The half marathon is the most popular of all road-running events, but many runners are confused about fueling—not just during the race but also the preparatory training sessions before the event. Should you be consuming in-race and in-training carbohydrate drinks and gels, or are they best reserved for the marathon distance?

Long-Term Burn

To answer this question, we need to understand two simple concepts about the energy demands of running a half marathon. For all athletic events lasting more than a few minutes, a large proportion of the energy is supplied from oxygen-fueled aerobic metabolism in the muscle cells, where carbohydrates and fat are combined to release the energy required for muscle contraction. Once the duration exceeds 30 minutes or so, nearly all energy is supplied this way. Given that the world record for the half marathon currently stands at 58:01, you can see that this event is fueled almost exclusively by aerobic metabolism—whether you’re an ultra-elite, middle-ranking or weekend novice runner.

Event Duration and Energy Source During Maximal-Effort Exercise

Event duration and energy source during maximal-effort exercise

Hybrid Fueling

The second concept concerns the ratio of fuels (carbohydrate and fat) that are oxidized in muscles during aerobic exercise. Research shows that once exercise intensity increases beyond “moderate,” the proportion of fat utilized for fuel declines, while carbohydrate utilization increases, until at hard or very hard intensities, nearly all of the energy to fuel exercise is derived from aerobic oxidation of carbohydrates.

Fat vs. Carbohydrate Burning at Different Exercise Intensities

Fat vs. carbohydrate burning at different exercise intensities
Illustration: Adapted from Asker Jeukendrup

More accomplished runners will almost certainly be working near lactate threshold throughout the half-marathon distance, relying mainly on stored muscle carbohydrates for their energy source. Novice and recreational runners with lower levels of aerobic fitness will tend to work at slightly less intensity, which in theory means they will derive a slightly higher proportion of energy from fat.

Just to confuse things however, the research also shows that more highly-trained an endurance athlete, the more fat he/she can utilize as fuel at higher exercise intensities compared to their less fit contemporaries. In reality, this means that both highly and lesser-trained athletes will utilize a little fat for fuel during a half marathon.

However, the bulk of energy will still be derived from carbohydrates—and this is what counts for performance. The pivotal role of carbohydrates—rather than fat— has been confirmed by research; during exercise at 85% of maximum aerobic capacity (not dissimilar to the intensity of a half marathon), carbohydrates are the primary fuel for exercise, contributing 83–91% to total energy expenditure—with only a small contribution from fat-based fuel.

Whether you’re a 1:15 or 2:15 half marathoner, carbohydrate fueling is critical to performance. That carbohydrate will consist mainly of stored muscle carbohydrates (glycogen).

Limited Storage Tank

The final factor to consider is the size of the body’s carbohydrate storage. Here the elite runner has a clear advantage because muscle glycogen storage is only enough to fuel around two hours’ worth of high-intensity exercise. The elite runner will still have plenty left in the tank as he/she crosses the finish line—unlike slower runners who will be on the road nearer to two hours and maybe longer. For these less-elite runners, ever lower muscle glycogen levels will likely result in heavy limbs and a noticeable increase in effort and heart rate for the same pace—inevitably forcing a reduction in pace.

Optimizing Carbohydrate Nutrition

Getting carbohydrate nutrition right is essential for running a good half marathon. Let’s see what this means in terms of the actual pre-race diet, the race itself and for training.

Pre-Race Diet

All runners, whether elite or non-elite should arrive at the start line with muscle glycogen stores fully topped up. This means consuming a carbohydrate-rich diet (high in bread, pasta, rice, potatoes, corn, oats, beans, lentils, etc.) in the two to three days before the race. This also assumes of course that you will be resting up during this period in order to conserve energy and be fresh for your race! And since glycogen is “fixed” into the muscles with water, it’s imperative that you also consume plenty of fluids and minimize or eliminate alcohol intake (which is dehydrating).

Needless to say, all runners should consume an easily digested but carbohydrate-rich breakfast on the morning of the race, preferably containing gently releasing energy foods, such as oat-based cereals, in order to avoid energy swings and troughs.

Exercise Duration and Carbohydrate Intake Recommendations

Exercise duration and carbohydrate intake recommendations
Illustration: Adapted from Asker Jeukendrup

In-Race Nutrition

Highly trained runners who are confident of completing the race in less than 75 minutes or so have no particular need to consume carbohydrate drinks or gels during the race to top up glycogen stores, although fluid might be desirable in hot conditions. Where race duration is expected to exceed 75 minutes, some modest use of carbohydrate drinks and gels (supplying around 30 grams per hour) is recommended to top up stores (see above). For race durations of 120+ minutes, carbohydrate supplementation supplying 60–75 grams per hour will help stave off the worst effects of glycogen depletion. The golden rule of any carbohydrate supplementation, however, is to try it in training first. Gels in particular may not be well tolerated by some runners, and research shows that the gastric distress they cause could lead to a slower rather than faster half marathon time.

Half Marathon Training Nutrition

During preparation, most runners’ training sessions will be of shorter duration than their anticipated race time—thus carbohydrate supplementation is usually not necessary. However, the use of carbohydrate drinks and gels is popular during training on the basis of helping to keep muscles “topped up” and so reduce the likelihood of fatigue in subsequent sessions. There’s some merit in this, particularly for slower runners performing longer sessions, or where it’s difficult to consume a carbohydrate-rich meal after training.

There’s good reason, however, not to habitually use carbohydrates during training: When carbohydrates are freely available, muscles will become less efficient at fat burning. While your fat-burning capacity is not (as we’ve seen) a deal breaker for half-marathon performance, performing some regular training sessions that enhance fat burning is good for reducing body fat and increasing sustainable power-to-weight ratio. All things being equal, lower levels of body fat (and therefore weight) will reduce your energy expenditure and oxygen consumption at any given pace—or, to put it another way, your maximum sustainable pace will be higher. It’s no coincidence that all elite runners have low body fat levels and excellent power-to-weight ratios!

Other Factors that Affect Fueling and Performance

Although carbohydrate nutrition is critical for half-marathon performance, don’t forget about the importance of hydration, especially in warm conditions. Drink plenty of fluids prior to the race to ensure you’re fully hydrated at the start. Studies suggest that you drink to thirst for any event lasting 60–90 minutes so runners completing the race in less than 75 minutes should have no real need to drink unless they feel thirsty. However, slower runners taking more than 90 minutes should plan to consume fluids on the run, even in more temperate conditions.

For both elite and non-elite runners, pre-race caffeine is a proven ergogenic aid, helping to stave off fatigue, thereby extending endurance, and should be considered as a useful nutritional tool for half-marathon performance.

Pre-race nitrate (in the form of beetroot juice) appears to enhance sub-maximal endurance performance, which could particularly benefit slower runners, but it seems less effective for more elite athletes. Don’t forget the golden rule though—try any supplementation strategies out in training before using them in a race!