Beetroot and citrulline: a recipe for combination

Evidence for citrulline

Like nitrate-rich foods and extracts (eg from beetroot) there’s good evidence that citrulline supplementation can produce performance benefits via an increased production of NO, despite there being fewer studies carried out:

·        A 2020 study found that six grams per day of citrulline supplementation for seven days increased exercise tolerance and total work on aerobic power performance⁽¹¹⁾.

·        A 2016 study found that 2.4 grams per day of citrulline supplementation increased VO2 uptake kinetics (the rate at which oxygen delivery gets up to speed when exercise is started) and reduced the time needed to complete a 4km time trial⁽¹²⁾.

·        Studies show that citrulline can stimulate a key signalling pathway called mTOR pathway, which helps increase the activity of genes involved in muscle protein synthesis⁽¹³⁾, helping to explain why citrulline ingestion has been shown to induce increases in isometric muscle strength levels when taken in conjunction with strength training⁽¹⁴⁾.

·        Along the same lines, a single dose of citrulline (8 grams) supplementation prior to exercise was shown to improve the number of bench press repetitions performed at 80% at 1-rep max⁽¹²⁾, and significantly reduce the sensation of fatigue⁽¹⁵⁾.

Are two supplements better than one?

The evidence for beetroot/nitrate and citrulline mediated performance gains (via increased NO production and muscle blood flow) is certainly persuasive. An obvious question then is whether a combination of the two offers even greater benefits – a kind of double-whammy effect? To date, there has been virtually no research to answer this question. Studies have been carried out aimed at comparing these two supplements for their ability to improve resistance training performance (with inconclusive results)^(16,17). However, there has only been one study to date looking at the effects of a combination of citrulline and beetroot on performance.

Published in the journal ‘Nutrients’, this study investigated the effects of long-term (nine weeks) combined oral supplementation with 3 grams per day of citrulline plus 2.1 grams per day beetroot extract (containing 300mg/day of nitrate) in 32 male well-trained triathletes⁽¹⁸⁾. In this study, the researchers split the athletes into four groups:

·        A citrulline group – consuming 3 grams of citrulline per day.

·        A beetroot extract group – consuming 2.1 grams per day of beetroot extract providing 300mgs per day of nitrate.

·        Citrulline + beetroot group - consuming 3 grams of citrulline plus 2.1 grams of beetroot extract per day.

·        A placebo goup – consuming inert cellulose capsules.

What they found was that the ‘beetroot’ and the ‘citrulline + beetroot’ groups significantly improved their aerobic performances in the 12-minute Cooper test compared to placebo and citrulline-only groups. The gains in the citrulline + beetroot group were particularly impressive. However, only the citrulline + beetroot group displayed a significant increase in a jump height test and a 1-minute abdominal sit ups endurance test.

Latest research

Since that study on triathletes, there have been no further studies on combining citrulline and beetroot to enhance performance. But just a couple of weeks ago, a team of Spanish researchers published their findings on combined citrulline plus beetroot supplementation in elite rowers⁽¹⁹⁾. Published in the European Journal of Apllied Physiology, this study was slightly different in that it used higher supplemental doses of beetroot/nitrate (3.5 grams per day delivering 500mgs of nitrate per day) and citrulline (6.0 grams per day), but for a shorter period – one week instead of nine weeks.

Another difference in this study design was that it compared placebo to beetroot-only to beetroot plus citrulline (ie no citrulline-only group). In addition, it used a crossover design, meaning that all 20 rowers participating went through the protocol three times, once in each condition (ie those who started with placebo switched to beetroot a week later and then switch to beetroot plus citrulline a week after that – and so on).Before and after each of the three trial weeks, the rowers were assessed for the their peak oxygen uptake (VO2max) in a rowing performance test, and their countermovement jump heights. Figure 2 shows a schematic of the study design. Blood tests were also conducted to measure lactate levels before and after exercise.

Figure 2: Rowing study design

What they found

When the data was analyzed, there were two main conclusions. Firstly, in the aerobic power test, the rowers were able to clear raised levels of blood lactate more rapidly than when they took beetroot alone or the placebo. Secondly and more importantly, in the rowing performance test, the beetroot-citrulline combination allowed the rowers to generate higher levels of peak power compared to the placebo condition (see figure 3). This was despite the fact that their heart rates and maximum oxygen uptake levels were not significantly different from the placebo and beetroot-only trials. When it came to the countermovement jump test, while the beetroot-citrulline condition produced slightly higher figures, the difference was insignificant (ie could have occurred just through chance).

