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Fat-loss supplements: fact or fiction?

Rob Hobson
Article written by Rob Hobson

Date published 04 December 2023

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In a fast-paced world, where quick results are highly prized, fat-burning supplements are increasingly popular – thanks to the promise of rapid weight loss and an effortless path to fitness goals.

But once you've cut through the flashy adverts and bold claims, are these supplements genuinely effective or just clever marketing playing on bodyweight insecurities and a desire for quick results?

From green tea extracts and caffeine to more exotic compounds like New Zealand blackcurrants, this article separates fat-burner fact from fiction.

🕒 9 min read

Green tea extract: FACT

Green tea extract can assist in fat burning, but the effect might be more modest than some claims suggest. The active ingredients in green tea, particularly catechins like epigallocatechin gallate (EGCG) and caffeine, have been studied for their potential effect on weight loss and metabolism.

How does it work?

Increased metabolism: Caffeine and catechins can help increase energy expenditure and fat oxidation, which means they can help the body burn fat more efficiently. An analysis of six studies showed that a combination of green tea and caffeine helped people burn 16% more fat than a placebo.

Increased fat burning while exercising: Some research indicates that green tea extract might be more effective at increasing fat burning during physical activity. One study of men who took green tea extract found that they burned 17% more fat than men who did not take the supplement.

Appetite suppression: There is some evidence that green tea can help in appetite regulation, although this is less conclusive.

Reality check

Modest effects: Although studies show some positive results, the effect of green tea extract on weight loss is relatively modest. It's not a miracle solution and works best when combined with a healthy diet and regular exercise. The greatest results were found when green tea was taken in combination with caffeine.

Variability in response: Individuals may respond differently to green tea extract. Genetics, lifestyle, and the starting level of body fat might influence its effectiveness.

Caffeine: FACT

Caffeine is one of the few natural substances proven to aid in fat burning. It's found in coffee, tea and various other foods and beverages, as well as in many fat-burning supplements. Here's how caffeine can contribute to fat loss.

How does it work?

Boosts metabolism: Caffeine can increase the body's metabolic rate, meaning it helps to burn more calories at rest. This thermogenic effect makes caffeine a popular ingredient in weight loss supplements. Research has shown that caffeine can help to boost your metabolism by up to 16% over 1-2 hours.

Enhances fat oxidation: Caffeine can stimulate the breakdown of fatty acids stored in adipose tissue (body fat). This process, known as lipolysis, involves releasing fatty acids into the bloodstream, where they can be used for energy. However, this appears to be more significant in people with a lower body weight versus those who are overweight or obese.

Improves exercise performance: By stimulating the central nervous system, caffeine can reduce the perception of effort during exercise, potentially leading to longer or more intense workouts and thus increased calorie burn.

Reality check

Diminishing effects: Over time, individuals may develop a tolerance to the effects of caffeine, reducing its effectiveness for weight loss.

Not a magic solution: Although caffeine can aid fat burning, it's not a standalone solution. It works best when combined with a healthy diet and regular exercise.

Potential side effects: High doses of caffeine can have side effects like jitteriness, insomnia, heart palpitations, and increased blood pressure. It's important to consume it in moderation.

Individual sensitivity: People have varying sensitivities to caffeine. Some may experience more pronounced effects, while others may need to limit their intake due to negative reactions.

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CurraNZ blackcurrant extract: FACT

There is emerging research suggesting that blackcurrant extract may have potential benefits in fat burning and weight management, but the evidence is still in its early stages and not conclusive.

Blackcurrant extract is derived from the blackcurrant fruit, known scientifically as Ribes nigrum. It's rich in antioxidants, particularly anthocyanins, which are responsible for the fruit's deep purple colour.

How does it work?

Increased fat metabolism: Human trials have found that compounds in blackcurrants can increase fat metabolism. This might help the body to burn fat more efficiently.

Exercise performance: There is evidence that blackcurrant extract can enhance blood flow and reduce muscle fatigue, potentially improving exercise performance. Better exercise performance can indirectly contribute to fat burning.

Antioxidant-rich: The high antioxidant content in blackcurrants might aid overall health and could potentially support weight-loss efforts, although this is a more indirect effect.

Reality check

Good-quality convincing research: Research indicates potential benefits from blackcurrant extract in enhancing fat utilisation during extended physical activity. This might be advantageous for endurance athletes, potentially reducing the need for carbohydrate replenishment during competitions.

In one investigation, 16 endurance-trained female athletes consumed either two blackcurrant capsules (CurraNZ) or a placebo before a two-hour cycling session at a moderate pace. The results showed a notable 27% increase in fat oxidation following the intake of the blackcurrant capsules, with the study ensuring sufficient carbohydrate reserves were available.

Similar outcomes have been documented in recent studies published in Dietary Supplements and Nutraceuticals. A two-week trial involving 16 healthy men revealed that CurraNZ blackcurrant extract enhanced fat oxidation rates both at rest and during physical activity. Participants who took a daily dose of 600mg experienced an average increase in fat oxidation of 21%, with some individuals showing increases as high as 61%.

Furthermore, the research highlighted that 75% of the subjects demonstrated improved fat-burning responses. These findings are not only beneficial for endurance athletes, who aim to train their bodies for increased fat utilisation during exercise, but they also suggest potential advantages for weight management.

