Caffeine supplementation is ergogenic in soccer players independent of cardiorespiratory or neuromuscular fitness levels | Journal of the International Society of Sports Nutrition

The athletic gut microbiota | Journal of the International Society of Sports Nutrition
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Group characteristics

The anthropometric and physiological characteristics of the participants, divided according to cardiorespiratory or neuromuscular fitness, are presented in Table 1. Groups differed significantly in the selection parameter (VO2max or CMJ) by design. Additionally, the high-fitness groups had significantly lower percentage body fat that the corresponding medium-fitness groups, and the HNF group had significantly higher VO2max than the MNF group. Groups did not differ significantly in habitual daily caffeine consumption, which was moderate (with an overall median of 0.9 mg∙kg− 1) and considerably lower than the experimental dose of 6 mg∙kg− 1.

Table 1 Characteristics of participants with high (HCF) or medium cardiorespiratory fitness (MCF); and with high (HNF) or medium neuromuscular fitness (MNF)

Time to exhaustion

There was a significant main effect of treatment (caffeine vs placebo) on time to exhaustion (Fig. 1) regardless of whether participants were classified based on cardiorespiratory fitness [F(1, 18) = 29.15, p < 0.001, ES = 0.561] or on neuromuscular fitness [F(1, 18) = 23.59, p < 0.001, ES = 0.567]. Time to exhaustion was longer with caffeine compared with placebo (719 ± 288 vs 469 ± 228 s). However, time to exhaustion was not different between groups, and there was no treatment-by-group interaction (p > 0.05).

Fig. 1
figure1

Means and SD of time to exhaustion with caffeine (solid bars) and placebo (hatched bars). Participants were divided according to cardiorespiratory (left) or neuromuscular fitness (right), with high groups shown in black and medium groups shown in grey. There was a significant treatment effect (p < 0.001)

Three of the participants who performed the caffeine trial after the placebo trial noticed their longer times to exhaustion and guessed they had taken caffeine. None of the other participants made any presumption related to what they were taking.

Jump performance and reaction time

There were significant treatment [F(1, 18) = 22.84, p < 0.001, ES = 0.559] and time [F(4, 72) = 16.13, p < 0.001, ES = 0.473] main effects on CMJ (Fig. 2a), as well as a time main effect on RT [F(4, 72) = 3.15, p = 0.02, ES = 0.149, data not shown], while there was no significant group effect or interaction (p > 0.05) for the cardiorespiratory fitness classification. CMJ was higher with caffeine compared to placebo (42.7 ± 4.2 vs 38.6 ± 4.4 cm overall).

Fig. 2
figure2

Means and SD of countermovement jump height with caffeine (solid bars) and placebo (hatched bars). Participants were divided according to cardiorespiratory (a) or neuromuscular fitness (b), with high groups shown in black and medium groups shown in grey. One to 5 correspond to time points as follows: 1, before the start of the trial; 2, between the 1st and 2nd periods; 3, immediately after the end of the 2nd period; 4, immediately before the start of the 3rd period; 5: between the 3rd and 4th periods. There were significant treatment and time effects in both a and b, as well as a significant group effect (by design) in b (p < 0.05)

For the neuromuscular fitness classification, (Fig. 2b) CMJ exhibited significant main effects of group [F(1, 18) = 16.91, p < 0.001, ES = 0.484, as expected, since groups were defined on the basis of CM], treatment [F(1, 18) = 25.38, p < 0.001, ES = 0.585], and time [F(4, 72) = 16.17, p < 0.001, ES = 0.473], while there was no interaction (p > 0.05). There was no significant main effect or interaction in RT (p > 0.05, data not shown).

Cardiovascular responses

When the data were analyzed based on cardiorespiratory fitness level, there was a treatment [F(1, 18) = 12.85, p = 0.002, ES = 0.417] and a time [F(2, 41) = 17.68, p < 0.001, ES = 0.495] main effect on MAP (Table 2). MAP was higher with caffeine compared to placebo (98 ± 8 vs 92 ± 10 mmHg overall). Similarly, when the data were analyzed based on for neuromuscular fitness level (Table 2), there was a treatment [F(1, 18) = 12.81, p = 0.002, ES = 0.416] and a time [F(2, 44) = 18.51, p < 0.001, ES = 0.507] main effect on MAP. There was no significant group effect or interaction for either classification (p > 0.05).

Table 2 Mean arterial pressure for the cardiorespiratory and neuromuscular fitness classifications (mm Hg, mean ± SD)

Average HR during each of the four periods of the treadmill protocol (Table 3) showed significant main effects of treatment [F(1, 16) = 17.77, p < 0.001, ES = 0.526] and time [F(2, 32) = 12.57, p < 0.001 ES = 0.440], as well as an interaction between treatment and time [F(3, 48) = 3.06, p = 0.04, ES = 0.161] for the cardiorespiratory fitness classification. HR was higher with caffeine than with placebo (163 ± 12 vs 157 ± 13 bmin− 1 overall). For the neuromuscular fitness classification (Table 3), there were only significant main effects of treatment [F(1, 16) = 17.07, p < 0.001, ES = 0.516] and time [F(2, 34) = 14.39, p < 0.001, ES = 0.473]. There was no significant group effect for either classification (p > 0.05).