Figure 3: Rowing test peak power, oxygen uptake levels and heart rates

Advice for athletes

What do these findings mean for athletes? If you use beetroot supplements, should you add citrulline to enhance the NO releasing effect? In the latest study above, peak power in the rowing test and lactate clearance improved with the beetroot-citrulline combination – despite oxygen uptakes and heart rates being no different. However, this is consistent with the improved muscle blood flow effect, which allows muscles to operate more efficiently.

It’s true that the researchers were expecting to see greater benefits from the beetroot-citrulline combination. However, they pointed out that the 1-week supplementation period was quite short compared to other studies, and that more time may have been needed to see greater benefits. Also, the rowers were not just highly trained (as in the triathlete study) but were rowing at the elite level. All twenty rowers were rowing in the first rowing professional Spanish league ‘Asociación de Clubes de Traineras’ with an average of 14 years of experience in rowing. This is an important point since beetroot/nitrate supplementation to improve muscle efficiency appears to be less beneficial in elite athletes, who already have muscles with superb blood flow, and which work extremely efficiently^(20,21)!

Given the sum of evidence to date, and that citrulline is natural amino acid, inexpensive and non-toxic even at high doses, athletes who want to try adding it to a beetroot supplementation routine would appear to have nothing to lose – especially recreational-level athletes. This is especially the case for athletes who often suffer from gastric distress; that’s because citrulline supplementation has also been shown to help reduce the phenomenon of ‘splanchnic hypoperfusion’ - a condition where the normal patterns of blood flow to the intestines and organs in the abdominal region are disturbed, for example during vigorous exercise⁽²²⁾.

If you decide to try adding citrulline to a beetroot supplementation, it may be better to opt for a lower dose (around 3 grams per day) over a longer time span rather than high doses for a few days. The most popular form of citrulline is ‘citrulline malate’, which can be purchased cheaply from sports shops and online. When purchasing however, it is recommended not to go for the very cheapest online product, but to stick with a good-quality brand name, where product purity can be guaranteed. If using a powder, citrulline malate can be dissolved fairly easily and has a not unpleasant ‘tangy’ taste, which makes it go well in fruit juices. Just be aware however that for those prone to heartburn, high doses can be a bit acidic on the stomach!

References

1.       Pharmacol Rev 1991;43:109-42

2.       J Appl Physiol. 2009 Oct;107(4):1144-55

3.       Med Sci Sports Exerc. 2011 Jun;43(6):1125-31

4.       Int J Sport Nutr Exerc Metab. 2012 Feb;22(1):64-71

5.       J Acad Nutr Diet. 2012 Apr;112(4):548-52

6.       Nutrients. 2020 Jun 28;12(7):1912

7.       J Sci Med Sport. 2021 Jan;24(1):80-84

8.       Sports Health. 2022 May 21;19417381221083590

9.       Nutrition. 2013 Mar; 29(3):479-84

10.     Amino Acids. 2005 Nov;29(3):177-205

11.     J Strength Cond Res. 2020 Mar; 34(3):647-652

12.     J Int Soc Sports Nutr. 2016; 13():6

13.     Am J Physiol Endocrinol Metab. 2012 Nov 1; 303(9):E1117-25

14.     Curr Opin Clin Nutr Metab Care. 2017 Jan; 20(1):92-98

15.     J Diet Suppl. 2016; 13(3):269-82

16.     J Strength Cond Res. 2019 Sep;33(9):2321-2329

17.     J Diet Suppl. 2020;17(6):698-717

18.     Nutrients. 2022 Jan; 14(1): 40

19.     Eur J Appl Physiol. 2024 Feb 10. doi: 10.1007/s00421-024-05415-4. Online ahead of print

20.     Scand J Med Sci Sports. 2013 Feb;23(1):e21-31

21.     Int J Sport Nutr Exerc Metab. 2017 Jun;27(3):255-263

22.     Med Sci Sports Exerc. 2014 Nov;46(11):2039-46