How Do Fat Burners Work? The Science of Weight-Loss Pills

What Are Fat Burners (Thermogenics)?

Thermogenic supplements are commonly referred to as fat burners and weight-loss pills. Thermogenesis is the process of your body using calories to produce heat (i.e. thermal energy), which is essential for homeostasis [1]. 

Thermoregulation (maintaining body temperature within a normal physiological range) intrinsically depends on metabolism, the summation of cellular processes that release and consume energy. For example, when you're sweating during an intense workout, your metabolism is working harder to cool the body down (again, as part of the thermogenic process). Likewise, your metabolism works hard to keep your body warm in cold environments; shivering is one way your body attempts to produce more heat in such circumstances.

Alterations in energy expenditure induced by dietary supplements or cold exposure are examples of adaptive thermogenesis, the type of thermogenesis that's amenable to lifestyle interventions [2]. The idea behind fat burner supplements is that taking specific ingredients can help your body naturally burn more energy (calories) throughout the day by modulating adaptive thermogenesis. In other words, they speed up your metabolism, thereby encouraging weight loss. However, there are several ways fat burners work, such as suppressing appetite, increasing motivation, and improving mood.

Yet, many consumers (rightfully) wonder, "Do fat burners work, or are they a waste of money?" The short answer: Yes, some fat burner supplements are backed by science and clinical studies. But, there are unfortunately tons of fat burner pills on the market that are unlikely to be effective. 

This article will show you which fat burners work based on research and how they can help you lose weight

How Do Fat Burners Work?

In short, fat burners work by boosting your blood pressure and augmenting your overall energy expenditure, potentially resulting in weight loss. With that said, if you have pre-existing high blood pressure, caution is advised since consuming these supplements may induce hypertension.

Supplement ingredients that increase metabolic rate (i.e. calorie burn) typically act as central nervous system (CNS) stimulants in the body. Caffeine is an example of a thermogenic stimulant since it is a ligand for adenosine receptors in the brain, which has downstream effects like bolstering the production of catecholamines (e.g. dopamine and adrenaline). Hence, caffeine is renowned for increasing vigilance, perceived energy, and resting metabolic rate [3]. 

But not all thermogenics are stimulatory; stimulant-free alternatives like Transparent Labs PhysiqueSeries Recomp Stim-Free can also help promote weight loss through independent mechanisms. For example, fat burners may contain ingredients that work at the molecular level by inhibiting or synergizing with chemicals that regulate lipogenesis (fat storage), lipolysis (fat breakdown), and fatty acid oxidation. 

Some ingredients also suppress appetite and modulate thyroid hormone levels [4]. As such, stim-free fat burners are becoming popular counterparts to their traditional stim-based brethren.

Primary benefits of fat burners may include:

  • Increase metabolic rate (calorie expenditure)
  • Enhance the use of fat (and other substrates) for energy
  • Boost energy and vigilance
  • Lower rate of perceived exertion (RPE) during exercise
  • Improve mood and reduce food cravings

With that in mind, let's look at which thermogenic ingredients to look for in weight-loss supplements and fat burner pills.


Caffeine is undoubtedly the best example of a stimulant-based thermogenic. By definition, stimulants are substances that elevate physiological or nervous activity in the body. They generally increase alertness, wakefulness, motivation, heart rate, respiratory rate, and nutrient metabolism. 

Naturally, caffeine can promote weight loss by allowing people to burn more calories than they usually would through voluntary activity [5]. It's also a great mood lifter before hitting the gym.

Taking caffeine before exercising might help you run a few extra miles on the treadmill, or bang out a few more reps and sets during a leg workout, making it easier to stay in a calorie deficit. There's also evidence that stimulants like caffeine and capsaicin (discussed below) can boost resting metabolic rate (RMR), which translates to your body expending more energy to maintain basic life-sustaining processes throughout the day [67]. 

