Weight loss is weird. Most people think it’s just about eating less or running until your lungs burn. But honestly, if you look at the biochemistry, it’s a lot more like a complex accounting system managed by a very stubborn hormonal boss.
So, how does losing weight work at its core? It isn’t just "disappearing" into thin air. When you lose weight, you’re actually exhaling most of it. Yeah, you read that right. Research published in the British Medical Journal by Ruben Meerman and Andrew Brown shows that when fat is broken down for energy, the byproducts are water and carbon dioxide. You pee out the water, but the vast majority of that former "fat" leaves your body through your lungs. You’re literally breathing your weight away, one molecule at a time.
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The Metabolic Engine: Calories and Chemistry
The basic rule everyone quotes is the First Law of Thermodynamics. Energy cannot be created or destroyed. In the context of your body, this translates to Calories In vs. Calories Out (CICO). If you take in less energy than your body needs to maintain its current mass, it has to find that energy elsewhere. It taps into the pantry. That pantry is your adipose tissue—fat.
But it's never that simple.
Your metabolism isn't a static number. It’s a moving target. You have your Basal Metabolic Rate (BMR), which is what you burn just staying alive—keeping your heart beating and your brain firing. Then you have the Thermic Effect of Food (TEF). This is the energy it takes to actually digest what you eat. Protein, for instance, has a much higher TEF than fats or carbs. It’s "expensive" for your body to process.
Then there’s NEAT. Non-Exercise Activity Thermogenesis. This is the secret weapon. It’s the calories you burn fidgeting, standing up, walking to the mailbox, or even just maintaining your posture. For many people, NEAT accounts for more daily energy expenditure than an hour at the gym ever could.
Why Your Hormones Are Calling the Shots
If weight loss were just a math equation, we’d all be thin. But your hormones, specifically insulin and leptin, act as the gatekeepers.
When you eat, especially refined carbohydrates or sugar, your blood glucose spikes. Your pancreas pumps out insulin to shuttle that sugar into your cells. Here’s the kicker: insulin is an anabolic hormone. It's a storage hormone. When insulin levels are high, your body is effectively locked out of its fat stores. It’s in "store mode," not "burn mode." This is why many clinicians, like Dr. Jason Fung, author of The Obesity Code, argue that the frequency of eating and the type of food matter just as much as the total calories. If you’re constantly snacking, your insulin never drops low enough to signal your body to start burning body fat.
Then there's leptin. Think of leptin as the fuel gauge on your car. Produced by your fat cells, it tells your brain how much energy you have stored. If you have plenty of fat, leptin levels are high, and your brain says, "Cool, we’re good, let’s burn energy and not be hungry."
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But in cases of chronic overeating, you can become leptin resistant. Your brain stops hearing the signal. It thinks you’re starving even when you’re carrying extra weight. It slows down your metabolism to "save" you. It’s a biological glitch that makes losing weight feel like fighting your own survival instincts. Because, in a way, you are.
The Role of Muscle in How Does Losing Weight Work
Muscle is metabolically "expensive." It takes more energy to maintain a pound of muscle than a pound of fat. This is why strength training is often more effective for long-term weight management than steady-state cardio.
When you do a lot of cardio while in a massive calorie deficit, your body sometimes panics. It thinks, "We’re traveling long distances but there’s no food. We should probably get rid of this heavy, energy-sucking muscle to survive." This is how people end up "skinny fat"—they weigh less, but their body composition is mostly fat and very little muscle. Their metabolism drops because they’ve lost their engine.
To keep the metabolic fire hot, you need resistance. Lifting weights or doing bodyweight exercises signals to the body that the muscle is necessary for survival. The body then prioritizes burning fat instead of muscle tissue to fill the energy gap.
The "Set Point" Theory and Metabolic Adaptation
Ever notice how the first 10 pounds fly off and then you hit a wall? That’s not just in your head. It’s metabolic adaptation.
As you lose weight, your body becomes more efficient. A smaller body requires less energy to move. Additionally, your mitochondria—the power plants in your cells—start to get stingy with energy. Your body tries to defend its highest weight. This is often called the "Set Point."
To break through a plateau, you can't just keep cutting calories indefinitely. Eventually, you’d be eating nothing. This is where "refeed" days or diet breaks come in. By occasionally bringing calories back up to maintenance levels, you can sometimes signal to your hormones (like thyroid hormone T3) that the "famine" is over, which helps prevent the metabolic slowdown from becoming permanent.
Misconceptions You Should Probably Ignore
Forget "fat-burning" supplements. Most of them are just overpriced caffeine pills. They might increase your heart rate slightly, but they won't do the heavy lifting of fat oxidation for you.
Also, "spot reduction" is a total myth. Doing a thousand crunches won't burn the fat off your stomach specifically. Your body decides where it pulls fat from based on genetics and hormonal profiles. Usually, the place you put it on first is the last place it leaves. It's frustrating, but it's just how the plumbing works.
How to Actually Apply This
Understanding the science is great, but it doesn't mean much if you don't change the behavior. Here is how to actually make the biology work for you instead of against you.
Prioritize Protein
Aim for about 0.7 to 1 gram of protein per pound of your goal body weight. This protects your muscle mass and keeps you full because protein suppresses ghrelin, the "hunger hormone."
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Focus on Fiber
Fiber isn't just for digestion. It slows down the absorption of sugar into your bloodstream, which prevents those massive insulin spikes we talked about earlier. Think beans, leafy greens, and cruciferous vegetables.
Move More, but Stress Less
High intensity is great, but high stress (cortisol) can actually cause your body to hold onto midsection fat. Walk more. 10,000 steps isn't a magic number, but it’s a great proxy for keeping your NEAT levels high enough to maintain a deficit without burning out.
Sleep is Non-Negotiable
When you’re sleep-deprived, your leptin drops and your ghrelin spikes. You literally wake up biologically wired to crave sugar and overeat. You can't out-diet a lack of sleep.
The Long Game
Weight loss isn't a straight line. Your weight will fluctuate based on water retention, salt intake, and even the time of month for women. Focus on the trend over weeks and months, not the number on the scale every morning.
Sustainable weight loss happens when you stop viewing it as a temporary punishment and start seeing it as a series of biological levers you can pull. It’s about creating an environment where your body feels safe enough to let go of its stored energy. Stop fighting your biology and start working with the chemistry of how your body actually functions.
Actionable Next Steps
- Calculate your TDEE (Total Daily Energy Expenditure) using an online calculator to get a baseline for your "maintenance" calories.
- Start a food log for just three days. Don't change anything yet—just see where your hidden calories (like oils, sodas, or large portions) are coming from.
- Incorporate two days of resistance training per week. This can be as simple as pushups and squats at home to signal to your body to preserve muscle.
- Audit your sleep. Aim for 7-9 hours. If you're consistently getting 5, no amount of dieting will overcome the hormonal chaos that follows.
- Increase daily movement through small habit changes, like taking the stairs or walking while on phone calls, to boost your NEAT.