Although the role of carbohydrates in moderate-to-high-intensity exercise is recognized, science is still exploring how their consumption before and during training influences performance.

Overview

The influence of carbohydrates on resistance training performance was investigated through an analysis of 21 studies.

The data analysis revealed that carbohydrate supplementation led to a significant increase in total training volume. This benefit was particularly observed in the subgroups of participants who performed long-duration workouts (more than 45 minutes) and those who had fasted for at least 8 hours.

To give you a better understanding: consuming carbohydrates during resistance workouts lasting more than 45 minutes, or after fasting for more than 8 hours, can optimize your training capacity.

What's the problem?

At rest, the body primarily uses fat as its energy source. However, at the beginning of physical activity, carbohydrates (CHO) become the primary fuel, especially during high-intensity exercises such as resistance training. CHO are transformed into ATP, the molecule that provides energy to cells. Since ATP is rapidly depleted during intense exercise, it is crucial to constantly replenish it to maintain performance. The body obtains glucose to produce ATP from three main sources: muscle glycogen, blood glucose (derived from liver glycogen or gluconeogenesis), and ingested carbohydrates.

Muscle glycogen depletion intensifies with increasing training volume and intensity. Studies indicate that typical exercise volume reduces glycogen stores by 30%. Intensity, by recruiting more type II muscle fibers, further accelerates this reduction. Recent research confirms this, showing a 38% decrease in glycogen in the vastus lateralis after leg exercises, with almost total depletion in half of the type II fibers.

During sleep, the liver regulates blood glucose by reducing its glycogen stores, while muscle glycogen remains stable. Since intense exercise depletes muscle glycogen and overnight fasting decreases liver glycogen, the question arises as to whether consuming carbohydrates (CHO) before training would improve performance and adaptations, a common practice in endurance sports. However, recent studies show no clear benefits of acute CHO ingestion on performance, although it may be useful during prolonged fasting and high-volume training. A meta-analysis is needed to clarify the effects of CHO on performance, due to the variability in study results.

Purpose and hypothesis

The researchers sought to determine the effects of acute carbohydrate (CHO) ingestion on resistance training (RT) through a systematic review of the available literature and assessment of the certainty of the evidence. No specific hypothesis was proposed, but the authors highlighted the potential benefits of carbohydrates on training performance and the need for a quantitative synthesis of the data.

What did they try?

Participants

Twenty-one studies were identified for the systematic review, with a total of 226 participants (predominantly men, with a mean age between 20 and 30 years) and more than 5 years of radiotherapy experience in some cases.

Table 1 Characteristics of Studies Included in the Systematic Review

Values are means ± standard deviations.  BM body mass, CHO carbohydrate, CON control, PLA placebo.

Study procedures

This systematic review included studies that met the following criteria: crossover design, evaluation of the acute effect of carbohydrate ingestion on performance during resistance training, comparison with a control group (placebo or water), and measurement of muscular strength, power, or endurance (1RM, isokinetic/isometric strength, repetitions, total work, session duration). Perceived exertion and blood markers (glucose and lactate) were also considered.

Random-effects meta-analyses were planned for each measure (strength, endurance, blood markers, etc.) if reported in at least two studies. Due to insufficient data, correlations were calculated using unpublished data from the investigator's laboratory for the measures of interest. We acknowledge this limitation, but the lack of published data restricted our options. Meta-regressions were performed for CHO dose (g/kg of body mass), load used (% 1RM), and total number of maximal effort sets, provided at least six effects per outcome were available.

Measures

Due to the limited information in the included studies, a meta-analysis was only possible with three measures. Of these, only one assessed muscle performance (training volume), while the other two focused on blood lactate and glucose levels.

To assess total training volume, repetitions completed to muscle failure were counted. This metric was selected due to its higher reporting frequency in the analyzed studies. The laboratory-based correlation analysis revealed a moderate positive association (r = 0.78, n = 5).

After exercise, blood lactate and glucose levels were measured. The correlation analysis revealed a strong positive association for lactate (r = 0.74) and a weak positive association for glucose (r = 0.26).

What did they find?

Total training volume

The meta-analysis presented in Figure 1(a) reveals that CHO intake produces a moderate increase in the effect size (SMD = 0.61; 95% CI: 0.11, 1.11) compared to placebo or control. This result, represented by the diamond in the diagram, suggests that CHO supplementation has a positive impact on performance, similar to that observed with other ergogenic aids such as caffeine or citrulline malate.

However, the analysis also indicates high statistical heterogeneity (I² = 79%), suggesting significant variability among the included studies. This heterogeneity is reflected in the dispersion of squares in the forest plot.

A subgroup analysis revealed that CHO intake had a significant effect in exercise sessions lasting longer than 45 minutes (SMD = 1.02; 95% CI: 0.07, 1.97), as shown in the large diamond in Figure 1 (a). Furthermore, a positive effect was observed in fasting periods exceeding 8 hours (SMD = 0.39; 95% CI: 0.06, 0.72), indicated in the lower forest plot (b) of Figure 1.

