Thermogenic’s, more commonly known as fat-burners, claim to work in various ways either by; increasing metabolic rate or energy expenditure, impairing fat absorption, or increasing fat oxidation during exercise, and thereby lead to enhanced fat loss (1).
There are multiple thermogenic’s on the market, many containing various ingredients which are believed to work synergistically to provide a “fat burning” effect. Common ingredients found in these supplements include; caffeine, carnitihine, green tea extract, and bitter orange extract. Despite the hype, there is little or no scientific evidence to back up most of the claims made by the manufacturers of these products (1,2).
Caffeine is a stimulant which is classed as a drug rather than a nutrient. It works by blocking adenosine, a chemical within the brain that induces sleepiness, thus increasing alertness. It is well recognised as having the ability to enhance performance in sports and exercise by increasing alertness and concentration, reducing the perception of effort, and therefore allowing exercise to be sustained at a high intensity for a longer period of time (2). While there is quite a lot of evidence suggesting that caffeine enhances performance, this effect has not been concretely linked to increased fat oxidation (3). Side effects are dose-dependent and vary between individuals. High doses can increase heart rate, anxiety, irritability, palpitations, and nausea. The European Food Safety Authority (EFSA) advise that the recommended daily intake of caffeine, from all sources, should not exceed 400mg and single doses should not exceed 200mg, for adults in the general population (4). Pregnant women should further cap their intake at 200mg a day in total. It is important to highlight that caffeine content varies greatly between supplements, foods, and drinks, but also between brands and preparations of the same product. For reference, the caffeine content of a cup of filter coffee can vary from 60-120mg and the caffeine content of many energy drinks can be up to 100mg (2).
Green tea extract
Green tea extract may have some value which is believed to be due to the active compound, and polyphenol, epigallocatechian gallate (EGCG). The evidence suggests that it appears to have a very small effect on fat oxidation and weight loss, and in order to achieve a significant effect, doses are estimated at approximately 300-500mg EGCG which equates to approximately 6-10 cups of green tea (1,2,5,6). Supplements are available, but again, dose and formulation varies and it has been cautioned that very high doses (>800mg EGCG/day) can be toxic to the liver (7).
Finally, a review of fat burning supplements cannot be complete without reference to ephedrine. Ephedrine is an extract derived from the Ephedra plant (or ma Huang). It is a potent stimulator of the central nervous system and is chemically similar to amphetamines (mood-altering drugs). It is considered a drug, not a food supplement, and is used at low concentrations in cold and flu remedies, but also as an ingredient in some dietary supplements marketed for weight loss and enhanced athletic performance. However, the U.S. Food and Drug Administration (FDA) banned the use of ephedrine in dietary supplements in 2004 because of the increasing number of reported adverse events due to serious side effects from the drug. This ranges from increased blood pressure, palpitations, heart attacks, seizure, stroke and, at very high doses, death. While there is a very small amount of evidence to support its efficacy for short-term weight loss, the increased risk of serious health problems, including death, heavily outweighs any potential benefit (8).
Thermogenic supplements are marketed as having the ability to boost metabolism and burn body fat. While some of these ingredients may have small, positive effects on metabolism, it’s unclear whether these effects are significant enough to help people lose significant weight or body fat. My personal view is that the financial cost incurred and the risk of potential side effects, certainly outweigh any measurable positive effect from taking such supplements. A cup of coffee can be a useful performance enhancer and is personally my chosen “pre-workout” of choice. It’s also much cheaper, safer, and enjoyable.
(1) Jeukendrup, A.E. and Randell, R., 2011. Fat burners: nutrition supplements that increase fat metabolism. Obesity reviews, 12(10), pp.841-851
(2) Bean, A., 2017. The complete guide to sports nutrition. Bloomsbury Publishing.
(3) Graham, T., Helge, J., MacLean, D., Kiens, B. and Richter, E. (2000). Caffeine ingestion does not alter carbohydrate or fat metabolism in human skeletal muscle during exercise. The Journal of Physiology, 529(3), pp.837-847.
(4) EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA), 2015. Scientific opinion on the safety of caffeine. EFSA Journal, 13(5), p.4102.
(5)Hursel, R.R.H.H., Viechtbauer, W., Dulloo, A.G., Tremblay, A., Tappy, L., Rumpler, W. and Westerterp‐Plantenga, M.S., 2011. The effects of catechin rich teas and caffeine on energy expenditure and fat oxidation: a meta‐analysis. Obesity reviews, 12(7), pp.e573-e581.
(6) Hursel, R., Viechtbauer, W. and Westerterp-Plantenga, M.S., 2009. The effects of green tea on weight loss and weight maintenance: a meta-analysis. International journal of obesity, 33(9), p.956.
(7) EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS), Younes, M., Aggett, P., Aguilar, F., Crebelli, R., Dusemund, B., Filipič, M., Frutos, M.J., Galtier, P., Gott, D. and Gundert‐Remy, U., 2018. Scientific opinion on the safety of green tea catechins. EFSA Journal, 16(4), p.e05239.
(8) Shekelle, P., Hardy, M.L., Morton, S.C., Maglione, M., Suttorp, M., Roth, E., Jungvig, L., Mojica, W.A., Gagne, J., Rhodes, S. and McKinnon, E., 2003. Ephedra and ephedrine for weight loss and athletic performance enhancement: clinical efficacy and side effects. Evidence report/technology assessment (Summary), (76), p.1.