Fed vs Fasted Cardio For Fat Loss

The debate is as old as cardio itself: Is it better to do cardio fasted or fed? Coaches and clients fight one another to prove once and for all which answer reigns supreme. Should you do cardio without anything in your system, or is something better than nothing?

In a study done in 2013(1), they took 16 women, and divided them into two groups: Fed and Fasted. Over the course of 6 weeks, researchers had them do 18 HIIT sessions (10 x 60 second cycling sprints at 90% of their max heart rate). What did they find after 6 weeks? There was no significant difference between the fasted groups and the fed groups for body composition. The fact that they were doing HIIT decreased their bodyfat and increase leg muscle mass, but those results were the same in both groups; it had nothing to do with food, and everything to do with the type of exercise.

In another study(2) using 8 healthy men, scientists wanted to tests the effects of fed/fasted cardio on hunger hormones after an intense bout of exercise. While they established that leptin and ghrelin levels weren’t significantly different, they also noted that:

  • The heart rate measured during exercise in the fasted state was about 10 beats x min lower then in fed subjects. This means the subject wasn’t working as hard.
  • Respiratory quotient (ratio of C02 produced to 02 used) was lower. RQ is used as an indicator of which metabolic fuel is used; meaning whether you’re burning carbs or fat as fuel.
  • Plasma lactate concentration (essentially, lactic acid) in the fasted state was significantly lower than in the fed state. Less lactic acid means less growth hormone, which means less muscle-building and fat-burning for you.
  • The fasting increased plasma IL-6 concentration at rest by a significant amount. Interleukin 6 (IL-6) is a pro-inflammatory protein typically secreted by the body after trauma, infection, or damage. So you have this inflammatory molecule coursing through your blood because your body things it’s under attack. No bueno!

If you want more proof of why fasted cardio isn’t as effective, look no further than medical journals. I picked two from reputable sources, but there are pages and pages of more information out there.

With all that said, however, some of you don’t have time to get food in before you’re off to the gym. You have to get your partner ready, your kids ready, and barely have enough time to brush your teeth before you’re out the door and off the to gym before work. You’re lucky if you can breathe, let alone inhale a meal. If that’s the case, don’t stress out. Fasted cardio isn’t the worst thing in the world, and you won’t lose your gains, but I would consider drinking BCAAs or a protein shake during your session to prevent some of the negative repercussions. So there you have it – get something in you before cardio in order to see the gains (and losses!) that you want!


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