Endurance Sports Nutrition: Are High Carb or High Fat Diets Better? Delving into the Science...

The following article will cover the current nutritional recommendations for endurance sports.

There is a growing theory that we will become a "metabolically efficient" athlete if we starve ourselves of carbohydrates (CHO) by eating a diet high in fat and low in CHO. This theory is based around that if we only have fat available to use for energy, our bodies will adapt and utilise more of this for energy production. Our glycogen (CHO) stores are limited to around 2,000 calories in the average individual (or 2 hours of high intensity exercise), whereas we have an infinite supply of fat stores. If we are able to utilise fat instead of CHO, we will be able to exercise for longer without "hitting the wall" or "bonking". This would absolutely be beneficial to endurance performance.

So what's the problem? Well, currently there is very limited scientific evidence to support this. I see a lot of articles stating that eating high fat, low carb diets will boost your fat metabolism, but I am yet to see one which explains why or how this occurs. It's all well and good for people to write this, but until somebody can explain to me the science behind how this occurs, I'm not sold.

How Does Metabolism Work?

Metabolism is determined by an individual’s oxygen consumption and carbon dioxide production. There are two primary fuels our body uses to create energy: CHO and fat. CHO creates energy quickly at a low oxygen cost, but produces an identical amount of CO₂. Fat, on the other hand, creates a larger energy yield, but requires 2.4x more oxygen to metabolise than a CHO. The percentage of fat and CHO metabolism is determined by dividing carbon dioxide production by oxygen consumption and is called the Respiratory Exchange Ratio (RER). An RER of 1 indicates 100% of our energy contribution is from CHO (eg 4000ml of oxygen consumption and 4000ml of CO₂ production), and an RER of 0.70 indicates 100% of our energy contribution is from fats (eg 3000ml of oxygen consumption and 2,100ml of CO₂ production). An RER of 0.85 indicates a 50% contribution from each fuel source and is referred to as the energy ‘cross over point’.


Energy released kcal/g

Oxygen required litres O2/g

CO2 litres/g












Percentage of fuel source contribution at different intensities. The 'cross over point' occurs at 65% VO₂ max.

Percentage of fuel source contribution at different intensities. The 'cross over point' occurs at 65% VO₂ max.

So what does this mean? Well, when we have plenty of oxygen available in our body and only a low-moderate oxygen demand (ie exercise intensity is low-moderate), we are able to utilise a lot of fat for energy. Why? Although we require 2.4x as much oxygen to burn a fat compared to a CHO, because our exercise intensity is low-moderate, we have plenty of time to metabolise this. As exercise intensity increases (oxygen demand increases), we shift towards metabolising predominantly CHO because we need energy quickly. CHO is the body's preferred fuel source above around 65% VO₂ max because it is broken down quickly and easily to release energy. CHO has a much smaller oxygen cost than fat so we use CHO at high intensities and most race paces.

The effect of exercise intensity of fuel utilisation. Source: Romijn et al. 1993

The effect of exercise intensity of fuel utilisation. Source: Romijn et al. 1993

What happens when you "hit the wall" during a race? You slow down significantly because you have run out of glycogen/CHO stores. What your body is doing is switching back to using predominantly fats as a fuel source. As we require 2.4x as much oxygen to burn fat, we have to greatly reduce our exercise intensity (reduce our oxygen demand) to give our bodies enough time to metabolise this. You can run solely on fats for days, but it will be at granny pace.

Cycling work output after consuming CHO compared to not consuming CHO.

Cycling work output after consuming CHO compared to not consuming CHO.

No matter who you are, you will ALWAYS burn 100% CHO at your VO₂ max. Always. Oxygen demand is very high and we need energy as quickly as possible. If you are taking up 4000ml of oxygen at VO₂, I guarantee you will be producing at least 4000ml of CO₂ and not burning any fat.

So What’s All The Hype About?

