1. Introduction
Metabolic disorders such as insulin resistance, hyperglycaemia, hypertension, dyslipidemia and visceral obesity, are the set of clinical markers of the Metabolic Syndrome (MetS) [
1]. Increased consumption of highly processed foods, including those rich in fructose, has contributed to an epidemic incidence of MetS in the last decades [
2], and now is a global problem [
3]. Although present in fruits in small amounts, the large quantity of fructose in the diet comes from processed foods, largely, sweetened beverages as soft drinks [
3]. The consumption of high fructose corn syrup (HFCS) is known for its role as a source of dietary fructose [
4].
Consumption of sweetened beverages and foods has increased in all age groups, with adolescents as their main consumers [
4,
5,
6]. Several studies have shown that stressors, mainly dietary factors, in the early stages of life, such as preconception, gestation, lactation and peri-pubertal period, can trigger the development of MetS in adult life [
7,
8]. This hypothesis is part of the DOHaD (Developmental Origins of Health and Disease) concept, which postulates that early insults can program for metabolic disorders in adult life [
9,
10,
11]. Current investigations already show that peri-pubertal period is a susceptible period that lead to metabolic programming [
12,
13], which can be induced by increased ingestion of high caloric drinks and foods.
Ingestion of fructose has been shown to cause hyperglyacemia and hyperinsulinemia [
14,
15], insulin resistance [
16,
17,
18,
19], as well as cardiovascular disorders [
20]. Fructose is first captured and metabolized by the liver, where it will act in the formation of lipogenic precursors, such as dihydroxyacetone phosphate (DHAP), and Acetyl-CoA [
21], leading to hepatic fat deposition, dyslipidemia and in hepatic insulin resistance [
17]. Moreover, animals supplemented with 10% fructose for two weeks exhibited hepatomegaly, moderate steatosis, disorganized liver histoarquitecture, altered triacylglycerol (TAG) synthesis and ectopic lipid accumulation [
22].
The promising effects of physical exercise on the prevention and treatment of metabolic disorders have been verified by many studies that point out its importance in weight loss, fat reduction and in the improvement of insulin sensitivity [
23,
24]. The moderate-intensity training exercise has been showed to be protective against development of the MetS [
25,
26]. Evidences from both human and animal model studies suggests that physical exercise performed at different ages facilitates neuroplasticity of certain brain structures, and some of these protective effects may be related to an attenuation of risk factors associated with MetS [
27]. Our laboratory recently demonstrated that the moderate intensity running when performed during adolescence is able to decrease the Walker-256 tumor growth and metastasis in adult male rats, due to improvement in glucose–insulin homeostasis and beneficial effects in biometric and metabolic markers [
28,
29].
The experimental model used in the present investigation is reported in the literature as effective in producing MetS signs, mainly in adult rats [
16]. However, the long-lasting effect of peri-pubertal fructose intake concomitant with physical exercise on adult metabolism remains unexplored, and also the probably effect of metabolic programming in this model is original and deserves to be elucidated. Therefore, the present study aims to investigate: (I) the effects of fructose intake during peri-pubertal period in the metabolism in adulthood and, (II) if the short-term moderate-intensity training performed at peri-pubertal period, concomitant with fructose intake, could protect the metabolism against MetS development in adult life.
3. Discussion
We found that chronic intake of 10% fructose in drinking water during peri-pubertal period programming male rats to the MetS-related phenotype at adulthood. Besides, for the first time, we demonstrated that moderate physical training applied during peri-pubertal period, simultaneously with fructose consumption, was able to preventing several MetS components, such as BWG, dyslipidemia, insulin resistance, hyperglycaemia and hyperinsulinemia during ipGTT and increased parasympathetic nerve activity in adult Wistar rats.
In this study, we obtained substantial data that early fructose supplementation is capable of induce the onset of MetS, such as impaired blood glucose-excursion following ipGTT in 60 days-old sedentary rats. Consistent with previous findings [
30], we also showed a reduction in FI and an increase in BWG and adiposity in these animals, a phenomenon unrelated to the TCI, but sufficient to disturb glucose tolerance at end of the 30 days of treatment. No alteration in fasting glycaemia was detected, in agreement with previous findings that introduced fructose in 21 days-old male rats [
31].
