The 500-Calorie Deficit: Where It Came From, Why It Won’t Die, and Why It’s Especially Wrong on GLP-1s

If you spend any time around weight loss conversations — especially online — you’ll see this advice everywhere:

Work out your TDEE and subtract 500 calories.

It’s usually said with a lot of confidence, like it’s basic, unquestionable science. And for many people, especially women in midlife or those using GLP-1 medications, it’s exactly where things start going wrong.

So let’s talk about where this idea came from, why it’s still so popular, and why it often doesn’t deliver what it promises.

First up: what is TDEE and why does everyone use it?

TDEE stands for Total Daily Energy Expenditure.
In plain English, it’s an estimate of how many calories your body burns in a day.

That estimate usually includes:

• Your resting metabolism (how much energy it takes to keep you alive)

• Daily movement and exercise

• The energy used to digest food - the thermic effect of food

TDEE calculators are popular because they feel personalised and reassuring. They give people a number — a starting point — in a space that often feels confusing and emotional.

And to be fair, TDEE isn’t useless.
It can be a reasonable estimate.

Where things go off the rails is what people are taught to do with that number.

Enter the infamous 500-calorie deficit

Once someone has a TDEE, the advice usually goes like this:

“Subtract 500 calories and you’ll lose weight.”

This comes from a very old bit of maths:

• One pound of fat is roughly 3,500 calories

• Divide that by seven days

• Result: a 500-calorie daily deficit equals one pound of fat loss per week

It’s neat. It’s tidy. And it assumes the human body behaves like a calculator.

This rule dates back to the 1950s and completely ignores:

• Hormones

• Stress

• Sleep

• Dieting history

• Muscle loss

• Metabolic adaptation

• And the fact that women's bodies behave differently to men's

TDEE itself is only an estimate — based on population averages. When we turn that estimate into a rigid rule and then apply an aggressive deficit on top, we’re stacking assumptions on assumptions.

And real bodies rarely play along.

Why Dr Tony Boutagy’s work matters here

Dr Tony Boutagy is an internationally respected exercise physiologist and metabolic health educator who has spent decades working with women — particularly those who are stuck, exhausted, plateaued, or doing 'everything right' with nothing to show for it.

His work underpins much of the modern understanding around:

• Energy availability

• Metabolic adaptation

• Stress physiology

• Why chronic dieting backfires, especially in women

I’m a student of Tony’s and a coach trained through his education frameworks, and one of the strongest themes in his work is this:

Fat loss is not about eating as little as possible.
It’s about giving the body just enough energy to feel safe while gently nudging change.

Energy availability: the piece most plans ignore

Tony focuses heavily on energy availability — essentially how much usable energy the body perceives it has access to after training, work stress, life stress, and basic survival needs are accounted for.

When energy availability drops too low for too long, the body doesn’t respond by burning more fat.

It responds by protecting itself.

That often looks like:

• Reduced metabolic output

• Increased cortisol

• Poor recovery

• Fatigue and flat training sessions

• Fat loss resistance, particularly around the middle

This response is stronger in women than in men, and stronger again in midlife women juggling hormonal changes, sleep disruption, and cumulative stress.

This is why rigid 500-calorie deficits so often lead to stalls, burnout, and the familiar 'my weight loss has plateaued' scenarios.

Why GLP-1 meds change the picture even more

GLP-1 medications already:

• Reduce appetite

• Lower spontaneous food intake

• Improve insulin sensitivity

• Slow digestion

In other words, many people on these meds are already eating less without trying.

So when someone:

1. calculates a theoretical TDEE (often overestimated)

2. automatically subtracts 500 calories, and

3. adds a medication that further suppresses appetite

they can end up unintentionally under-eating by a significant margin.

The consequences aren’t always immediate weight gain. They’re quieter:

• Loss of lean muscle

• Poor training performance

• Fatigue, coldness, hair loss

• Plateaus that feel confusing and unfair

Tony describes this as stacking stressors — and stressed systems don’t give up fat easily.

What about smaller deficits?

You’ll often hear science-backed experts working in this space talk about much smaller deficits. Sometimes 100–300 calories. Sometimes even less.

This isn’t about being cautious or 'soft'.
It’s about being physiological.

A smaller, sustained deficit:

• Sends a weaker stress signal to the body

• Preserves lean mass

• Keeps training quality high

• Works with GLP-1 appetite suppression rather than fighting it

• Is far more sustainable for real women with real lives

Ironically, this approach often leads to better fat loss over time than aggressive restriction.

What I actually advise my clients

Rather than TDEE minus 500, I coach using a framework grounded in energy availability and real-world outcomes:

Start at estimated maintenance.
Let appetite settle before making changes.

Watch what the body does, not what an app predicts.
Is weight trending down slowly?
Is strength training holding steady or improving?
Is energy reasonable?
Is hunger manageable?

If yes — you’re already doing enough.

If an adjustment is needed, go small.
Think 100–200 calories, not dramatic cuts.

Protein and resistance training are non-negotiable.
On GLP-1s, protecting muscle matters far more than chasing rapid scale loss.

TDEE is a tool, not a prescription.
The 500-calorie deficit is a mechanical rule applied to a biological system.

Fat loss works best when the body feels safe enough to allow it.

Smaller, smarter deficits aren’t being soft.
They’re being informed.

And for midlife women, especially those using GLP-1 medications, this approach is often the difference between steady progress and yet another round of frustration.

Sometimes the most effective thing you can do is stop eating like your body is the problem and start fuelling it like it’s on your side.

Because it is. 💛

XO Jane