Of all the supplements out there, fat burners are arguably the ones with the most questionable effectiveness. Despite that, the market is full of products promising to help you finally burn that stubborn fat.
Some products even argue to have scientific evidence backing their claims. This might look quite promising for the regular person but the studies they cite are often sponsored, biased, or cherry-picked.
The terrible truth is that most of the products are not only a complete waste of money, but some can be downright dangerous for your health.
In this article, we decided to take a deeper look at the science of fat burners and reveal what truly works and what doesn’t.
TL; DR: What You Need to Know
- Some fat-burners work but not in the exact way you expect
- Most fat-burners are useless and based on cherry-picked studies
- Some fat-burners can have side-effects
- A few fat-burners can be even dangerous
- Our top fat-burner picks
If you’d rather just see our picks, below is a comparison table. Otherwise, you can keep reading to discover why we picked them and why most fat burners do not work.
Top 5 Choices for Best Fat Burners
Hydroxycut Hardcore Elite
Contains anhydrous caffeine - the most ergogenic form of caffeine + green coffee, L-theanine and Coleus forskohlii (Forskolin)
Capsaicin can increase fat oxidation, although the effect is minimal. Furthermore, Thermodiamine, aka Evodiamine has shown promising effects similar to capsaicin.
The product contains 5-HTP (5-Hydroxytryptophan) which is an effective appetite suppressing fat burner, leading to significantly greater weight loss than placebo in all studies.5-HTP shouldn’t be combined with any neurological drug such as antidepressants.
Lipo-6 Black UC
Yohimbine is an alkaloid that can be found in the bark for the Pausinystalia Yohimbe tree. It is a stimulant similar to caffeine and boosts your noradrenaline production and thus can slightly increase your energy expenditure and help you burn more fat.
A combination of caffeine and chlorogenic acid which has potentially powerful ergogenic and fat-burning effects
Do Fat Burners Work?
The short answer is yes – but not in the way one might expect them to work.
There is no magic pill when it comes to fat loss. How much weight you will lose depends only on your energy balance.
That is a basic principle of thermodynamics and it applies to everything, including our bodies.
We have known this fact for decades and there is solid evidence to back it up.
Negative energy balance can be achieved in two ways – increasing the amount of calories burned and reducing the amount of calories consumed.
We expect from fat burners to boost our caloric expenditure, but this strategy is not as effective as restricting the caloric intake.
If you still eat as much as you feel like, without controlling the energy intake, you can easily compensate for the increased caloric expenditure with more food.
Your body is trying to maintain balance (homeostasis) and you will likely feel hungrier once you start burning more calories.
That is why simply eating fewer calories is the better strategy for fat loss and it is well supported by studies.
This is why most fat burners today are blends and focus on appetite and calorie absorption control rather than trying to boost “fat-burning” itself.
How Do Fat Burners Work?
There are 3 main mechanisms through which fat burners can help you stay in a caloric deficit to burn fat:
- Appetite suppression
- Decreased calorie absorption from food
- Increase in resting energy expenditure
People taking fat burners usually expect to benefit from the 3rd mechanism but ironically that is the least successful one for fat loss.
Boosting fat oxidation is another effect that weight loss pills and fat burners claim.
Unfortunately, it is quite useless, because your body can achieve amazing rates of fat oxidation on its own. As long as it is a caloric deficit, of course.
Even a study in Korean women with metabolic problems (type 2 diabetes) the maximum fat oxidation could reach 0.38 g/min during exercise. Calculated for a week, that is almost 4kg (8.5 lb) of fat loss.
This shows that your body’s ability to burn fat is not the limiting factor for weight loss. The limiting factor is the amount of energy deficit you can maintain.
Unfortunately, no exercise program or fat burner alone will help you lose weight if you are not eating less energy than you burn.
How Did We Research Fat Burners?
Many ingredients are often labeled “supported by science”. However, biased studies are quite common.
While there might be some evidence supporting their effectiveness, it is important to avoid “cherry-picking” studies. Instead, we looked at all the available research.
As it turns out, plenty of fat burner ingredients are not as well supported by evidence as they claim…
It is a type of soluble fiber that can absorb water and turn into a gel-like substance.
Thus, it creates volume in the stomach and intestines which could aid with fullness and satiation. It is added to fat boosters as an appetite suppressant but some manufacturers claim that it also boosts fat loss.
The subjects were randomly assigned to either placebo or glucomannan supplementation. According to the study, everyone had the same caloric deficit but the glucomannan group lost significantly more weight.
Unfortunately, it turns out that the study has a high attrition bias. If you take a deeper look you will discover that only 80 subjects completed the study.
Furthermore, only 50 of those had an eligible food diary to monitor their caloric intake. The dropout rate was highest amongst the glucomannan group because of adverse effects such as gastrointestinal problems.
