The adaptive response to training is determined by the type of exercise performed and by the training load applied (e.g., duration, intensity frequency) [
8] but also by other factors such as nutritional status [
9,
10]. Thus, nutritional intake can modulate exercise-induced adaptations.
The availability of energy during exercise modulates the physiological response to training [
11]. Nowadays, glycogen is not only considered as an energy substrate but also as a regulator of the signaling pathways that regulate the adaptations in response to exercise [
12,
13]. Thus, manipulating the endogenous (glycogen stores) and exogenous (CHO intake) availability of CHO before and/or after training sessions may be used to alter exercise-induced adaptations [
14].
2.1. Carbohydrates Availability and Resistance Training-Induced Adaptations
It has been observed that the anabolic response after resistance training depends on the status of glycogen stores. This anabolic response is regulated by complex signaling pathways, one of the most important being the mTOR pathway [
15]. Glycogen availability exerts a negative regulatory activity over AMPK. Thus, in situations of energy deficit (low glycogen availability) the AMPK pathway, which is involved in the generation of ATP, is activated, whereas the mTOR pathway, which requires energy for its activity, is inhibited [
16,
17]. Although the evidence is not unanimous, low levels of glycogen could inhibit the hypertrophic response to resistance training [
18,
19] and increase proteolysis [
20,
21]. For this reason, it would be convenient to start resistance exercises with replenished glycogen stores [
16,
17].
The hypertrophic adaptations to resistance exercises could be also facilitated by an increase in the post-exercise hormonal response. Insulin is an anticatabolic, which achieves muscle protein anabolism by the inhibition of muscle protein breakdown [
22], so CHO intake after exercise could be beneficial to potentiate exercise-induced adaptations. However, evidence is mixed in this regard, and it seems that supplying an optimal amount of high-quality proteins, specifically proteins that contain an adequate dose of leucine (e.g., 3–4 g/dose) [
23] after exercise can be enough to achieve a positive protein balance and stimulate an anabolic response [
24]
In addition to allowing better rates of work, another goal of resistance training programs consists of maintaining or stimulating a positive nitrogen balance [
25]. Thus, CHO ingestion maintains higher levels of insulin concentration, which is a hormonal response that stimulates the uptake and storage of CHO in tissues and decreases the use of amino acids [
26]. Therefore, CHO ingestion during the effort could be useful to reduce amino acid degradation, favoring a positive nitrogen balance.
2.2. Carbohydrates Availability and Endurance Training-Induced Adaptations
Classical studies have already demonstrated the beneficial role of a high-CHO diet for endurance exercise performance [
27,
28]. A high CHO availability is beneficial in endurance exercise during competitions or in training sessions in which a high level of intensity is required [
8,
29]. Thus, a high CHO intake (10 g/kg) has been recommended the day before a competition (more precisely ~36 h from the last training session) and physical inactivity to ensure glycogen supercompensation [
1,
9]. However, it may be unnecessary in sessions where exercise intensity is not important [
1]. According to the last position of the American College of Sport Medicine, nutritional recommendations have traditionally focused on optimal nutrition during competitions, but a universal recommendation of an optimal intake of CHO might not be appropriate as there is a great diversity of goals and training loads across training microcycles and macrocycles [
1].
The acute and chronic adaptations to endurance exercise have been widely described, including mitochondrial biogenesis, a shift in the fiber-type prevalence toward a slower-twitch phenotype, improved-oxidative metabolism, and increased-capillary density [
16,
18,
30]. Several studies demonstrated that during periods in which CHO availability was strategically reduced (known as “training low”) there was an increase of molecular markers related to endurance training-induced adaptation [
31,
32]. Considering these responses, it could be important to differentiate between the optimal nutrition to compete and to train [
32]. Thus, whereas during high-intensity sessions or competitions it is essential to train with a high CHO availability, it might be beneficial to exercise with a reduced CHO availability in low-intensity ones [
33]. This paradigm is known as “train low, compete high” and could be applied specifically when exercise intensity is close to or below the first ventilatory threshold (VT1) as this intensity matches with the maximum recruitment of slow-twitch motor units [
34].
