This article aims to clarify once and for all what is the ideal heart rate for weight loss, given the much confusion and summary information circulating on the subject.
Too often instructors and personal trainers solve the problem by recommending a heart rate between 60 and 70% of the HRmax, since at this level of intensity there is the greatest consumption of fat, therefore theoretically the maximum lipolytic activity of the exercise. A theory, this, rather archaic, which must necessarily be revisited.
Looking at the image above, we open a small but very important parenthesis on the body's energy fuels. As shown in the figure, under normal conditions intramuscular sugars amount to about 300-500 grams, to which are added about 100-150 grams of liver and the small amount of glucose circulating in the blood, where it is kept at fairly constant levels (glycaemia ). Another very important fuel is given by fats, packed in very high quantities in the adipose tissue and to a lesser extent among the muscle fibers. The image does not show the third fuel of the organism, used above all when the reserve warning light turns on. sugars; we are talking about muscle amino acids and the amino acid pool present in the blood.
At this point, before talking about the ideal heart rate for weight loss, it is essential to illustrate two key concepts:
carbohydrates, also called carbohydrates or sugars, are the most effective fuel, capable of bringing the human machine to maximum performance levels;
weight being equal, lipids provide more energy than sugars in absolute terms (9 Kcal / g against 4 for carbohydrates), but not in relative terms, because the greater the amount of oxygen needed to generate this energy. It follows that the lipid fuel produces lower performance levels than those that can be generated with the use of sugars.
The limiting factor for the consumption of lipids for energy purposes is therefore given by the availability of oxygen at the level of the individual muscle fibers; the more energy is required from the effort, the more oxygen is consumed. But what does the amount of oxygen that can be used by the muscles depend on? The main limitation is not located at the pulmonary level, but at the peripheral level; this means that larger lungs or larger airways do not guarantee large increases in performance. Rather, the plasma concentration of red blood cells and hemoglobin significantly affect performance, and at the muscle level the density of the capillary bed, the fiber composition (white and red), as well as the number, size and efficiency of enzymes that catalyze energy reactions. The more efficient these systems are and the greater the percentage of lipids burned during high intensity efforts, sugars are in fact limited, so the body tries to save them by using mainly fats.
From the above it can be easily deduced that:
the higher the intensity of the exercise, the greater the percentage contribution of the fuel to sugars becomes. Conversely, in order not to attack the limited reserves of these nutrients, in the lightest efforts the body burns mainly fat.
Some significant data:
IF THE PHYSICAL "ACTIVITY" IS "LOW INTENSITY" AND SHORT LASTED (FOR EXAMPLE, RUN SLOWLY, "WITHOUT FIATONE" FOR AT LEAST 20-30 MINUTES), LIPIDS AND CARBOHYDRATES CONTRIBUTE TO THE COVERAGE OF THE ENERGY DEMAND. VICE VERSUS, IF THE PHYSICAL "ACTIVITY" IS OF "LOW INTENSITY" BUT IT EXTENDS FOR AT LEAST ONE "HOUR, THERE IS A PROGRESSIVE DEPUPERATION OF THE GLYCOGEN RESERVES, CONSEQUENTLY A GREATER USE OF LIPIDS, WHICH ARRIVE TO COVER 80%" OF THE ENERGY REQUEST.
In particular, it is estimated that 60 - 90 minutes of very intense exercise are sufficient to significantly reduce carbohydrate stores. If at the end of the exercise these are close to exhaustion it will then take 24 - 48 hours to reconstitute them.
For what has been said, a theoretically effective way to lose weight would be to train when carbohydrates have already been significantly reduced by a previous workout, a low-calorie diet or night fasting. A potentially useful solution, but with a series of limitations that we have analyzed in the dedicated article.
The points just illustrated are the basis of the aforementioned theory according to which the "optimal heart rate for weight loss would be around 60-70% of the HRmax (at this intensity of exercise the mixture of fuels used is very rich in fat). Unfortunately, this is a simplistic and fundamentally wrong reasoning, for a variety of reasons:
- if we want to be able to burn fat for energy purposes we must first of all take care of increasing the amount of oxygen that reaches the muscles. How, then, to raise the density of mitochondria, enzymes and capillaries at the muscle level? It is enough to simply engage in long-lasting workouts (minimum 50 ") with a heart rate equal to 60-70% of the HRmax; in practice, it is enough to do what most of the instructors recommend to lose weight. This rule applies especially to sedentary people or to those who come from a long period of suspension of training. The program will be maintained for a period ranging from four to ten weeks, during which, however, it is possible to insert some stretches and short stretches of slightly higher intensity. The purpose of this first phase is be able to run for at least 40 minutes at low intensity without breaks and with relative ease.
- It is now known that the caloric consumption of walking is equal to about 0.5 KCal per kg of body weight, just over half of what is burned by running. Therefore, a 100 kg person running for 10 km burns about 1000 KCal , regardless of the "intensity of exercise". In fact, the caloric consumption per km of the slow run is very similar to that per km of the run carried out to the maximum of one's ability; what changes, in these cases, is only the mixture of fuels used : richer in fats in the first case and richer in sugars and amino acids in the second. Hence the aforementioned recommendation to maintain a heart rate equal to 60-70% of the HRmax, in order to maximize the slimming power of running. are the important objections in this regard, the first is very simple, but no one seems to think about it: in the example we talked about mileage, not time. The question therefore arises spontaneous and provocative: "If we only have one hour in the gym to train, will we do more kilometers running at low or high intensity? The answer, of course, is obvious. But then, if in this period of time our 100 kg individual travels three km more, for what has been said, he also burns 300 KCal more. For the same reason, fat consumption will be lower in relative terms (grams per km), but not that much in absolute terms (total grams). Furthermore, we must not forget the extra 300 KCal burned, which, due to the laws of thermodynamics, are undoubtedly useful for losing weight.
- Then there is a third element, little known, which explains why the heart rate to lose weight must still be challenging. We are talking about the oxygen debt. At the end of a physical exercise, metabolic activities do not immediately return to their resting level, but require a more or less long time depending on the intensity and duration of the exercise. The more demanding the effort, the longer this period is. In practice, therefore, we are underlining how the athlete continues to burn more calories than normal even for a certain period after the end of the exercise, a period which will be the longer the more intense and lasting the effort that preceded it was. This phenomenon is explained by the need to restore energy supplies, dispose of lactic acid, transform it into glycogen (Cori cycle), re-oxygenate the myoglobin and repair the macro and microscopic structures damaged by exercise. Furthermore, the contribution of hyperthermia (the basal metabolism increases by 13% for each degree of increase in body temperature) and of the hormonal structure, with activation of stress hormones, acute (catecholamines) and chronic (glucocorticoids) should not be underestimated.
- If we want to make the most of what has been explained in the previous point, we must rely on "interval training, which consists in alternating stretches of high intensity with others for recovery." In this way a very large oxygen debt is created, partially recovered and a new one created; the "intensity of" training reaches the stars and with it the calorie consumption. Rightly so, with this technique the percentage of fat used will be quite low, but the calories burned will skyrocket both during and after exercise. For all these reasons, we believe it is time to abandon the old theories that the ideal heart rate for Losing weight should be between 60 and 70% of the HRmax; therefore ask your instructor to prepare you some cards also focused on intensity work, without forgetting the right recovery periods.
