As a running coach, one of the most asked questions I get (especially among my race trainees) is, "What's the best fuel, fat or carbs?"
I learned along time ago, that like politics, food can be a very emotional topic for people. So, I'm not here to say one fuel is bad and one fuel is good. I can share from personal experience and from the research which tends to be better for a runner. As an individual, you an decide what's best for you. And as always, consult with your physician and/or registered dietitian when making any dietary changes, especially if you have specific dietary needs or a physical condition dependent on a specific diet.
In my race training plans I provide my runners with basic fueling guidelines to support their pre-, during-, and post run fueling and refueling. These guidelines provide basic carbohydrate, protein, and fat recommendations based on my education from RRCA, ASCM, and NPTI. From my running coach's perspective, carbohydrates are a runners main source of fuel.
A lot of recent popular diet programs use a fat-for-fuel approach. A big draw to these programs is initial quick weight loss. Most of the fat-based diets severely restrict carb to as little as 10% while increasing fat up to 75% of calories. The bases for his shift is that this type of diet for the athlete will prevent bonking or crashing. This type of diet is supposed to provide a bigger reserve of energy sustaining activity for longer periods of time. Research has been done showing that ultra marathoners and Ironman triathletes who had a high fat diet of 70% fat and 10% carbs for 9-36 months burned fat at much greater rates during an 180-min run than those who followed a lower fat diet of 25% fat, 59% carbs. This outcome seems pretty good.
More research, however, shows that for sports where you need to kick it up a notch at times, a high-fat diet, isn't going to provide that umph you're looking for. Why? Well, as your body learns to burn fat more efficiently, it impairs your body's ability to burn carbohydrates. Your body uses fewer chemical reactions to convert carbs to energy. So, with a proper carb store and your body being able to quickly convert carbs to energy, you can get that boost you need to charge that hill, or pull out of a fatigue slump, or change gears as you near that finish line. With conversion of fat to fuel, your body is more like, "Oh okay, you want to go faster. Wait, hang on. I'm working on it."
Idea Fitness Journal (April 2018) shares that in a review of 61 studies, the vast majority showed that diets relying on more calories from carbs than fats were optimal for athletic performance (Stellingwerff & Cox 2014). None of the studies showed that a carb-rich diet hurt performance. Most research shows that decreasing carbs in favor of fat isn't likely to get you to the finish line faster. (Burke et at. 2017; Havemann et al. 2006; Urban et al. 2017; Zinn et al. 2017).
Another thing to keep in mind if you desire to go to a high fat/low carb diet is that it takes weeks to adapt to. I do not recommend a runner who wants to try a high fat diet switch to it while at the same time starting a race training program. Start the transition several months prior to starting race training. Why? When switching to a high fat diet one can expect to experience fatigue, constipation, and mood swings none of which would be great to experience while starting up a race training plan.
Personally, I'm a proponent of eating a well balanced diet. I'm a strong believer in a good mix of complex carbs (including whole grains, fruits and veggies), healthy fats, and protein is key. Then when kicking in race training, I believe slightly increasing healthy complex carbohydrates is key in supporting your workouts.
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