This post is taken from the Vitamin. Every Thursday, I drop some knowledge bombs on your face to help you reach your goals faster while avoiding all the bullshit.
Note: This article is by Andrew King, who joined the Physiqonomics research team a few months back.
Andrew is currently completing his master’s degree in Sports and Exercise Science out in New Zealand at the Auckland University of Technology. His current research projects are looking at pre-exercise fuelling and how it affects resistance training performance.
Alongside his interest in sports nutrition, Andrew is also a competitive powerlifter, so strength and hypertrophy research is another interest.
Carbohydrate from our diet, which is absorbed into the bloodstream as blood glucose, is an important fuel source during exercise (along with fats), especially as the intensity of exercise increases. 1
The blood glucose that’s not immediately used for energy can be stored in the liver and skeletal muscle as glycogen. Liver glycogen can be considered a ‘backup’ carbohydrate store, broken back down into blood glucose when supply is strained, like during exercise. On the other hand, the blood glucose stored as glycogen in our muscles is there for one specific purpose only: to fuel muscle contractions (especially higher intensity contractions). 2
Beyond carbohydrates’ established role as a fuel source during exercise, they can also affect our central nervous system, which could influence performance.
For instance, there are receptors in our mouth that sense the presence of carbohydrates (fun fact: the specific type of receptor hasn’t even been conclusively identified by scientists yet!), which relay messages to the regions of the brain associated with reward, motivation, and motor output. 3 Just swishing some carbohydrate-containing solution around in your mouth can improve exercise performance in some circumstances! 4
While previous scientific literature has shown the beneficial effects of carb ingestion on improving endurance exercise performance 5, much less is known about the ergogenic effects of carbs on resistance training performance.
Where endurance exercise is often continuous and prolonged in nature, resistance training is intermittent and of higher intensity. Given the different demands, carb needs may differ between these exercise modalities.
To answer this, the study we’re looking at this month investigated this question and is the first publication from my own PhD! 6
We investigated whether having carbohydrates before and/or during a resistance training session improved the amount of training volume you could complete.
Note: The paper is open access––meaning you have free access to the article by copying and pasting the following link into your browser: https://doi.org/10.1007/s40279-022-01716-w. Nonetheless, it can be a little scientifically dense to read and decipher, so we’ll break it down here.
What did we (the researchers) do?
- This study was a systematic review (i.e., we comprehensively searched several academic databases for potential studies to be included) with meta-analysis and meta-regression (i.e., once we had decided which studies fit our inclusion criteria, we applied statistical methods to quantify how carbs affected training performance across the available literature and identify which factors influenced this effect).
- Several academic databases were searched for studies that compared the effects of carbohydrate ingestion (typically a sports drink) to a placebo (water + artificial sweetener) on outcomes that quantified resistance training performance (e.g., number of reps completed to failure, barbell velocity).
- The studies also needed to be a cross-over design, so participants always completed at least two training sessions in the included studies: one with carbs, the other with the placebo.
- We also looked for secondary outcomes, such as blood (e.g., glucose and lactate) and perceptual (e.g., perceived exertion) measures.
- Once we had all the studies we wanted to include, we assessed their methodological quality using a tool called Cochrane’s Risk of Bias.
- We grouped all the outcomes (e.g., total session volume, maximal strength) from the included studies to see what we could meta-analyse (you can’t do a meta-analysis if only one study reported on it!). Where there was sufficient data for an outcome, we performed a random-effects meta-analysis.
- The GRADE tool was used to assess the certainty of evidence overall.
What did we find?
- There was enough data to meta-analyse three outcomes: total session training volume, post-exercise blood glucose, and post-exercise blood lactate.
- Carbohydrates improved resistance training volume performance in the pooled analysis (i.e., when all the studies measuring total session volume were lumped in together).
- Carbohydrates were found to improve total session volume during longer duration training sessions (>45 mins) and where the fast before training was equal to or greater than 8 hours.
- However, carbs did not improve performance during shorter duration sessions (less than 45 mins) and where the fast duration was less than 8 hours.
- The number of sets completed moderated the effect of carb ingestion, but the training load (%1RM) and the carb dose did not.
- Carb ingestion had a large effect on blood glucose, but it is unclear whether elevated blood glucose is needed for performance.
- Carb ingestion elevated post-exercise blood lactate (probably due to the increased volume performed).
The meta-analysis found that carb ingestion improved resistance training volume performance.
Essentially, participants in the included studies completed more session volume (most often measured as total session repetitions completed) to failure when consuming carbs versus a placebo.
However, there was quite a bit of variability in the results (i.e., some studies reported improved performance with carbs, others didn’t), which is quantified in the study as ‘heterogeneity’ (I2), so a deeper dive using sub-group analysis was needed to figure out under what conditions carb ingestion can benefit performance.
The sub-group analysis indicated that carb ingestion before and during longer duration training sessions (greater than 45 mins), and where the fast duration before training was 8 hours or longer, is beneficial for volume performance.
Additionally, the number of sets completed seems to be an important factor for modifying the effect of carbs––so the more sets you do, the more important carbs become. However, the load used (%1RM) did not seem to influence the results.
We also saw a trend where studies including lower body exercises reported improved performance with carb ingestion, whereas those including upper body only did not. It’s unclear whether this was an effect of lower body exercise selection per se or the studies using upper body exercises weren’t high enough in volume (they tended to be a bit lower, and we found the amount of volume completed is important).
Post-exercise blood glucose was robustly increased with carb ingestion compared to the placebo. This makes sense. Most of these studies basically gave their participants a sports drink, which are simple and easily absorbed sources of carbs. Blood glucose could be useful as a readily available fuel source during training, but it wasn’t clear from our review whether this is needed for improved performance. It’s often postulated carbs could prevent decreases in blood glucose (hypoglycaemia) with resistance training, but we didn’t see this in our review. So if there were to be an effect of increased blood glucose on training performance, it would be because of its role as a readily available fuel source for muscle contractions.
We also saw an increase in post-exercise blood lactate with carb ingestion. It should be noted that blood lactate is not responsible for muscular fatigue (and is, in fact, an important fuel source for exercise). Still, it does serve as a useful surrogate for monitoring metabolic fatigue. Post-exercise blood lactate was likely higher because of the increased volume performed with carb ingestion (more work = more fatigue!).
As with all studies, there are limitations to our analysis.
Likely of most importance is that we compared carbohydrates to a zero/low kcal placebo. It’s possible that energy alone could be the reason why carbohydrate is important for training performance (rather than a metabolic or central effect of carbohydrate per se), and this is something we’re also trying to answer with a cross-over trial of our own in which data collection is just wrapping up.
A few studies have found where feelings of satiety are promoted and hunger is attenuated, carbs are not necessary for improving performance during resistance training. 7 8
Henselmans et al. (2022) also wrote a review recently that both agrees and disagrees with our conclusions. The interested reader can compare our findings––there’s probably a little too much detail to go into here, but both studies are open access. 9 6
Additionally, we only found enough data in the literature that allowed us to look at the effects of carb ingestion on total session volume performance, there wasn’t enough data to look at other indices of training performance (e.g., maximal strength expression, power).
- Carbs before and during training are beneficial for volume performance, especially where the session is longer than 45 mins and where the fast before training is longer than 8 hrs.
- You could get carbs from a pre-exercise meal or from something like a sports drink during your training. Make yourself feel full for the training session!
- 25g of carbs have been demonstrated to improve high-volume resistance training performance, so this may be a good start in terms of minimum dose.
- Eating something will be better than nothing, sorry those who train fasted in the morning!
Thanks for reading. If you enjoyed this, you’d love the Vitamin
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