Last year I began to explore the topic of fat-loading, a method used to enhance ones running performance in endurance events lasting longer than 2 hours. The purpose of fat-loading is to teach the body to burn its own fatty acids as fuel (to a greater extent than it already does), thereby conserving glycogen stores. Conserving glycogen stores as much as possible will help to prevent ‘bonking’ or ‘hitting the wall’ on race day.
Plus, fat-loaders tend to appreciate one of the side effects of fat-loading is a tendency to ‘lean out’ considerably – helping those who struggle to reach their racing weight to achieve it. Certainly this was a benefit I personally was very pleased with when I tried fat-loading for the first time last July.
There are two, potentially three, primary methods we can utilize to help our bodies learn to burn more fatty acids for fuel:
- The first is simply by way of endurance training. The more we train and the fitter we get, the more efficient our bodies become.
- The second is by strategically implementing carb-depleted runs. A 2014 review in Sports Medicine confirmed that carb-depleted runs can improve our bodies ability to burn fatty acids for fuel along with other adaptations that may allow us to improve race performance.
- And the third potentially possible way is by practicing fat-loading.
With fat-loading being a rather large topic, I wrote in part one on WHY you’d want to fat-load, including many stories of those who have achieved success using this method. Today, in part two of three, I’ll explore the science behind fat-loading; and in part three, how to fat-load.
A 1994 study published in the European Journal of Applied Physiology demonstrated improved performance in cyclists who had fat-loaded for 10 days. Similarly, this 2001 study published in the International Journal of Sport Nutrition showed enhanced performance. Over a 20km time-trial, the cyclists who had fat-loaded (followed by a 3 day carb-load) were 4.5% faster.
Following fat-loading with the carb-load is an important step – a 2003 study published in the International Journal of Sport Nutrition demonstrated a high-fat diet of 6 weeks resulted in weight loss but a decrease in performance (no carb-load was completed before this cycling test).
Looking at runners (instead of the cyclists used in the above studies), this 2001 study published in the Journal of the American College of Nutrition suggested that higher levels of fat in the diet increases endurance running time in well trained endurance runners.
However, this 2002 study published in Medicine and Science in Sports and Exercise underlined the dramatic increase in fatty acid utilization, yet concluded that despite the favourable physiological changes, performance benefits only followed for some athletes.
As sports scientists have looked at utilizing various dietary strategies for performance including fat-loading for many decades, this 2011 review published in Applied Physiology, Nutrition and Metabolism sums up the evidence to date nicely. In all the studies they reviewed, fat-loading did indeed improve the bodies capacity to burn fatty acids for fuel (in well-trained athletes), exceeding levels achieved by endurance training alone. However, despite this adaptation, only some studies reported performance benefits while others did not.
One of the possible reasons for this discrepancy is that they found some athletes to be ‘responders’ to fat-loading (thereby experiencing performance enhancements), while others were ‘nonresponders’ (either experiencing no benefits or decreased performance).
Another reason for the decrease in performance that was experienced for some may have been because the athletes were working at 80-85% of their VO2max (the speed most runners can hold in a race for about 1-hour). Fatty acid utilization, and thereby the performance benefits of fat-loading, may be elevated when working at lower V02max levels. However, given that marathon racing intensities vary from 75% V02max (trained runners) to 84% V02max (elite runners), you can see why the benefits of fat-loading may apply to some runners and not others.
Additionally, it appears that the bodies ability to oxidize carbohydrate is decreased following fat-loading (as per this 2006 study published in the American Journal of Physiology – Endocrinology and Metabolism) – which means performance would be hindered in any sport requiring the athlete to perform at 90-100% V02max. For example, surging past a competitor in the final stages of the race may not be possible if the muscles ability to oxidize carbohydrate is impaired.
If you do decide to try fat-loading, please note that it’s typical for energy levels to drop and therefore performance in workouts may be compromised while fat-loading. This is temporary and upon carb-loading your energy will return quickly and fully for race day, while the physiological adaptations for burning more fatty acids as fuel remain in place.
As you can see, thus far studies show that fat-loading works for some athletes but not others. Knowing this, some runners may be able to intuitively decide that fat-loading isn’t right for them.
However, for distance runners who feel they have a capacity for increasing their body’s ability to burn fatty acids for fuel and/or who are hoping to achieve racing weight for race day, this strategy may be something they’d like to explore. Certainly for myself and others whose stories are featured in part one, fat-loading has worked very well.
Which leads us to the how. How would one go about fat-loading correctly? I’ll explore this topic next week in part three.
Eat clean, run strong, be well… Cheers,
Sarah J Cuff, RHN