INVESTIGATION OF THE EFFECT OF L-CARNITINE SUPPLEMENTS IN ATHLETES ON MUSCLE DAMAGE AND DELAYED ONSET MUSCLE PAIN (DOMS) AFTER HIGH INTENSITY INTERVAL TRAINING
Phase: NA
Conditions: Recovery Methods, Exercise and Recovery, Exercise Fatigue, Carnitine Ingestion, Exercise Ergogenics
Sponsor: Istanbul Sabahattin Zaim University

Study Type:  Interventional  
Actual Enrollment:  205 participants  
Allocation:  Randomized  
Intervention Model:  Parallel Assignment  
Masking:  Double (Participant, Investigator)  
Primary Purpose:  Treatment  
Official Title:  Effects of Carnitine Supplementation on Exercise-Induced Muscle Damage: A 
Randomized Controlled Trial 
Actual Study Start Date:  May 9, 2025 
Actual Primary Completion Date:  May 15, 2025 
Actual Study Completion Date:  June 5, 2025 
 

 
METHOD 
Participants 
Initially, 30 participants volunteered to take part in the study. However, participants numbered 
17, 18 and 30 withdrew before the pre-test assessments. Participant number 25 withdrew after 
the second set of the high-intensity interval exercise protocol during the pre-test session, and 
participant number 12 withdrew before the post-test measurements. Therefore, the study was 
completed with 25 participants (mean age = 20.48 ± 1.61 years; mean training age = 9.04 ± 
3.78 years). Participants were informed about the study protocol before the study began and 
were informed that they could withdraw from the study at any time during any part of the study. 
Participants who had clinically significant circulatory, respiratory, digestive, urinary, and 
nervous system disorders, who had a history of any muscle damage within the last week before 
the study day, using “cold and flu” medications and vitamins/foods/supplements containing 
antioxidants, who took food and supplement products with anti-inflammatory effects, or who 
used any commercially available supplements/ food/ nutritional products (creatine, protein 
drinks, amino acids, vitamins, etc.) for performance enhancement were not included in the 
study. Participants were instructed to avoid unusual exercises and resistance exercises 
(especially exercises involving eccentric contractions) for 1 week prior to the study protocol to 
prevent any muscle damage. Participants were randomly assigned to receive either 3 g/day of 
L-Carnitine (Group LC; n=13) or a placebo (Group PLS (CHO); n=12) for 21 days. The 
experimental design process for conducting the study is presented in figure 1. Before the study, 
participants were asked to fill out a questionnaire regarding their eating habits, medical history 
and supplement use (Supplementary File 2). This study was conducted in accordance with the 
Declaration of Helsinki and was approved by the Istanbul Medipol University Clinical Research 
Ethics Committee (approval number/date: E-95961207-604.01.01-2244/30 March 2023).  This 
research was supported by Kutahya Dumlupınar University Scientific Research Projects 
Coordination Office under grant number #2024-24. 

 
Figure 1. Consolidated standards of reporting trials (CONSORT) flow diagram of study participation from 
enrollment to analysis.  
LC: L-Carnitine 
 
Study Design 
This randomized, double-blind, placebo-controlled study was conducted to investigate the 
effects of L-carnitine supplementation on markers of muscle damage and delayed onset muscle 
soreness following high-intensity interval training in trained athletes. The study was reported 
in accordance with CONSORT guidelines for randomized controlled trials. The exercise 
protocol, anthropometric measurements ( and supplement protocol of the study were carried 
out by the researchers at the Exercise Laboratory of Istanbul Sabahattin Zaim University 
Faculty of Sports Sciences. The study consisted of two stages. On the first day of the visit 
(Figure 2), the participants' height (Tarti Telescopic Height Meter) and weight (Tanita MC 780 
ST Black) measurements were determined. In the next stage, before the supplementation period, 
resting venous blood samples of the participants were taken from the antecubital vein by a 
specialist nurse working at Istanbul Sabahattin Zaim University Health Unit. Following the 

blood collection process, the randomization processes of the participants were carried out. For 
this purpose, written numbers from 1 to 24 were placed inside a box in a way that the 
participants could not see, and the participants were asked to choose one of the numbers at 
random, accompanied by an independent observer other than the researcher. This process was 
recorded by the observer both on the survey form and digitally. The second day of the visit took 
place after a 21-day reinforcement period (Figure 2), blood samples were collected from the 
participants by a healthcare professional at rest, 2 h, 24 h, 48 h and 72 h after exercise, and were 
centrifuged by the researcher and stored at -20 degrees. 
 
