Introduction
When it comes to carbohydrates, opinions can swing from one extreme to the other. Some claim you don’t need carbohydrates, while others promote complex pre-, intra-, and post-workout strategies for a 30-minute gym session. What does the research say about carbohydrate timing? The answer is more nuanced than you might expect. This article will help you understand how carbohydrates support your training and when to time them.
Let’s dig in.
A brief look at how carbohydrates can fuel training
Let’s start with how carbohydrates fuel exercise.
When you eat carbohydrates, they break down into glucose, which enters your bloodstream and can be used for energy during training. Carbohydrates are particularly efficient compared to fat (they yield more ATP per oxygen molecule during aerobic metabolism), and they provide a substrate for anaerobic metabolism as exercise intensity increases even further.
Your body stores carbohydrates as glycogen in your muscles and liver, which is an energy source during exercise. As glycogen stores deplete, performance tends to suffer. Consuming carbs before or during training can help maintain glycogen levels and sustain your ability to train. This is especially helpful when dealing with high-volume or prolonged training sessions.
The main takeaway: Carbohydrate intake before and during exercise helps you make better use of stored glycogen and keeps performance from tanking –– especially when training volume is high.
Note that carbohydrate timing is somewhat individual. Some people may benefit from tweaking their intake around workouts. In contrast, others will see similar results by focusing on overall carbohydrate and Calorie intake. For longer or more intense training, intra-workout fueling can help you sustain performance more effectively.
These next few sections will cover a few details to help you make the best decision regarding your type of training and carbohydrate needs.
Resistance training
Carbohydrate intake is less of a concern if your program involves lower volume, fewer sets, and long rest periods between lifts. This is especially true if your resistance program allows you to rest without much high-rep or cardio/conditioning work involved. So, the first thing to consider regarding carbohydrate timing is how much volume you’re getting in your training sessions (or if you’re throwing in conditioning or cardio within your sessions).
A meta-analysis found that for most resistance sessions — particularly those with 10 or fewer sets per muscle group — carbs didn’t provide a noticeable boost to performance. However, when training volume increases above 10 working sets, the benefits of carbohydrate intake become more evident. In these cases, athletes could perform more reps and sustain higher total work output, especially when glycogen levels were low, or they were in a fasted state. This aligns with findings from keto studies during more strength-based sessions, suggesting that carbohydrates overall may not play as significant a role in low-to-moderate volume resistance training programs.
In muscle building, it could get a little more nuanced. There’s an ongoing debate around training volume, set/rep ranges, and their interaction with glycogen stores. Higher set and rep ranges, or taking multiple sets to failure, can lead to greater glycogen depletion. So, the variability in depletion depends on factors like training intensity and exercise selection, which is why the evidence on carbohydrate timing is mixed.
A systematic review and meta-analysis on the effects of carbohydrate ingestion on resistance training performance found that carbohydrate intake enhances total training volume, particularly in sessions lasting longer than 45 minutes or following an 8-hour fast. This suggests consuming carbohydrates before longer resistance training sessions (e.g., 45+ minutes) could improve performance.
Lastly, studies indicate that consuming carbs within a 1-4 hour window before a workout — versus training in a fasted state — can offer advantages. Fun fact: Even consuming something as simple as a viscous placebo goo before a session can help enhance performance. So, we are still learning the depths of these details.
Ultimately, the takeaway is that while carb timing isn’t always essential for general resistance training, it becomes increasingly important when the training volume ramps up. For muscle-building athletes focusing on higher-volume sessions, paying attention to carbohydrate intake around the workout window could help support both performance and recovery.
Endurance
As stated, glycogen is a key fuel source for endurance performance, especially as exercise volume and intensity increase. Low-intensity sessions under 40 minutes likely don’t require additional carbohydrates. However, higher-intensity or longer sessions, where glycogen depletes more quickly or to a greater extent, benefit from carbohydrate intake. In other words, short, low-intensity workouts may not need extra carbs, but as intensity and duration increase, so does the need for fueling — ranging from 0g/hour for low-intensity sessions to up to 80g/hour for longer, race-pace endurance events.
A quick caveat: This article doesn’t delve into the wide variety of specific protocols of carbohydrate loading or training strategies leading up to events or races. While there is clear evidence that maintaining elevated glycogen levels can enhance performance, the focus here is on broader dietary considerations rather than prescribing exact strategies. For that, you’d really need to work with a dietetic advisor in that sport.
Regarding carbohydrate timing and amounts, Newell et al found that trained cyclists who consumed between 20-64g of carbohydrates per hour during a two-hour ride showed improved performance compared to those who consumed none. While higher intakes, like 64g/hour, provided marginal benefits, they also increased the risk of gastrointestinal discomfort.
Smith et al found that around 78g/hour of carbohydrates maximized performance benefits for endurance athletes, but going beyond that could lead to diminishing returns and digestive issues. Similarly, Podlogar et al showed that higher intakes, up to 120g/hour, increased carbohydrate oxidation but didn’t spare glycogen or result in significant performance gains compared to 90g/hour.
For more on optimizing performance, a systematic review, meta-analysis, and meta-regression found that glycogen depletion plays an important role in endurance performance, particularly during moderate to high-intensity exercise lasting over two hours. Carbohydrate intake was more beneficial in these longer efforts, with a greater impact seen in less-trained participants. Note this may be because more trained athletes are better adapted to storing glycogen and may rely more on fat for fuel during prolonged activity.
Take home? The longer or more intense your endurance training, the more important carbohydrate intake becomes. For most people, 30-60g of carbohydrates per hour should suffice during prolonged endurance efforts. Some individuals may tolerate up to 90g/hour but should monitor for gastrointestinal discomfort and diminishing returns.