Considering this huge dropout rate, this study cannot claim to be proof of anything, but let’s look even deeper.
Another major bias is the fact the subjects in the control group were significantly lighter and shorter in height which means they had lower resting energy expenditure. Besides, the glucomannan group was in a higher caloric deficit, which is why it lost just over a kilogram of more weight compared to the control group.
However, another systematic review and meta-analysis of 18 studies didn’t find any statistical relationship between glucomannan and weight loss. Adverse effects such as abdominal discomfort, diarrhea, and constipation were common.
And to make everything worse, Health Canada stated the following: “glucomannan products have a potential for harm if taken without at least 8 ounces of water or other fluid. The risk includes choking and/or blockage of the throat, esophagus, or intestine, according to international adverse reaction case reports. It is also important to note that these products should NOT be taken immediately before going to bed.”
Synephrine (Bitter Orange Extract)
Synephrine is an alkaloid occurring naturally in some plants such as bitter oranges (Citrus Aurantium). It has adrenaline-like effects, which is why it is speculated to boost metabolic rate and fat oxidation.
Most of the research on synephrine is focused on its safety rather than its effectiveness. That is because reports raised concerns that synephrine might be related to hypertension, arrhythmia, or even myocardial infarction.
However, the alkaloid is often in combination with caffeine and other ingredients, which might be accountable for some side effects.
A review from 2011 found no adverse effects due to supplementation with the natural form – p-synephrine. The researchers suggest that studies showing adverse effects are performed with m-synephrine (the artificial form) or intravenous applications of high doses.
Another 60-day double-blind, placebo-controlled study from 2013 also found no adverse effects after supplementation.
Unfortunately, studies on its effectiveness are not as optimistic. The majority of clinical trials with people have very poor design and quality.
As studies suggest, the active component capsaicin found in chili peppers can increase metabolism by activating brown adipose tissues.
Evidence shows that it can promote fat-oxidation and support your metabolism while in energy restriction.
As an extra benefit, studies reveal that capsaicin improves the feeling of fullness and thus can reduce your desire to eat after dinner.
One trial suggests that capsaicin can lead to a 20% increase in energy expenditure but the effect lasts for 10-30 minutes.
It seems like even if there is a short-term boost in thermogenesis, it is not likely to result in an overall increase in your caloric deficit or rate of weight loss.
In addition, regular consumption might have long- term risks. Studies performed in Mexico suggest that capsaicin might increase the risk of gastric cancer but more studies are needed to test this hypothesis.  
Acetyl-L-carnitine is an amino acid with a role in energy production and fat oxidation. Our body can produce it on its own from other essential amino acids.
The function of carnitine is to transport fatty acids into the mitochondria for oxidation. It seems logical that supporting the transport and mitochondrial function itself would lead to higher fat oxidation.
On the bright side, studies suggest that supplementation with it can benefit healthy aging, brain function, liver function, insulin sensitivity, blood pressure, and even sperm quality.    
If you are vegan and not consuming enough essential amino acids, then your body can have trouble producing enough L-carnitine on its own.
Then supplementation might have additional benefits for weight loss. Otherwise, you are getting enough L-carnitine through your diet and you won’t burn any extra fat by supplementation.
Common side effects due to supplementation include nausea and stomach discomfort.
Natural Fat Burners That Work
The best natural fat burner without a doubt is coffee.
It boosts your metabolism, reduces your appetite and (green coffee) reduces the amount of calories you absorb from carbs. Those effects are thanks to not one but two ingredients – caffeine and chlorogenic acid.
Other studies suggest that high caffeine intake is associated with weight loss in women, via increased thermogenesis and suppressed leptin.
Caffeine can improve physical performance as well, which can lead to even higher energy expenditure.
However, you can develop tolerance towards all the effects of caffeine, which makes them ineffective at that point. Tolerance occurs even quicker in higher doses.
Chlorogenic acid is found in green coffee. The process of roasting significantly reduces its levels. Despite the name of the acid, there is no chlorine in it.
Chlorogenic acid inhibits one of the enzymes responsible for breaking down carbohydrates. Thus, it reduces the amount of calories absorbed from carbs in the digestive system.
A 2011 systematic review and meta-analysis of randomized clinical trials, suggests that the effectiveness of Chlorogenic acid as a weight-loss supplement is promising, although the quality of the studies is poor.
Thanks to the effect on carbohydrate metabolism, chlorogenic acid supplementation could help control your blood pressure and blood sugar levels.
In addition to all the benefits, coffee is also the biggest source of antioxidants in typical western diets.
Side Effects of Fat Burners
Most fat burners have side effects that range from mild stomach discomfort to serious conditions.