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L-carnitine: Fact but with caveats

L-Carnitine, a naturally occurring amino acid, is often marketed for its potential fat-burning capabilities. Although there is some scientific basis to these claims, the reality is more complex and less conclusive.

L-Carnitine plays a crucial role in energy production by transporting fatty acids into the mitochondria, the cell's powerhouses, where they are burned for energy. This function has led to the hypothesis that supplemental L-Carnitine could enhance fat burning.

How does it work?

Exercise performance: Some studies have shown that L-Carnitine supplementation may improve exercise performance, which in turn could lead to increased fat burning.

Modest fat loss: Research has demonstrated modest fat loss in certain groups, particularly in those with lower levels of L-Carnitine to begin with or among elderly populations.

Varied results: Overall the evidence is mixed. Many studies have not found significant weight-loss or fat-burning effects from L-Carnitine supplementation, particularly in younger individuals and athletes.

Reality check

Supplementation versus natural production: The body produces L-Carnitine naturally, and it is also found in foods, particularly in meat. Supplementing may not provide added benefits if your levels are already sufficient.

Long-term effects and safety: The long-term effects of taking high doses of L-Carnitine are not well understood, and there may be side-effects or interactions with medications.

Weight-loss strategy: Although L-Carnitine might offer some benefits, it's unlikely to be a magic solution for fat loss. Effective weight management typically requires a combination of dietary changes, increased physical activity, and overall lifestyle adjustments.

Raspberry ketones: FICTION

Raspberry ketones, a compound found in raspberries and other fruits, have been touted for their fat-burning properties. However, the scientific evidence supporting these claims is weak and primarily based on animal or test-tube studies.

Raspberry ketones are the natural compounds that give raspberries their distinct aroma. In recent years, they have been extracted and concentrated for use in weight-loss supplements.

How do they work?

Mechanism of action: In theory, raspberry ketones could influence a hormone called adiponectin, which plays a role in regulating metabolism and fat storage. However, this effect hasn't been convincingly demonstrated in humans.

Animal studies: Some of the enthusiasm for raspberry ketones comes from studies in rodents, where high doses appeared to affect fat metabolism.

Lack of human studies: There is a significant lack of reliable scientific evidence to support the effectiveness of raspberry ketones in humans. The studies in rodents cannot be directly applied to human metabolism.

Reality check

Safety and dosage: The safety of raspberry ketones at the high doses required for weight loss is not well established. Supplements contain levels of raspberry ketones far higher than you would get from consuming the fruit.

Marketing versus science: The promotion of raspberry ketones for weight loss is largely driven by anecdotal and testimonial evidence, rather than robust scientific data.

A pile of blackberries

Find out more: The performance benefits of blackcurrant supplements

More than just weight loss: Nutritionist Rob Hobson dives into the other performance benefits of blackcurrants, from blood flow to immunity.


Read more

Protein powder: Fact, with context

Protein powder can be a beneficial tool in a fat-loss diet, but it's not inherently a fat-burning supplement. Its effectiveness lies more in how it supports overall dietary habits that contribute to fat loss.

Protein powder is a concentrated source of protein, usually derived from foods like whey, casein, soy, or peas. It's commonly used to supplement protein intake, especially among athletes, bodybuilders, and those looking to increase their protein consumption for various health reasons.

How does it work?

Satiety and appetite control: Protein is more satiating than carbohydrates or fats, meaning it can help you feel full for longer. Using protein powder to increase your protein intake can reduce overall calorie consumption by curbing appetite.

Increased metabolism: Higher protein intake has been associated with a slight increase in metabolism. This is because the body uses more energy to metabolise protein compared to carbs or fat.

Muscle maintenance and growth: Protein is essential for muscle repair and growth. Maintaining muscle mass is crucial during weight loss because muscle tissue burns more calories than fat tissue, even at rest.

Weight management tool: For those replacing meals or snacks with a protein shake, it can be a convenient and controlled way to manage portions and calorie intake.

Reality check

Part of a balanced diet: Protein powder should be used as a supplement to a balanced diet, not as a sole source of nutrition.

Quality matters: Not all protein powders are created equal. Some may contain added sugars, artificial ingredients, or unhealthy additives.

No magic bullet: Protein powder on its own doesn't burn fat. It aids in fat loss as part of a calorie-controlled diet and regular exercise.

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An all-round approach

The world of fat loss supplements is filled with both fact and fiction. Although some supplements like caffeine and green tea extract have shown moderate promise in aiding fat loss, it's clear that there is no magic pill.

Supplements can play a supportive role, but only diet and regular exercise are going to offer significant and sustainable fat loss. It's also crucial to approach these products with a critical eye, understanding that marketing claims are not always backed by scientific evidence. Ultimately, the journey to effective and sustainable fat loss lies in an all-round approach to health and nutrition.

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Rob Hobson

About Rob Hobson

Rob Hobson MSc RNutr is an award-winning registered nutritionist (AFN) and sports nutritionist (SENR) with over 15 years of experience. He founded London-based consultancy RH Nutrition, and has degrees in nutrition, public health nutrition and sports nutrition.

robhobson.co.uk