Switching to fat

This article is designed to provide you with a brief look at our energy systems and why eating a diet low in carbs and high in fat may help recreational runners like you and me in our running performance.

First, we need to take a look at our energy systems.

Next, we see what happens when we eat carbs.

Finally, we summarise.


In order to keep this piece easy to understand (I hope!), I won’t be delving too deep into the science. However, I will be providing links where you can obtain more information should you wish to. There is a great resource on Peter Attia’s website that gets pretty technical if that’s your thing: https://peterattiamd.com/category/ketosis-and-fasting/


Energy Systems

Any discussion about energy systems has to include a mention of ATP (Adenosine Triphosphate). ATP is commonly referred to as the energy currency and has two vital functions:

  1. Ion transport (pumping sodium into our cells and potassium out of our cells.)
  2. Muscle contraction

In the first case (ion transport), I am not going to go into any more detail here but suffice to say that if this process does not occur (i.e. the pumping in/out) we die.

So, on to muscle contraction which is probably what most of us are far more familiar with. ATP is required more for the release of the muscle contraction rather than the contraction itself. Of course, without a contraction being released it cannot be repeated. A good example of this is in rigor mortis where the muscles have stopped releasing due to the fact that we have stopped producing ATP – because we are dead!

ATP, therefore, is a vital molecule that our body must produce. It also has to get to our cells in some way, and that’s where our energy systems come in.

Most of us have heard of the terms aerobic and anaerobic, but many not be familiar with them in respect to our energy systems. In addition, there is actually another energy system that is not often talked about. So let’s take a look:

  • Creatine-Phosphate System
  • Anaerobic Pathway (often referred to as the alactic system in sports)
  • Aerobic Pathway

We can see from the following picture (figure 1) how each energy system works:

Figure 1: Energy Systems.

Adapted from Peter Attia’s presentation on UCAN


Creatine-Phosphate System

This system works in quite a different way than the other two in that creatine is used to convert ADP (Adenosine Diphosphate) to ATP. There is no requirement for an external fuel source such as glycogen or fat for this to take place, so at first glance it looks very good and doesn’t have all the issues that the other two systems have.

The rather large downside to the Creatine-Phosphate System is that it is very limited. This is due to us not having very much creatine in our bodies. In fact, typically this system only has between 10 and 30 seconds worth of high intensity effort.

Look back at evolution, this energy system was very useful for providing a rapid burst of energy; in hunting or escaping for example. In terms of athletic performance, it is the system used when sprinting, during high intensity activity and explosive bursts of energy such as in competitive weightlifting.


Anaerobic/Alactic Pathway

This energy system is one of the two commonly known systems, and operates in the absence of oxygen. That is, oxygen is not needed for ATP to be produced in this system. What is needed, however, is glycogen. Glycogen is the stored form of glucose and we will learn more about this in the section on carbohydrates. 

We can see from figure 1 above that:

  • Glycogen is turned into lactic acid (through the use of pyruvate)
  • This is then turned into ATP

The Anaerobic energy system comes into use when we cannot take in oxygen at a fast enough rate to use it in the manufacture of energy. In terms of running (and other exercise), this is called the Lactate Threshold and is the point where we start to produce more lactate and hydrogen ions (acidic environment) than we can clear from the cells. We typically see this in high intensity interval training, and for doing those sprint finishes once the Creatine-Phosphate system has gassed out.

Like the Creatine-Phosphate system, the Anaerobic Pathway is limited in that we cannot sustain energy output at this level for long periods. Unlike the Creatine-Phosphate system though, we can maintain it for up to 4 minutes or so.

The point at which we cannot maintain that high level of intensity any more is known as our VO2 Max (Maximum Ventilation (consumption rate) of Oxygen). This is the point of failure when we are exercising.

The Anaerobic system can only use glycogen and not fat as its fuel source.

