One of the most popular nutritional supplements are known as ‘fat burners’, they are by far the most popular supplement. They have been linked to health and performance improvements, as well as weight loss. Fat burners generally claim to highly increase fat metabolism, weight loss and fat oxidation during exercise, alongside reducing fat absorption. However, it is important to note that the supplements available on the market are made out of a combination of ingredients, each with its own mechanism of action which leads to the proposed benefits. Many advertisements for these products use ‘too-good-to-be-true’ before and after photos of individuals who have used them, without enough evidence for their efficacy. Here, we’ll look at the most popular ingredients of these supplements and how they induce fat metabolism and weight loss, together with the evidence for their efficacy.
Caffeine
Caffeine is naturally found and added to a vast range of food and drinks. The most caffeinated products are those directly derived from coffee, cacao beans, and tea leaves. Studies of the effect of caffeine on metabolism were first performed in the 1970s, scientists showed that consuming coffee or pure caffeine before exercising increased fat oxidation, performance and switched muscle energy metabolism from carbohydrates to fat.
Since then, research has shown other effects of caffeine on metabolism. For example, caffeine indirectly increases adrenaline levels, in turn making fat more available as an energy supply for muscles. This is a sympathetic nervous system response, inducing ‘fight-or-flight’ where our muscles get ready to respond to a threat to survival. Caffeine has also been shown to increase resting metabolic rate 3hrs after consumption, even at low doses. However, it has also been shown that acutely consuming caffeine (e.g. through supplements) may not be potent enough alone to induce weight loss, particularly if consumed over long periods of time or in habitual high-caffeine consumers.
In summary, research to date has demonstrated that in some cases, caffeine can increase metabolism (rest) or fat oxidation (rest and low-intensity exercise), but these effects are less well-founded in moderate or high intensity activities.
L-cartinine
L-cartinine has an important role in metabolism. It helps transport fat to the mitochondria (the powerhouse) of the muscle cells, where it is then used as an energy source during overnight fasting and low to moderate intensity exercise.
Theoretically, taking L-cartinine as a supplement would increase this fat metabolism, as you increase the transport of fat to muscle cells. However, studies have shown that consumption of L-cartinine does not actually increase its concentration in muscle cells, as muscles have more than enough of it to carry out fat metabolism efficiently, therefore, any L-cartinine consumed is just excreted. On the other hand, few studies have shown that cartinine may have an effect on fat metabolism if in combination with a consumption of a large amount of carbohydrates. However, this research is at it’s early stages and we cannot draw any major conclusions. Furthermore, if used for weight loss purposes, it seems counterintuitive to consume large amounts of carbohydrates in order to see a small improvement in fat metabolism.
As it stands, there is not enough evidence to recommend L-cartinine for weight loss or highly increased fat metabolism, but some athletes may see small improvements in these areas.
Green tea
Green tea has caffeine, as found in all teas, and high quantities of a substance known as catechin polyphenol, which is not found in black tea.
Consumption of green tea extract (GTE) has been well investigated to date. Those studies have shown that GTE ingestion at rest and chronic GTE ingestion in combination with exercise is likely to increase fat metabolism. GTE seems to work by inducing increased activation of fat metabolising enzymes. However, scientists are still unsure about the amount and dose of GTE that is most effective, and whether it should be taken together with a pure caffeine supplement or not in order to see the best effects.
In summary, GTE has the potential to increase fat metabolism and weight loss, both at rest and during exercise, however the mechanism is not completely understood and there are doubts as to which dose would be most effective.
Conclusion
The effects of caffeine and GTE on fat metabolism and weight loss have been studied through many decades and there is more available data to support their fat metabolising properties. It is important to note that effects in humans have been reported to be small and generally more consistent in people who do not usually consume caffeine. Other supplements have also been studied, such as chromium and conjugated linoleic acid (CLA), but there is a lack of human evidence on their efficacy. There is an increasing list of ‘fat burners’ being promoted in the industry and research around them is likely to grow, but for now caffeine and GTE are the ‘safest bet’ for fat metabolism and weight loss.
Paul Rimmer (BSc, MSc, PhD)
