High-fat, low-carb diets good for you and your cycling cyclingtips gas x tablets himalaya


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.

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.

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 depleted 1.

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.

• 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.

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 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.

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% VO 2max. 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”.

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.