High-fat, low-carb diets: good for you and your cycling?

by Matt de Neef

For decades the use of carbohydrate in a cyclist’s diet has been a given. We know from research findings that carbohydrate is necessary for improving both high intensity and endurance performance. But recently this theory has been challenged by a number of endurance athletes and researchers.

In this first part of a two-part series, Joe McQuillan and Alan McCubbin introduce us to high-fat, low-carb diets, discuss the benefits of such diets and look at how you can try one for yourself.

Some athletes claim that following a low carbohydrate diet — with a greater proportion of energy coming from fat — has allowed them to consume less carbs during exercise without any loss of performance. Not only that, but they’ve seen additional benefits to overall health and body fat levels.

In a recent blog post successful endurance coach, veteran athlete and author Joe Friel noted:

“The bottom line is that last fall I lost 8 pounds in 9 weeks by eating more fat and less carbohydrate. That was 5% of my body weight (160 pounds – at the time I was well on my way to my normal winter weight). I was never hungry. In fact, it seemed like the more fat I ate, the more weight I lost.”

To understand why this is interesting we first need to look at existing approaches to fuelling athletic performance.

The traditional approach: carbs = performance

There’s been plenty written on CyclingTips in the past about the importance of carbohydrates in training and in race situations. These recommendations stem from research showing that the reliance on carbohydrate (as opposed to fat) to provide energy increases with the intensity of the exercise.


Studies have shown that beginning endurance exercise with more carbohydrate stored (as glycogen in muscles and the liver) improves performance when the duration is more than 2 hours long and when the exercise is performed at a moderate-high intensity.

Consuming additional carbohydrate during exercise further improves performance by adding to the total amount of carbohydrate available to the muscles.

A recent study showed that increasing the amount of carbohydrate consumed during endurance exercise (2 hours of constant moderate intensity cycling followed by a 20km time trial) improved performance (see Table 1, above right).

Sports scientists and dietitians working with pro cycling teams have adopted these recommendations, with the pros often consuming upwards of 90g/hr carbohydrate on the bike.


The need for such volumes of carbohydrate stems from the need to avoid running out of muscle glycogen during periods of high-intensity training or racing. When this occurs (without additional carbs coming in from food) the muscles draw on blood glucose as the only remaining source of carbs in the body. If the body draws too much then blood glucose levels fall, resulting in hypoglycaemia.

Most of us know this as ‘hitting the wall’ or ‘bonking’, and the longer and more intense the event, the more likely carbs will become key in how much power you can produce.

You might remember Cadel Evans’ implosion on stage 17 of the 2002 Giro d’Italia, which possibly cost him the overall win. And then there was Lance Armstrong’s fade on the last climb of the 16th stage of the 2000 Tour de France showing, if nothing else, that even EPO cannot prevent a performance loss if you’re completely glycogen depleted1.

Low-carb diets and energy production

At lower-intensity exercise our body requires very little energy to move the bike forwards. When the body can keep up with demand for oxygen, fat can be used as the major energy source.

Our body’s stores of fat are far greater than carbohydrate — this is likely the result of evolution because one gram of fat provides 38 kilojoules of energy, whereas one gram of carbohydrate only provides 17kJ (and requires water to be stored along with it). This makes fat a far more weight-efficient way of carrying stored energy in the body.

So while the body’s glycogen stores are fairly limited, fat stores are near inexhaustible for any given period of continuous exercise. If we could better access this pool of energy at higher exercise intensities we might be able to reduce our dependence on carbohydrate (dietary and stored) and prevent bonking during a race.

Several factors affect the body’s use of carbohydrate and fat as energy sources:

  • Genetics — Some people appear to be much better suited to using fat as opposed to carbohydrate as an energy source at any given exercise intensity
  • Training adaptations — Any exercise training that improves cardiovascular fitness will reduce the reliance on carbohydrate as an energy source at any given power output
  • Diet — Avoiding carbohydrate prior to and during exercise also reduces the body’s use of carbohydrate as a fuel source. Studies from several labs have shown that training with less carbohydrate available to the muscles increases the body’s ability to use fat at higher exercise intensities.
  • Disease states — Type 2 Diabetes, in particular, has a dramatic effect on the body’s flexibility to change between using mostly fat or mostly carbs for energy.

Terms such as “metabolic efficiency” have been thrown around by people who eat low carbohydrate diets, to describe the goal of preferentially using more fat than carbs at any given exercise intensity. They’re also often described as being “fat adapted”.


The following data are taken from my (Joe’s) lab at the Auckland University of Technology, and show how individuals differ dramatically in their use of fat and carbohydrate as fuel sources.

In both cases the athletes rode for 20 minutes at 100 watts; thereafter the wattage increased by 25W every 5 minutes with heart rate and blood lactate measurements taken at the end of each 5 minute stage.

The 20-minute warm-up was used to allow the athlete to increase their reliance on fat as a fuel. This was at a very low intensity (100W) in which athletes reported 7/20 score (extremely light) on the Borg’s Rating of Perceived Exertion (RPE) scale.

As you can see in figure 1, the first athlete preferentially used greater relative amounts of carbs over fat even at low intensities, despite not eating for four hours prior to the test. At best this is a contribution of 72% carbs and 26% fat. It is fair to say that Athlete 1 is heavily carb dependant even at lower intensity exercise.

Figure 1: Percentage fat and carbohydrate use during incremental cycling exercise (Athlete 1).
Figure 1: Percentage fat and carbohydrate use during incremental cycling exercise in Athlete 1.