Regarding training volume, the data in Figure 2 show that three studies found no improvements in total repetitions to failure in lower-body workouts of 2 to 4 sets. This suggests that the benefit of CHO intake may be more evident in workouts with a higher number of sets. Indeed, mixed-effects meta-regression indicates that the total number of sets to failure is a significant moderator of SMD, with an apparent increase in the effect after four sets.

Figure 1 Meta-analysis of the effect of CHO intake on total training session volume.

Random-effects meta-analysis of acute CHO ingestion on total training session volume compared to a placebo or water only.  Sub-group analyses were based on session and fast duration.  CHO carbohydrate, CI confidence interval, SMD standardized mean difference.

Figure 2 Mixed effects meta-regression controlling for the effects of maximal effort bouts completed.

Larger data points received greater weighting than smaller data points.  Solid lines represent the estimated relationship and dotted lines represent the upper and lower 95% confidence intervals.  BM body mass, CHO carbohydrate, 1RM 1-repetition maximum, RT resistance training.

Blood lactate

Carbohydrate (CHO) intake significantly influenced training volume. Consequently, and as evidenced in the meta-analysis in Figure 2 (SMD = 0.58; 95% CI: 0.03, 1.14), a significantly greater accumulation of blood lactate would be expected and was observed with CHO consumption. This post-exercise increase suggests that CHO intake facilitated a greater training volume, leading to greater fatigue and, consequently, elevated lactate levels.

Figure 2 Random effects meta-analysis of the effect of acute CHO ingestion on post-exercise blood lactate concentration.

Random-effects meta-analysis of acute CHO ingestion on post-exercise blood lactate concentrations compared to a placebo or water only.  Sub-group analyses were based on session and fast duration.  CHO carbohydrate, CI confidence interval, SMD standardized mean difference. 

Blood glucose

The results confirm that carbohydrate (CHO) consumption after exercise causes a significant increase in blood glucose levels, with a considerable impact (SMD = 2.36; 95% CI: 1.17; 3.55). This effect was also significantly observed in fasting periods of less than 8 hours (SMD = 2.83; 95% CI: 1.09; 4.57) and in exercise sessions of more than 45 minutes (SMD = 2.94; 95% CI: 1.67; 4.21), as illustrated in Figure 3.

Figure 3 Random effects meta-analysis of the effect of acute CHO ingestion on post-exercise blood glucose concentrations.

Risk of bias and other considerations

The risk of bias assessment identified one study with a high likelihood of randomization bias due to a lack of blinding. Participant and investigator knowledge of group assignment compromised the validity of its results. Three other studies presented a low risk of bias, while two were excluded due to insufficient data. Consequently, the analysis focused on the remaining studies, which were considered legitimate for evaluating the benefits of CHO in training. While the reduction in the number of studies is a limitation, it strengthens the reliability of the data by avoiding the influence of potentially misleading results.

What do the findings mean?

A meta-analysis can be conducted with as few as two published studies on a topic. Although these are often considered the highest level of evidence, their validity is intrinsically linked to the quality of the studies they comprise. I found the publication of this article interesting, as it underscores both the need for more research in this area and the potential for pre- and intra-training carbohydrate ingestion to offer greater benefits than previously estimated.

This analysis reveals a positive effect of pre- and/or intra-training CHO supplementation on performance. However, the high heterogeneity observed suggests that the effectiveness of CHO ingestion varied considerably across studies. Essentially, while there is a benefit, it appears to be conditional on certain circumstances, which were identified in the subgroup analyses. Specifically, a clear benefit was observed when training sessions exceeded 45 minutes and when training occurred after fasting for more than 8 hours. The dose of CHO ingested, within the range of 0.4 g/kg of body mass to 2.4 g/kg of body mass, did not appear to influence the magnitude of the benefits.

How can we apply these findings?

This study effectively summarizes the current landscape of CHO intake and its impact on performance, offering practical recommendations for optimizing muscle mass and strength.

To optimize muscular adaptation in workouts lasting more than 45 minutes after an eight-hour or longer night's rest, carbohydrate consumption before or during the session is suggested. The magnitude of this benefit varies according to individual nutritional status. A substantial caloric intake the night before modifies glycogen metabolism. In individuals with low glycogen stores due to restrictive diets, pre-/intra-workout carbohydrate ingestion could be crucial, considering adequate caloric intake during exercise (approximately 0.2 g/kg of muscle mass to avoid excessive liquid calories). In contrast, advanced bodybuilders during pre-season training on high-calorie diets can use these easily digestible carbohydrates to increase training volume and total caloric intake. An interesting finding suggests a possible specific benefit for lower-body training, although further studies focusing on the upper body are warranted. Finally, maintaining adequate blood glucose levels through rapidly absorbed carbohydrates (such as sports drinks) could support greater training volume by providing readily available energy.

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