In reality, the Anaerobic Pathway is activated right from the beginning of our activity, as are the other two systems. However, at different points (timescales and intensities) one of the energy systems is the dominant one. This table, taken from brianmac.co.uk , illustrates which energy system is used at what point in time when working at 95% effort:


Figure 2: Energy Systems in use at 95% intensity

Duration Classification Energy Supplied By
1 to 4 seconds Anaerobic ATP (in muscles)
4 to 10 seconds Anaerobic ATP + CP
10 to 45 seconds Anaerobic ATP + CP + Muscle glycogen
45 to 120 seconds Anaerobic, Lactic Muscle glycogen
120 to 240 seconds Aerobic + Anaerobic Muscle glycogen + lactic acid
240 to 600 seconds Aerobic Muscle glycogen + fatty acids

Note: CP is the Creatine-Phosphate System

Longer periods of exercise (and at a lower intensity level) require the oxidation (use of oxygen) of either glycogen or fat. In other words, it requires the Aerobic Pathway.


Aerobic Pathway

This energy system utilises oxygen and under certain circumstances can provide energy up to our lactate threshold level for several days.

Other than the use of oxygen, the Aerobic system differs from the Anaerobic system in that it can also use fat as a fuel source to create energy. In terms of our running, this is where it gets really exciting!

We can see from figure 1 above that:

  • Glycogen or fat is turned into phosphate
  • This is then turned into ATP


How we use Carbohydrates

Now is a good time to offer a very simplistic (but very useful) view of what happens when we consume carbohydrates.

Carbohydrate essentially comes in two forms:

  • Starch
  • Simple sugar

Starch is stuff like flour, wheat, grains, etc, and is commonly referred to as complex carbohydrate.

Simple sugars are glucose, fructose, lactose etc.

In most cases, starch is broken down by the body into glucose and is first used by any cells that need it, and it is then stored in the liver and the muscles.

The stored form of glucose is called glycogen, a term many runners and other athletes will be aware of. The body only has a very limited capacity to store glycogen, and this amounts to between 1,200 kcal to 1,600 kcal (what we commonly term as calories) of glycogen between our muscles and liver. Any remaining glucose that cannot be stored is turned into body fat. 

In order for the glucose to be stored in the muscles, insulin is triggered. And it is the rollercoaster effect of eating carbs followed by a crash that can lead to insulin resistance and metabolic syndrome.

Simple sugar can also be stored as glycogen, especially glucose. Due to the fact that glucose is already in a form that can be used by the body, it can be absorbed up to 15 times quicker than the complex carbs. We can easily see then, that by consuming energy drinks and gels we can flood our system with glucose. Be aware, however, that we also have a limited capacity to absorb that glucose and this is one of the reasons why it is a contributor to metabolic syndrome that can lead to obesity and Type-II diabetes.

Interestingly, fructose hardly triggers any insulin at all. This means that it does not get stored in the muscles. Some fructose can be stored in the liver as glycogen, but only if there is capacity (i.e. not already full from glucose/fructose consumption). And in the same way that excess glucose gets turned into body fat, excess fructose also gets turned into body fat.

In fact, because fructose doesn’t really trigger insulin it has, until recently, not been seen as a contributory factor to metabolic syndrome.  However, it is now being viewed as far more dangerous than glucose, especially as it is found in so many foods that we eat. Simple table sugar (sucrose) is 50% glucose and 50% fructose. So any foods that say they have sucrose in them should be treated with caution.

So when looking at the energy systems, the important points to remember are:

  • Carbs can be starch (complex) or sugars (simple)
  • Starch is broken down to glucose which is used immediately, stored in the liver and muscles as glycogen, and any excess is stored as fat

We will ignore fructose for the purposes of this article, but please remember that fructose is quite likely to be stored as body fat. This is especially the case if consuming a general high carb diet.

How are carbs used for energy

We know now that in most cases, carbohydrate is broken down into glucose. We also know that this glucose is stored in both the muscles and the liver in limited quantities.

Both the Aerobic and Anaerobic energy systems use (or can use) glycogen to create ATP to power our muscles. So far so good.

