Hi friends~
I’m really happy you’re here! My mission with every post is to share stuff you can use, and I’d love to hear from you if there’s anything you’re thinking about.
In a recent virtual ROW+STRENGTH session we got talking about calories, energy, and fat because instead of programming meters or time in the workout, we were using calories as the metric. I always stress that calories on any machine are units of energy, and we can see on the monitor how efficiently we’re using energy use it as we work.
Calories on a machine monitor shouldn’t be considered food, but it’s hard not to of course. So as we chatted about calories, stored energy/fuel/fat, the question came up, “where does the energy/fat go when we burn it?”
Members in the class guessed “poop”, “pee”, and “sweat”…
Let’s get sciencey.
Fat isn’t released by the body in pee, poop, or sweat…
Nope – fat doesn’t leave your body as poop, pee, or sweat (well… a tiny bit in sweat, but not the main way). The real answer is:
The simple answer to how the body burns fat: you breathe it out.
Yep. When your body burns fat, it’s turned into carbon dioxide (CO₂) and water (H₂O).
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The CO₂ goes out through your lungs when you exhale.
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The H₂O leaves your body through breath, sweat, and urine.
What’s Happening Inside?
Think of it like a tiny fire in every muscle cell. Here’s the super-simplified process:
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Fat is broken down into smaller pieces (called fatty acids).
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Those pieces enter the Krebs cycle – a spinning, energy-making wheel inside your cells.
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The cycle needs oxygen to run (which is why you breathe more when you work out).
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As it spins, it creates energy (called ATP) to power your body.
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The leftovers are CO2 and H2O, which your body gets rid of mostly through exhaling.
Why It Matters in a workout – the “fat burning zone”:
When you’re moving – walking, lifting, rowing – you’re burning calories – more specifically, you’re breathing fat out through respiration. When you’re working hard your breath becomes more labored, and the longer and harder you work the more challenging your breathing becomes.
Many cardio machines (like treadmills, ellipticals, and bikes) display a “fat burning zone” on their monitors, suggesting that exercising at a lower heart rate (around 50–65% of your max heart rate) burns more fat.
This is technically partly true but very misunderstood:
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At lower intensities, a higher percentage of the calories you burn comes from fat (maybe 50–70%).
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At higher intensities (like running hard), a lower percentage comes from fat (maybe 30–40%), and more from carbohydrates.
But what matters more is total calories burned.
At higher intensities, you burn way more calories overall – even if a smaller percentage is from fat – meaning you still end up burning more fat and improving your fitness more efficiently.
Example:
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Slow walk: burn 200 calories in 45 minutes, 60% from fat → 120 fat calories.
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Hard run: burn 500 calories in 45 minutes, 40% from fat → 200 fat calories.
Bottom line:
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Low-intensity cardio is fine (especially for beginners, active recovery, or long-duration endurance work).
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But if your goal is fat loss, overall calorie burn and building muscle (which boosts metabolism) are far more important than staying in the so-called “fat burning zone.”
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Strength training + a mix of moderate and high-intensity cardio is usually more effective for fat loss.
Fat is fuel – premium fuel – it’s easy to fill the tank, but the body doesn’t want to give it up easily.
Why we store fat:
Our bodies evolved to survive, not to look good in leggings. Back when food wasn’t guaranteed, storing energy as fat was a smart survival tool. Fat is long-term energy storage – like a savings account. It’s there for when you’re “running from the saber-tooth tiger” or going through famine, aka, skipping meals or stress. Note: this is why super low calorie diets lead to yo-yo dieting (more on that another time).
Why fat is hard to burn:
The body protects that fat like it’s gold. It prioritizes burning quick-access fuel first – mostly carbohydrates (from food or stored as glycogen in muscles and liver). Only when those run low, or when you’re moving steadily for a longer time, does it say, “Okay, let’s tap the fat reserves.”
What helps us tap into fat stores:
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Consistent movement: especially moderate-intensity exercise (like walking, rowing, or strength training with good volume).
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Building muscle: More muscle means a higher resting metabolism. It demands more energy, 24/7—including from fat.
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Staying active throughout the day: NEAT (non-exercise activity thermogenesis)—like walking, cleaning, standing—burns fat quietly and steadily.
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Nutrition habits: Managing insulin levels (e.g., spacing meals, not constantly snacking) gives your body better access to fat stores.
The bottom line:
Your body wants to protect fat, but it will release it when it feels safe and it’s consistent. The body is so smart and efficient – movement, muscle, and smart habits signal, “It’s okay – we’ve got energy coming in, we can use what we’ve stored.”
The human body is a miracle.
Think of fat like battery backup. The energy in the battery can be accessed with consistent training demand on the body over time. If it isn’t being demanded, it will be stored as fat for when we need the energy to run from that saber-tooth tiger!
Onward!
x
Polli