At 60 days-old, trained animals exhibited different chronic adaptations. The moderate physical training was able to improve functional and metabolic capacity in control rats, as measured by FWL and RQ. Interestingly, the VO
2max, EE, glucose tolerance and total cholesterol levels improves only in rats that were treated with fructose and training during peri-pubertal period. Fructose-rich diet was shown to not impair the ability to perform aerobic trainings in a similar study [
32]. It has been shown that fructose co-ingestion increases carbohydrate availability and post-exercise glycogen replacement rates, maximizing endurance performance in athletes during prolonged moderate- to high-intensity exercise, as well as for acute post-exercise recovery [
33]. Furthermore, combined treatments in this study may have enhanced the energetic demand and lipolysis, preventing weight gain and dyslipidemia in young rats, in accord to previous data [
34].
High levels of plasma insulin concentration has been associated to increased insulin secretion and/or decreased insulin clearance [
35]. Our results showed that the high fructose consumption during peri-pubertal period promoted an increase in fasting glycaemia and insulinemia in sedentary adult rats compared to equivalent control. It is important to highlight that the hyperinsulinemia during ipGTT persisted even 60 days after the interruption of fructose consumption, despite normal blood glucose during in the same test. Consistent with previous findings, our data also showed that the administration of fructose induces insulin resistance in adult rats, according the HOMA IR and/or TYG index [
15,
36,
37]. TYG index making a relation between glycaemia and lipid profile, being strongly associated with diabetes, nonalcoholic fatty liver disease and metabolic syndrome [
38,
39].
Fructose consumption in 5 month-old rats increases the risk of cardiovascular disease by alterations in fat liver and blood lipid parameters [
17]. Cardiovascular disease, visceral adiposity and type 2 diabetes is also present in young and adult humans [
40,
41,
42]. In our study, with only 30 days of fructose intake, we detected an increase in TCI, excessive TC and TAGs levels, and elevated periepididymal and retroperitoneal adiposity in sedentary adult rats that received fructose during peri-pubertal period, compared to equivalent control. It was recently reported that the ingestion of 20% fructose elevates circulating levels of fibroblast growth factor 21 (FGF21) in C57BL/6J mice, causing resistance to FGF21 and, subsequently, an increase of lipid droplets in the liver and the size of adipose cells in white adipose tissue (WAT) [
43].
Although weight gain remained unchanged in adult rats, our data suggest the high potential of fructose to alters lipid metabolism and promote adipogenesis-induced hyperinsulinemia [
44]. Similar studies in literature that used 10% fructose in rats showed no increase in body weight [
1,
2]. Harrel et al. [
45] showed that the fructose consumption by adolescents increased fat stores without the BWG. Contrasting BWG outcome was observed after consumption of 20% fructose [
4]. We did not evaluate the hepatic lipogenesis, which also seems to contribute to a large production in TAGs levels [
36,
42,
46]. The livers were weighed, but no statistical difference was found.
Nutritional insults during critical developmental periods are known for their plastic long-term impact on metabolism and described on literature as being responsible for several adult health outcomes [
9,
11,
13,
47]. On the other hand, interventions using physical exercise were reported to attenuate their disturbing effects [
48,
49,
50]. Accordingly, the present data revealed that adolescent rats that received fructose and training concomitantly, presented a reduction in vagal electrical activity compared to the sedentary counterpart, and this mechanism could be explained, at least, a better metabolic profile in these animals. A high activity of the parasympathetic nervous system has been related to metabolic disorders such as obesity [
51], hyperinsulinemia and insulin resistance [
7].
We also showed that fructose treatment programmed a large reduction in sympathetic nervous system activity in trained and non-trained adult rats. It has been shown that sympathetic nervous system activity is decreased in obese animals and humans [
52,
53]. Our observation is in agreement of “MONA LISA” (Most Obesities kNown Are Low In Sympathetic Activity) hypothesis, proposed by Bray, which says that the resistance to leptin, related with the obese state, is linked to a decreased local sympathetic activity [
52,
54,
55].
