High-Intensity Training, Gut Health & Hormones: How to Move Your Body Without Burning Out

When your gut is flaring, the idea of running, lifting, or doing anything “high intensity” can feel… impossible.

Maybe you:

- Get urgent bathroom trips mid-run

- Feel bloated and crampy after harder workouts

- Have a history of doing intense programs + fasted training + under-eating, and now your hormones feel fried

That’s exactly where Leigh Merotto, RD, comes in. Leigh is a registered dietitian, long-distance runner, and founder of Gut Fit Nutrition. She helps active people with IBS and gut issues move their bodies and feel strong without their digestion holding them back.

In this episode turned blog, we break down:

• Why running and intense workouts can trigger gut symptoms

• What “high intensity” actually means (hint: most of us are not in true Zone 4–5 as often as we think)

• How exercise impacts cortisol and hormones

• How to use high-intensity training strategically - so it supports, not sabotages, your gut and hormone health

Why Do Hard Workouts Make Gut Symptoms Worse?

First, you’re not imagining it. Gut symptoms during exercise are very common, even in people without IBS. They’re just more noticeable when your gut is already sensitive.

A few key reasons why harder training can stir things up:

1. Blood flow gets diverted away from digestion

During high-intensity or longer sessions, your body shunts blood toward your working muscles and away from your gut.

Less blood flow to the gut = slower digestion, which means:

• Food sits heavier

• Anything harder to digest (high fibre, high fat, very large meals) is more likely to cause bloating, cramps, or urgency

So if you’ve ever had a rich, fast-food-style meal an hour before a long run and then regretted everything… this is why.

2. Running “jostles” your GI tract

Compared to cycling or swimming, running adds impact and pelvic jostling. That bouncing can:

• Exacerbate urgency

• Trigger “runner’s trots” in people who are already prone

• Make existing IBS symptoms feel louder

3. Dehydration and electrolytes

Training in heat or not drinking enough can lead to dehydration, which can cause:

• Short-term irritation and inflammation in the gut lining

• Loose stools or urgency during or after your workout

If you’re only sipping plain water and not replacing sodium, that can make things worse.

4. Taking in carbs during longer sessions without “gut training”

For endurance sessions (typically 75–90+ minutes), most athletes are told to take in carbs during the workout things like gels, chews, or sports drinks.

If your gut isn’t used to digesting carbs while you’re working hard, you may have fewer carb “transporters” available. The result:

• Gas

• Cramping

• Urgency

Just like your legs and lungs, your gut needs training and practice too.

Simple Tweaks to Support Your Gut and Your Training

The good news: GI trouble during exercise is common, but not inevitable. A few strategic shifts go a long way.

1. Time fibre and fat away from your hardest sessions

You don’t have to give up fibre or healthy fats. But timing matters.

• Aim to keep the pre-workout meal lower in fibre and fat, especially within 4 hours of a hard run or interval session.

• Load the bulk of your fibre (veggies, whole grains, legumes, seeds) into other meals in the day when you’re not about to train.

Example:

• Morning runner → small, lower-fibre carb snack before the run, big fibre-rich breakfast after.

• Evening runner → fibre-rich breakfast and lunch, lighter/low-fibre snack 1–2 hours before your workout.

2. Hydrate like it matters (because it does)

Support your gut and performance by:

• Starting your workout already hydrated

• Using fluids + electrolytes (especially sodium) during longer or sweaty sessions

• Not waiting until you’re dizzy or desperate to drink

3. Train your gut gradually

If you need carbs during longer runs or rides:

• Start with small amounts of an easy-to-digest carb source (e.g. ½ gel, small sips of sports drink)

• Introduce it during easier or shorter sessions first

• Slowly increase as your gut adapts

This “gut training” is just as legitimate as hill training or speed work.

What Is High-Intensity Training, Actually?

On social media, “high intensity” sometimes gets slapped onto anything that feels tough. Physiologically, it’s more specific.

Leigh uses two key ways to define it:

1. RPE (Rate of Perceived Exertion)

Think of a 0–10 scale:

• 0–3: Very light / easy (stroll, gentle movement)

• 4–5: Moderate (you’re working, but can still talk in full sentences)

• 6–7: Hard (talking is choppy; breathing is heavier)

• 8–9: Very hard (a few words at a time)

• 10: Max effort (you couldn’t maintain it much longer)

True high-intensity work usually lives in the 7–10 range, often in short intervals with rest breaks in between.