Very different findings are seen with Athlete 2, tested under the same protocols (figure 2). You’ll notice a greater use of fat as an energy source in the early to mid-stages of the assessment.

The Total Energy Expenditure (TEE) was very similar for both athletes, however the substrate (fat or carbs) contribution to TEE is markedly different (figures 3 and 4).

Figure 2: percentage fat and carbohydrate use during incremental cycling exercise (Athlete 2).
Figure 2: Percentage fat and carbohydrate use during incremental cycling exercise in Athlete 2.

As well as Athlete 2 having a 50W higher peak power output, it is obvious that they use a far greater percentage of fat as an energy source compared to Athlete 1. To this end, Figures 3 (fat comparison) and 4 (carbohydrate comparison) compare the two athletes’ data by way of percent VO2max given that this is a relative measure.

Note that ~70% VO2max represents a moderate exercise intensity, around 85% VO2max represents closer to a tempo type effort (representative of a hard bunch ride). Here the difference in “metabolic efficiency” is very clear between the two athletes, and may be due to differences in genetics, training status (especially given the different peak power outputs) and daily diet.

Figure 3, left: Percentage of fat utilisation vs VO2max. Figure 4, right: Percentage of carbohydrate utilisation vs VO2max.
Figure 3 (left): Percentage of fat utilisation vs VO2max. Figure 4 (right): Percentage of carbohydrate utilisation vs VO2max.

Why would low carb diets be beneficial for endurance athletes?

Being dependent on carbohydrate as the major energy source during exercise has some obvious limitations (limited supply, depletion results in hypoglycaemia), and therefore adapting the body to utilise more of our body fat stores to fuel exercise makes practical sense.

This may not be achievable at very high exercise intensities, as athletes usually approach 100% reliance on carbohydrate at 100% VO2max. But if we are able to increase “metabolic efficiency” and reduce carbohydrate use at moderate intensities, then we may be able to avoid the dreaded bonk while also reducing the requirement to eat during exercise, carry less food, reduce the likelihood of gut issues and the cost of buying or making gels, bars and sports drinks.

It’s important to note that even though Athlete 2 in the example had better “metabolic efficiency” than Athlete 1, neither of them were consuming a low-carbohydrate diet at the time of this initial assessment. Both have subsequently done so, and the following article in this series will present findings and individual anecdotes from their journey to becoming more “fat adapted”.

So how might being “fat adapted” benefit you? Other than the benefits mentioned above, people devoted to this approach (athletes and non-athletes) have anecdotally reported:

  • weight and body fat loss (because they can get away with eating less total calories/kilojoules each day, partly due to the effect of low carb diets on appetite)
  • a perception of increased and sustained energy throughout the day
  • improved sleep patterns
  • improvement in blood lipid profiles. A greater intake of (unsaturated) fat has lead to a decrease in LDL (so-called bad cholesterol) and an increase in HDL (so-called good cholesterol) and a reduction in total cholesterol
  • no afternoon “crash” that may be due to a reduction of blood glucose levels
  • no change (reduction) in VO2max or peak power
  • reduced or complete cessation of craving sweet foods

Fat adaptation and cycling performance?

While the concept of fat adaptation and low carb diets for athletes has only risen to prominence recently, research in this area goes back almost two decades. In 1995 the term “fat loading” was described as potentially “the next magic bullet” for endurance performance.

Five years later a string of studies on the topic were conducted by husband and wife team John Hawley (RMIT University, Melbourne) and Louise Burke (AIS Sports Nutrition). In 2000 they published data showing that as little as five days of a high fat, low carb diet altered the body’s use of fat and carbohydrate during exercise, although there was no benefit to performance. Several papers followed in the next few years, all showing the same result.

They also found that athletes struggled to perform high intensity training intervals after a period of high fat, low carb eating. A review paper on the topic at the time concluded:

“Overall, there is evidence to suggest that endurance performance at best can only be maintained after long term adaptation to fat-rich diets when compared with carbohydrate-rich diets, and therefore long-term fat diet usage cannot be recommended as a tool to improve endurance performance”.

The issue of lowered carbohydrate availability in training re-surfaced in the mid-late 2000’s, this time looking at the effect of performing every second training session with low glycogen stores, over a period of 10 weeks. Again these studies found evidence of fat adaptation, but no difference in performance at the end of the training block.

They also noticed that athletes who undertook every second session carb-depleted actually performed less work in training, but interestingly performed just as well at the end of the training block.

It can be argued that studies such as these didn’t change the athletes’ diets for long enough, and didn’t restrict carbohydrate severely enough to see the true benefits of “fat adaptation”. Some people who support the approach acknowledge it’s actually not about improving performance, but about other health benefits that can be achieved without a loss in performance compared to the traditional high carb approach.

Here’s Joe Friel again:

“Eating a LCHF [low-carb, high-fat] diet has not directly improved my performance. I’m not faster now than I was before. This is common in the research I’ve read on the topic. What it has improved is getting to and staying at race weight without calorie counting or hunger.”

But until someone completes well controlled studies of low carb diets, over longer periods of time and with a variety of performance measures (i.e. long, evenly-paced time trials as well as high intensity sprint or hill climb efforts following a prolonged ride) no-one can say for sure whether “fat adaptation” is the next evolution in endurance sports nutrition.

In part 2 in this series Joe and Alan show how the two test subjects responded to high-fat, low-carb diets, discuss what a high-fat, low-carb diet might look like, and explain what you can do if you want to try such a diet for yourself. Click here to read part 2.


1. Having said that, EPO will reduce your reliance on carbohydrate at any given power output, meaning you can go harder for longer before you run out.

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