Caffeine is the most commonly used and effective fat burning supplement but it can have serious side effects if overdosed. Some people are more sensitive to those effects than others.
The recommended dose for adults is up to 400 mg per day. For adolescents, it is up to 100mg daily. Pregnant women should also limit their caffeine intake because the effects on the baby are not completely known.
The symptoms of overdose include dizziness, headache, irritability, vomiting, irregular or fast heartbeat, and convulsions. Allergic reactions to green coffee extract are also possible.
In extreme cases, fatal heart arrhythmia has been reported.
Depending on the dose and time of consumption, caffeine can have negative effects on sleep, which potentially negates the weight loss benefits.
Are Fat Burners Dangerous
While most natural ingredients in fat burners are safe in recommended doses, there are some exceptions.
Such an example is usnic acid. It naturally occurs in certain lichen species and used to be an ingredient in some fat burners.
That was one of the ingredients in the infamous LipoKinetix – a weight loss supplement that caused liver failure in numerous patients according to several reports.
Usnic acid is not only considered hepatotoxic but it could interfere with various medications.
Another blend of herbal extracts – OxyELITE Pro has also been linked to cases of acute liver failure.
A product called “Exolise” has also been linked to cases of acute liver failure and eventually withdrawn from the market.
This product contained another natural ingredient – Chinese green tea extracts (Camellia sinensis).
Epigallocatechin-3-gallate (EGCG) is the main polyphenol (antioxidant) in green tea extract and it is quite common in many fat burners.
However, it is yet unclear if the previously reported hepatotoxicity is due to a possible presence of a pesticide called endosulfan which is now banned.
Best Fat Burner Choices
Best Fat Burner 2020: Hydroxycut Hardcore Elite
A great product that contains anhydrous caffeine which seems to be the most ergogenic form of caffeine. The green coffee, which contains chlorogenic acid can reduce the amount of calories absorbed and aid weight loss.
L-theanine is a relaxing agent that doesn’t induce sedation. It is a great synergistic combination with caffeine which promotes attention and cognition. The most important effect is that it attenuates the “edge” of stimulants so they won’t cause anxiety or an increased heart rate.
Coleus forskohlii (Forskolin) is still under research for its effects on fat loss.
Best Thermogenic Fat Burner: Burning Man
Capsaicin can increase fat oxidation, although the effect is minimal. Furthermore, Thermodiamine, aka Evodiamine has shown promising effects similar to capsaicin, but all the studies are in mice or in vitro.  Human studies are needed to confirm its efficacy.
Unfortunately, L-carnitine is completely useless as a fat-burner unless you are vegan and not consuming enough protein. However, it has multiple other benefits for healthy aging, brain function, liver function, etc. Gamma Butyrobetaine is its precursor.
The only benefit from Grains of Paradise (Aframomum Melegueta) extract is when combined with cold therapy according to a study. More research is needed to confirm any benefit at all. Chromium is completely useless for appetite control and fat loss according to more than 11 studies.
Best Fat Burner for Women: PhysiqueSeries Fat Burner
The product contains 5-HTP (5-Hydroxytryptophan) which is an effective appetite suppressing fat burner, leading to significantly greater weight loss than placebo in all studies.   All of them were in overweight individuals.
There are two types of hunger: physiological and emotional. It seems likely that 5-HTP suppresses emotional hunger and food cravings but not physiological hunger signals.
5-HTP shouldn’t be combined with any neurological drug such as antidepressants. Selective serotonin reuptake inhibitors (SSRI) usage in combination with 5-HTP is potentially lethal.
Best Fat Burner for Men on Intermittent Fasting: LIPO-6 BLACK UC from NutreX
Yohimbine is an alkaloid that can be found in the bark for the Pausinystalia Yohimbe tree. It is a stimulant similar to caffeine and boosts your noradrenaline production and thus can slightly increase your energy expenditure and help you burn more fat. It also improves erections according to studies.
Studies show that its fat-burning effect is mainly noticeable during exercise and it is completely suppressed by the consumption of food. This why yohimbine is likely effective only when combined with intermittent fasting. Rauwolscine effects are similar to yohimbine.
It is advisable to start with low doses, otherwise it can lead to side effects such as high blood pressure and anxiety.
The supplement should be taken in cycles of 2-4 weeks to avoid tolerance. Otherwise, the stimulation of noradrenaline can be lost in 15 days of using 12 mg a day, according to studies.
Best Green Coffee Fat Burner: Bulk Supplements
The Take-Home Message
The harsh truth is you cannot lose weight without being in a caloric deficit and the most reliable way to achieve it is with a diet.
Indeed, some natural products can help you maintain a deficit easier by boosting the amount of energy you burn and suppressing your appetite but the effect is not always significant.
Besides, you should pick fat burners carefully, because many can be more harmful than helpful.