Table 3 Heart (HR) and rating of perceived exertion (RPE) for the cardiorespiratory and neuromuscular fitness classifications (mean ± SD)

Perception of effort

There were significant main effects of treatment [F(1, 18) = 13.16, p = 0.002, ES = 0.422] and time [F(2, 39) = 101.83, p < 0.001, ES = 0.850], as well as an interaction between treatment and time [F(2, 36) = 4.58, p = 0.006, ES = 0.203] for the cardiorespiratory fitness classification on RPE (Table 3). RPE was lower with caffeine than with placebo (12.6 ± 1.7 vs 13.3 ± 1.6 overall), as seen in Table 3. For the neuromuscular fitness classification (Table 3), there were also significant main effects of treatment [F(1, 18) = 13.49, p = 0.002, ES = 0.428] and time [F(3, 54) = 107.01, p < 0.001, ES = 0.856], as well as an interaction between treatment and time [F(2, 34) = 3.35, p = 0.05, ES = 0.157] on RPE. There was no significant group effect for either classification (p > 0.05).

Energy expenditure and fuel oxidation

No differences between treatments or groups were found in energy expenditure, fat oxidation, or carbohydrate oxidation (p > 0.05). The overall energy expenditure was 15.74 ± 1.76 kcal·min− 1, fat oxidation was 1.13 ± 0.19 g·min− 1, and carbohydrate oxidation was 1.41 ± 0.47 g·min− 1.

Blood metabolites

When data were analyzed based on cardiorespiratory fitness level, plasma glucose (Fig. 3a) exhibited significant main effects of treatment [F(1, 18) = 14.30, p = 0.001, ES = 0.443] and time [F(3, 57) = 13.73, p < 0.001, ES = 0.433], as well as an interaction between treatment and time [F(3, 47) = 5.38, p < 0.001, ES = 0.230]. Plasma glucose was higher with caffeine than with placebo (overall 5.6 ± 0.7 vs 5.3 ± 0.6 mmol∙l− 1). Similarly, when analysis was based on neuromuscular fitness level, there were also a treatment effect [F(1, 18) = 13.15, p = 0.002, ES = 0.422], a time effect [F(3, 60) = 14.24, p < 0.001, ES = 0.442], and an interaction between treatment and time [F(3, 45) = 5.19, p < 0.001, ES = 0.224]. There was no significant group effect with either classification (p > 0.05).

Fig. 3
figure3

Means and SD of plasma glucose (a), lactate (b), glycerol (c), NEFA (d) and epinephrine (e) with caffeine (full circles) and placebo (open circles). Participants were divided according to cardiorespiratory (left) or neuromuscular fitness (right), with high groups shown with solid lines and medium groups shown with dashed lines. One to 7 correspond to time points as follows: 1, 75 min before the start of the trial; 2, 15 min before the start of the trial; 3, between the 1st and 2nd periods; 4, immediately after the end of the 2nd period; 5, immediately before the start of the 3rd period; 6: between the 3rd and 4th periods; 7, at exhaustion. *Significant treatment effect (p ≤ 0.01). †Significant time effect (p < 0.001)

Within the cardiorespiratory fitness level analyses, there were significant main effects of treatment [F(1, 18) = 8.12, p = 0.01, ES = 0.311] and time [F(2, 33) = 65, p < 0.001, ES = 0.782], as well as an interaction between treatment and time [F(3, 58) = 4.31, p < 0.007, ES = 0.193] in plasma lactate (Fig. 3b). As with glucose, lactate was higher with caffeine than with placebo (overall 3.3 ± 1.2 vs 2.9 ± 1.2 mmol∙l− 1). Within the neuromuscular fitness level analysis, there was also a treatment effect [F(1, 18) = 8.43, p < 0.001, ES = 0.319], a time effect [F(2, 30) = 62.12, p < 0.001, ES = 0.775], and an interaction between treatment and time [F(3, 61) = 4.31, p < 0.006, ES = 0.193].

With regard to plasma glycerol (Fig. 3c), NEFA (Fig. 3d), and epinephrine (Fig. 3e), the only statistically significant outcomes of the factorial ANOVA were a main effect of time for the cardiorespiratory fitness classification [F(2, 35) = 129.65, F(3, 45) = 116.78, and F(3, 59) = 22.54, respectively; p < 0.001 for all; ES = 0.878, 0.866, and 0.556, respectively] and for the neuromuscular fitness classification [F(2, 35) = 134.05, F(3, 47) = 110.06, and F(3, 51) = 20.78, respectively; p < 0.001 for all; ES = 0.882, 0.859, and 0.536, respectively], as well as a main effect of group on epinephrine for the neuromuscular fitness classification [F(1, 18) = 6.25, ES = 0.258, p = 0.022]. The time effect was due to a general increase in all three parameters from the beginning to the end of the trials, and the group effect was due to epinephrine being higher in the MNF group.

Influence of body mass and body fat

When body mass, body fat, or both variables were added to the analysis as covariates, all statistical outcomes but one were qualitatively the same. That is, all significant outcomes of ANOVA remained significant with ANCOVA and all non-significant outcomes of ANOVA remained non-significant with ANCOVA, with the exception of NEFA, which, although not significantly different between the HCF and MCF groups according to ANOVA, became significantly different (specifically, higher in the HCF group) when body fat [F(1, 17) = 4.49, ES = 0.209, p = 0.049] or body mass and body fat were used as covariates [F(1, 16) = 5.22, ES = 0.246, p = 0.036]. These finding show that body mass and body fat had no influence on the caffeine responses.



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