The surprising truth about exercise and fat burn

Today I wanted to address an often-asked question, with some long-believed myths, and truths: do I have to work out for hours to start burning fat?

When it comes to exercise and fat burn, the belief is, you have to work out for over an hour if you want to switch your body into fat burning mode. So, I am going to look a little into this, and hopefully make it all a bit clearer for you!

truth about exercise and fat burn

Where does this idea of fat burning come from?

Our body uses different energy sources at different times, depending on how much energy we need, how quickly, and how we have fuelled ourselves. At any given time, our body will be burning some fat as fuel, some glucose as fuel. This will vary depending on your metabolic rate, what activities you’re doing and what is easily available to your body.

When you start moving more vigorously, say, you start jogging, your body immediately needs some more energy. You will use the most readily available fuel, which is glycogen (coming from carbohydrates). But you are likely still using some fat for fuel, just at a lesser ratio.

If you were to continue running for a prolonged period of time, your body would switch to predominantly using fat for fuel, as we store just a limited amount of glycogen and the body needs to preserve this reserve. Depending on how well trained you are, your body may switch sooner to using fat for fuel (see fat adaption below).

So, there is some truth in it when people say you need to work out for longer in order to tap into your fat stores.


If your goal is fat loss, you can still lose fat if you are burning carbohydrate stores. How, I hear you ask?

Two things – ultimately, you want to burn the highest number of calories in your workout. If you are walking for 90 minutes, this low intensity, endurance activity will likely draw mostly on fat for fuel. However, total calories burned will be roughly 300 calories, for example (and let’s say 60% are from fat). If, however, you chose to do a 45-minute-high intensity workout, you will predominantly use carbs for fuel (for example with a 60% carbs, 40% fat ratio), however you will be burning more calories in total, around 500 for example. This calorie deficit becomes more important to your weight loss goals, and 40% of 550 (220cals) is still more than 60% of 300 (180cals). (Check out this article for more of an explanation on this, and an explanation of the after-burn effect!).

And the other thing… when we use glycogen for fuel, your body still needs to replenish energy sources. If there is no glycogen available, or replenishment from carbs in your diet, it will run off fat for fuel. So yeah, you end up burning that fat anyway!

A very restrictive low-carb diet of 50 or fewer grams daily does not provide enough carbs to restore liver or muscle glycogen. But, you don’t need to, because your body shifts into ketosis, where it runs off a different fuel source consisting of fatty acids and ketones. Ketones are compounds your body naturally produces when too little external glucose is available for energy. Doing this leads to fat adaption.

What is fat adaption?

Ahhh, the buzzword of the century. We all want to be fat adapted, right? But what does it actually mean? Becoming fat adapted is when your body will naturally turn to fat for fuel. It is the idea that we restrict carb intake heavily, and eat a whole load of healthy fats, and as such our body becomes so used to using fat as fuel, it doesn’t know whether we just ate it, or it’s coming from the spare tyre around our waist.. but either way that fat is getting burned!

How to become fat adapted?

There are a couple of ways you can train your body to rely on fat for fuel. If you are an endurance athlete and training long distances regularly, your body may already be pretty good at this (and women are especially good at it – hooray!). But if you aren’t trained to burn fat as fuel, when people talk of ‘hitting the wall’ in an endurance event, it is generally when their body is making the switch to using fat for fuel, and it doesn’t like it!

Train fasted

This is one way that we can help our body adapt to burning fat for fuel, without having to be 45 minutes into a work out for it to happen. If you approach your workout with depleted glycogen stores, then already your body will have to start using fat for energy pretty early on. You can do this by training in the morning on an empty stomach. And it’s going to be more effective if you’ve had a carb light meal the night before i.e not a pizza! Some argue that you can’t train as hard fasted as you could if you were well fuelled with glycogen stores topped up, but if you regularly train your body in this state, your body becomes better at using fat as fuel, and you can still work out pretty damn hard!

Decrease carbs, increase fats in your diet

When I studied nutrition, I attended a particularly interesting lecture by Mark Sisson, created of The Primal Blueprint. Here’s something he said that really stood out to me:

“We’ve evolved to be fat-burners (must be why we’re so adept at storing it on our bodies!). It’s easy to see why. Fat burns slow and evenly, providing all-day steady energy levels. Carbohydrates burn quickly, and they’re gone in an instant, leaving you groggy and depleted unless you “carb up.” Furthermore, carbohydrates are an inherently unreliable and fleeting source of energy for our body, with most people only able to store about 400-500 grams of carbohydrates on the body at any one time. Our storage capacity for fat, on the other hand, is virtually endless. Just 10-15lbs of body fat, which is the bare minimum available on even the leanest individuals, can provide tens of thousands of calories. Luckily, reducing carbohydrates and increasing fat intake sends the epigenetic signals necessary to help us revert back to fat-burning. And it only takes a week or two to get things moving in the right direction.

Become fat-adapted, enjoy boundless energy. Free yourself from the shackles of a carbohydrate-based metabolism/dependency.”

So perhaps we are actually meant to use more fat as fuel and rely less on carbs. Maybe this will stop the mood swings, cravings and ups and downs of sugar highs and lows?!

So I digressed… sorry that tends to happen when I get Mark on the brain ; )

So Sarah, back to the question at hand: do you have to train for hours to burn fat?

In short, no. You do not need to exercise for two hours to start burning your fat stores. Eat smart, train smart, eat tasty food, train in a way you enjoy, and you’ll see the fat melt off.

Want to try it a high fat diet for a day and see how you feel? Here’s a good place to start.

For training and nutrition advice, be sure to get in touch and see how I might be able to help you by contacting




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