends
Segmental muscle changes
Bone mass trends
Overall body composition

This gives us far more useful information when making decisions about:

Nutrition
Training
Recovery
Weight-loss strategies

Should We Throw BMI Away?

No.

And this is where many fitness professionals get carried away.

BMI still has value.

It's quick.

It's inexpensive.

It's useful for population-level health screening.

The problem isn't BMI.

The problem is treating BMI as the entire story.

Think of BMI like looking at a car's speedometer.

It's useful information.

But it doesn't tell you:

Fuel levels
Engine condition
Tyre pressure
Oil levels

You would never evaluate a car using only one measurement.

The same principle applies to your body.

The Better Question

Instead of asking:

"What is my BMI?"

A more useful question is:

"What is my body made of?"

Because if your goal is improving health, performance, or body composition, that's usually the question that matters most.

What We Tell Members At The Training Ground

BMI can be a starting point.

It should rarely be the finish line.

Whether your goal is:

Weight loss
Muscle gain
Improved health
Better performance
Healthy ageing

Understanding your body composition provides information that BMI simply cannot.

Because the scales measure weight.

BMI categorises weight.

Body composition helps explain weight.

And that's where meaningful decisions become possible.

The Bottom Line

BMI is not useless.

It is simply incomplete.

What BMI tells us:

How heavy you are relative to your height.

What BMI does not tell us:

How much fat you carry.
How much muscle you carry.
Whether your weight is improving your health or harming it.
What type of weight you are gaining or losing.

At The Training Ground, we believe the goal isn't simply to weigh less.

The goal is to become healthier, stronger, and more capable.

And to do that, you need more information than a height and weight calculation can provide.

Because the most important question isn't:

"How much do you weigh?"

It's:

"What is that weight made of?"