This is probably one of the most commonly asked questions which is why it’s worth addressing. This method is born from bodybuilding subculture, because bodybuilders who are getting towards the low-end of body fat levels and nearing contest shape, use this tactic to lose the last percentages of body fat before getting on stage. This provides some of the answer because the body fat levels of the individual is going to be the main factor in determining whether doing cardio fasted in the morning is worthwhile or not.
A small amount of physiology needs to be addressed because there are three primary steps involved in losing fat. They are mobilizing the fat, the transport of the fat, and the oxidation of the fat. All three of these must occur in order to lose the fat. It is possible to mobilize, transport, and then have the fat re-deposited somewhere else. That is why all three steps need to be completed in order to completely eliminate fat stores.
Mobilization refers to actually getting stored fat, or fatty acids, out of the fat cell. This process is under the primary control of insulin and the catecholamines although hormones such as growth hormone, cortisol and others play secondary roles as well. Transport refers to the actual moving of the fatty acids, which are bound to albumin within the bloodstream. This step can be difficult when dealing with stubborn body fat, such as lower abdominal fat and low back fat in men, and hip and thigh fat in women. One of the reasons for this is that blood flow is often impaired in those areas. The third step is oxidation which is the actual using of fatty acids within tissues such as skeletal muscle, liver and heart for energy.
In lean individuals, relatively lean being around 12 to 15 percent body fat for men, and about 19 to 22 percent for women, fat mobilization becomes more difficult. Blood flow is often an issue as well as mentioned above because now the areas where the most fat is stored are the stubborn fat areas. This is why the easiest weight to lose happens at the beginning of a diet. As folks get leaner, the body undergoes a series of adaptations that occur to make getting fat out of the fat cells more difficult. Oxidation, or ‘fat burning’, is the easiest part of the three-step process for the lean individual.
At the other extreme, that is in the very obese individual which would be 35 percent or more body fat for men, and 40 percent or more body fat for women, then the opposite issue is present. There is an abundance of fatty acids floating around in the bloodstream, but oxidation has become impaired. So the third step is the more difficult for the obese individual.
Between those two extremes is the average person. Mobilization is usually not a problem since the body hasn’t started to fight back, transport isn’t an issue since stubborn fat isn’t being targeted, and oxidation is rarely a problem since the defects which show up at the extremes of obesity generally aren’t present. Everything is going to work very well at this stage of any diet.
For the lean trying to get very lean, strategies including fasted cardio are probably going to be required to offset the mobilization and blood flow issues that present themselves. This is the reason why bodybuilders began using fasted early morning fasted cardio decades ago, and with great results.
Another possibility is fasted HIIT. I’ve seen suggestions by coaches who recommend doing five to ten minutes of fasted sprints to mobilize the stubborn fat followed immediately by LISS to oxidize the fat. I have seen no study to back this up and doubt I ever will as getting extremely lean isn’t much of an interest for science, but the strategy does seem logical. The brief period of HIIT will produce the desired biochemical reaction to mobilize the fat and the following session of steady state will burn the mobilized fat.
For folks who aren’t that lean yet, the folks in the middle range of body fat levels, it really doesn’t matter. The best time to do cardio will be whenever it will most consistently get done. It’s more important in the situation of the average person that it gets done, more than what time of day it gets done and what is in your stomach at the time. Until next time,