During your cardio, caffeine stimulates fat oxidation as much as p-synephrine
It's a trend. Companies are introducing more and more weight loss supplements with synephrine from Citrus aurantium. Most of these products are intended for athletes. Do they work? The human study, published by Spanish sports scientists in Medicine & Science in Sports & Exercise, does not provide a complete answer to that question. But the end results are pretty disappointing.
Citrus aurantium contains p-synephrine. To be precise: Citrus aurantium contains (-)-p-synephrine. You also have (+)-p-synephrine. Laboratories have found (+)-p-synephrine more than once in fat loss supplements with Citrus aurantium, but scientists assume that this substance is probably of synthetic origin - and secretly added by fraudulent companies.
The Spanish researchers experimented with synthetic p-synephrine. Whether this was (-)-p-synephrine, (+)-p-synephrine or a racemic mixture of those 2 p-synephrines, they do not tell, but we suspect the researchers used a racemic mix.
In addition to p-synephrine you also have m-synephrine and o-synephrine, and each of these two substances has two isomeres. The synephrine in Citrus aurantium is a mixture of several types of synephrines - and the composition of this mixture is always a bit different.
On 4 different occasions, the researchers got 13 active people cycling. The subjects started cycling with an intensity of 30 percent of their maximal oxygen uptake [VO2max], and every 3 minutes that intensity increased by 10 percent of the VO2max - until an intensity of 90 percent of the VO2max was reached.
The subjects cycled in the afternoon after having received an identical lunch. An hour before the test, they had taken capsules.
On one occasion there was nothing in these capsules [Placebo], the other time the capsules contained 3 milligrams of caffeine per kilogram of body weight. On yet another occasion, there were 3 milligrams of p-synephrine per kilogram of body weight, and on a fourth occasion, the subjects took both caffeine and p-synephrine.
An imaginary test person weighing 80 kilos would therefore have used 240 milligrams of caffeine, or 240 milligrams of p-synephrine, or 240 milligrams of p-synephrine plus 240 milligrams of caffeine.
The dose of p-synephrine the researchers used is high. In Sweden, supplements may not contain more than 160 milligrams of p-synephrine per daily dose, in Canada no more than 40-50 milligrams and in France no more than 20 milligrams. In Germany, a daily intake of supplements may maximally contain 6.7 milligrams of syneprines in total.
No supplementation increased the total energy consumption. However, caffeine, p-synephrine and the combination of these two did increase the oxidation of fat, and reduced the oxidation of carbohydrates. What stimulant the test subjects used, did not matter at all.
Med Sci Sports Exerc. 2018 Sep;50(9):1899-1906.
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