Spring is in the air and many of us will be unearthing our runners to hit the pavement, or starting a new exercise regimen to prep for bikini time. To accelerate fat loss, some people may have considered trying one hot trending diet and fitness method called fasted cardio.
After a night’s sleep, much of our carbohydrate stores have been used up. Since we don’t eat overnight while we sleep, waking up and doing a morning workout without food is called fasted cardio.
The rationale behind fasted cardio is that training when glycogen stores are low shifts fuel utilization from stored carbohydrates to stored fat, thus promoting a higher fat burn and weight loss. Makes sense.
Some studies comparing training in the FED state versus the FASTED state showed promise that this was true, and furthermore, that consumption of carbs prior to aerobic activity actually inhibited fat utilization. And so, fasted cardio has become a popular tactic to get at those pesky fat stores. But, there are a couple caveats.
Caveats to Fasted Cardio Success
- The results are specific to low intensity cardio.
- Other studies actually contradict this result, showing that training in a FED state increases the thermic effect of exercise (TEE), and fat use may actually be increased by eating prior to exercise. This effect remained significant in the 24 hours following the activity, too.
To have a discussion about the pros and cons of fasted cardio, it’s important to understand some of the basics of exercise science.
When we exercise we always use a combination of fuel sources: a certain ratio of fat, carbs and protein. This ratio depends largely on how much oxygen is available. Basic physiology. Low intensity aerobic activity, when oxygen is readily available, allows for more fat use. As intensity rises and metabolism becomes anaerobic, less oxygen is available, and we are less able to use fat for energy. Why is this relevant in the context of the fasted cardio argument?
Well, at higher intensities there’s a limit to how much fat the body can use for fuel due to decreased oxygen needed to oxidize fat. So, unfortunately, whether fasted or not, the body will need to access carb stores to continue at some point. But what happens if we don’t have those carb stores available?! We will be forced to take the intensity down to adapt. Translation: shitty performance and fewer calories burned. Oh, and we might need to borrow some energy from the body’s protein stores to continue, which is obviously less than ideal.
Bottom line, trying to do fasted cardio at higher intensities is counter-productive.
Once we reach that point of breathlessness, fasting results in a lack of energy and inability to work as hard. This compromises our training capacity in the short and long term and, ultimately impedes weight loss.
When is fasted cardio useful?
It may have some potential advantages for highly trained physique athletes in their shedding phase to lose the last couple pounds, and it may also be okay if your morning routine includes a moderate walk, a very light jog or an easy bike ride. Low intensity is usually defined as working at less than 60% of our max heart rate. We should not be out of breath or feeling a great deal of muscular fatigue.
The other useful application is for endurance training. Specifically, marathon runners or long distance cyclists have effectively used fasted cardio techniques in their training program to force their body to adapt to having low carb stores on board, simulating the end-of-race environment. If muscles can get more efficient at fat oxidation, they can conserve glycogen a bit better. Often, this allows endurance athletes to last a little longer without feeling gassed and minimizes the need to refuel as often which has its own set of advantages. While there’s no weight loss goal here, there may be specific performance advantages when it comes to stepping up your game in long duration endurance sports.
From a weight loss or fat loss perspective, be careful with fasted cardio. Burning a little more fat versus carbs as fuel doesn’t necessarily translate into actual fat loss. In scientific terminology, there’s a difference between mobilizing intra muscular triglycerides (fuel) versus subcutaneous fat (think muffin top and pot belly). Ultimately, it’s more important to look at how hard we can work during our training, and for how long because total calories burned will always win out when it comes to weight loss.
So, before venturing out without food, we have to consider our goals as well as our type of training.