• Steve Cronshaw

Is Morning Fasted Exercise Beneficial?

Updated: Aug 1

Should you undertake morning Fasted Exercise?

Some Science behind Fasted Exercise.

If you asked me 5-years ago, if going out on the bike in the morning without breakfast in a fasted state beneficial?

I would have said “NO Way”, Don’t do it!

I would have gone on to tell you the disadvantages far out way any benefits.

Muscle wastage, blood sugar lows and eating the entire contents of the fridge on returning from the ride, are things that would jump out at me.

However, after studying the science, I have completely changed my mind over the last two years researching the subject on my Sports Nutrition M.Sc. The science has convinced me that there are definitely good benefits to be gained from fasted exercise for most athletes. But also equally important reasons for non-athletes to undertake this practice. However, there are defiantly good “ways to do it” and “ways NOT to do it”.

In this article, I will look at the science and try to demystify it, to look at the benefits for all of us to gain some advantages.

Let me start by saying if you are a young, fit, slim, elite endurance 20-year old athlete, then the benefits are not going to be as pronounced, but there are still benefits to be had in this group.

However, If you are someone who is trying to lose weight or a master’s athlete, or simply attempting to improve your aerobic endurance then read on, as this is defiantly for you. You can undoubtedly get more “bang for your buck” in doing aerobic exercise and improved body composition occasionally in a fasted state.

A reminder first, of how we produce energy for sport and exercise that our bodies should be flexible enough to obtain energy from multiple sources, the food we eat, mainly carbohydrates and fats. Additionally, protein, but this is a convoluted route to produce it. Furthermore, energy comes from stored sources, this is the glycogen in our muscles and liver, from circulating essential fatty acids in our blood, and from our bodies own fat reserves. This last one, “our own fat reserves”, most people have considerable reserves to go for days using this fuel source, even for very lean athletes.

Three Good Benefits

1. Metabolic Flexibility, what’s that you ask? Well, it’s our bodies ability to produce energy from different fuel sources depending on what is available. Most people these days’ and athletes consume large quantities of carbohydrates. Carbohydrates are essential for very top-end performance. Nothing wrong in this, you say! But yes, there can be if you are not in an energy balance, the obesity epidemic illustrates this.

But even for fit athletes if they over-consume carbohydrates and sugar, then over the years and decades they build up a condition called insulin resistance that creates a dependency on glucose for energy. This condition increases blood sugar levels and fluctuations that in turn, make the body release more insulin to bring down the blood sugar levels. Our bodies are designed to only have a teaspoon full of sugar in our blood at any one time. So the surplus blood sugars should go into our muscles and liver to store it for energy. Once we have this condition of insulin resistance, then our bodies are not very efficient at storing it in muscles and liver due to the high insulin levels. The high insulin levels are very effective at driving this blood sugar “wham”, straight into our fat cells for storage. When we have continuously high blood sugar levels and high insulin, then the hormone insulin prevents us from accessing energy from our fat cells and makes us ever more reliant on carbohydrates for fuel. The result can be an increasing accumulation of fat around our middle, energy highs and lows and a decreased desire to exercise.

2. Mitochondria Biogenesis

This is the process by which cells increase Mitochondria mass)

Mitochondria are the power plants in our cells and muscles that produce the energy to power our aerobic energy system. Improving our aerobic conditioning is mostly about endurance training that induces higher mitochondrial content levels; a benefit of this is leading to higher glucose uptake by our muscles.

When we train in a fasted state at the zone-2 intensity with low blood lactate levels, the research has shown that the body can be more effective at building higher mitochondria density in our muscles to the benefit of our performance. This could be for an endurance athlete trying to achieve higher endurance performance, or for a power and sprint athlete that may only do one aerobic session per week. Both types of athlete want to get the most benifit in terms of spending less time achieving a more significant benefit.

3. Improved Body Composition

Losing weight for most athletes is not good if you are losing your muscle mass, or reducing your power to weight ratio. However, if you are reducing your body fat and increasing your power to weight ratio, this can have health benefits and sporting performance benefits.

Research has shown, when we train in a fasted state at zone-2 intensity with low blood lactate, this is a good way of getting into our maximum fat burning zone (Fatmax). This produces less fatigue at this intensity, so we can do more of this type of training, hence remain in this Fatmax zone for longer to the benefit of our body composition. There is a huge difference in where this maximum fat burning zone is for different people, and every athlete needs to know where their own Fatmax zone is so they can effect and train to improve it or to take advantage of it.


So to sum up, there are definitely benefits from fasted morning exercise, the three main ones are, improved fat burning, building new mitochondria and improved body composition .

Try it with a coffee and straight out on the road if you cycling or running, it is very time efficient.

However, it is IMPERATIVE to stay in your correct heart rate zone-2 personal to you, with low blood lactate levels.

If you come back from the session super hungry and you want to eat the entire contents of the fridge! Then that is a sign that you went too hard Or it could be a sign you have a Metabolic syndrome condition that is not good for your endurance performance or your long term health. The good news is this can be reversed to the benefit of performance, but also long term health risks.

An ex-pro cyclist and now a mitochondria researcher Dr Iñigo San Millán who has produced lots of research in this area, a good research article on this subject is the San-Millan and Brooks (2018) as below.

Other References

San-Millán, I. and Brooks, G.A., 2018. Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports medicine, 48(2), pp.467-479

Edinburgh, R.M., Hengist, A., Smith, H.A., Travers, R.L., Betts, J.A., Thompson, D., Walhin, J.P., Wallis, G.A., Hamilton, D.L., Stevenson, E.J. and Tipton, K.D., 2019. Skipping breakfast before exercise creates a more negative 24-hour energy balance: A randomized controlled trial in healthy physically active young men. The journal of nutrition, 149(8), pp.1326-1334.

Jeukendrup, A.E., 2017. Periodized nutrition for athletes. Sports Medicine, 47(1), pp.51-63.

Burke, L.M., Hawley, J.A., Jeukendrup, A., Morton, J.P., Stellingwerff, T. and Maughan, R.J., 2018. Toward a common understanding of diet–exercise strategies to manipulate fuel availability for training and competition preparation in endurance sport. International journal of sport nutrition and exercise metabolism, 28(5), pp.451-463.