American teenager consumes approximately 72.8 g/day of fructose [
45]. Although the difficulty in developing experimental models that present all the characteristics of humans MetS, in the present study we used a dose equivalent to a diet containing 48–57% fructose [
14] that also mimics fructose consumption by human adolescents [
4], we suggest that 10% fructose ingestion by pubescent rats in this investigation was associated with may result in a long-last programming composed for an onset of MetS-related disorders in adult life [
16,
36,
56,
57,
58].
Several studies have demonstrated an effectiveness of the physical exercise on prevention of MetS risk factors [
22,
56,
59] and a direct correlation between moderate exercise and improved insulin sensitivity in young and adult individuals [
60,
61], even after cessation of training [
26,
28]. Our laboratory has already demonstrated that the short-term physical training when applied in adolescence, protects the organism from the MetS caused by a high-fat diet (HFD) in adolescence [
25,
62]. Frantz and colleagues [
2,
22] reinforce the beneficial effects of physical exercise as an important regulator of fructose dyslipidemia, especially in plasma TAGs levels reduction, corroborating with our results in TF animals.
Consistent with previous findings on the protective ability of moderate-intensity physical exercise on glucose-insulin homeostasis, lipid metabolism and autonomic parameters [
25,
34,
63], our results showed a significant lower insulin, TC and TAGs fast levels, as well as in HOMA-IR and TYG index in adult rats that were supplemented with fructose and submitted to the moderate physical protocol during peri-pubertal period, compared to sedentary group. Likewise, after 6 weeks of a moderate exercise program, Dupas and coworkers found reductions in BW, fat stores and improved glucose-insulin metabolism in fructose-fed rats. For the respective authors, the results were associated with exercise-triggered improvement in muscle insulin sensitivity [
37].
Our data also showed that moderate-intensity training performed at peri-pubertal period could reduce the parasympathetic nerve activity in adult rats treated with fructose. This result corroborates with previous results that related beneficial effects for vagus nerve activity in HFD-fed exercised animals [
25,
62]. Nevertheless, it is notable that the effect of moderate-intensity training was observed in the present study, even when it was interrupted after 60 days before the experiments, suggesting some physiological adaptations and/or plasticity of the young organism, as it was carried in a sensitive window to programming health or disease [
7] in order to prevent components of MetS. When moderate physical training have been started early, the effects of discontinuation are not so deleterious [
25,
26,
28,
29].
This work predominantly focuses on a non-pharmacological strategy to prevent or attenuate several compounds of fructose-induced MetS in male Wistar rats. Overall, the main results reveal the ability of a young trained organism to recover, at least in part, to its previous metabolic pattern, as observed in supplemented and non-supplemented rats. Despite some limitations, we suggest that the expression of some proteins should be investigated to identify other mechanisms, in addition to glucose-insulin homeostasis and ANS activity experiments performed, the way an organism responds to fructose supplementation, as well as the potential beneficial effects of moderate-intensity training on programming metabolism especially in fructose-supplemented rats. This needs to be considered over the long term when thinking about translational research.
Figure 1.
Food intake, weight gain and total caloric intake. The values are presented as the mean ± SEM of 8-10 rats per group. Food intake (A), weight gain (B) and total caloric intake (C) from 30 to 120 days-old. Repeated measures ANOVA with Sidak’s post hoc test was performed to analyze the evolution of food intake, weight gain and total caloric intake. Two-way ANOVA and Tukey’s post hoc test were used for AUCs comparison in each figure. SC, sedentary rats subjected a normal diet; SF, sedentary rats subjected a fructose diet; TC, trained animals subjected to a normal diet; and TF, trained animals subjected to a fructose diet. F, fructose factor; T, training factor; and I, interaction between fructose and training factors.
Figure 1.
Food intake, weight gain and total caloric intake. The values are presented as the mean ± SEM of 8-10 rats per group. Food intake (A), weight gain (B) and total caloric intake (C) from 30 to 120 days-old. Repeated measures ANOVA with Sidak’s post hoc test was performed to analyze the evolution of food intake, weight gain and total caloric intake. Two-way ANOVA and Tukey’s post hoc test were used for AUCs comparison in each figure. SC, sedentary rats subjected a normal diet; SF, sedentary rats subjected a fructose diet; TC, trained animals subjected to a normal diet; and TF, trained animals subjected to a fructose diet. F, fructose factor; T, training factor; and I, interaction between fructose and training factors.
Figure 2.
