‘Carb loading’ is a practice that athletes have performed for decades in order to maximise their glycogen stores ready for training or an event. It is most commonly used in the endurance sector of sport where glycogen storage and glucose availability is paramount to exercise capacity. Glycogen is a major limiting factor when it comes to endurance activity, in other words if glycogen stores run low an athletes ability to run, cycle, swim or move is significantly hindered.
So what of ‘fat loading’?
Endurance athletes, particularly ultra endurance athletes can come to rely heavily on fat as an energy source, this is well understood. As glycogen stores begin to deplete your body shifts from using glucose as an energy source to fat via beta oxidation. However many athletes have taken this ‘fat loading’ strategy to the extreme, whereby fat comprises around 60% of the diet (almost double the recommended daily percentage of fat). The consequence of this is that carbohydrate intake has to go down in order to keep calorie intake stable.
So does it make sense to fat load?
Well, in the case of reducing your carb intake to allow for more fat in your diet prior to an event, the answer appears to be a resounding no! Fat is more energy dense than carbohydrate, but the bodies ability to use it as energy is less efficient making carbs the bodies preferred energy source. Research has shown that time to exhaustion i.e. how long it takes to run out of energy is reduced (i.e. you run out of energy faster). Studies suggest that consuming a high fat diet beyond 4 weeks has been shown definitively to have a detrimental effect on endurance. The studies that support regular high diets for endurance events do not consider all facets. Many of the studies were working with a cohort of subjects who trained at below a VO2 max of 70%, and it is well documented that most endurance athletes will be working in excess of a 70% VO2 max, meaning the results of such studies are not applicable to most endurance athletes.
Where might fat loading be beneficial
Although this remains relatively speculative, it is surmised that ultra endurance athletes may well benefit from fat loading. Ultra endurance athletes have to perform at a relatively slower pace (below 70% of VO2 max), usually lasting longer than 4 hours, meaning they could well benefit from a diet high in fat (if the aforementioned studies are anything to go by). If the body is used to a higher fat diet then it may well be trained to be more efficient at converting fat to energy, in turn sparing glycogen (carbs) for energy.
Loading with fat safely
Although extreme dietary manipulations are risky, there are occasions (particularly in the sporting world) where they may be beneficial, even necessary. Should you choose to load with fat some sources suggest consuming a 50-60% fat diet for 6 days followed by one day of carb restoration. Is this a practice I would recommend, well not for the lay public, but if you were to do it then be sure to do it under the expert guidance of a Dietitian or sports nutritionist.
A diet that is excessively high in fat is ill advised in any walk of life, the health risks attached to large amounts of fat in the diet vastly outweigh the negatives. Eberle, (2014) states that following a diet that is way above the recommended maximum of 30-35% of total daily calories is senseless for most endurance athletes. She feels that such a diet reduces the amount of carbohydrate consumed lowering muscle glycogen stores. Low glycogen stores can reduce energy delivery, endurance and an athletes ability to perform at high intensity…so in short, if you’re going to load anything then make it carbohydrate.
Eberle, S, G. (2014). Endurance Sports Nutrition. Fuel your body for optimal performance. 3rd Ed. IL: Human Kinetics.