Why I’m Cutting Body Fat Before Marathon Training (And NOT During)

I’m currently in the middle of a 3-week calorie-cut off the back of marathon training.

My goal is simple: shed as much body fat as possible before I start “formal” training for Chicago this October.

I’m already pretty lean (~10% body fat), and people that I’ve spoken to over the last 10 days have been surprised about my “diet.”

But this goal isn’t about body image or chasing numbers on the scale.

It’s about performance.

Why “Cutting” Matters for Endurance Runners

If you scan the elite field at the Chicago Marathon, here’s what you'll see: sinewy limbs and hollow cheeks from start to finish.

The fastest marathoners in the world aren’t just fast. They’re ruthlessly lean. And that’s not a coincidence.

The legendary marathoner, and winner of the Paris Olympic Marathon, Sifan Hassan.

Their leanness isn’t a by-product of heavy training. It’s an important consideration when racing at the elite level.

Some researchers have demonstrated that weight and body fat have a higher correlation with finishing times in races than any other training variables (such as weekly training time).

That makes total sense.

In an endurance event, whether a 5K or a marathon, you’re loading every gram of weight with every stride.

Research proves that as you gain weight, the energy you need to run grows proportionally.

For example:

• A 68kg (150lb) runner will burn ~68 kcal per km.

• A 73kg (160lb) runner will burn ~73 kcal per km.

What that means, according to Matt Fitzgerald, is:

A 160lb runner needs 6.5% more energy to run at the same pace as a 150lb runner.

How do you think that will impact you over 26 miles?

In fact, the implications of excess weight may be even more problematic.

According to Jay Dicharry, the body deals with 2.5-3 times its body weight with each stride.

That extra weight isn’t just costing energy and impacting performance — it’s increasing load on your joints and soft tissues every step.

So there are multiple reasons to cut weight:

• Reduced energy requirements, leading to improved race performance

• Less load on joints and soft tissue, meaning less toll on the body

But not all body weight is born equal. Some body weight is more important than others.

It’s Not Just About Weight — It’s About Body Composition

“Excess body fat is the enemy of performance.” — Matt Fitzgerald

I’m not trying to lose weight per se — I want to lose fat.

Why do I want to lose fat, and not muscle? Doesn’t muscle weigh more than body fat?

Yes — muscle is denser than fat, meaning it weighs more per unit of volume.

But muscle is functional, whereas body fat is not.

A strong, efficient runner needs muscle for propulsion, posture, and injury resistance.

Obviously you don’t want to bulk up. Excess muscle is going to add weight that is also going to hinder performance. But you still need some strength.

On the other hand, body fat can hinder performance.

Excess body fat:

• increases your oxygen requirements at rest, meaning a potentially reduced aerobic capacity

• reduces thermoregulation, making it harder to cool down in warm conditions

• adds weight without adding value: humans need a baseline of body fat for hormones, survival, and general health. But anything beyond that is essentially dead weight.

To elaborate on that final point: yes, we need some body fat to stay alive. But even the skinniest elite runners still carry enough essential fat to function perfectly.

If those lean athletes have enough body fat, then chances are you have some to lose.

My approach to cutting weight

So, how can you actually cut as runner?

First, before you consider cutting, here’s something to consider: you should probably never cut during a training block.

Cutting inherently means less calories. Less calories means poorer recovery, which in turn impacts performance. What’s the objective of a training block? Performance.

Cutting is essentially the enemy of performance.

Another thing to consider: cutting is not sustainable:

• According to Jay Dicharry, even a deficit as low as 350kcal can eventually lead to stress fractures.

• Chronic caloric deficits can lead to RED-S.

• Let’s be honest: it’s miserable. Your energy levels are literally depleted. You can experience low-mood, brain fog and a host of other things that impact your quality of life. (Yes, I’m experiencing those right now. It’s day 10 for me, and I’ve had at least 7 hard days thus far.)

With all that said, here’s my approach:

• Moderate kcal deficit: 500kcal is considered a healthy, moderate deficiency in the short term. Anything higher will increase your injury risk.

• More weight training: you should increase strength training to 2-3x per week to ensure that you maintain muscle mass while losing body fat.

• Maintain protein intake: your protein intake should remain the same, therefore your relative intake should increase to something around 30% of your calories. If you don’t eat enough protein, you risk losing muscle mass alongside fat.

• Smart training:

  • Power intervals to promote EPOC.

  • Fasted runs at moderate intensity to increase fat burn.

  • Otherwise, easy efforts. Maintain a long run once-a-week.

  • But don't go too hard. A reduced kcal will impede your recovery, and if you do too many hard workouts, you may end up injured.

• Measure your weight: take daily measurements of your weight. If you can get hold of them, scales that measure body fat are especially useful.

This is what’s working for me right now. I’m not a coach or dietitian, just a runner experimenting with what my body responds to.

Chicago will be my first WMM, and what better way to start the series off than as lean as possible?

TL;DR: Cutting for Performance

• Leaner = faster. Every gram matters.

• Cut fat, not muscle. Body composition > scale weight.

• Don’t cut during training blocks — do it during downtime.

• Use strength training + high protein to preserve muscle.

• Fat loss is a short-term, strategic tool — not a lifestyle.

UPDATE 26.06.25: 3 weeks into my Chicago training, disaster struck: a stress fracture. It isn't clear exactly how this happened, but I pin this down to not following my own advice.

Stupidly, I tried to operate in a 1,000 kcal deficit, and I ran my efforts (especially my power intervals) way too hard. I've updated the post to emphasise these risks.