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  • Writer's pictureCourtney Ustrzycki

Fasted Training & Cardio

I think the top question I get from all (or about 90% of my clients) is something to do in terms of fasted training and/or cardio. When we think of fasted training (or cardio; but let’s just call it ‘training’ from here on out for ease of reading), we’re thinking about waking up first thing in the morning, no breakfast and just hitting the gym. In simplest terms, if that’s what works better for the client, I’m not going to be overly concerned about how they do their workouts. I have some clients in my home gym that arrive at 7:30am to workout, and it’s way too much to wake up even earlier to sit down, have a hearty meal, and have enough time to digest before hitting the weights. At the end of the day, to each their own. But here’s some simple information that you can understand and possibly apply for future training.

At any given point in time, whether we are taking about exercise or not, our body uses carbohydrates as its main energy source, so without them in your body before you hit the heavy iron, your body needs to take its energy sources from somewhere else. This is where most people think that if their carb sources are low (or depleted), the body will take from the fat sources. Yes and no. Not how we would imagine it to do so anyway.

During rest and light intensity exercise, your body uses about 50-60% of its energy source from fats, and about 30-40% of its energy source from carbs. Simply put, if you were to do fasted cardio, then doing steady state would be optimal in this situation.

High intensity sprints or intervals require about 90-90% carb energy, with the rest separated between fats and protein. So if you’re planning to do fasted sprints, you’re really not going to benefit much from the process.

Similar to high intensity intervals, if you plan on hitting the heavy weights (strength and/or endurance training) in a fasted state, you’re going to need about 75% carb energy in order to perform optimally. You will not be able to sustain intensity because muscle glycogen will deplete too quickly.

Expecting to perform optimally for a challenging morning workout without any food is like trying to drive to work on an empty tank of gas. Sure, you may get to the finish point, but was that good for your engine/body?

A study (referenced below) showed that consuming carbs pre-training will not slow down lipolysis (the process of fat breakdown to generate energy) in an individual and in the fasted state even going after the breakdown of more fat, the body will take those excess fats that aren't oxidized and basically be stored back into fat. Eating food (carbs) before training increases the thermic effect of exercise (the ‘fat burning’ state).

Another negative factor to training in the fasted state is the impact on proteolysis (breaking down proteins, AKA losing muscle), nitrogen losses more than doubled in the fasted state as compared to having energy (from carbs) within the body. So you’re actually breaking down that hard-earned muscle.

Training in a fasted state will cause lower energy levels, therefore decreasing the fat burning process. Performance will suffer, especially when trying to do the far superior style of cardiac training, which is HIIT. The more energy that your body has when doing cardio means the more calories burned both during and after physical activity, thus more fat burned. The process of fat burning is not immediate; it can take days (not hours), so as you burn through carbs during your workout, the body will continue to burn fat after you finish exercising.

From this information, we can come to the conclusion that steady state cardio (light walking) in the morning in a fasted state is totally fine, and probably recommended, but if you want to really use your body and burn fat, I would highly recommend that you eat little meal and use BCAAS to help retain lean muscle and burn more fat!

Reference: Craig Yarnall, CSCS, CPT, WNBF Pro;

Strength and Conditioning Journal (vol. 33, Feb/March ’11)

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