Elite marathoner Tina Muir is gunning for a personal best this December when she toes the line at the California International Marathon in Sacramento. This will be her fifth go at the distance. She continuously works to dial in exactly the training and nutrition approaches that will help her shave precious minutes off the clock. Her latest addition to the regimen? Caffeinated gum.

One of the newer entrants to the burgeoning sports-nutrition market — expected to reach $52 billion by 2020, according to Transparency Market Research — caffeinated gum promises to deliver a faster burst of energy than other ergogenic aids because it doesn't have to pass through the digestive system. Muir is a believer.

"I have tried caffeinated gels before and liked them, but they took a while to take effect," she says. "It's tough to wait 10 minutes when you are tired and in need of a boost in the later miles of a race."

The gum, she says, hits her system right away. "I took the gum at mile 14 of a 20-mile progression run, where I had to continuously pick up my pace," she explains. "I noticed an immediate, significant difference with the gum. In particular, I was clear-minded and could tackle any doubts I had at that point in the run."

Muir is experiencing exactly what she should with the gum, says Kristina LaRue, an Orlando-based registered dietitian specializing in sports nutrition. "Caffeine is a well-studied performance enhancer," she says. "It can make the load feel easier and does help with focus and clarity."

Up until 2004 the World Anti Doping Agency placed limits on caffeine intake, but it is so ubiquitous that it dropped the limit. It is still, however, banned at the NCAA level.

Carl Paton, an associate professor at New Zealand's Eastern Institute of Technology, has conducted three studies on caffeinated gum and its impact on performance in endurance sports. "You get similar effects to gels and tablets, but it's absorbed faster, and you may not need as big of a dose," he says.

He concurs on Muir's also experiencing a mental boost. "Caffeine has the benefit of reducing the perception of fatigue, so you can potentially run harder with it in your system," he says.

In a 2014 study on the impact of the gum on the race performances of male and female cyclists, Paton found that the biggest gain came in the final 10 kilometers of a 30K time trial. "The gum improved both endurance and sprinting power at the end of the effort, most likely through an increase in nervous system activation," he says.

A separate study he conducted in 2010 on male cyclists also found that the caffeinated gum delayed fatigue during repeated, high-intensity sprint exercise. In this study, researchers measured testosterone and cortisol concentrations and determined that the gum elevated the former while reducing the latter. Higher levels of testosterone are shown to improve performance, while cortisol is a stress hormone that has been shown to hurt it.

It is for all these reasons that Olympic runner Nick Symmonds developed a caffeinated gum targeted to endurance athletes. He likens his "Run Gum" to an energy drink such as Red Bull, without any of the potential digestive distress that sometimes accompanies such performance aids. "You want to improve performance, but you don't want to fill up," he says. "Gum is a better delivery vehicle."

Like Muir, 52-year-old runner Matt Ingram, of Plano, Texas, is a recent convert to the gum. "I've never been a big supplement person," he says, "but the gum can make the first few steps a little bit easier for me."

Ingram competes in 400-meter and 800-meter masters' races on the track, with the occasional 5K on the docket in the off-season. "I use the gum both in training and in racing," he says. "I'll chew it about 10 to 15 minutes ahead of time and can feel its effects right away."

For all its upsides, however, Paton cautions athletes that they shouldn't depend solely on caffeinated gum.

"In a major endurance event, you can't survive on the gum alone," he says. "You'll need carbohydrates, too."

Muir understands this and is using the gum as an additional aid to her established carbohydrate intake.

"I use my normal drink as my fuel and the gum as an energy boost later in the run," she says.

For Muir, the only downside was that she found the gum to be a bit crumbly.

"It could have been easy to panic when trying to chew it and breathing hard as it crumbled," she says. "But I was able to push it to the side of my mouth, catch my breath, and then chew it again. When you're going long, the effects of a cup of coffee are long gone at mile 20. This allows you to benefit from those effects just when you need them."