It’s well known that carbohydrates fuel moderate-to-high intensity exercise, but there’s still much to learn about their impact on training performance with pre- and/or intra-exercise ingestion.

Overview

What did they test? Twenty-one studies were analyzed to determine if and to what degree carbohydrate ingestion influences resistance training performance. 
What did they find? Pooled results indicated a significant benefit of carbohydrate ingestion compared to a placebo or control for total session training volume.  Sub-group analysis revealed a significant benefit of carbohydrates during sessions longer than 45 min and after a fast of 8 h or more.
What does it mean for you? If you’re performing resistance training longer than 45 minutes or after a fast of > 8 h, ingestion of carbohydrates can promote greater training volumes. 

What’s the Problem?

At rest, your body's primary fuel source is fat. As you stand and begin using your muscles for movement, carbohydrates (CHO) increase their contribution to energy expenditure. With greater contributions from CHO at moderate to high-intensity exercise like resistance training (RT). The primary function of CHO is to provide energy for cells to do work. Your body begins to break down CHO to form adenosine triphosphate (ATP). The cell's energy currency ATP is limited in supply, requiring rapid replenishment during high-intensity exercises like resistance training. There are three primary sources the body can pull from when more glucose is needed to make ATP 23:

1. Muscle glycogen
2. Blood glucose derived from liver glycogen
   a. Through gluconeogenesis (formation of glucose from noncarbohydrate sources)
   b. Through liver glycogen breakdown
3. Blood glucose derived from ingested carbohydrates

Greater training volumes result in greater decreases in muscle glycogen stores, with most studies showing a roughly 30% decrease in total muscle glycogen stores from typical training volumes 23 24 25 26 27. Since greater training intensities recruit more muscle fibers (fast twitch type II, precisely), this results in an exponential increase in glycogen breakdown with a linear increase in training intensity 23. A recent study supports this claim, showing modest decreases in total glycogen stores of the Vastus Lateralis (38%) after lower body training, including roughly half of type II fibers near complete depletion of intra-myofibrillar stores of glycogen 29.

While you sleep, blood glucose is tightly regulated by your liver glycogen, resulting in significant decreases of liver glycogen but minimal effect on muscle glycogen 34 35. So, when you consider that higher training volumes increase the rate of muscle glycogen depletion and overnight fasting reduces liver glycogen, looking at the above-bulleted list that's tapping into two of your sources for making more ATP. Wouldn't it be logical to assume a benefit to supplementing with CHO right around training to maximize training performance and, therefore, adaptations? After all, this has been common practice and recommendation for aerobic/endurance-based performance individuals 33. As highlighted in a recent systematic review 28, most studies show no benefit to acute CHO intake on training performance. Still, there seems to be a trend for potential benefits of CHO intake for longer pre-exercise fast durations and training programs with more than ten sets completed 28. However, not only is the data limited (typically the case for these studies), but the methods behind the study design are also much different. Hence, the need for the current meta-analysis to summarize the effects of CHO intake on training performance 1.

Purpose & Hypothesis

Researchers wanted to determine the effects of acute carbohydrate (CHO) ingestion on resistance training (RT) by systematically reviewing the available literature and assessing the certainty of evidence.  There wasn’t a specific hypothesis, but the authors highlighted the potential benefits of carbohydrates on training performance and the need for quantitative synthesis of the data.