The AMPK-PGC-1α signaling pathway plays a key role in the adaptations to endurance exercise through inducing mitochondrial biogenesis and improving oxidative capacity [
35,
36]. A decrease in glycogen content and an increase in the AMP:ATP ratio are two important signals that trigger the activation of the AMPK pathway [
31,
37]. The adaptive response to endurance exercise with a low glycogen availability is regulated by both AMPK and p38MAPK, converging both pathways in the regulation of key transcriptional factors and co-activators, such as PGC-1α, p53, and PPARδ [
37,
38]. Although AMPK and p38MAPK, and subsequently PGC-1α, are activated by endurance exercise per se, these molecular signals seem to be amplified when glycogen stores are low [
31].
The first study analyzing the adaptations to exercising with a low CHO availability possibly took place in 1983, when explorers in the Arctic and Antarctica seemed to adapt to these kind of diets [
39]. Nowadays, several studies have showed how performing endurance exercise with a low CHO availability induced greater physiological adaptations than performing the same training session with a normal CHO availability [
31,
40]. For instance, “training low” augmented the expression of genes related to mitochondrial biogenesis and oxidative metabolism [
41,
42,
43]. Moreover, a reduction of CHO availability during 3–10 weeks improved enzymatic activity, increased the content of proteins involved in oxidative metabolism, and increased the capacity to oxidize lipids in endurance athletes [
44,
45,
46].
Despite the aforementioned physiological adaptations, controversy exists regarding the effectiveness of periodized nutritional strategies for the improvement of exercise performance with some studies reporting beneficial effects [
32,
38,
47,
48,
49] and others showing no benefits or even performance impairments [
44,
45,
46,
50,
51]. Burke et al. recently reported that elite race walkers that had followed a very low-CHO diet during 3 weeks increased their fat oxidation rate and decreased their body mass, which could be considered a beneficial physiological adaptation [
50]. However, their performance was impaired in comparison with diets supplying a higher amount of CHO [
50]. Similarly, another study analyzed the effects of CHO restriction during four weeks in elite endurance athletes and did not find beneficial effects on their performance [
51]. By contrast, Marquet et al. performed a study in which athletes altered their CHO intakes attending to the intensity of their training sessions, reducing glycogen availability during low-intensity training sessions (which were performed the day after a high-intensity training session with no posterior CHO reposition) and increasing glycogen availability (i.e., increased CHO intake) when higher intensities were required. The authors observed that this strategy yielded performance benefits (improved 10-km running performance, improved cycling efficiency, and increased time to exhaustion at 150% of the maximal aerobic power in a cycle ergometer) compared to a group that consumed an even distribution of CHO during all training sessions. An important aspect of this study was that the total CHO intake was the same in both experimental groups, and they just differed in the timing of this intake. These results suggested that the timing of a CHO intake should be individualized and periodized every day depending on the intensity of the training session [
38]. Thus, nutritional periodization might be beneficial for athletes’ performance [
1,
10].
2.3. Protocols for Achieving a Low CHO Availability
Different protocols for the manipulation of CHO availability have been studied, notably training twice on the same day [
46,
49,
52], exercising in a state of fasting [
41], or restricting CHO intake during [
44] or after exercise [
41]. Another strategy that is growing in popularity is ‘sleeping low’, which consists of performing a high-intensity training session in an afternoon/night with a high CHO availability followed by a period of CHO restriction during that night and a low-intensity training session while fasting on the following day [
38]. This strategy has been reported to enhance exercise-induced adaptations and performance in endurance athletes [
38], and neither the immune system nor the exercise intensity during the high-intensity training session seem to have been affected [
53].
Despite the theoretical basis supporting ‘training low’ strategies, there are some potential risks associated with the long-term maintenance of a low CHO availability, including an impairment of the immune system, a reduced CHO oxidation rate during exercise—and consequently an inability to exercise at high intensity, and an increased oxidation of muscle proteins [
37]. These limitations can be even more exacerbated if athletes and coaches incorrectly interpret the term “training low” as “training zero”, which means training chronically with an extreme CHO restriction [
31]. In order to avoid the deleterious effects of energy deficits on protein synthesis, consuming a leucine-rich protein close to the time of the training session has been proposed [
32]. Some strategies can also be used to avoid lowering training intensity when exercising with a low CHO availability. For instance, studies have suggested that caffeine intake, mouth rinsing with a CHO solution during exercise, or even mouth rinsing with a caffeinated solution can attenuate the reduction of training intensity in these situations [
54,
55].