We mentioned that our liver and muscles have limited glycogen storage capacity, but in fact the issues with using glycogen as our primary fuel source are a little more complex. Glycogen that is stored in the muscles (a greater capacity than the liver) can only be used by those muscles. That is, they cannot share that glycogen with any other cells, not even other muscle cells. This is because they lack the required enzyme to break the glycogen right down, and this is a good thing as in most cases we want to keep the glycogen in the muscle where it is needed. The liver on the other hand can send its glycogen to other cells, primarily the brain.

This means that as we use up our glycogen stores, we must replenish them in order to keep producing ATP (energy). This is why traditional sports advice says we should carb load before endurance events, and why we need to use sports drinks and gels during endurance training.

Whilst we are talking about endurance, I classify distances from 10k as ‘endurance’ in the context of energy systems and fuel sources. This is mainly due to my own experiences racing 10k events, and in particular the fact that we are trying to run at the highest maintainable pace possible over that distance. If you recall from our discussion on energy systems, this pace is just below the lactate threshold. At this level of intensity we are using our glycogen stores up faster than at lower levels of intensity. We’ll see why this is important soon.


Generating ATP

Each of the three energy systems results in the creation of ATP for energy, this much we already know. What we haven’t yet looked at is how much ATP is generated by each system, and therefore how efficient each system is with respect to energy creation.

For each glucose molecule, we get 2 units of ATP.

Additionally, in the Anaerobic system, lactate is utilised to create an additional 2 units of ATP. Therefore, the Anaerobic system generates 4 units of ATP from each glucose molecule.

The trade-off for this is that as lactate travels around the body (and builds up as part of the anaerobic system), it takes with it a Hydrogen Ion. This hydrogen ion is thought to lead to muscle stiffness and give us that burning sensation in our muscles when we do an all out session.


As an aside, lactic acid tends to get the blame for this muscle stiffness and burning sensation, but it is in fact the hydrogen ion that is the culprit.


This is a rather simplistic view, as there are other theories about what else may be contributing to muscle fatigue and that burning sensation (such as nerve ending exhaustion etc).

The Aerobic system works much more efficiently and actually produces an additional 34 units of ATP from the same single glucose molecule. Therefore, the total units of ATP in this system is 36!

As part of this process (the bit that generates 34 units of ATP) we produce carbon dioxide and water, which is why we exhale both when we breathe.

Remember that the Aerobic system can also use fat (fatty acids) as well as glucose to generate energy. Well, if we use fatty acids instead of glucose we get 34 units of ATP instead of te 36. This may seem like we are losing 2 units of ATP by using fat, but also remember that our glucose (glycogen) stores are very limited. If you recall from earlier, we have the capacity to store between 1,200 kcal and 1,600 kcal in glycogen. Compare that to around 100,000 kcal from fat! Therefore, one huge advantage fat has over glucose is that we do not need to refuel (top up) anywhere near as often (days not tens of minutes).

What this actually means in our running, is that for every hour a person exercises (based on a fairly fit individual) at a pace they can maintain for a long time (marathon pace for example), they would use up about 750 kcal per hour (or 12.5 kcal per minute).

This would mean that at this pace we should have enough energy from our glycogen stores to last about 2 hours. This sounds pretty good, especially when we are talking about 10k, 10 mile and half marathon distances. However, I would refer back to my own experience that we usually attempt to race these distances at pace quicker than the “all day” pace.

Also worth noting at this stage is that when glycogen stores become depleted, the resulting crash in energy is significant and immediate. Generally, there is no real warning that this is going to happen and is what we commonly refer to as hitting the wall or bonking.

Contrast this to utilising fat as our energy source. We know from earlier that we can store up to 100,000 kcal in fat. At the rate of energy use noted above (750 kcal per hour), we could last for over 5 days!

