Cases of liver carcinomas are the fifth most common in the global ranking of cancer diagnoses , growing in line with the growth of NAFLD [13, 14]. Because the latter disease may be due to the insulin resistance found in obese individuals and because the incidence of obesity has been increasing worldwide, affecting children, youths and adult men and women , the importance of the prevention and treatment of obesity, which can be initiated in childhood by physical exercise, is important to minimize the inconvenience caused by this condition in adulthood.
Because the current findings surrounding the effect of moderate physical exercise (80% of Lan) on the development of NAFLD are not clear, the present study examined the effect of a protocol of moderate training (80% of individual aerobic capacity) from childhood to adulthood on fat metabolism in the liver.
A thorough knowledge of the appropriate exercise intensity is of paramount importance. There are numerous studies that show different responses of an organism to different intensities of exercise [16–18]. According to Dubé et al. , there is a high correlation between the dose/response of physical exercise intensity with insulin sensitivity, with the recovery of individuals after stroke  or acute myocardial infarction  and with fibromyalgia . These authors also showed that the higher the intensity of exercise, the better the body's response to that exercise. Therefore, a precise determination of the individual aerobic capacity prior to start of a physical training program is important.
To determine the aerobic capacity, the maximum lactate steady state (MLSS) test is considered the gold standard, but it requires several days of tests, and its application can be limited in some special cases. Therefore, in 1993, Tegtbur et al.  developed a test to determine the anaerobic threshold (AT) called the lactate minimum test, which has been considered a method to determine the exercise intensity corresponding to the equilibrium between the production and removal of blood lactate [23, 24] or MLSS [25, 26]. In addition to this protocol being considered a valid method to determine aerobic capacity, it also enables the identification of the intensity corresponding to the MLSS in a single test session [25, 26].
Moreover, studies of the metabolic effects of exercise in rats are often challenged by a lack of information about the intensity of the effort performed by the animal during exercise. Therefore, Gobatto et al.  adapted the MLSS test from human adults to Wistar rats, and after understanding that human tests could be adapted to animal models, Araujo et al.  adapted the lactate minimum test for adult Wistar rats in swimming exercise. These authors suggested that regardless of the overload in the test, the minimum blood lactate concentration was approximately 5 mmol/L. Because the application of overloads to rats during exercise in the water is relative to body weight, these tests may appear inappropriate for rats that are too light or in an accelerated growth phase, such as weanling rats. However, this study successfully determined the AT in weanling rats, which was previously unknown in the literature. This study also confirmed the data previously described by Gobatto et al.  and Araújo et al. , which showed that the lactate concentration at AT, regardless of the intensity obtained, was approximately 5 mmol/L.
After standardization of the lactate minimum test because the AT is often assumed to be characterized by a higher intensity of exercise in which the blood lactate does not oscillate more than 1 mmol/L from 10 to 30 min of exercise with a constant overload , four days after the lactate minimum test, the rats were submitted to exercise with constant overloads corresponding to those found in the test. The lactate behavior was similar to that found in the MLSS test, and thus the lactate minimum test can be an important tool to determine the AT for weanling rats in a single test.
The exercise intensity in cases of obesity and fat accumulation in the liver is a matter that has much been discussed currently because if the sustained intensity is high, there is a high fat oxidation during the exercise session. However, to restructure the organization for the future to sustain wear, this oxidation is increased after this practice, a condition known as excess post-exercise oxygen consumption (EPOC) in which the consumption is directly related to the intensity supported during the physical exertion; thus, the greater the exercise intensity, the greater the oxygen consumption as long as this condition persists [30–32].
When exercise is performed with the intensity ranging from mild to moderate, during which the energy demand is not required quickly, there will be time enough to metabolize the fatty acids (FA) used to feed this energy demand, and thus there will be a greater activation of the enzyme lipoprotein lipase. In turn, there will be an increase in the rate of lipolysis, and the FAs from adipose tissue will predominantly be oxidized to sustain exercise for a long time, causing a consequent decrease in fat mass and fat intake by the liver. This area of work is known as Fatmax [32, 33].
In 2010, Da Silva et al.  showed that the intensity of exercise performed at 70% of AT decreased insulin resistance due to a decrease in inflammatory processes and also showed that rats swimming freely, without an overload but for a long period (6 hours), had improvements in insulin sensitivity. These results indicated that very light exercise can be compensated for by increasing the volume of exercise. However, this high volume training may not be able to be supported by obese individuals, especially in those activities that require the body weight to be supported, indicating that training protocols with shorter durations are important.
A study performed by Lima et al. , in which the effects of different exercise intensities in type 2 diabetic subjects were analyzed, revealed that diabetic patients who consumed high carbohydrate solutions oxidized even more fat compared with healthy subjects after high-intensity exercise (90% Lan). This protocol can improve insulin sensitivity and minimize the damage caused by insulin resistance in obese individuals.
If the exercise intensity is not known, the exercise can act as a villain in combating obesity. According to Mayer et al. , when exercise is performed at a very low intensity that is not accompanied by a decrease in hunger, the body mass can increase. According to these authors, the effect of physical exercise on satiety occurs at intense levels, in which elevated levels of catecholamine suppress the feeling of hunger. In this study, subjecting the animals to a moderate-intensity exercise (80% of AT) caused these animals to have a lower weight gain, leading to a lower amount of adipose tissue and thereby reducing the intake of fat by the liver, ultimately preventing the development of NAFLD.
The stored energy in the form of fat can act in a toxic way on the liver and lead to lipotoxicity of the liver  and even organ failure by activating apoptotic pathways . Analyzing the accumulation of hepatic fat through the lipogenic rate and triacylglycerol concentrations in the tissue, the insulin resistance was shown to generate a favorable environment for the greater accumulation of hepatic fat, which can be explained by the fact the activation of the lipogenic transcription factor hepatic SREBP (sterol regulatory element-binding proteins) by high concentrations of insulin [39–41]. The state of hyperinsulinemia increases the activity of SREBP in the liver, thereby increasing the levels of lipogenic genes, such as fatty acid synthase (FAS), acetyl-CoA carboxylase (ACC) and Stearoyl-CoA Desaturase1 (SCD1) contributing to the buildup of hepatic fat. Recently, Cintra et al.  showed in mice fed a high-fat diet that regular physical exercise that reduces SREBP levels was also effective in reducing the levels of FAS and SCD1 and increasing the levels of ACC, thereby contributing to the reversal of hepatic steatosis.
Because unsaturated fatty acids can activate proinflammatory pathways through membrane receptors, such as the toll-like receptors (TLRs) , reducing the supply of lipid substrates as a result of the increased resistance to insulin-shaped fatty acids is necessary. In this study, moderate physical exercise, by promoting a decrease in concentrations of FFA, was effective in improving insulin sensitivity, which may be associated with increased insulin activity in insulin-sensitive tissues. According to Boden & Shulman , the responsiveness of peripheral tissues to insulin may be damaged due to an increase in the concentration of FFA, affecting the insulin signaling pathway and thus leading to type 2 diabetes. Another factor that may have contributed to the best response of the peripheral tissues to insulin was the strong power of muscle contraction, which increases the expression of proteins that participate in the insulin signaling pathway, causing glucose uptake through mechanisms independent of intracellular insulin in rats and mice, whether they are obese or lean, during one or more physical exercise sessions, thereby making the body less glucose-intolerant and insulin-dependent [42–49].