How Does Fat Leave the Body

Have you ever wondered how does fat leave the body when we lose weight? It’s a question that rarely crosses people’s minds. Most assume that fat simply gets burned off, or that it exits through our urine or faeces. However, the science tells a very different and fascinating story. In fact, only about 10% of fat leaves through bodily fluids like urine. So, where does the rest go? 

As a nutritionist and food scientist, I discuss these details with my clients to help them better understand weight management. So, let’s demystify the process and explore the real science behind how fat leaves the body. 

The Basics: How Fat is Stored

Before we tackle how fat leaves the body, it’s important to know how it’s stored. Fat is stored in cells called adipocytes (pronounced A-duh-pow-sites). These cells contain triglycerides – molecules formed from excess calories that your body didn’t use for immediate energy. Interestingly, the number of adipocytes you have is largely set during childhood and adolescence (Harvard Health Publishing). 

When we gain weight, these cells don’t multiply; instead, they expand to accommodate more fat. This means when we lose weight, the fat cells don’t disappear – they simply shrink. Think of them like balloons inflating and deflating. This insight helps explain why maintaining weight loss can be tricky. Sadly, your existing fat cells are always ready to store more if you consume excess calories. 

The Process of Fat Loss: It’s All About Exhaling

So, how does fat actually leave the body? When we lose weight, our body breaks down the fat molecules (triglycerides) for energy. This process produces carbon dioxide (CO₂) and water (H₂O) as by-products. 

• Carbon dioxide is expelled when you breathe out. 

• Water exits through sweat, urine, and even breath moisture. 

According to research from the National Institutes of Health (NIH), a staggering 84% of fat loss occurs through breathing out carbon dioxide (NIH – How Fat Leaves the Body). The remaining 16% leaves as water. In other words, most of the fat you lose literally leaves your body when you exhale! 

This is one reason exercise plays a crucial role in weight loss. The more you move, the more you breathe, and the more carbon dioxide you expel. Whether it’s a brisk walk, a yoga session, or a high-intensity workout, focusing on deep, steady breathing can help maximise fat loss (British Journal of Sports Medicine). 

Why Exercise Matters for Fat Loss

When people talk about “fat-burning” exercises, they’re not entirely wrong – but the term is misleading. You’re not setting fat on fire; you’re breaking down the fat molecules (triglycerides) and expelling the resulting carbon dioxide and water. 

During exercise, your body requires more energy, which prompts the breakdown of fat stores. The harder you work, the more carbon dioxide you exhale. This is why aerobic activities like running, cycling, and swimming are particularly effective for weight loss (American Council on Exercise (ACE)). They encourage deeper and faster breathing, helping you get rid of fat more efficiently. 

But it’s not just about intense workouts. Even gentle activities like walking or stretching can contribute to fat loss over time – as long as you’re consistent. 

Sleep and Fat Loss: The Overnight Effect

Whilst working with The Sleep foundation I came to understand perhaps a lesser-known fact that we’re still losing fat while we sleep. In fact, a significant amount of fat loss happens during deep sleep when your body is repairing itself and maintaining essential functions. Your breathing rate remains steady, and you continue to expel carbon dioxide. 

It comes back to my mantra – Good quality sleep is key as it regulates our hormones like leptin and ghrelin, which control hunger and fullness. Evidence from research conducted by  Harvard T.H. Chan School of Public Health suggests that disrupted sleep can interfere with these hormones, leading to overeating and weight gain. So, prioritising sleep is crucial for maintaining a healthy weight.

If you would like to arrange a team’s call to discuss our sleep presentations or learn more about our sleep presentations and webinars click here 

Key Takeaways 

• Fat loss primarily happens through breathing. About 84% of fat is expelled as carbon dioxide, and 16% as water (NIH – Fat Metabolism).

• Fat cells shrink during weight loss but don’t disappear. This makes maintaining weight loss challenging (Harvard Health-Fat burning process).

• Exercise boosts fat loss by increasing your breathing rate and expelling more carbon dioxide (ACE Fitness).

• Good sleep supports hormone balance and fat loss (Sleep Foundation).

• If you’re wondering how fat does leaves the body, remember: you’re literally breathing it out!

 

References 

• National Institutes of Health – How Fat Leaves the Body 
• NIH – Adipocyte Function
• Harvard Health Publishing – Why We Gain Weight-  Fat burning process