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They’re one of the most popular and contentious category of sports supplements. Fat burners come in all shapes and sizes and its seems there's a new one every week. But with fat loss being one of the most desirable health changes for individuals, fat burners have earned themselves a bit of a reputation for the range of side effects they can elicit.

Just like the different forms they come in, the side effects of fat burners can be wide and varied. This is largely because everyone is biochemically unique meaning that two people can have totally different reactions to the same ingredient or supplement.

Fat burner supplements are often recommended to be taken on an empty stomach, which is another factor that can affect an individual’s reaction or response to the supplement. Co-ingesting food with a supplement tends to slow its absorption, which in the case of adverse reactions can be a good thing. So if you have experienced a negative side effect when taking a fat burner on an empty stomach, it might be an idea to try taking it with food. Rather than analyse every fat burner supplement or ingredient on the market, this article will review the safety data for a number of the most popular fat loss ingredients as well as touching on some safety data from a multi-ingredient product.

History of Fat Burner Side Effects

Before launching into a review of fat loss ingredients, it’s helpful to review some of the history concerning fat burners and their side effects. Some of the most serious side effects ascribed to fat loss supplements relate to one particular notorious fat loss ingredient in the form of ephedra. Ephedra or ephedrine alkaloids were normally listed on the label in the form of ma huang. Numerous studies have been published documenting several hundred cases of adverse reactions1. Generally speaking, side effects were limited to the cardiovascular or central nervous system1-3. More specifically high blood pressure was the most frequent side effect, with heart palpitations and tachycardia also being common1-3. In extreme cases, side effects severe as stroke, seizures and even death were reported. As a result, the American Food and Drug Administration (FDA) banned the sale of any supplement containing ephedra in April 20043. Australian food authorities followed suite soon thereafter.

Synephrine Side Effects

Since the ban of ephedra, synephrine has become its most common replacement. This is largely because synephrine is structurally related to ephedra, in that it also falls within the class of compounds referred to as 'sympathomimetic amines'. While the term itself sounds long and complex, sympathomemetic agents simply mimic the effects of products of the sympathetic nervous system, such as adrenaline and noradrenaline – the ‘flight-or-fight’ hormones.

Citrus Aurantium & P-Synephrine

P-synephrine is the specific naturally occurring alkaloid in Citrus aurantium (i.e. bitter orange) largely responsible for its documented thermogenic effects. P-synephrine is widely considered safer than ephedra, but is not without its own risk; especially when combined with caffeine. Caffeine is thought to potentiate the cardiovascular sympathomemetic  effects of synephrine such as increased blood pressure and heart palpitations4. Despite these concerns however, a number of recent scientific reviews dealing specifically with the issue of safety of p-synephrine have not listed any major concerns5, 6. When considered together, these reviews have largely come to the conclusion that bitter orange extract alone (p-synephrine) or in combination with other herbal ingredients does not produce significant adverse events such as an increase in heart rate or blood pressure5, 6.

Green Coffee Bean Extract

Green coffee bean extract is an increasingly popular ingredient in fat burners in keeping with expanding research on its efficacy for fat loss. It is actually one of the few fat loss ingredients to have a large body of quality published safety data on it. For example, a study published in 2012 explored the safety and efficacy of a green coffee bean extract in overweight subjects using aggressive dosing of between 700mg and 1050mg per day7. The extract used in the study was standardised to 46% chlorogenic acids – which is one of the highest grades of green coffee bean available. In welcomed news for fat burner supplement users, the study found the green coffee bean extract to be void of any adverse side effects7. So green coffee shapes as a relatively safe ingredient in fat burners that can be dosed quite aggressively with no ill iffects.

Garcinia Cambogia (HCA)

Garcinia cambogia has is a long standing fat loss phytonutrient that has enjoyed a recent resurgence in popularity. Compared with most other popular fat loss nutrients, garcinia cambogia has a large body of scientific evidence exploring its effects on weight loss. A major review specifically exploring the safety of garcinia cambogia from existing animal and human studies concluded there are no credible reports to suggest there are any significant adverse side effects from the oral use of garcinia cambogia8.

Green Tea

Green tea (camellia sinensis) is largely viewed as a safe and effective ingredient when it comes to fat burners. However, some may be surprised to learn that there are some adverse side effects reported in the scientific literature relating to the use of green tea. While the reports are very sparse, they suggest there may be a risk of liver toxicity with large doses of green tea in susceptible individuals. It’s worth noting however, much of the interest in green tea liver toxicity came after the discontinuation of a single weight-loss product containing an extract of green tea in France and Spain. So it’s possible much of the hysteria was caused by one bad product. Nonetheless, the United States Pharmacopeia have suggested, but not mandated, a warning , stating symptoms of liver injury be placed on any green tea extract monograph produced9. While there are no official Australian warnings regarding green tea consumption, it may be worth exercising caution (in the form of medical advice) if you’re an individual with any sort of liver disease that’s considering supplementing with green tea.

Red Pepper Fruit Extract (Capsicum annum)

Human studies involving CapsiMaxTM (a patented extract of capsicum standardised for capsaicinoid content) have revealed that oral administration does not result in significant changes in epinephrine or norepinephrine levels. What’s more, supplementation with CapsiMaxTM was not found to alter heart rate or blood pressure10. While this study found no significant adverse side effects, any products containing CapsiMaxTM come with a general warning to avoid intake if the particular individual is intolerant to spicy foods or prone to stomach upset and/or GI burning when consuming spicy foods.