2. Heart rate zones

If you use heart rate data:

• Zone 4 ≈ 80–90% of maximum heart rate

• Zone 5 ≈ 90–100% of maximum

Most of your training shouldn’t live here, and many people who fear they’re “always in high intensity” are actually sitting in Zone 2–3 (moderate).

So when reels say “never do HIIT if you have hormone issues,” remember:
a lot of movement that gets demonized isn’t even true high-intensity by these definitions.

Cortisol, Hormones, and Why Intensity Isn’t the Enemy

Cortisol is often labeled the “bad stress hormone,” but it’s actually:

• Essential for alertness and focus

• Involved in inflammation regulation

• Tied to your circadian rhythm (naturally higher in the morning, lower at night)

High-intensity exercise does temporarily raise cortisol, but that short spike followed by a drop is:

• Normal

• Adaptive

• Part of how your body mobilizes fuel (glucose + fatty acids) for performance

Where cortisol becomes a concern is when it’s:

• Elevated all day, every day

• Combined with chronic under-fuelling, poor sleep, high life stress, and zero recovery

In other words: it’s not “doing intervals twice a week” in a nourished, well-rested body that burns people out.  It’s everything piled together.

Benefits of well-programmed high-intensity training

When used thoughtfully, high-intensity work can:

• Improve VO₂ max (a powerful marker of longevity and cardiovascular health)

• Boost insulin sensitivity (huge for PCOS and blood sugar balance)

• Increase brain-derived neurotrophic factor (BDNF), which supports mood, learning, and brain health

We don’t need to fear intensity. We need to respect it, program it well, and fuel for it.

How to Use High-Intensity Training Without Overloading Your System

Leigh shared four big pillars to keep intensity feeling supportive, not destructive.

1. Balance your training week

Less can truly be more here. General guideposts:

• Newer to exercise / lower training age

• Focus on building a base: walking, easy runs, moderate strength

• Add maybe 1 high-intensity session per week once your base feels stable

• More advanced / seasoned athletes

• Usually 2–3 high-intensity sessions per week max

• Never stack two hard days back-to-back

• Hard day → easy day (walk, gentle cycling, yoga, or truly easy run)

2. Schedule real rest

At least 1 rest day per week, often 2 for many people.

“Rest” can still include:

• Gentle walking

• Stretching or mobility

• Non-structured, low-pressure movement

You don’t have to earn rest. It’s part of the training plan, not a failure.

3. Avoid fasted high-intensity training

Fasted training (especially for women) can:

• Increase cortisol more than necessary

• Put you in a low energy availability state

• Increase gut permeability and GI distress during or after the session

• Make it harder to actually hit true high intensity

If you’re going into a hard run, heavy lifting session, or intervals, try to have at least a small carb-containing snack 30–75 minutes beforehand (and more if there’s time).

Fasted, low-intensity walks under ~45–60 minutes may be okay for some, but that’s very different from fasted HIIT.

4. Zoom out: total stress load matters

High-intensity workouts are one form of stress. Others include:

• Work deadlines

• Caregiving, parenting, emotional labour

• Poor sleep

• Under-eating (especially chronic dieting or “clean eating” that’s secretly low-calorie)

If it’s been a brutal week and you’re sleeping badly, that might be a time to:

• Swap your HIIT for a walk + light strength

• Or shorten the intervals and dial down the RPE

You don’t need to abandon movement; you can adjust the dial.

Where Gut Health, Hormones, and Movement Meet

For people with PCOS, IBS, or hormone concerns, this can feel complicated. But the good news is:

• Movement is powerful medicine for insulin sensitivity, mood, digestion, and long-term health

• You can run, lift, and even train hard with IBS or hormone issues (when it’s paced, fuelled, and supported)

• You do not have to choose between “protecting your hormones” and “being athletic”

At Nest & Nurture, registered dietitians and clinicians work with clients to balance training, nutrition, gut health, and hormones so exercise remains supportive instead of overwhelming the body.

The work is in the how:

• Timing fibre and fat away from hard sessions

• Staying hydrated with electrolytes

• Training your gut just like you train your muscles

• Programming high-intensity work sparingly and strategically

Listening to your body and looking at the whole stress picture