When it comes to fitness-boosting amino acids, BCAAs seem to get all of the attention. However, two other amino acids deserve some spotlight, too: l-arginine and l-citrulline. Here’s what you need to know about their unique benefits—and which to incorporate into your workout routine.
The Arginine And Citrulline Basics
Arginine and citrulline are both amino acids, the molecules that make up proteins. So, unsurprisingly, you’ll find both amino acids in a number of different foods.
While many protein-rich foods, like fish and nuts, contain arginine, citrulline is found in fruits and vegetables. (The food that contains the highest concentration of citrulline is watermelon.)
Though arginine and citrulline are different amino acids, there’s a reason we often talk about them together. In the body, they constantly convert back and forth from one into the other.
When we consume arginine, an enzyme called nitric oxide synthase (NOS for short) breaks it down into a gas called nitric oxide (NO for short) and citrulline. That citrulline then heads to the kidneys, which recycle it back into arginine so the cycle can continue.
When we consume citrulline, it travels straight to the kidneys to transform into arginine and start the same cycle.
The Performance-Enhancing Effects
That nitric oxide gas both arginine and citrulline eventually produce holds the ticket to their health and fitness benefits. Why? Because it increases blood flow throughout the body.
“Greater NO creates wider blood vessels,” explains Marie Spano M.S., R.D., C.S.C.S., C.S.S.D., sports nutritionist for the Atlanta Hawks, Braves, and Falcons. “This allows for more blood—and more oxygen and nutrient—delivery to muscles.” It also helps the body remove ammonia, a waste product created when we break down protein. Together, these factors help muscles function at their best and delay fatigue.
What does that mean for your workouts? Improved endurance, performance, and recovery, says Antonio Castillo M.S., R.D.N., L.D.N., sports dietitian for the Toronto Blue Jays.
Science backs this up: According to a 2016 study of 22 male cyclists published in the Journal of the International Society of Sports Nutrition, athletes who supplemented with 2.4 grams of citrulline for seven days saw an improvement in trial-time performance and reported less soreness after exercise. Meanwhile, 2017 study of 56 male soccer players published in the European Journal of Clinical Nutrition found that athletes who supplemented with two grams of arginine for 45 days saw significant improvements in their VO2 max (a measure of how efficiently the body uses oxygen).
The NO-producing amino acids offer similar benefits for strength-training athletes, too. One 2010 Journal of Strength and Conditioning Research study, for example, found that lifters who supplemented with eight grams of citrulline saw more than a 50 percent greater increase in barbell bench press repetitions than those who took a placebo. They also reported 40 percent less muscle soreness 24 and 48 hours after exercise.
Plus, bonus! Outside of the gym, research suggests nitric oxide also plays an important role in the bedroom. Case in point: One 2017 study published in Andrology found that a significant portion of men with erectile dysfunction had low arginine and citrulline levels.
Is One Better Than The Other?
Though research has found both arginine and citrulline to boost levels of nitric oxide in the body, most recent research—like this The Journal of Nutrition study—shows that citrulline actually delivers the most benefit.
“Citrulline expands blood vessels to a greater extent than arginine,” says Spano. Why? The body use arginine for a variety of functions, so it doesn’t use all of the arginine it absorbs to produce NO.
Plus, unlike citrulline, higher doses of arginine have been linked to gastrointestinal problems, says Castillo. Because it tends to be poorly absorbed, arginine can even lead to diarrhea when consumed in large amounts.
How To Pump Up Your Workouts With Citrulline
To pump up your performance—and how your muscles look in the mirror—consider adding extra citrulline to your pre-workout supplement routine. (There’s probably some in your go-to pre-workout formula already.)
Spano recommends starting with one gram of citrulline malate and working your way up to five or six grams if you don’t notice much effect at first. This form of citrulline may also help enhance the body’s ability to use oxygen to create ATP, the chemical it uses to produce energy.