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Introduction

Supplements that help with weight loss are part of the most popular category of nutritional supplements known as “fat burners”.

To be part of this category a supplement has to claim to achieve one of the following:

  • Increase energy expenditure
  • Increase fat metabolism
  • Impair fat absorption

Most supplements in this category have not been remotely proven to help with any of these factors and are purely a marketing ploy to entice the masses into purchasing the latest “breakthrough ingredient”.

However, few supplements have some good research behind them to potentially aid weight loss diets.

1. Caffeine

Caffeine is an alkaloid derivative that is found naturally in certain foods and drinks, but also added to many different products.

Early sport science studies revealed the ability for caffeine to increase fat oxidation rates during an exercise session [1].

Since then it has become even clearer that caffeine causes a “shift” in substrate metabolism during exercise to becoming more reliant on fat as a fuel source (as opposed to carbohydrate) when taken pre-exercise. In fact this is one of many reasons why caffeine can spare carbohydrate stores (muscle glycogen) during exercise and ultimately improve endurance performance [2].

This may be as a result of caffeine increasing the activity of the central nervous system and the amount of circulating adrenaline levels, thereby upregulating the release of fatty acids from adipose tissue. This mechanism allows for an increased amount of fatty acids to potentially become available for energy use.

In addition, caffeine may have a more direct role on lipolysis. In lab studies, caffeine has shown to inhibit an enzyme called phosphodiesterase, thus increasing circulating cyclic AMP levels, which then increases the metabolic pathways involved in fat metabolism and breakdown [3].

Lastly, there is some evidence that caffeine is a thermogenic agent that may increase daily energy expenditure [4]. This is likely due to it being a stimulant, and thus increasing ones daily activity levels as people will become more mentally stimulated.

However, high doses of caffeine (8mg per kg bodyweight) have even shown to significantly increase resting metabolic rate – the amount of calories burnt at complete rest to support internal functioning – in the 3 hour period after ingestion [5]. A similar effect has been seen with much lower doses of caffeine (100 mg), such as a 3-4% increase in resting metabolic rate 2-3 hours after consumption [6].

Whether such small difference in resting metabolic rate will actually make a noticeable difference to fat loss is unknown, but probably unlikely.

At the moment, no reliable evidence is available to suggest that caffeine use causes weight loss, or that caffeine users have a lower bodyweight compared to non-caffeine users.

It is also important to consider that people who habitually consume caffeine have a reduced sensitivity to the substance and will begin to have a “blunted” physiological response when it is consumed.

Based on this, although acute caffeine ingestion has the potential to enhance metabolism, it is probably not potent enough alone to act as a weight loss product [7].

At best, it will slightly enhance the extent of fat loss if someone is in a caloric deficit – eating less calories than they burn per day.

2. L-Carnitine

L-carnitine plays an important role in fat metabolism.

The primary function of l-carnitine is to transport fatty acids across the mitochondrial membrane and into the mitochondria where they can enter energy-producing cycles [8].

Once inside the mitochondria, fatty acids are degraded to acetyl-CoA through beta-oxidation and proceed to the Krebs cycle.

More importantly, the availability of carnitine within the muscle cell is extremely important for determining the rate of carbohydrate metabolism and fat metabolism [9].

In simple terms, if carnitine availability is limited then fatty acids will not be able to enter the mitochondria, and fat metabolism will significantly slow down.

Understanding this concept led to the introduction of carnitine supplementation, as it was assumed that regularly consuming carnitine will increase the muscle carnitine concentration, and therefore increase fat metabolism during exercise.

Some evidence suggests that 2 grams of l-carnitine per day for 6 months can effectively reduce carbohydrate dependency during exercise, and increase fat as a viable fuel source to support activity [10].

However, the big issue with l-carnitine supplementation to stimulate fat loss is that l-carnitine needs a significant amount of carbohydrate (~40 grams carbohydrate per 1 gram of l-carnitine) to be consumed alongside it to “shunt” it into muscle cells. Depending on the situation, it may be counterproductive to consume large amounts of carbohydrates during a weight loss phase purely just to allow carnitine to enter muscle cells and stimulate fat metabolism [11].

Overall, the practical implication of l-carnitine as a fat burner is up for debate, but there is enough evidence to recommend l-carnitine as a supplemental aid during a weight loss phase.

3. Green Tea

Green tea is processed from specific leaves and contains high amounts of catechin polyphenols.

The main catechin polyphenols it contains are epicatechin, epigallocatechin, epicatechin-3-gallate and epigallocatechin-3-gallate (EGCG), with the latter thought to be the most pharmacologically active.

Many researchers believe that EGCG can stimulate fat oxidation through the direct inhibition of an enzyme that degrades adrenaline, called catechol-O-methyltransferase [12].

This may cause short-term stimulation of the nervous system and result in increased concentrations of catecholamines, which may potentially increase fat mobilization and oxidation.

Some human studies have found that green tea has significantly increased 24 hour fat oxidation, and does so by ~20% more than caffeine has shown to do alone [13].

Meta-analysis’ investigating the effects of catechin–caffeine supplementation, such as green tea, also have concluded that they can increase fat oxidation rates over a 24 hour period by ~15% [14].

Similarly, green tea drinkers seem to have a lower respiratory quotient – indicating the ratio of carbohydrates to fat used for energy – than non-green tea drinkers after 8 weeks, which indicates a greater reliance on fat metabolism [15].

In general, long-term green tea supplementation may lead to reductions, or at least a maintenance of body weight [16]. On average, catechin interventions cause ~1.5kg more weight loss over a 3 month period compared to placebo [17].

However, it is important to know that green tea does not always seem to provide additional benefits to body composition when combined with a low calorie diet [18].

As with caffeine, the effects appear to be relatively small, and may become negligible when the diet as a whole is in place to support weight loss – such as reducing calorie intake and keeping protein intake elevated.

4. Conjugated Linoleic Acid (CLA)

Conjugated linoleic acids (CLA) are a group of isomers of the essential omega-6 fatty acid linoleic acid.

Although a high intake of omega-6 is closely linked to chronic inflammation and certain health problems, CLA has been suggested to act as an anti-obesity agent through its ability to reduce hunger and fat storage, and increase energy expenditure and fat oxidation.

Animal studies have observed alterations in body weight and composition when an ad libitum (eat as required) diet is supplemented with CLA. Specifically, the additional supplementation has caused a ~60% reduction in body fat and ~15% increase in lean body mass [19].

Similarly, other animal studies have found a near 10% reduction in body fat mass after 1 year of CLA supplementation compared to placebo [20].

The exact mechanism which appears to be having a beneficial effect is not clear although there is limited evidence for the following effects:

  • Reduction in food intake [21]
  • Increased energy expenditure [22]
  • Increase in CPT activity which allows more fatty acids to enter the energy-producing mitochondria [23].

However, when CLA has been studied in humans it only appears to be effective in diabetics, and does not cause fat loss in healthy adults [24].

Conclusion

The “fat burner” category of supplements is overpopulated and most supplements have not been shown to directly cause weight loss, however when used in the context of an energy restricted diet and exercise, they can potentially be helpful.

However, a few supplements which may either increase energy expenditure or fat metabolism are caffeine, l-carnitine, green tea, and conjugated linoleic acid.

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This article has been reviewed and fact-checked by a certified nutritionist, and only uses information from credible academic sources.