In general, most active people should limit their daily caffeine intake to one or two doses of 100-400 mg. This amount will provide plenty of ergogenic benefits without leading to harsh side effects. 

Be aware that the more caffeine you take, the higher your tolerance becomes; it's imperative to take periodic breaks from caffeine to avoid becoming dependent on it.

Cayenne Pepper Extract (Capsaicin)

Cayenne pepper contains capsaicinoids, notably capsaicin, which have been shown to increase energy expenditure by elevating epinephrine and norepinephrine [8]. Research also indicates that capsaicin regulates hunger signaling pathways and stabilizes insulin levels, leading to favorable nutrient partitioning and fewer food cravings between meals [9].

Capsaicinoids also increase lipolysis and fat oxidation. More simply, capsaicinoids help mobilize stored triglycerides from fat cells so fatty acids can be burned for energy [10]. In addition, the capsaicin in cayenne pepper activates TRPV1 receptor channels that convert white adipose tissue to brown adipose tissue, the latter of which is metabolically demanding body fat [11]. 

Green Tea Leaf Extract

Green tea leaves are a rich source of catechins, a class of promising polyphenol antioxidants that have numerous health benefits [12]. Green tea leaves also contain a low amount of natural caffeine, but they can be made stimulant-free through processing techniques. 

The primordial green tea catechin is epigallocatechin gallate (EGCG), which has been shown to increase energy expenditure, fatty acid oxidation, and caffeine's thermogenic effects [13]. Recent evidence also suggests that green tea extract favorably alters anti-inflammatory biomarkers of exercise, specifically adiponectin and c-reactive protein [14]. 

An effective dose of green tea catechins ranges between 100-800 mg per day based on the current research [15].


Forskolin is an eponymous diterpenoid found predominantly in the roots of the plant Coleus forskohlii [16]It works by increasing the activity of the secondary cellular cyclic AMP (adenosine monophosphate), which is an integral regulator of fat storage, fat release, and hormonal control [1718].

Animal studies have shown that forskolin supplementation reduces fat accumulation by stimulating the release of stored triglycerides from fat cells and liver cells [19]. Furthermore, forskolin appears to increase circulating thyroid hormone levels, likely through cAMP-dependent mechanisms [20].

Thyroid hormones have ubiquitous effects on metabolism and thermogenesis. It's common for people with an underactive thyroid gland to struggle to lose weight, while those with an overactive thyroid can seemingly eat whatever they want and stay lean. 

After long periods of low-calorie dieting, use of stimulants, and excessive amounts of physical activity, your metabolic rate will naturally decline. The body does this as a defense mechanism to ensure you survive during calorie deprivation. The decrease in resting metabolic rate is directly related to reducing thyroid hormone activity [21]. 

Other health benefits of forskolin may include relaxing the arteries, lowering blood pressure, and reducing pro-inflammatory biomarkers [22].


Yohimbine is a stimulant compound derived from the plant species Pausinystalia Yohimbe. Research suggests that yohimbine induces lipolysis by inhibiting alpha-2a adrenergic receptors in adipose tissue and stimulating the production of catecholamines [23]. 

A 21-day study of 20 elite male soccer players found that subjects who supplemented with 20 mg of yohimbine daily lost significantly more body fat than those taking a placebo [24]. A separate study of obese subjects demonstrated similar results [25].

As a "bonus," yohimbine is a natural aphrodisiac that promotes sexual vitality in men and women [26]. However, yohimbine is known to have potent psychoactive properties and is more likely to cause adverse effects in high doses (e.g. >20 mg per day) [27]. When taking yohimbine with other stimulant-based fat burners, stick to a lower dose of 2.5-5 mg.


L-Carnitine is a naturally occurring quaternary ammonium compound found in meat and dairy products. Sufficient carnitine is vital for fat metabolism because it transports long-chain fatty acids into mitochondria, where they are burned (oxidized) for energy [28].