Serious athletes will often get a metabolism test (or metabolic efficiency test) to determine their cross over point between predominantly using fat to predominantly using CHO (ie RER=0.85). As mentioned before, most individuals hit their crossover point at about 65% VO₂ max, and this point does not change. Let’s say, for example, an athlete is tested and has a VO₂ max value of 3000ml/min and they reached this while cycling at 300 watts. They will reach their cross over point at around 1,950ml/min (65% VO₂ max) and at 195 watts. They then go off and do some endurance training for 3 months and come back again. This time they have improved their fitness and their VO₂ max value is now 4,000ml/min which was reached at 400 watts. Their cross over point still occurs at 65% of VO₂ max, but 65% of VO₂ max is now at 2,600ml/min of oxygen consumption (4,000x0.65 = 2,600) and at 260 watts. This athlete now burns 50:50 fat:CHO at 260 watts, predominantly burns fat below 260 watts, and predominantly burns CHO above 260 watts. If we go back and cycle at 195 watts (which used to be 65% VO₂ max but is now significantly lower), we will utilise a higher proportion of fat because we are able to supply more oxygen (through endurance training) to process fat and break it down into energy, not because we have magically changed the type of fuel we use at the same relative intensity. The cross-over point is still 65% of VO₂ max, however this value just occurs at 260 watts after training instead of 195 watts because the individual has increased their fitness. The take home message? Improve your aerobic capacity if you want to burn more fat.

Endurance Sports Nutrition:

So we’ve determined that the CHO is the nutrient of choice for endurance athletes according to the current guidelines, but what type of CHO and how much do we want?

CHO can be broken down into low, moderate, and high GI carbs. Glycaemic Index (GI) is a rating between 0-100 on how quickly a food increases blood glucose.

Low GI foods (whole grain bread, brown rice etc) increase blood glucose slowly, allow sustained energy release, and are digested slowly. These foods are best days before and 3+ hours prior to training and racing.

High GI food increase blood glucose quickly (eg gels), spike our energy levels, and are digested quickly. These foods are best during racing and immediately post-event within 15 minutes. If we consume 1-1.2g/high GI carb per kilo of body weight within 15 minutes post a long endurance session, we will replenish our glycogen stores up to 3 times quicker than if we delay eating by 2 or more hours.

Moderate GI foods sit in the middle of the two. These are best 1-2 hours prior to an event.

Guidelines for daily CHO intake according to training habits.

Guidelines for daily CHO intake according to training habits.

Carbohydrate Loading

Carbohydrate loading is a strategy which involves consuming a large quantity of CHO in the 1-3 days leading up to an endurance event accompanied by a 10-12 day exercise taper. This maximises your muscle glycogen stores so you come into a race with more than your regular 2,000 calories of glycogen, thereby increasing the time taken to reach glycogen depletion and fatigue.

The recommendations are as follows:

  1. Consume 7-12g of CHO per kilo of body weight in the 2-3 days lead up to a long distance event.

  2. Reduce your training volume by 50% for 10-12 days while maintaining training intensity.

  3. Consume 30-60g of high GI carbs (gels, sports drink etc) every hour during an event.

Something to Consider...

Scientific studies have shown that high concentrations of circulating insulin (which is released when we eat high GI CHO), inhibits fat metabolism to an extent. This would mean that if we consume a lot of gels, sports drink etc during or directly before exercise (ie within 30 minutes before start time), that we will rely more heavily on CHO stores for energy during the event.  Note this is NOT anything to do with metabolic efficiency, this is to do with food choices during an event or training session. My recommendation? Still consume high CHO diets in the lead up to training and an event, and directly afterwards. Play around with the types of foods (still CHO based) you eat during low-moderate intensity training and see if lower GI alternatives (such as a banana or sandwich) prolong your performance by avoiding large insulin spikes. There is still research to be completed, but modifying your in-event food consumption (rather than before and after) may assist to avoid bonking. The major consideration is whether eating foods lower in GI cause you gastric disturbances and stomach cramps as they take longer to digest.

Summing It Up

As mentioned in our first blog post The 7 Core Components of Successful Endurance Performance: An Overview, there are still a lot of unknowns about the body and how it functions. This article attempted to explain our current understandings of sports nutrition and describe how the proportion of fat and CHO is determined in metabolism. There is a lot of research currently being undertaken to answer the question of whether high fat, low carb diets improve fat burning capacity. As yet, there is no solid evidence which I have come across. If you are currently having success with such diet, then great. Just ask yourself whether you think have found performance benefits because you are more "metabolically efficient" or whether you are just a better fat burner because you have increased your aerobic capacity through training and can therefore supply your muscles with more oxygen. Building your aerobic capacity has been covered in our VO₂ Max article. Consider the effect of insulin spikes during an event and play around with trying lower GI alternatives during training, while maintaining high CHO intake before and after. In the meantime, I will be prescribing high CHO diets to athletes as there is undeniable proof that it is effective in prolonging endurance performance.

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Written by Luke McIlroy – Director of Sport Science at METS Performance Consulting