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Signs of Fat Burning in Urine

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You’ve been on your healthy eating diet for a couple of weeks now and your gym sessions are progressing nicely.

You’re feeling stronger, fitter, healthier and above all, slimmer.

But how do you know if you’re really losing fat or whether you’re feeling slimmer because you’ve dropped water or muscle?

Can you tell that you’re burning fat from your urine?

In this article we tell you everything you need to know about the sign of fat burning, and if your urine can be a good indicator of fat loss. 

Where Does Fat Actually Go When You Lose it?

You gain fat when your body senses that it has more energy coming in than it needs and has to store some out of the way for a later day.

It’s a simple balance between incoming energy and how much you burn off that decides whether or not you burn fat or store it.

The way that your body uses fat as fuel involves a number of complex physiological chain reactions. Scientists, researchers and clinicians are still learning more and more about fat loss on a daily basis.

But what they do know is that if you eat more than you burn off each day you’ll start to pile on the excess pounds. Burn more off and you’ll melt fat quicker than butter in a hot pan.

Lucky for you, you only need to understand the basics of fat loss science to see what happens to fatty acids when you lose energy from your fat cells.

Fatty acids are a fuel source

You store fat in specific cells called adipose cells.

These cells hold on to little droplets of lipids called triglycerides – a molecule made up of glycerol and three fatty acids.

When you trigger fat loss, the adipose cell allows the glycerol component to split from the fatty acids so that they can travel to specialized organelles called mitochondria where they are used as fuel for energy – a bit like throwing coal into an open furnace.

This process is called the Kreb’s cycle, but is also referred to as the citric acid cycle too.

The thermodynamics of weight loss – fat can’t just disappear

Regardless of whether the fat you consume is a solid, a liquid, it comes from avocado or you get it from bacon, it provides energy for your body.

Fat is an energy source, and as with any energy, it can’t be created or destroyed, only destroyed. 

That means that once you’ve used those extra fatty acids as energy, they can’t just cease to exist. They just transform.

You’re losing fat – only it has to turn into something, not just disappear from the face of the earth.

So what happens to fat when you lose it?

Surprisingly, when you oxidize fatty acids in the mitochondria, you eliminate them by breathing them out.

Yep, you heard that right – over 80% of the fat you burn is breathed out [1].

As part of the Kreb’s cycle, those fatty acids are metabolically converted into usable energy called adenosine triphosphate (ATP) by taking part in numerous stages where it is broken down into smaller and smaller parts until all that’s left is the actual ATP and some by-products which can then be breathed out as surplus to requirement gases.

What happens to the rest of the fat?

The remaining components leave the body through your sweat, tears, bodily fluid and of course, your urine.

Muscular bodybuilder guy standing and posing triceps muscle

Can Your Urine Signal Fat Burning?

Not really.

Urine is the by-product of human metabolism. It is the waste left from numerous chemical reactions that is mostly water, but also includes some nitrogenous wastes – urea, uric acid and creation.

It is excreted from the kidneys to the bladder, from where it exits your body via your urethra. It is the responsibility of your kidneys to filter excess waste products from your blood and then eliminate it from the body before it can cause damage.

You don’t technically urinate out your fat

The way in which fat is broken down for energy involves a number of chemical reactions, ultimately leading to some left over hydrogen, oxygen and carbon.

So what you’re left with is essentially water, which leaves your body through your urine, and carbon dioxide, which you simply breathe out.

In a way you could say that urinating more frequently could be a sign of fat burning – but then it could also be a sign of drinking more fluids.

So it’s not that you urinate out fat, it’s more that you urinate out the by-products left over from the breakdown of fat in the body.

You Can Measure Ketones in Urine

The ketogenic diet is a high fat, low carb approach to eating.

During keto, you essentially replace your carbs with fats and protein as a way of increasing ketones in your blood.

Ketones are produced by the liver when you restrict carbs and are water-soluble acids left over from fat burning.

They provide you with an alternative energy source to glucose and trigger an increase in fat oxidation because of the lack of carbohydrates in your diet.

Many refer to the keto diet as a shift from glucose burning to fat burning – a little like a metabolic on-switch.

It’s not a diet for everyone and can leave many people feeling tired, lethargic and run down. You run the risk of suffering from low blood sugar and the side effects can outweigh the benefits for most people.

But for the few that do like it, it can help to control calorie intake and lead to fat loss. Whether that’s because of the actual diet or just that you are low on calories overall is up for debate.

Increased ketones in your urine could signal more fat burning

When you drop carb intake your ketone levels increase.

There are three different types of ketone bodies that can be measured – these are called acetone, acetoacetate and beta-hydroxybutyrate.

One way to measure ketones is to test your urine. You’re not measuring fat per se but you are measuring the increase in ketones – a sign of potential fat metabolism increase.

Testing your urine using specialized (and often expensive) ketone strips helps to give you a rough idea of how far into ketosis you are.

By holding the strip in your urine you’ll note a color change if the number of ketones in your body has increased excessively – specifically acetoacetate because it is expelled from the body via urine, as opposed to acetone which is expelled via breathing (one of the reasons why those in ketosis often report having sweet smelling breath).

Cyclist peeing in the bushes during a Race


Whether you feel that such a complicated method as urine strips is worth the time and effort or you’d rather just use a tape measure to check that you are losing inches, or use before and after photos to check your progress is ultimately up to you.

But the bottom line is that you can’t really measure fat loss with your urine.

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  1. Meerman, R et al. When somebody loses weight, where does the fat go? BMJ. 2014; 349: 7257