The most effective and legal slimming supplement in the EU is caffeine. Take 600 mg per day and you'll burn about 110 kcal extra. [Am J Clin Nutr. 1989 Jan;49(1):44-50.] But unfortunately caffeine doesn't agree with everyone. We dug up a study from the late nineties which shows that you can boost your fat burning with just a small amount of caffeine too – if you combine it with green tea.
The study we're talking about was published in 1999 in the American Journal of Clinical Nutrition and was written by researchers at the University of Fribourg. It was partly financed by Arkopharma Laboratories, the manufacturer of the green tea extract Exolise.
At the time of the study slimming supplements containing caffeine and ephedra were still legal. They were known to be effective but caused side effects in a large minority of users. Green tea supplements don’t have side effects. But the researchers wanted to know whether green tea is a suitable ingredient for slimming supplements.
They assumed that a slimming supplement based on green tea would only work if it also contained a small amount of caffeine. They regard 50 mg as a 'small amount' of caffeine, about the amount in a cup of coffee.
The researchers put 10 healthy males in their twenties in a respiration chamber for 24 hours so that they could measure exactly how much kcals the subjects burned.
On one occasion the subjects were given a fake supplement with no active ingredients [Placebo]; on another occasion the subjects were given 50 mg caffeine to take at breakfast, lunch and supper; on yet another occasion they were given 50 mg caffeine again but this time with half a gram of green tea extract as well. So on the last round the men took 150 mg caffeine and 1.5 g green tea extract per day.
The green tea extract the researchers used consisted for 25 percent of catechines, of which 72 percent of those consisted of EGCG [structural formula shown here]. That's a high proportion: in most green tea extracts the catechines consist for about half of EGCG. According to scientists, EGCG is the most powerful flavonoid in green tea.
The table below shows that caffeine boosted energy expenditure [EE] by a mere 15 kcal per day. The combination of caffeine with green tea worked better, boosting total energy expenditure by 79 kcal.
Now 'higher energy expenditure' doesn't necessarily mean 'more fat burning' so the researchers also measured the amount of protein, carbohydrates and fat them men burned.
"In conclusion, oral administration of the green tea extract stimulated thermogenesis and fat oxidation and thus has the potential to influence body weight and body composition via changes in both energy expenditure and substrate utilization", the researchers concluded.
Am J Clin Nutr. 1999 Dec; 70(6): 1040-5.