Is it beneficial to fast before working out?


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Many athletes wonder whether they should work out on an empty stomach or whether it’s crucial to have a snack beforehand. As “fasted cardio” gains popularity, some are opting to skip the pre-workout snack altogether. But is this no-fuel strategy advised for athletes?

The short truth to the story is… (Are you ready for it?) it depends! There’s not a one-size-fits-all approach to nutrition and fueling for sports performance. There may be a time and a place for skipping that pre-workout meal or snack; however, whether or not you should do it depends on many factors.

While traditional sports nutrition guidelines promote carbohydrate-rich fueling strategies, there may be a benefit for some athletes to go against the grain, pun intended. Some endurance athletes take the approach of “training low,” or training on minimal to no glycogen stores for longer endurance runs. This is done in an effort to improve metabolic efficiency, or a greater utilization of fat for fuel.

Rikki Keen, MS, RD, a certified specialist in sports dietetics and certified strength and conditioning specialist says, “science has shown placing the muscle in a stressful state of low glycogen levels during selected aerobic training sessions can trigger a cascade of hormonal and gene signaling that further enhance training adaptations within the muscle cell.” Because stored carbohydrates are in limited supply and fats are plentiful, this metabolic shift could benefit endurance athletes, allowing them to go longer before crashing or bonking.

However, “fasted endurance training won’t show immediate performance gains,” says Scott Sehnert, MS, RD, a sports dietitian at Auburn University. “Instead it causes metabolic changes that may produce optimal performance later, when the athlete is well-fueled.”

It’s also important to understand that increased fat burning during exercise does not directly equate to bodyfat loss; calorie deficits still comes into play when aiming to lower body fat.


When snacking makes sense

If performance is the goal, there are clear benefits to eating shortly before exercise. For example, don’t skip a snack before a competitive event. Instead, consume a meal or snack that’s high in carbohydrates, low in fat and moderate in protein to keep energy levels high and fuel optimal performance. Eating before exercise is intended to delay fatigue, enhance endurance and support performance; promote mental clarity; and prevent low blood sugar and hunger.

If you’re looking for strength and muscle gains, foregoing the pre-workout snack is not the way to go. Consuming a meal with protein and carbohydrates before a workout will increase your body’s ability to burn carbs needed for energy to perform and utilize amino acids from protein to increase lean muscle mass.

And because high-intensity interval training relies on the anaerobic energy system, which requires carbohydrates to be burned as fuel, it’s not recommended to skip pre-workout nutrition. “Because HIIT is so reliant on carbs, and because most people doing HIIT (CrossFit and the like) want to see muscle gains, they need to be well-fueled for that and not break down their lean tissue for energy,” says Sehnert, a certified specialist in sports dietetics and certified strength and conditioning specialist.


Does fasting increase fat burn?

Keep in mind these fueling recommendations are geared toward serious athletes with intense training regimens and performance goals. When exercising moderately for fitness benefits, it may not be necessary to follow the same guidelines.

“In general, if someone is going to do moderate cardio simply to burn calories or improve fitness, then I don’t think a snack prior is necessary, especially if they’ve eaten within the last 3-4 hours,” says Sehnert. If you’re doing a light early morning sweat sesh and skip a pre-workout snack, just make sure to fuel up after to recover quicker and reduce muscle soreness.

Turning to fasted cardio for weight loss? “It can be used a method to enhance fat oxidation during low-moderate exercise, assuming the individual still gets a quality workout in and controls for calories for the remainder of the day,” says Keen, the team sports dietitian for Orlando City Soccer Club. “It’s one tool that can be used to support weight management; however, it is not for everyone and could actually backfire with the person overeating at the next meal.”

Skipping meals may also lead to overeating later in the day, emotional eating and mood swings, and it can promote eating disorders. People susceptible to or experiencing these issues likely wouldn’t benefit from fasted exercise.


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