Metabolism boosters—also called thermogenic supplements or fat burners—are widely marketed to increase the rate at which the body burns calories. Typically containing caffeine, green tea extract, capsaicin, and various herbal compounds, these products promise enhanced metabolic rate and weight loss. However, their safety and efficacy remain contentious. In the UK, most are regulated as food supplements rather than medicines, meaning they do not undergo the rigorous testing required for pharmaceuticals. This article examines the evidence on metabolism boosters, their potential risks, and safer, evidence-based alternatives to support metabolic health.

What Are Metabolism Boosters and How Do They Work?

Metabolism boosters, also known as thermogenic supplements or fat burners, are products marketed to increase the body's metabolic rate—the speed at which calories are burnt at rest and during activity. These products typically contain a combination of ingredients including caffeine, green tea extract, capsaicin, L-carnitine, conjugated linoleic acid (CLA), and various herbal compounds such as guarana or bitter orange (synephrine).

The proposed mechanisms of action vary depending on the active ingredients. Caffeine and similar stimulants work by increasing sympathetic nervous system activity, which can temporarily elevate heart rate, blood pressure, and energy expenditure. Green tea extract contains catechins, particularly epigallocatechin gallate (EGCG), which may modestly enhance fat oxidation. Capsaicin, derived from chilli peppers, is thought to increase thermogenesis—the production of heat in the body—thereby burning additional calories.

However, it is important to understand that the metabolic effects of these supplements are typically modest and temporary. Clinical studies suggest that any increase in metabolic rate is usually small, and effects may diminish as the body develops tolerance to stimulant-based products over time.

In the UK, most metabolism boosters are classified as food supplements rather than medicines. This means they are regulated under food law by the Food Standards Agency (FSA) and local Trading Standards, while advertising claims are regulated by the Advertising Standards Authority (ASA). They are not subject to the same rigorous testing and regulation as pharmaceutical products. The Medicines and Healthcare products Regulatory Agency (MHRA) becomes involved if products are found to contain medicinal ingredients or are adulterated with illegal substances. This regulatory framework means that product quality, ingredient accuracy, and contamination risks can vary considerably between brands.

These supplements are generally not recommended for pregnant or breastfeeding women, people under 18, or those with pre-existing cardiovascular conditions.

Potential Risks and Side Effects of Metabolism Boosters

Metabolism boosters carry several potential health risks, particularly when they contain high doses of stimulants or unregulated herbal ingredients. The most commonly reported adverse effects are related to cardiovascular and central nervous system stimulation.

Cardiovascular risks include elevated heart rate (tachycardia), increased blood pressure (hypertension), palpitations, and in rare cases, more serious events such as arrhythmias, myocardial infarction, or stroke. These risks are particularly concerning for individuals with pre-existing cardiovascular conditions, hypertension, or those taking other medications that affect heart function. Products containing synephrine (bitter orange extract) have been associated with cardiovascular events in case reports.

Central nervous system effects commonly include anxiety, restlessness, insomnia, tremor, and headaches. High caffeine content—often exceeding 200–400mg per dose—can lead to caffeine toxicity, characterised by severe agitation, seizures, and in extreme cases, death. The European Food Safety Authority (EFSA) recommends that single doses of caffeine should not exceed 200mg, and daily intake should remain below 400mg for most adults. Pregnant women should limit total caffeine from all sources to 200mg per day.

Gastrointestinal disturbances such as nausea, diarrhoea, and abdominal discomfort are frequently reported. Some products have been linked to hepatotoxicity (liver damage), particularly those containing green tea extract in high concentrations. EFSA has identified risks associated with high doses of EGCG from green tea extracts. Warning signs of liver problems include yellowing of the skin or eyes (jaundice), dark urine, upper right abdominal pain, or unusual fatigue.

Additionally, there are concerns about product contamination and adulteration. Some metabolism boosters have been found to contain undeclared pharmaceutical ingredients, including banned substances such as ephedrine, sibutramine, or 2,4-dinitrophenol (DNP), which carry serious health risks. The MHRA regularly issues safety alerts regarding contaminated weight-loss products.

Drug interactions are another significant concern. Metabolism boosters can interact with prescription medications including antihypertensives, anticoagulants, antidepressants (particularly MAOIs), and diabetes medications, potentially reducing their effectiveness or increasing adverse effects.

If you experience chest pain, severe palpitations, collapse, severe headache with neurological symptoms, seizures, or severe shortness of breath after taking these products, call 999 or go to A&E immediately. If you develop jaundice, dark urine, or severe abdominal pain, seek same-day medical attention.

