Effect of p-Synephrine on Fat Oxidation Rate during Exercise of Increasing Intensity in Healthy Active Women

Abstract

p-Synephrine is the principal alkaloid of bitter orange (Citrus aurantium). Several recent investigations have found that the intake of 2–3 mg/kg of p-synephrine raises fat oxidation rate during exercise of low-to-moderate intensity. However, these investigations have been carried out only with samples of male participants or mixed men/women samples. Therefore, the aim of this investigation was to study the effect of p-synephrine intake on fat oxidation during exercise of increasing intensity in healthy women. Using a double-blind, randomized experiment, 18 healthy recreationally active women performed two identical exercise trials after the ingestion of (a) 3 mg/kg of p-synephrine and (b) 3 mg/kg of a placebo (cellulose). The exercise trials consisted of a ramp test (from 30 to 80% of maximal oxygen uptake; VO2max) on a cycle ergometer while substrate oxidation rates were measured at each workload by indirect calorimetry. In comparison to the placebo, the intake of p-synephrine increased resting tympanic temperature (36.1 ± 0.5 vs. 36.4 ± 0.4 °C p = 0.033, d = 0.87) with no effect on resting heart rate (p = 0.111) and systolic (p = 0.994) and diastolic blood pressure (p = 0.751). During exercise, there was no significant effect of p-synephrine on fat oxidation rate (F = 0.517; p = 0.484), carbohydrate oxidation rate (F = 0.730; p = 0.795), energy expenditure rate (F = 0.480; p = 0.833), heart rate (F = 4.269; p = 0.068) and participant’s perceived exertion (F = 0.337; p = 0.580). The maximal rate of fat oxidation with placebo was 0.26 ± 0.10 g/min and it was similar with p-synephrine (0.28 ± 0.08 g/min, p = 0.449, d = 0.21). An acute intake of 3 mg/kg of p-synephrine before exercise did not modify energy expenditure and substrate oxidation during submaximal aerobic exercise in healthy active women. It is likely that the increase in resting tympanic temperature induced by p-synephrine hindered the effect of this substance on fat utilization during exercise in healthy active women.

Does the Time of Day Play a Role in the Acute Effect of p-Synephrine on Fat Oxidation Rate during Exercise in Women? A Randomized, Crossover and Double-Blind Study

“…The results of this study are different to previous investigations carried out in samples of men or mixed samples where women were only a minor portion of the sample, where p -synephrine in the same dosage and form of administration as the one used in the current investigation was effective in enhancing the rate of fat oxidation during exercise at 30–80% of VO 2 max [ 7 , 8 , 9 , 10 , 11 ]. However, the lack of effectiveness of p -synephrine to enhance fat oxidation in women is not novel, as it has been recently reported despite maintaining dose, form of administration and fitness level of participants [ 12 ]. Collectively, all this information suggests that p -synephrine may be a less effective substance that modifies substrate oxidation in women compared to men.…”

“…After this, the tympanic temperature was measured in triplicate by using an infrared tympanic thermometer (Thermoscan 7, Braun, Kronberg, Germany; [ 19 ]). Thereafter, participants voided, and a urine sample was obtained to measure pre-exercise urine concentrations of p -synephrine and 4-hydroxymandelic acid, as previously described [ 12 ].…”

“…Additionally, it has been found that p -synephrine intake increases the total amount of fat oxidized during an acute bout of aerobic exercise (i.e., 60 min of cycling at moderate intensity) [ 11 ]. All these investigations have been carried out in samples of athletes or active individuals with a predominance of men, while a recent investigation in women has disputed the effect of p -synephrine to enhance fat oxidation during exercise [ 12 ]. Only in women has an increase in body temperature with p -synephrine been reported that may affect its potential benefits on fat oxidation during exercise [ 12 ].…”

“…All the above studies were conducted in a stable morning environment [ 7 , 8 , 9 , 10 , 11 , 12 ]. Previous investigations on circadian rhythms showed that there is a diurnal variation in the rate of fat utilization during exercise that may interfere with the effectiveness of p -synephrine to enhance fat oxidation in women.…”

“…However, a similar experiment has not been carried out on p -synephrine, and it remains unknown whether there is an interaction between the effect of p -synephrine and the time of the day on fat oxidation during exercise. Additionally, the lack of effect of p -synephrine to improve fat oxidation in women may be because this effect has only been tested in the morning in women [ 12 ]. This study aimed to evaluate the effect of the acute ingestion of 3 milligrams of p -synephrine per kilogram of body mass (mg/kg) on fat oxidation during exercise of increasing intensity when the exercise is performed in the morning vs. the evening.…”