What does not work (and sells very well)
Fat burners, testosterone boosters, glutamine, BCAAs, ketones and company: what the evidence says about each, and why they still get the best shelf space.
This is probably the chapter in the guide that saves the most money. None of these products is a scam in the legal sense: they contain what they say and are sold with permitted claims. They simply do not do what people think they do.
Fat burners and thermogenics
The real effect is the caffeine inside them
Jeukendrup and Randell reviewed the usual "fat burner" ingredients — carnitine, green tea, HCA, CLA, synephrine, raspberry ketone — and found that almost none meaningfully increases fat oxidation in humans, and that none produces significant fat loss beyond the calorie deficit. The only one with a measurable effect on expenditure is caffeine, and it is small.
And it is the riskiest category
Fat burners account for a disproportionate share of detected adulterations: withdrawn sibutramine, amphetamine analogues, thyroid hormones. It is where most cases of acute liver injury have been documented (why it happens).
What does work for losing fat is in the training guide: a sustainable deficit, high protein and maintained strength work. There is no legal chemical shortcut.
Testosterone "boosters"
Neither the composition nor the claims hold up
Clemesha analysed the best-selling supplements in this category and their claims: 90% claimed to raise testosterone, and fewer than a quarter had any data behind it; several contained zinc and other minerals well above the tolerable upper limit. Tribulus, the historic headline ingredient, has shown no effect on testosterone or body composition in healthy men.
The nuance worth understanding: correcting a zinc or vitamin D deficiency in someone who is deficient can normalise hormones. That is not "boosting testosterone": it is no longer having it suppressed by a shortfall. In someone with normal levels, nothing happens.
The list, with what the evidence says
| Product | What it promises | What there is |
|---|---|---|
| BCAAs | Anti-catabolism, recovery | Six of the nine essentials are missing; they cannot sustain protein synthesis (detail) |
| Glutamine | Recovery, immunity | No effect in healthy athletes; its demonstrated use is clinical |
| L-carnitine | Burning fat | Oral carnitine does not raise muscle carnitine except with months-long protocols alongside large amounts of carbohydrate; no useful effect on body fat |
| CLA | Reducing fat | Tiny, inconsistent effects in humans |
| Tribulus, ZMA, D-aspartic acid | Testosterone | No effect in healthy men with normal levels |
| Exogenous ketones | Alternative fuel, performance | Real mechanism, negative or neutral performance results in most trials; expensive and with GI upset |
| HMB in trained people | Muscle mass | The large effects appear in untrained people and in muscle wasting; they shrink in people who already train |
| Multivitamin without a deficiency | Performance, energy | No demonstrated effect on performance or body composition |
| High-dose antioxidants | Recovery | May attenuate training adaptations (why) |
| Shark cartilage, alkaline water and similar | Various | No plausible mechanism and no evidence |
Exogenous ketones have not delivered what the mechanism promised
Evans, Cogan and Egan explain the physiological basis well: ketone bodies are a fuel muscle can use and that spares glycogen. The problem is what happened when it was measured: most performance trials find null or negative effects, with frequent gastrointestinal upset. It is a good example of a convincing mechanism that does not translate into a result.
Why they keep selling
No bad faith needs to be assumed to explain it. Four mechanisms work at once, and they are worth recognising, because the next product will use the same ones.
- 1
The study exists, but does not say that
In vitro, in rats, at impossible doses, or in clinical populations. The citation is real; the extrapolation to the gym is not.
- 2
Sensation gets mistaken for effect
The tingle of beta-alanine, the pump from citrulline, the heat from a thermogenic. Feeling something is not working, and working is not always felt.
- 3
It coincides with the moment people get serious
Someone who buys a fat burner usually starts a diet and trains harder in the same week. The product takes credit for the other two.
- 4
The margin pays for the advertising
A product with no evidence is defended with sponsorships, athletes and influencers — which is exactly the budget a generic creatine does not have.
The one-sentence filter
If a product promises a big, fast result, it is either false or a drug. The supplements that work promise very little: 2%, one more repetition, two minutes off a marathon. A modest claim is a quality signal.
Takeaways
- In a fat burner, the only thing doing anything is caffeine, and not much.
- Testosterone boosters do not raise testosterone in people with normal levels.
- BCAAs, glutamine, CLA, carnitine, tribulus and ZMA have no useful effect in healthy athletes.
- Exogenous ketones are the textbook case of good mechanism, null result.
- A big, fast claim is the best signal that something does not work.
Sources for this chapter
- [1]Jeukendrup AE, Randell R (2011). Fat burners: nutrition supplements that increase fat metabolism. Obesity Reviews. review
- [2]Clemesha CG, Thaker H, Samplaski MK (2020). 'Testosterone Boosting' Supplements Composition and Claims Are not Supported by the Academic Literature. World Journal of Men's Health. review
- [3]Wall BT, Stephens FB, Constantin-Teodosiu D, Marimuthu K, Macdonald IA, Greenhaff PL (2011). Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. Journal of Physiology. trial
- [4]Wolfe RR (2017). Branched-chain amino acids and muscle protein synthesis in humans: myth or reality?. Journal of the International Society of Sports Nutrition. review
- [5]Gleeson M (2008). Dosing and efficacy of glutamine supplementation in human exercise and sport training. Journal of Nutrition. review
- [6]Evans M, Cogan KE, Egan B (2017). Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation. Journal of Physiology. review
- [7]Wilson JM, Fitschen PJ, Campbell B, et al. (2013). International Society of Sports Nutrition Position Stand: beta-hydroxy-beta-methylbutyrate (HMB). Journal of the International Society of Sports Nutrition. position stand
- [8]Merry TL, Ristow M (2016). Do antioxidant supplements interfere with skeletal muscle adaptation to exercise training?. Journal of Physiology. review
- [9]Australian Institute of Sport (2025). AIS Sports Supplement Framework: clasificación ABCD de suplementos. Australian Institute of Sport. official guideline
- [10]Cohen PA (2014). Hazards of hindsight — monitoring the safety of nutritional supplements. New England Journal of Medicine. review