P-synephrine is an alkaloid found in a wide variety of citrus fruits such as oranges, mandarins, and grapefruits. Because of its chemical similarities to ephedrine, a nervous system stimulant, it has become a popular food supplement and is included in many weight loss products despite very little existing research that proves its effectiveness.
This study determined to examine the effects of p-synephrine on energy metabolism and the rate of fat and carbohydrate oxidation during rest and exercise. In a randomised, placebo-controlled, double-blind, experimental study, 18 subjects underwent two experimental trials. The first group consumed a dose of p-synephrine, whilst a control group took a placebo. An hour after ingesting the substance, energy expenditure and arterial tension were measured before and after physical activity on a static bike.
It was found that acute p-synephrine ingestion had no effect on energy expenditure, heart rate, or arterial pressure. However, the p-synephrine did increase the rate of fat oxidation at low and moderate intensities.
So wait a minute. Weight loss pills do work a fat-burning miracle after all?
Not exactly. The data suggested that p-synephrine supplements could be useful to increase fat oxidation by 7 g (0.07kg) per hour of exercise. The maximum rate found for fat oxidation during exercise was 0.7 g/min. That would suggest that in a very best-case scenario, an individual could burn 42 g of fat after an hour of exercise at that level of intensity with p-synephrine. But that is really hauling your ass in a workout by their measure, and the data merely suggests that p-synephrine was behind that and not different individual differences in metabolism between the study’s subjects.
The researchers themselves remained convinced that there was no case for the substance increasing fat loss without exercise, especially not for more extreme weight loss goals. As one of the lead researchers commented:
“[This] should be the aim: to lose a kilo [of fat] per month. It’s less attention-catching than miracle diet slogans, but scientifically speaking, effective change would be at that rate. The rate of loss could increase with p-synephrine, but always [when] combining the substance with exercise.”1
The authors also highlighted the need for further study to determine the long term effects of p-synephrine in the future.
1. Jorge Gutiérrez-Hellín y Juan Del Coso. ‘Acute p-synephrine ingestion increases fat oxidation rate during exercise’. British Journal of Clinical Pharmacology (2016).