How does fat leave the body? The science of weight loss explained

'A minute on the lips, forever on the hips' might be a popular saying, but we’ve all seen enough people thriving on their weight loss journeys to know that it’s fundamentally untrue.

Still, if you’re in the early days of trying to lose weight yourself, the saying might resonate, with that excess body fat feeling like it’s here to stay.

It's a normal feeling - trust us - but nobody's shape or number on the scales is set in stone, and understanding how and why fat leaves the body can offer some guiding principles to help you on your own weight loss journey.

What is fat?

When it comes to understanding how you lose fat, it’s first important to understand just what fat is and how it’s retained in the body.

When you eat, your body breaks down food to its main components of protein, carbohydrates and dietary fat. It does this to take the nutrients from what you've ingested, but also to metabolise the glucose in those components as energy to fuel your body to function.

While it’ll start to use this energy right away, it doesn’t always need all of it at that very moment. When that happens, your body will pack away that unused energy as a type of lipid, otherwise known as a fatty molecule, for future use and insulation, which is ultimately what makes up body fat [1].

How is fat stored?

Those lipids are stored inside fat cells known as adipocytes throughout the body. While everyone has these fat cells, they grow and shrink up to a factor of 50 depending on how many lipids are inside them at any one time [1].

Adipocytes can be anywhere in your body, ultimately varying in placement depending on your genes, lifestyle and body type, so in other words, don’t fret if you carry weight more on your hips while others carry it in their arms or thighs. That said, there is one really important distinction when it comes to where your body fat is stored, which can have different health implications.

Generally speaking, 90% of a person’s body fat is subcutaneous fat, which is just underneath the skin. This is the type you’ll find that you can generally feel when you’re moving, getting changed or feeling yourself. Visceral fat, which makes up the other 10% in most people, is stored fat beneath the abdominal wall and in spots around the liver, intestines and other internal organs.

Of the two types of fat, visceral fat can sometimes be cause for concern as it has been found to increase the risk of several serious diseases [2].

Luckily, both types of body fat can be burned and reduced.

How is fat burned?

Ultimately, burning fat and losing weight is a simple equation. When you consume fewer calories than your body needs to function, it will start to burn stored fat to provide the energy you need.

You can prompt this process by reducing the amount of calories that you eat, ramping up the exercise you’re doing so that your body needs more fuel for all the activity, or both [1].

When your body needs to burn body fat for energy, it'll do this by releasing an enzyme that breaks down the fat tissue into its basic components - namely carbon dioxide and water - until the waste can be excreted from your body [5].

How does fat leave the body when you lose weight?

Once your body has used the energy it needs from those fat cells, that lipid has to go somewhere. People tend to guess that we sweat out our fat during a workout, and while some of the water from the fat tissue does come out in sweat, urine and other bodily fluids, we actually mostly exhale it. The main component that fat tissue is broken down to after all is carbon dioxide, the very thing we breathe out.

It’s important to remember, though, that as you lose weight, you’re losing the fatty molecules, not your fat cells. In other words, your fat cells shrink as fat loss occurs and your body weight reduces, but those cells don’t go away. Maintaining weight loss is a lifelong journey, as a result, because those fat cells can always be refilled and lead to weight gain [4].  

How long does fat take to leave the body?

How long fat takes to leave the body is, like many things when it comes to weight loss, dependent on a whole host of factors, from your starting body weight and body fat percentage to your metabolic rate, genetics, medical history and lifestyle.

Generally speaking, though, it takes about 30-60 minutes of exercise for people to start to burn body fat and build muscle mass because your muscles tend to use stored sugar for energy before they use your stored fat.

How much fat you burn, too, will ultimately depend on your caloric intake and your activity, but a healthy and sustainable weight loss journey tends to offer 500g to 1kg of fat loss per week [5].

How do I know if my body is burning fat?

Unfortunately, there's no real way to know for sure if your body is burning fat outside of the general hallmarks of weight loss. In that sense, if your measurements are getting smaller, clothes are fitting better, muscle mass is more defined, and the number on the scales is getting lower, you can generally assume you're burning body fat. Hello, non-scale victories!

Does sweating burn more fat?

When your body burns fat, one of the components it breaks down to is water, and one of the ways water leaves the body is through sweat.

That said, you're also going to be sweating out water generally, and when you're working out, you should be drinking a lot of it. This can make sweating a bit of a misnomer when it comes to fat loss because a lot of that sheen on your chest or forehead, or that moisture pooling at the small of your back, is actually just temporary water loss and not a reflection of the fat-burning process.

Keeping your fluids up during physical activity is important, though to keep you hydrated, so keeping that water bottle at the ready is a must.

The effect of sleep on fat loss

We all know that sleep is vital for a healthy body, but you might be surprised to hear that getting enough is also vital for losing weight and weight maintenance.

There have been increasing links to sleep deprivation of under 6 hours a night and obesity, along with other medical conditions. One of the reasons for this is that people eat more when they don’t get enough sleep because they require a higher energy intake. This can affect people’s circadian rhythm, which has been shown to affect the weight loss process, making it a pretty challenging situation for weight management [3].

As a result, ensuring you get the recommended 7-9 hours of sleep a night will promote weight loss and help support you to maintain that weight loss long into the future.

A sustainable approach to weight loss

When you’re just starting on your weight loss journey, it can be easy to look for shortcuts to shed that excess fat or to make sweeping changes to your lifestyle that are simply not sustainable. Crash dieting, fitness challenges or social media weight loss trends can often appeal in the short term for rapid weight loss, but result in weight regain in the weeks and months that follow. Losing weight and maintaining weight loss as a result needs to be approached not just holistically, but realistically.

If you’re working full-time with kids at home, committing to a two-hour workout every day probably isn’t going to be possible, but Park Run on the weekend and a mid-week swim might slot right into your schedule, just as meal planning and prepping on the weekend might see you avoiding the frazzled evenings where fast food feels like the only option. The important thing is to remember that at its core, weight loss is about eating more nutrient-dense food with fewer calories and making regular cardio and strength training exercise a part of your routine.

Make lifestyle changes that you know you can maintain, and set healthy habits that you can get excited about. Not sure where to start? Juniper’s Weight Reset Program can help with that. Led by dieticians who offer comprehensive health coaching and support, this medically-backed treatment comes with an online community ready with advice and cheerleading to get you started and keep you going.

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