Recap on carbohydrate recommendation for timing in a maintenance state during different training programs
| Training type | Carbohydrate timing importance | Recommendations |
|---|---|---|
| Resistance training (low-volume) | Carbohydrate intake before longer, higher-volume sessions (e.g., 45+ minutes or after fasting) can improve performance. Aim for roughly ~3-4 g/kg of body weight daily. Consider consuming carbs within a 1-4 hour window before glycogen-depleting sessions. | Carbohydrate timing is less important due to longer rest periods and lower total volume. Focus on meeting daily carbohydrate and Calorie needs rather than pre-workout timing. |
| Resistance training (higher-volume) | Moderate to high | Carbohydrate intake before longer, higher-volume sessions (e.g., 45+ minutes or after fasting) can improve performance. Aim for roughly ~3-4 g/kg of body weight daily Consider consuming carbs within a 1-4 hour window before glycogen-depleting sessions. |
| Endurance training | Moderate to high | Aim for 30-60g of carbohydrates per hour during exercise and increase only if you’re noting performance enhancements and no digestive disturbance. Aim for roughly ~6 g/kg of body weight daily Carb intake is particularly important for sessions lasting over 2 hours. |
Calorie restriction: Overall performance
In an energy-restricted state, glycogen levels begin to deplete. While timing meals and carbohydrates around workouts can help replenish those stores, it may become harder to restore glycogen over time. Interestingly, short-term performance doesn’t seem to suffer too much. However, it’s important to note that early on, glycogen can be drawn from either the liver or muscles, but as you continue to rely on these stores, performance can eventually be affected, even if only intermittently.
As I already discussed with resistance training, at least in the short-term, energy restriction has fewer detrimental effects on performance. Regarding muscle gain, it’s widely accepted that overall caloric restriction negatively impacts muscle growth. However, even in older populations, resistance training can help offset muscle loss. So, while you’re not in the most optimal state for growth during caloric restriction, resistance training remains important for muscle preservation.
But the question is: Would timing carbohydrates around workouts, even if it’s just to boost circulating plasma glucose, help during energy deficits?
It falls into the category of, “It might help, and it probably won’t hurt.” So it’s worth giving it a shot. Even in the previously discussed meta-analysis that framed carbohydrate timing as not being particularly important for resistance performance, the authors concluded that consuming carbs before a workout could boost available blood glucose and reduce the need to tap into depleted glycogen stores, which may support performance, especially in energy-restricted states.
For endurance athletes, performance can be more affected as energy intake becomes restricted over time. A mini-review on male endurance athletes found that long-term energy restriction led to worsened performance and decreased recovery. For female endurance athletes, reduced energy intake not only impacts performance but can also increase distress markers.
So what’s the take-home for carbohydrate timing and caloric restriction? Carbohydrate timing might be helpful when glycogen is low and your training shifts toward higher volume or endurance work. However, the overall energy deficit will have a more substantial effect over time, regardless of carbohydrate timing.
Carbohydrate timing and sleep quality
Another factor to consider with carbohydrate timing is its potential effect on sleep. Evidence suggests that eating a large meal before bed can negatively impact sleep, mainly by causing digestive discomfort or leading to more frequent night awakenings. This really applies to any macronutrient, not just carbohydrates. It’s fairly intuitive — if you eat too much before lying down, you might fall asleep, but you’ll likely wake up during the night.
Carbohydrate timing throughout the day and evening becomes more nuanced. The dose and timing of carbohydrate intake can influence sleep quality, but the effects can vary. For example, a study by Yajima et al found that consuming a low-fiber, high-carbohydrate meal four hours before bedtime reduced slow-wave sleep (a phase important for recovery) and increased sleep latency (the time it takes to fall asleep). However, other studies have found conflicting results, showing the complexity of how carbohydrate intake can affect sleep.
A systematic review, meta-analysis, and meta-regression on carbohydrates and sleep revealed a similarly complex relationship. It found that higher carbohydrate intake can prolong REM sleep, but this may come at the expense of reducing deep sleep (slow-wave sleep), which is important for physical recovery. On the other hand, lower carbohydrate intake tends to increase the amount of slow-wave sleep but reduces REM sleep. So, mixed.
It’s a bit of a trade-off: while higher carbohydrate intake might extend overall sleep duration in some cases, there’s also evidence that it could lead to more frequent awakenings and reduce overall sleep quality. While carbohydrates might enhance the amount of sleep you get, they could compromise how restful sleep is by impacting sleep efficiency. However, so could having too few.
Carbohydrates can affect sleep differently depending on the dose, timing, and even the specific sleep phase you’re trying to optimize. The presence of carbohydrates seems more net positive for sleep, but timing them a little earlier in the evening, mixed with a fibrous intake, would probably get you the best outcome. This way, you get the benefits of a diet with a solid presence of carbohydrates, but maybe not too much, too close to bed.
Recap
Carbohydrate timing is unique to each person’s situation. Its importance depends on your training goals, overall diet, and what works best for your schedule.
Key takeaways:
- Resistance training: Carbohydrate timing is less important, especially with lower volume and longer rest periods. Hitting your daily carbohydrate and Calorie targets is usually enough. However, carbohydrates may become more relevant when training volume increases.
- Endurance training: Carb timing is more important here. Aim for 30-90g of carbohydrates per hour during longer sessions, with a daily floor target of around 6g/kg of body weight to maintain glycogen stores.
- Calorie restriction: In a Calorie deficit, timing carbohydrates around workouts can help maintain performance when glycogen levels are low, particularly for endurance athletes or during high-volume training. However, the overall energy deficit will ultimately have a greater impact.
- Sleep quality: Carbohydrates might increase total sleep duration, but they could also disrupt sleep. The right balance and intake timing might be earlier in the evening with fiber to maximize the benefits.
