Breakfast and Fasted Cardio – Is it Really Worth it?

We see it, read it and perhaps even witness it – fasted cardio for weight loss. This approach is currently trending in many exercise circles, but does it really stand up to all the hype?

Let’s examine this concept through a series of practices – say for example you consume a dinner this evening containing carbohydrates. This will replenish (to varying levels) your two primary glycogen stores; the muscles and the liver within the ensuing hour or two. While we all understand the reason for storing carbohydrates within muscle, one might ask why we store carbohydrates within the liver? The reason lies with the fact that once carbohydrates enter the muscle, they cannot be released back into circulation (1, 3).

By contrast, liver cells can release glucose into the blood which is critical to our survival given how little glucose is stored in the blood at any time. The blood serves as a medium to constantly deliver glucose to various cells (e.g., brain, central nervous system) and as a source of glucose for our red blood cells which can only fuel with glucose. In other words, it is the role of the liver to preserve blood glucose, but the challenge lies with the fact that the liver only stores about 75-100g of glycogen (50g / Kg or mass) or 300 – 400 kcal worth of energy which can be depleted relatively quickly. After eating, you may not retire to sleep, but may spend a few hours awake, skimming glycogen from both muscle and liver before bed as illustrated in Figure 1. However, during your overnight sleep, while your muscle stores do not deplete, your liver tank empties due to ongoing metabolism. Lowering your glycogen stores within the liver overnight triggers the release of cortisol, a glucose-preserving hormone that responds to the biological stress of lowered liver glycogen stores).

Let’s use an analogy to help explain concepts – imagine driving around a strange part of town and your gasoline warning light comes on – how will you modify your driving? I imagine by driving more conservatively to preserve gasoline and avoid being stranded. Think of your body in the same manner. When our liver nears empty, our body strives to do the same by conserving energy and sparing glucose. This is achieved by elevating cortisol levels which promotes greater levels of lipolysis (fat breakdown) which helps spare blood glucose (remember, this has nothing to do with muscle glycogen stores which are adequately stocked, but with the rest of your body which relies on liver glycogen). Unfortunately, cortisol also promotes gluconeogenesis, the breakdown of proteins to manufacture glucose which could prove to be an undesirable consequence. Furthermore, cortisol can also have a temporary supressing effect on your metabolism, resulting in fewer calories being expended by your body.

Let’s put this in perspective. The primary idea behind fasted cardio is to promote greater fat utilization while performing a moderate-intensity workout. For example, a 300 kcal fasted session at that moderate intensity may shift your fat calories from 180 kcal to 240 kcal, netting an additional 60 more fat calories. While this may seem impressive at first, consider that 3,500 kcal represents one pound of fat, which would require an additional 58 more sessions (59 x 60 = 3540) to lose a pound of fat. That is hardly impressive, but let’s also examine the other costs associated with elevated cortisol levels:

• As previously discussed, elevated cortisol may increase unwanted breakdown of muscle tissue, albeit small.
• Elevated cortisol can suppress metabolic rates during those fasted hours until food is eaten, reducing the calories expended in the exercise session and the early part of the day.
• The accumulation of ketone bodies in the blood also decreases blood pH and to manage acidosis, the body uses its lactate buffer to maintain normal blood pH – this reduces the capacity for higher-intensity exercise.
• Elevated ketones can also reduce levels of serotonin and dopamine in the brain, which is associated with greater levels of anxiety and depression.