Conjugated Linolenic Acid

CLA has a good safety profile with one of the major human studies citing only mild and minor symptoms included gas, acid regurgitation, diarrhea, nausea, and vomiting. However, the authors of the study note that there were no serious side effects were reported and of the subjects who withdrew, none reported any adverse side effects as the reason for withdrawal11.

Acetyl L-Carnitine

Acetyl-L-carnitine is likely safe for most adults. However some reports suggest it may cause some side effects including stomach upset, nausea, vomiting, and restlessness. It has also been known to cause a "fishy" odor of the urine, breath, and sweat.

Raspberry Ketones

Raspberry ketones are a popular ingredient in fat burners. However most of the data on their fat loss properties has come from animal studies12-14. As such, there’s very little safety data on raspberry ketones, but there are no reasons to suspect any adverse side effects when taking raspberry ketones orally.

Side-Effects of a Multi-Ingredient Fat Burner

While the information above cites specific safety data for individual nutrients and extracts, in reality, most fat burners are multi-ingredient supplements. As such, there’s always the possibility that ingredients may have adverse effects when taken together. While the safety data on such multi-ingredient fat loss supplements are very scarce, there is one recent published study that measured the safety and efficacy of a multi-ingredient fat loss supplement containing many of the nutrients above. The table below shows the supplements facts panel for the particular fat loss supplement in question.

Supplement Facts Panel for Multi-Ingredient Fat Loss Supplement

Subjects in the study were instructed to take 2 capsules twice a day with breakfast and lunch as part of a supervised diet and exercise program for 8 weeks. The supplement was shown to be safe and effective for the weight loss program by augmenting improvements in body composition. No significant adverse side effects were noted in any of the subjects. While the findings of this study cannot be applied to all multi-ingredient fat burners, it does provide some assurance that your average multi-ingredient fat burner is unlikely to carry significant side effects when used according to directions.

In closing, it can be said that there are a number of popular fat loss ingredients that have some credible safety data on them to support their use as part of a properly devised weight loss program. But because of the vast number of fat burner supplements and the complexities involved in studying multi-ingredient supplements, there is very little safety data on multi-ingredient supplements. As with most issues concerning safety and side effects, individuals need to exercise healthy caution and appropriate diligence relevant to their age, health, environment, existing medication and previous experience with supplements. In most instances, adverse side effects should be minimal when consuming any given supplement according to directions. Supplements with high amounts of caffeine and p-synephrine (citrus aurantium) can carry an increased risk of cardiovascular issues such as heart palpitations and therefore individuals overly sensitive to caffeine or who are on any existing medications for blood pressure should steer clear of such supplements, unless advised otherwise by a suitably qualified healthcare practitioner.


1. Haller CA, et al. Adverse cardiovascular and central nervous system events associated with dietary supplements containing ephedra alkaloids. N Engl J Med 2000;343:1833-1838
2. Geiger JD. Adverse events associated with supplements containing ephedra alkaloids. Clin J Sport Med. 2002;12(4):263.
3. Maglione M, et al. Psychiatric effects of ephedra use: an analysis of Food and Drug Administration reports of adverse events. Am J Psychiatry. 2005;162(1):189-91.
4. Stohs SJ, et al. Effects of p-Synephrine alone and in combination with selected bioflavonoids on resting metabolism, blood pressure, heart rate and self-reported mood changes. International Journal of Medical Sciences. 2011;8(4):295-301.
5. Stohs SJ, et al. A review of the human clinical studies involving Citrus aurantium (bitter orange) extract and its primary protoalkaloid p-synephrine. Int J Med Sci. 2012;9(7):527-38.
6. Kaats GR, et al. A 60day double-blind, placebo-controlled safety study involving Citrus aurantium (bitter orange) extract. Food Chem Toxicol. 2013 May;55:358-62.
7. Vinson JA, et al. Randomized, double-blind, placebo-controlled, linear dose, crossover study to evaluate the efficacy and safety of a green coffee bean extract in overweight subjects. Diabetes, 8. Metabolic Syndrome and Obesity: Targets and Therapy 2012:5 21–27.
8. Ma´rquez F, et al. Evaluation of the safety and efficacy of hydroxycitric acid or garcinia cambogia extracts in humans. Critical Reviews in Food Science and Nutrition. 2012;52:585–594.
9. Sarma DN, et al. Safety of green tea extracts : a systematic review by the US Pharmacopeia. Drug Safety. 2008;31:469–484.
10. Bloomer RJ, et al. Effect of oral intake of capsaicinoid beadlets on catecholamine secretion and blood markers of lipolysis in healthy adults: a randomized, placebo controlled, double-blind, cross-over study. Lipids Health Dis. 2010;9:72.
11. Chen SC, et al. Effect of conjugated linoleic acid supplementation on weight loss and body fat composition in a Chinese population. Nutrition. 2012;28(5):559-65.
12. Parks KS. Raspberry ketone increases both lipolysis and fatty acid oxidation in 3T3-L1 adipocytes. Planta Med. 2010;76(15):1654-8.
13. Wang L, et al. Raspberry ketone protects rats fed high-fat diets against nonalcoholic steatohepatitis. J Med Food. 2012;15(5):495-503.
14. Morimoto C, et al. Anti-obese action of raspberry ketone. Life Sci. 2005;77(2):194-204.
15. Lopez HL, et al. Eight weeks of supplementation with a multi-ingredient weight loss product enhances body composition, reduces hip and waist girth, and increases energy levels in overweight men and women. J Int Soc Sport Nutr. 2013;10:22.

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