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For decades, the benefits of fasting exercise have been discussed, to increase the rate of fatty acid oxidation and, secondarily, to reduce body fat mass. Does this have any scientific basis?
Before continuing, it is necessary to know that in a training regimen considered normal, only a maximum of 25% of the total kilocalories burned in a day are through physical activity, and that up to 75% of these kilocalories are expended through basal metabolism, therefore it seems much more interesting to undertake the loss of fat weight through the elevation of the metabolic rate than through the burning of fatty acids during exertion. Even so, the subject that concerns us today is the latter.
Much progress has been made in recent years in understanding the regulation of carbohydrate and fat metabolism during exercise. The regulation of the oxidation of fatty acids is very complex and is determined by several factors, among which are:
- The rate of lipolysis, or conversion of triglycerides into fatty acids and glycerol
- Plasma fatty acid concentration
- The transport of fatty acids into the muscle cell
- The uptake and transport of fatty acids within the cytoplasm
- The synthesis / lysis ratio of intramuscular triglycerides
- The transport of fatty acids into the mitochondria
There are genetic determinants, type of training or exercise intensity, that greatly influence the oxidation of fatty acids, but dealing with that goes beyond the objectives of this paper. Of the reported factors, the last 4 are highly conditioned by the availability of carbohydrates in the form of blood glucose or muscle glycogen, so that the greater the availability of carbohydrates, an inhibition of these 4 episodes occurs, linked to the final oxidation of The fats. In particular, the availability of muscle glycogen appears to be a very intense stimulus that largely determines the type of substrate most commonly used during exercise: fats or carbohydrates.
Thus, it seems logical to suppose that a prolonged period of fasting (such as night) will decrease the availability of plasma carbohydrates, and therefore increase the rate of fatty acid oxidation. But as we have said, this is not enough, it is also necessary that the availability of muscle glycogen is also compromised. And for this it takes more than just not having breakfast. To achieve this effect, it is necessary to have produced a glycogen emptying in the muscles that I will train the next day in the morning and not allow this replacement, so we need to intervene also in the post-workout diet of the previous day, in addition to fasting. It would not do much good to fast if the dinner the night before had been rich in carbohydrates, which, helped by her friend, insulin, had filled muscle glycogen stores to the brim.
In other words, the key to achieving a greater oxidation of fatty acids DURING the effort is not to exercise in the morning on an empty stomach, but to exercise with certain characteristics in a situation of low availability of carbohydrates, which can be very easily adjustable through training and diet, and do it at any time of the day, not just in the morning on an empty stomach.
And why do we raise this question? Because we know that exercising first thing in the morning has other types of consequences, such as an anabolic environment less conducive to being the time when the testosterone / cortisol ratio is lower. But above all there are 2 powerful reasons to reconsider the suitability of this type of strategies.
- Neuromuscular activation
Both the nerve impulse frequency, the fibrillation synchronization capacity, and the muscle tone are compromised for at least the two hours following rising. This entails the following limitations during that period of time:
- Lower peak force output
- Lower MFI, explosive strength and power
- Less ability to process proprioceptive information and send consequent efferent information.
- In short, it seems likely that during the two hours following getting out of bed, in addition to being compromised in the production of muscle tension, the risk of injury is higher, so any maximum effort or a certain risk of injury should be avoided. .
2. Vertebral structure
Sleeping in a horizontal position, in discharge and with a smaller range of movements in relation to wakefulness, involves a change in the length of the vertebral chain, and can be increased by up to 19 mm at night, as recorded by Reilly, Tynell and Troup ( 1984). Furthermore, sleeping in decubitus leads to the osmotic pressure exceeding the hydrostatic pressure of the disc. All this causes a structural alteration such that, in front of flexion, lateral or rotation, Adams, in one of his well-known works in Bristol, in 1987, documented increases in disc stress of up to 300% and increases in ligament stress of up to 80%. Ligament restructuring and normalization of disc hydrostatic pressure takes at least 90 to 120 minutes, so again, we should restrict flexion, lateral flexion, or spinal rotation movements, as well as repeated impacts on the vertical axis. for 2 hours after getting out of bed. It has even been shown mechanically that for this reason, the extended habit of starting the day with a few stretches is not a good idea if deep spinal flexion movements are included, as it can jeopardize disc integrity during this time period.
Given this, from the point of view of the prevention of disc problems, if you want to carry out a long duration and medium-low intensity exercise to promote the oxidation of fatty acids at this time of day, there are two types of exercise that we should avoid. o Restrict: running (due to its repeated impacts on the vertical axis at the vertebral level) and rowing (due to its continuous solicitation of flexion of the spine).
- To achieve continued and stable fat weight loss, the best strategy is to increase metabolic rate, rather than fat intake during exercise.
- There are many factors that determine the relationship between fat and carbohydrate metabolism during exercise.
- The most important factor seems to be the availability of, firstly, intramuscular glycogen, and secondly, blood glucose.
- There are several ways to manipulate diet and training to modify the availability of muscle glycogen and blood glucose.
- Fasting exercise can be a way of exercising in situations of low carbohydrate availability, so it can promote fat consumption
- However, there are reasons derived from less neuromuscular activation and a change in the vertebro-disc structure during the first hours of the day, to consider that certain types of exercise (maximum intensity, high speed, or those involving flexion, column, rotation of the column or impacts on it) should be done at another time of the day.