Glucose during the ipGTT. The values are presented as the mean ± SEM of 6-8 rats per group. Glucose during the ipGTT from 60 (A) and 120 (B) days-old. Repeated measures ANOVA with Sidak’s post hoc test was performed to analyze glycaemia after glucose injection. Two-way ANOVA and Tukey’s post hoc test were used for AUCs comparison in each figure. SC, sedentary rats subjected a normal diet; SF, sedentary rats subjected a fructose diet; TC, trained animals subjected to a normal diet; and TF, trained animals subjected to a fructose diet. F, fructose factor; T, trained factor; and I, interaction between fructose and training factors.
Figure 2.
Glucose during the ipGTT. The values are presented as the mean ± SEM of 6-8 rats per group. Glucose during the ipGTT from 60 (A) and 120 (B) days-old. Repeated measures ANOVA with Sidak’s post hoc test was performed to analyze glycaemia after glucose injection. Two-way ANOVA and Tukey’s post hoc test were used for AUCs comparison in each figure. SC, sedentary rats subjected a normal diet; SF, sedentary rats subjected a fructose diet; TC, trained animals subjected to a normal diet; and TF, trained animals subjected to a fructose diet. F, fructose factor; T, trained factor; and I, interaction between fructose and training factors.
Figure 3.
Insulinemia during the ipGTT. The values are presented as the mean ± SEM 6-8 rats per group. Insulin during the ipGTT from 120 days-old. Repeated measures ANOVA with Sidak’s post hoc test was performed to analyze insulinemia after glucose injection. Two-way ANOVA and Tukey’s post hoc test were used for AUCs comparison in each figure. SC, sedentary rats subjected a normal diet; SF, sedentary rats subjected a fructose diet; TC, trained animals subjected to a normal diet; and TF, trained animals subjected to a fructose diet. F, fructose factor; T, trained factor; and I, interaction between fructose and training factors.
Figure 3.
Insulinemia during the ipGTT. The values are presented as the mean ± SEM 6-8 rats per group. Insulin during the ipGTT from 120 days-old. Repeated measures ANOVA with Sidak’s post hoc test was performed to analyze insulinemia after glucose injection. Two-way ANOVA and Tukey’s post hoc test were used for AUCs comparison in each figure. SC, sedentary rats subjected a normal diet; SF, sedentary rats subjected a fructose diet; TC, trained animals subjected to a normal diet; and TF, trained animals subjected to a fructose diet. F, fructose factor; T, trained factor; and I, interaction between fructose and training factors.
Figure 4.
Parasympathetic and sympathetic electrical nerve activity. The values are presented as the mean ± SEM of 5-9 rats per group. Parasympathetic (A) and sympathetic (B) nerve activity at 120 days-old. Two-way ANOVA and Tukey’s post hoc test were used. SC, sedentary rats subjected a normal diet; SF, sedentary rats subjected a fructose diet; TC, trained animals subjected to a normal diet; and TF, trained animals subjected to a fructose diet. F, fructose factor; T, trained factor; and I, interaction between fructose and training factors.
Figure 4.
Parasympathetic and sympathetic electrical nerve activity. The values are presented as the mean ± SEM of 5-9 rats per group. Parasympathetic (A) and sympathetic (B) nerve activity at 120 days-old. Two-way ANOVA and Tukey’s post hoc test were used. SC, sedentary rats subjected a normal diet; SF, sedentary rats subjected a fructose diet; TC, trained animals subjected to a normal diet; and TF, trained animals subjected to a fructose diet. F, fructose factor; T, trained factor; and I, interaction between fructose and training factors.
Table 1.
Oximetry and treadmill parameters for Wistar rats at 60 days-old.
Table 1.
Oximetry and treadmill parameters for Wistar rats at 60 days-old.
Table 2.
Oximetry and treadmill parameters for Wistar rats at 120 days-old.
Table 2.
Oximetry and treadmill parameters for Wistar rats at 120 days-old.
Table 3.
Tissues and biochemical parameters for Wistar rats at 60 days-old.
Table 3.
Tissues and biochemical parameters for Wistar rats at 60 days-old.
Table 4.
Tissues and biochemical parameters for Wistar rats at 120 days-old.
Table 4.
Tissues and biochemical parameters for Wistar rats at 120 days-old.