Are fasted long runs beneficial for performance?

As runners, we all probably know someone who runs fasted – no food before or during the run. Some may even complete a 20 miler with no fuel. Many who run fasted swear it is the way to go – so is it beneficial? Let’s dive into what the research says.

Increased fat burning – Running with low carb availability forces our bodies to oxidize more fat for energy. But, fat is a slow fuel source. We cannot run as fast when running primarily off fat. A review of 46 studies found carbs during long runs enhances performance.

Recruit more muscle fibers – When running in a no-fuel state, we attempt to recruit more muscle fibers because our muscles are starved of energy + exhausted. Exhaustion of muscle fibers directly coincides with glycogen depletion, aka no fuel long runs. Starting a long run with exhausted muscle fibers can increase injury risk.

Improve aerobic fitness –  In the lab, we do see improved markers of aerobic fitness from fasted training, however, these markers are not translating into improved race performance. Why? We’re not entirely sure but may be related to down downregulation of carb metabolism on race day.

Mental toughness – We don’t need to add nutritional suffering to our training to gain mental toughness. We teach ourselves to hang on when it hurts, and stay tough when our mind tells us to stop and keep running on tired legs – but we don’t need to starve our muscles + minds on long runs to get tough. 

More glycogen storage post run – ALL exercise opens the window for increased glycogen storage post-exercise. If the well is dry, it’s going to hold more water. 

Not only are the beneficial claims questionable, but the research has shown that we should be cautious when it comes to running while fasted. Concerns include:

• Decreased immunity 
• Altered hormones 
• Muscle breakdown 
• Delayed recovery
• Will this suffering actually improve performance? 

Improved performance – A 2014 review of all studies on runners and carb intake during endurance events found 82% of studies showed significant performance improvements from carbohydrate fueling during endurance running. 

Decreased perceived exertion –  One study of cyclists riding for 90 minutes at moderate to high intensity found that taking carbs halfway through this ride significantly increased carb oxidation and lowered the rate of perceived exertion which resulted in faster sprint performances at the end of their trials. 

Extended time to fatigue – Well-fueled runners can store enough carbs in the form of glycogen to sustain 90 minutes of activity. At this point, without fuel, we have to rely predominantly on fat for fuel. Fat is a slow fuel source. We cannot turn fat into energy as quickly and paces suffer, fatigue sets in, efforts feel harder, and we ‘hit the wall.’

A fueled body is a strong body. A strong body can get the work done that you’ve been training for. Give your body the fuel it needs!