BY DEAN KARNAZES
Let’s start with the basics, just what the heck is fasted cardio? Fasted cardio is nothing other than a catchy buzz-phrase for working out while in a fasted state. And what defines a fasted state? Generally speaking, the body needs a 8 to 12-hour period of not eating to be “fasted.” So, unless you had a midnight snack, most people are approaching a fasted state when waking in the morning.
Of course, you might be wondering why anyone would want to run or workout on an empty stomach? Personally, I prefer training on an empty stomach because I don’t like the feeling of food in my gut while exercising. That’s one reason. But the main reason people do cardio while fasting is because it shifts the fuel the body burns to a higher percentage of fat. Case in point, one study  found that when subjects were fasted during a morning cardio session they burned 20% more fat than when they had a meal prior to exercise. Research has demonstrated that we utilize our fat stores more readily when fasted, so we rely more on oxidized fat if we haven’t eaten anything 8 to 12 hours beforehand. Meghann Featherstun, R.D., states: “If we run fasted, we tap into our fat stores as a fuel source sooner, so we’re running more on oxidized fat versus glycogen or carbohydrates.” 
There are a couple advantages to burning a higher percentage of fat while working out. One is that you’re burning a higher percentage of fat while working out. For people looking to shed some excess body fat, this is one way to go about it. The other reason is that, according to some research , consistently training in a fasted state can improve your endurance. Fat is a more concentrated source of energy than carbohydrates, so that could be one factor. Additionally, the body can become more efficient at burning fat if that is a consistent source of energy it’s receiving.
Are there downsides to fasted cardio? Potentially, yes. While we can enter the workout in a fasted state, thus improving our fat-burning potential, we need to remember that some carbohydrate intake during exercise—not prior, during—is necessary for fat to be efficiently used as an energy source. Dr. Rick Kattouf puts it this way, "Not only do carbohydrates provide energy for working muscles, they also assist in enabling fat metabolism. In short, carbohydrates need to be present in order for fat to be utilized for energy."  Additional research found that “the decreased availability of hepatic glycogen with fasting led to decreases in the availability of plasma glucose, thus causing decreased glucose oxidation, thereby inhibiting fat oxidation.” 
Lastly, as we previously wrote about on page 14 in “The Ketogenic Diet Revisited”, when the body is depleted of calories, research  has shown that a substance known as pyruvate—which is formed during glucose metabolism—is unable to perform its job in the process of creating energy using oxygen (i.e. cellular respiration). When pyruvate is unable to perform its tasks—which it can only do in the presence of carbohydrates—the body’s ability to use fat as a fuel source significantly slows down or halts altogether.
This same research  shows that without replenishment of adequate amounts of carbohydrates during exercise, including high-intensity exercise, the body—through a process known as gluconeogenesis—will utilize protein as a fuel source, be it from a small protein donation via your energy fuel (ideal), or from the cannibalizing of specific amino acids (BCAAs, alanine) from muscle tissues (not ideal). The latter not only slows down metabolism, it also causes excess production of fatigue-causing ammonia.
The takeaway: having some carbohydrates during a fasted workout is essential for both preventing muscle breakdown and shifting the body to burn more fat.
Interestingly, there’s a hack that can achieve the same shift to a higher percentage of fat utilization as fasting achieves. It’s called caffeine. Two studies [7, 8], of many, have demonstrated that caffeine have shown that caffeine consumption prior to a workout increases the release of stored fat for fuel. But use it or lose it (back into storage). Hammer Nutrition 53X11 organic coffee is the perfect partner for cranking up the engine and burning more fat.
Sometimes during my fasted cardio I also plop a Perpetuem Solid in my mouth and just suck on it while working out. In a study of one (i.e., me) I feel better recovered the next day when I do this. My theory is that the slight bit of protein and carbohydrates in Perpetuem Solid caps help to prevent muscle breakdown.
As a runner, I’m a proponent of working out fasted, at least periodically. To me, the benefits outweigh the potential downfalls, and that comes from over a decade of self-experimentation. As I always counsel people when it comes to exercise and nutrition: listen to everyone, follow no one. I encourage you to try new things and seek what works best for you. And perhaps, if you’re like me, fasted cardio will become part of your fitness regime (along with Hammer Nutrition organic coffee!).
Dean Karnazes is an ultramarathoner and Hammer Nutrition Athlete. His latest book is, A Runner’s High.
 Wolfe RR. Metabolic interactions between glucose and fatty acids in human. Am J Clin Nutr
 McArdle WD, Katch FI, Katch VL. Exercise Physiology Nutritional. Energy, and Human Performance seventh, edition. 2010
 Dulloo AG, Geissler GA, Kangas AJ (2009) Normal caffeine consumption: influence on thermogenesis and daily energy expenditure in lean and post obese human volunteers. Am J Clin Nutr 49(1):44–50
 Acheson KJ, Zahorska MB, Pittet PY, Jéquier SD (2012) Metabolic effects of caffeine in humans: lipid oxidation or futile cycling? Am J Clin Nutr 33(5):989–997