So we can see that utilising fat as our primary energy source is more efficient in terms of the amount of time we can exercise for. However, you may be reading this thinking “yeah, but I can just keep topping up my glycogen stores. I can take gels and sports drinks for that.” Of course you are quite correct, but this carries with it some important disadvantages:

  • The amount of glycogen we can replace in a given time frame
  • Your body’s reaction to frequent re-fueling with carbs
  • The inconvenience of needing to refuel frequently

We cannot physiologically ingest more than about 60 gm of carbs per hour.  It has been suggested that this is independent of body weight.

Consuming more than this has an affect on the GI system and often leads to diarrhea, nausea and vomiting.

There is also the rollercoaster effect mentioned near the beginning of this article. Whenever we consume glucose (even complex carbs as they get broken down into glucose – but to a lesser extent) we trigger insulin. As you may recall, the insulin’s job is to “mop up” the glucose by storing it in the muscles in the form of glycogen.

If we flood our system with glucose the insulin spikes in order to mop it up faster. This state is known as hyperglycemia, and is when glucose is too high (compared to normal blood sugar levels).

However, as the insulin removes the glucose, it does it so efficiently that it causes the glucose to be way too low. This is a state known as hypoglycemia.

In order to try and correct the low blood glucose, the body makes us crave more and the cycle starts again. We can see this in figure 3 below. The dotted lines are where our normal blood glucose levels should be, and the blue line represents the blood glucose in this scenario. The aim is to keep the blue line within the dotted lines, but as you can see insulin doesn’t manage this very well.


Figure 3: Glycemic Rollercoaster

Adapted from Peter Attia’s presentation on UCAN


We can see from figure 3 that this rollercoaster ride can lead to headaches, sweating, lethargy, shaking, and hunger. Clearly, none of these things are desirable when training or racing.

As was hinted at a couple of paragraphs ago, it is the simple forms of carbohydrate that cause the most dramatic spikes in insulin. Complex carbs still raise insulin levels, but not anywhere near as much.

One side effect of raised insulin levels is that the breakdown of fat (lipolysis) is blocked. That is to say that while insulin levels are raised, the body cannot use fat for energy. If part of your goal for exercising is to lose weight (body fat in particular) and you consume sports drinks and gels, you are actually stopping that very process. In other words, you are not in fact burning that body fat.


So you may have noticed by now that we have four good reasons to prefer using fat to glycogen (carbohydrate) for our aerobic training:

  1. Storage capacity. We can store more fat that can be used for energy than we can store glycogen.
  2. Physiologically we cannot keep up with the required glycogen intake required for sustained high level activity.
  3. Most of the sports drinks, gels and other glycogen replacing supplements we take cause a spike in insulin levels which in turn leads to the glycemic rollercoaster and a whole bunch of undesired effects. Add to this the discomfort from attempting to consume more glucose than we can actually ingest.
  4. Raised insulin levels also block the utilisation of fat for energy.

This is a clear case why switching to fat instead of carbs can be far better for us and for our training. But wait! What about Anaerobic training?

An excellent question. There is certainly a slight tradeoff in that reaching the same VO2max may not be possible when we are using fat as our primary fuel source. However, this probably amounts to only 5% to 10% degradation of performance at the very top end of this. In training we may notice it slightly when doing high intensity intervals/reps. In racing we may notice it because getting that sprint finish in may be harder. That said, there is a solution to this that we won’t go into here, but you can find out more by checking out the video on Peter Attia’s site, or by looking at Generation UCAN. You can save 10% on Generation UCAN in their UK store by using the discount code RunTeach.

For most of us though, we won’t notice any difference in this area at all. Remember, you will still have the glycogen stores there when you need them. You just won’t need to access them until you are doing that anaerobic work, so there will be plenty to supply you.

One thing I have noticed is that both my easy pace and threshold pace have got faster. That is, I seem to have pushed both of these paces up by adapting to using fat rather than carbs. So, using this logic, if I can run more of a distance at an overall higher pace, my race times will improve naturally and I won’t have to rely on that sprint finish quite as much.

In conclusion, at our level of running we have everything to gain from switching to fat (even over shorter distances) and nothing to lose – except unwanted body fat!


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