Along with HIIT (high intensity interval training) fasted cardio has become a popular way to try and burn more fat during exercise. The theory goes that by exercising after period during which you don’t eat, the lack of immediately available energy from food makes the body start to break down fat to make energy instead. The most common form of fasted cardio is usually exercising before breakfast, or skipping breakfast altogether and doing a workout before lunch.
In theory this all sounds good, but does fasted cardio burn more fat? As with most elements of physiology, exercise and nutrition the answer is more complex and contradictory.
Here’s a typical example. You have dinner this evening, which contains some carbohydrates. This carbohydrate is converted in the gut into the fuel we use – glycogen or blood sugar. Within an hour or two this will replenish your two main “fuel tanks,” the muscles and the liver. You eat carbohydrate, it’s converted into glycogen and then stored in the liver and the muscles. Although many people know about storage of glycogen in the muscles, what they don’t know is this – energy stored in the muscles cannot be released back into the blood stream. It’s solely for use by the muscles. Which leaves us with just the liver.
Liver cells can release glycogen back into the blood, which we depend on for survival. Unlike some other animals we store very little energy in the blood itself. In addition our large brain needs a constant supply of glycogen; but despite this high demand for blood glycogen, even the liver only stores so much. In fact it can only store 75-100g (or 300-400 kcal of energy).
Back to the example. After dinner the liver and muscles are replenished with glycogen. But only the liver is of any use for general metabolism. After dinner you stay up for a few hours during which time energy is skimmed from the liver. During sleep the liver stores deplete even more through ongoing metabolism.
As glycogen stores in the liver get really low overnight, they trigger the release of a hormone, cortisol. This chemical messenger tells the body to be careful with energy and to use glycogen sparingly. This is achieved by greater levels of lipolysis – breaking down of fat for energy. So far so good – you’re burning more fat. Carry this on into a fasted workout and even more fat will be burned. Excellent. Or is it?
Unfortunately cortisol has some other effects. Known as our “stress hormone” it also promotes gluconeogenesis which is a breakdown of proteins to make glucose. Where do these proteins come from? The muscles. Keep in mind our muscle mass plays an important role in maintaining (or elevating) our metabolic rate. Less lean muscle = a lowering of the metabolic rate.
Cortisol can also temporarily lower our metabolic rate regardless of protein (muscle) breakdown. It also has the other effects: it reduces levels of dopamine and serotonin in the brain which is associated with anxiety and depression; it can suppress the metabolic rate not just in the fast but during your workout; it makes the blood slightly more acidic which uses lactate to re-balance the blood, making higher intensity exercise harder.
Let’s do some maths. The main idea behind fasted cardio is to burn more fat. It does indeed achieve this, but there is a cost involved. It involves a small step forward at the cost of several steps back. Example: A moderate fasted workout burns 300kcal. Fasting gets you increased fat burn, from 180 kcal to 240kcal. An extra 60 calories from fat. This looks good, but remember the 1lb of fat is approximately 3,500 cals. So through exercise alone you’d need to do another 58 sessions like this (in a week) to lose one pound of fat. 58 x 60 = 3,480.
So although you did increase the amount of fat burned, in the bigger picture it’s negligible as part of the bigger weight loss picture. Take into account all the backward steps happening at the same time and fasted cardio begins to look like a bad idea.
So what’s the solution?
Thankfully there is a way to maintain fat burning while mitigating the negative impact of fasted cardio and the presence of cortisol. Small meals. The body only needs a small amount of carbohydrate to restore cortisol levels back to normal – 20-25g of moderate to high GI carbohydrate – apples, bananas, brown rice, porridge, oats, whole grain bread and beans would all fit the bill. A small quantity will restore balance but maintain normal fat burning. The addition of a small amount of lean protein will prevent any muscle breakdown and again not be enough to overfill the fuel tanks which might prevent fat burning.
In summary – although fasted cardio will make you utilise a bit more fat, always consider the additional costs. A small meal of just 25g of carbohydrate and 10-20g of protein will restore normal fuel usage and correct the hormonal imbalances that occur during your overnight fast.