Some research suggests that L-carnitine may enhance endurance since fat is the primary metabolic fuel for long-term aerobic exercise [29]. There is also data that L-carnitine supplementation can significantly improve recovery from intense training [30]. 

Effective doses of carnitine range between 2,500 - 5,000 mg once per day; acetyl-L-carnitine (ALCAR) may be equally effective in smaller doses (1,000 - 2,000 mg) [31]. 


Naringenin is a distinct flavanone found in various citrus fruits (primarily grapefruit) with several metabolic and antioxidant properties in humans [32].

Several clinical trials have shown that consuming 150 - 900 mg of naringenin per day, either from whole orange juice or Citrus sinensis extract, leads to marked improvements in blood pressure readings, blood lipid profiles, and antioxidant activity [3334]. 

Another benefit of naringenin is that it inhibits caffeine metabolism, thereby prolonging caffeine's stimulatory effects [35]. Studies have also shown that naringenin improves nutrient partitioning and insulin sensitivity, reducing the risk of metabolic syndrome and type-2 diabetes [36]. 

5-HTP (Hydroxytryptophan)

5-hydroxytryptophan (5-HTP) is a derivative of the essential amino acid L-tryptophan; it is commonly extracted from the plant species Griffonia simplicifolia

5-HTP is classified as a nootropic, meaning it beneficially alters brain chemistry to support a better sense of well-being. 5-HTP readily traverses the blood-brain barrier and is converted into serotonin — a "feel-good" neurotransmitter that promotes calmness and relaxation. 

By increasing serotonin levels in the brain, 5-HTP can reduce appetite and cravings for sweets [37]. Several clinical trials have shown that 5-HTP supplementation (500 - 1,000 mg per day) significantly reduces overall food intake [38]. Ultimately, this means that a 5-HTP supplement may help people stay in a calorie deficit as part of a weight-loss program

Fat Burner FAQs

Q: Will I lose weight if I take a fat-burner and eat too many calories?

A: Absolutely not. Dietary supplements are not magic pills, so don't expect them to make up for poor nutritional habits and a lack of exercise. You need to be in a calorie deficit to lose body fat; there's no way around that.

Q: Do I need a fat-burner to lose weight?

A: Certainly not, but if you buy a properly formulated product, such as Transparent Labs Fat Burner, it can undoubtedly help you burn body fat over time.

Q: How long should I take a fat-burner to notice the best results?

A: It depends on the ingredients in the fat burner you're taking. In any case, you should limit your use of a thermogenic supplement to a few months at a time, particularly if it contains large doses of stimulants.

Q: What about using diuretics for weight loss?

A: Diuretics are not weight-loss supplements in the same sense as thermogenics. Instead, diuretics work by eliminating excess water weight. They are not something you should be relying on to lose bodyweight.

However, diuretics might be useful for short-term changes in body composition and muscle definition, especially in the context of competitive bodybuilding or getting ready for a photoshoot. 

As noted above, fat burners/thermogenics work to increase your metabolism and enhance fatty acid oxidation. 

Are Fat Burners Safe?

The safety of a fat burner supplement depends on the formula. Does scientific and clinical research support the ingredients? Where do the ingredients come from? How reliable is the supplement company that makes the product? These are all questions you have to consider when purchasing weight-loss supplements. 

While everyone is unique and may respond differently, the side effects of taking a quality fat burner formula in proper doses will be nonexistent to mild for the majority of healthy individuals. If you take a fat burner with a high amount of stimulants, you may experience side effects like shaking, heart palpitations, tachycardia, or anxiousness. 

Follow the dosing instructions on the label of the specific thermogenic supplement you're using. When in doubt, start with a 1/2 serving to assess your tolerance. 

Transparent Labs PhysiqueSeries Fat Burner pills are best taken at four capsules per day, ideally on an empty stomach earlier in the day. You can also rest easy knowing that TL fat burners are third-party tested for purity, potency, and authenticity. Plus, full-disclosure labeling means you don't have to guess what the doses are of individual ingredients.

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