When to use fasted cardio for fat loss.

fat adaptation

Fasted cardio, the key to getting shredded. If you believe the Bros, that is.

If you follow the commonly held belief that doing your cardio in a fasted state is necessary to getting leaner then I’m afraid the bros were wrong.  A study by Schoenfeld et al in 2014 found that for body composition changes, there was no significant difference between the fasted versus fed cardio groups.

Basically, for cardio benefits and a little extra calorie burn, do whichever option you prefer. You like to have a little food before you cardio, go for it. If you want to do it fasted, have at it.

So, is there any potential benefit to doing fasted cardio?

Metabolic flexibility is your body’s ability to switch between carbs and fat for fuel. If you spend a lot of time eating carb heavy food, your insulin levels are constantly elevated, pushing your body to use a higher percentage of glucose to fuel your activities. You end up further to the right side of the pic below and you tend to stay there for the most part.

CArb adapted v fat adapted

Improving your body’s ability to use fat not only improves a number of health markers but also improves your ability to switch between fuel sources on demand. No more crashing hard if you haven’t had a cookie in a couple of hours.

By setting up the metabolic situation to use fat for fuel, you can move away from constantly being up at the right end of the diagram above to using more of the whole range of fuel combos.

2 things to note!

  1. No, you don’t have to go into ketosis for this to happen.
  2. You never burn 100% carbs or fat, it’s always a mix of the 2.

For some folks, going full keto is where it’s at. If it’s fat loss you’re looking for, keto works just like any other diet, by creating a calorie deficit. No deficit, no fat loss.

There are other health benefits from following a keto diet, mostly for traumatic brain injury, epilepsy and moving away from being pre-diabetic etc. However, there are few performance benefits to be had, particularly from a power/ strength perspective. You may well be able to maintain low to moderate intensity efforts for a long time, but when it comes time to kick up a gear, you’re going to want carbs in the mix. And while it is certainly possible to get stronger on a keto diet, repeated efforts are limited by the slow regeneration of ATP using fat as fuel. It’s simply too slow compared with glucose led ATP regeneration.

The other issue with going keto, is that most folks spend all their time up at the carby end of the fuel spectrum, with very inhibited abilities to use fat. Come next Monday and they decide keto is the answer. Carbs drop to around 50g per day (most folks don’t really understand how low this is, about 3 slices of bread. Yes, for the entire day). This is going to suck. You won’t die, but you’ll feel like you might. Adherence is low and back to hold habits they go.

Is there a smarter way to get fat adapted?

I’m glad you asked.

Since fat adaptation is simply a better ability to use fats for fuel when appropriate, we should look at insulin. Think of it as a fuel selector switch. Insulin levels are high, you use more carbs for fuel. Insulin goes low, fat is the predominant fuel source. Remember, that this just means you use a higher % of fat, you still have glycogen in the liver and muscles, but at low intensities, the primary fuel source is fat.

Fasting is just not eating for a period of time. This has been shown to reduce insulin levels lower over a period of around 18 hours. They stay low until you eat something either protein or carb based. Both are insulinogenic, (which is why both carbs and protein intake is lower on a keto diet).

By performing some kind of low-intensity cardio towards the end of a fasting period you can start to improve your body’s use of fat. This is due to the low level of insulin, the fuel selector switch. Remember, insulin is low, fat is fuel. This may well be difficult to begin with, until you up-regulate your ability to metabolise fat. But your body is really good at adapting to the conditions you find yourself in repeatedly. This is after all the principle that training follows, Specific Adaptation to Imposed Demands, ie you get good at what you repeatedly do.

How to implement this into your lifestyle

First of all, do you need to? A simple test that I learned from an interview with Dr Mike T Nelson, is this:

At the end of a fasting period of 18-24 hours, go for a low-intensity jog of about 20 minutes. This can also be a fast-paced walk, but keep the intensity up as much as you can. Since you are in a fasted state, insulin is low, fat is the preferred fuel source. If you aren’t too great at mobilising and using fat as a fuel you are going to get to a point where your fatigue pretty quickly. Your legs will get heavy and you’ll need to stop. This suggests you need to improve your ability to use fat.

Step one – Build up to a 12-18 hour fast once or twice a week. Then perform some kind of low-level cardio, this could be on the treadmill, cross trainer or bike at the gym, or it could be getting out for a walk/ walking to work. Gradually you’ll improve your ability to use fat and rely less on carbs to fuel all your activities.

Step two – reduce your total carb intake. This isn’t going keto necessarily, it’s simply reducing the percentage of carbs in your diet except around training times or periods where you have much higher energy demands. This helps by providing the right fuel when you need it most, and by reducing your dependence on carbs to fuel all of your activity. As a bonus you’ll also probably reduce the calories you eat, helping with fat loss…

This might look like this:

Sunday dinner at 6 pm.

Monday, no breakfast and walk to work.

Eat lunch at around 12, a mix of proteins, fats and carbs.

Train at night, possibly with a pre-workout snack or shake of protein and carbs.

Go home eat dinner.

Tuesday, Wed and Thurs as normal.

Thursday night, dinner around 6 pm.

On Friday, repeat Mondays routine.

Etc, etc, etc.

If you are new to fasting, start slow. Maybe around 12 hours is a good start, if you have dinner at 7pm, don’t snack before bed, bed at 1030 and sleep for around 7 and a half hours. All you need to do is go one extra hour to make the 12 hour fast. Gradually build this to 18-24 hours as you become more used to it. If you feel light headed and/ or want to punch a colleague for talking too much, you may well need to finish early and eat a little something…

Even without the cardio aspect of this, you can start to improve your insulin levels and start becoming more fat adapted.

If you have any questions, feel free to shoot me an email and I’ll be happy to help.

Stay healthy,


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