Suspected adverse reactions to supplements can be reported through the MHRA Yellow Card Scheme.

Evidence-Based Alternatives to Boost Your Metabolism

Rather than relying on supplements of uncertain safety and efficacy, there are several evidence-based lifestyle interventions that can support healthy metabolic function without the associated risks.

Regular physical activity, particularly resistance training and high-intensity interval training (HIIT), is one of the most effective ways to increase metabolic rate. Muscle tissue is metabolically active, burning more calories at rest than fat tissue. The NHS recommends at least 150 minutes of moderate-intensity aerobic activity or 75 minutes of vigorous-intensity activity per week, combined with strength exercises on two or more days. Building lean muscle mass through resistance training can modestly increase resting energy expenditure, primarily by increasing fat-free mass.

Adequate protein intake supports muscle maintenance and has a higher thermic effect of food compared to carbohydrates or fats—meaning the body uses more energy to digest and process protein. The UK Reference Nutrient Intake (RNI) for protein is approximately 0.75g per kilogram of body weight daily for adults. Some weight management approaches may include slightly higher protein intakes to support satiety as part of a balanced, calorie-controlled diet.

Sufficient sleep is crucial for metabolic health. Sleep deprivation disrupts hormones that regulate appetite and metabolism, including leptin and ghrelin, and is associated with weight gain and insulin resistance. The NHS recommends that most adults need between 7 and 9 hours of quality sleep per night.

Staying well-hydrated is important for general health. While drinking water has been suggested to have a very small temporary effect on energy expenditure, this effect is minimal and should not be relied upon for weight management.

Managing stress is important, as chronic stress elevates cortisol levels, which can promote fat storage, particularly visceral fat. Stress management techniques such as mindfulness, cognitive behavioural therapy (CBT), or regular physical activity can support overall metabolic health.

Avoiding very low-calorie diets is essential, as severe caloric restriction can actually slow metabolic rate as the body adapts to conserve energy. A modest caloric deficit of 500–600 calories per day, as recommended by NICE for weight management, is more sustainable and less likely to cause metabolic adaptation.

When to Speak to Your GP About Metabolism Concerns

If you are concerned about your metabolism or are considering using metabolism boosters, it is advisable to consult your GP for personalised advice and to rule out underlying medical conditions.

You should seek medical attention if you experience:

• Unexplained weight changes—either significant weight gain or loss without changes to diet or activity levels, which may indicate thyroid disorders, diabetes, or other metabolic conditions

• Persistent fatigue despite adequate rest, which could suggest hypothyroidism, anaemia, or other systemic conditions

• Symptoms suggestive of thyroid dysfunction, including cold intolerance, dry skin, hair loss, constipation (hypothyroidism), or heat intolerance, palpitations, tremor, and anxiety (hyperthyroidism)

• Difficulty losing weight despite sustained lifestyle modifications, which may warrant investigation for conditions such as polycystic ovary syndrome (PCOS), Cushing's syndrome, or insulin resistance

Your GP can arrange appropriate investigations, which may include thyroid function tests (TSH, free T4), full blood count, liver and kidney function tests, fasting glucose and HbA1c to assess for diabetes, lipid profile, and other relevant blood tests based on your symptoms. If an underlying condition is identified, evidence-based treatment can be initiated.

It is particularly important to inform your GP if you are currently taking or considering metabolism boosters, especially if you have pre-existing health conditions such as cardiovascular disease, hypertension, diabetes, liver disease, or psychiatric conditions, or if you are taking prescription medications. Your GP can advise on potential interactions and safer alternatives.

Seek urgent medical attention (call 999 or go to A&E) if you experience chest pain, severe palpitations, collapse, severe headache with neurological symptoms, seizures, or severe shortness of breath after taking metabolism boosters.

For individuals with obesity (BMI ≥30 kg/m² or ≥27.5 kg/m² for people of South Asian, Chinese, or other Asian family origin), NICE recommends a multicomponent weight management programme involving dietary modification, increased physical activity, and behavioural interventions. In some cases, pharmacological treatment may be appropriate under medical supervision. Options include orlistat (for BMI ≥28 kg/m² with risk factors or ≥30 kg/m²) and, where specific criteria are met, GLP-1 receptor agonists such as semaglutide or liraglutide. Your GP can refer you to NHS weight management services if needed, ensuring you receive safe, evidence-based care rather than relying on unregulated supplements.

Frequently Asked Questions