 
Figure 2. Experimental Design Scheme for conducting the study 
High-Intensity Interval Training Protocol 
During the pre- and post-supplementation period, participants performed a high-intensity 
interval training protocol to exhaustion on a Wingate-based cycle ergometer (Monark 894E) at 
a workload of 0.75 g/kg watts, consisting of 30-s bouts of work divided by 4-min recovery 
intervals (Figure 2). Blood samples were taken from the fingertip to measure blood lactate 
levels immediately before the test and at the 1st, 3rd, 5th, 7th, 9th and 11th minutes after the 
end of the test. A lactate analyzer (LactateScout (+)) was used to measure blood lactate levels. 
In addition, participants were asked to rate the feeling of fatigue they felt on the Borg Scale 

(BorgScale 6-20) to determine the exercise intensity level at the end of each HIIT set (Figure 
2). 
Supplementation Protocol 
Although the optimal dose and form of L-carnitine administration are unclear, in a review 
article examining the pharmacokinetics of L-carnitine, the Alsuntangled Group (2020) and 
Evan and Fornasini (2003) reported that L-carnitine would be relatively ineffective in 
increasing plasma levels unless doses of 2–4 g were used daily due to its low bioavailability 
[36,37]. Additionally, pharmacokinetic data support the administration of multiple daily doses 
of LC. Therefore, in our study, participants in group LC were given 3 g/day of supplementation 
(1 g LC capsules three times daily at 8-hour intervals) for 3 weeks (SepeNatural, L-Carnitine 
L-Tartrate, China). The product contains L-carnitine (% 68), L-Tartaric acid (% 31) and a very 
small amount of chloride. This carnitine dose was chosen to maximize plasma carnitine 
concentrations without exceeding the renal threshold for carnitine [29]. The PLS group was 
given powdered sugar at the same rates and intervals (1 g CHO capsules three times daily at 8-
hour intervals). The usage instructions for each participant were written inside the numbered 
boxes. The capsules containing L-carnitine and placebo to be given to the participants were 
filled manually by the researcher using a precision scale (Neck; MH 3000 Model, 0.01 Gr 
sensitivity). All capsules belonging to the PLS and LC groups had the same appearance, color 
and dimension. During the study, the researcher made reminders every 8 hours via WhatsApp 
to monitor the participants' use of carnitine and placebo, and additional precautions were taken 
with alarms set by the participants every 8 hours. The products used in each session were taken 
after a full stomach. Additionally, no adverse effects related to the supplement were reported 
by the participants in this study. 
Determination of Muscle Pain Sensation 
Muscle pain sensation in the leg extensor and leg flexor muscles was determined using the 
Numbered Pain Rating Scale (NPRS), one of the most widely used scales due to its simplicity 
and practicality [38-40]. The NPRS is a scoring method where a score of zero represents no 
pain and a score of 10 represents maximum pain. While the NPRS has been validated on 
numerous patients as a measurement tool for pain intensity [41], its use as a screening tool has 
not yet been tested [41]. In the application, the patient is asked to rate the severity of their pain. 
This score is used as the pain score. In this study, we classified the scale scores as mild (1-3), 
moderate (4-6), or severe (7-10) as previously described [39,42].  Following 21 days of 
supplementation, participants were asked to quantify their pain sensations using the NPRS 

during blood sample collection at 2, 24, 48, and 72 hours after the HIIT protocol. Feedback 
from participants was recorded on the questionnaire by the supervisor. 
Blood Sampling and Analysis 
Total carnitine blood sample was collected from the antecubital vein at rest before the 21-day 
LC supplementation period by a specialist nurse at Istanbul Sabahattin Zaim University Health 
Unit, as well as immediately before HIIT and at 2 hours, 24 hours, 48 hours and 72 hours after 
HIIT following the 21 days of supplementation (CK,LDH,). The collected blood samples were 
kept at room temperature for approximately 30 minutes and then centrifuged at +4°C, 3000 
rpm, for 20 minutes using a centrifugal machine (Hitachi Koki Himac CR22N High-Speed 
Refrigerated Centrifuge, cCS Aus model/913350C2). After the centrifugation process, the 
transparent liquid remaining in the tube was transferred to 2 ml Eppendorf tubes, then stored at 
-20 C and analyzed in the laboratory. In addition, following 21 days of supplementation period, 
blood lactate levels were determined from the fingertip immediately before the HIIT test and at 
the 1st, 3rd, 5th, 7th, 9th and 11th minutes after the end of the test.  
 
Human Total Carnitine Measurement from Serum 
Total carnitine levels in serum samples were measured using a Human Total Carnitine ELISA 
kit (Bioassay Technology Laboratory, Shanghai, China). Absorbance readings were obtained 
with a Chromate 4300 microplate reader (Awareness Technology, Inc., Martin Hwy., Palm 
City, USA). Results were expressed in nmol/ml. Data were calculated using linear regression, 
with absorbance measured at 450 nm.  
Measurement of CRP, CK, LDH and Uric Acid 
Serum CK and LDH were analyzed on a fully automatic analyzer (Roche Cobas Integra 
400 Plus, Roche Diagnostics GmbH, Mannheim, Germany) using ROCHE kits (Mannheim, 
Germany).  
Statistical Data Analysis 
Data obtained using a double-blind placebo control group experimental design was analyzed 
using SPSS 25.0 statistical software. The analysis process began by assessing whether the data 
exhibited a normal distribution. Normality of the data was assessed using the Shapiro–Wilk 
test, as well as skewness and kurtosis values. Homogeneity of variances was examined using 
Levene’s test. A two-way repeated measures ANOVA (condition × time) was performed to 

determine the effects of L-Carnitine (LC) and placebo (PLS) supplementation on BMI, body 
fat percentage, lean body mass, resting heart rate, blood lactate concentration, delayed onset 
muscle soreness (DOMS), and muscle damage markers (CK, LDH) across different time points. 
When significant main or interaction effects were found, pairwise comparisons were conducted 
using the Bonferroni post-hoc test. All data are presented as mean ± standard deviation (SD). 
Statistical significance was accepted at p < 0.05.