Evidence-Based Fitness Supplements: What to Take (and What to Skip)

The sports supplement market moves billions every year, and almost all of it is marketing smoke. Only a handful of molecules have solid evidence (dozens of meta-analyses, not a single brand-sponsored study). In this guide we review supplements with real scientific backing, classify them by evidence level, and give you direct links to buy them conveniently.

Before we start: no supplement replaces a diet with enough protein, a well-planned calorie deficit or surplus, rest, and progressive training. If the foundation isn't there, supplements won't save you. If it is, some of them do make a difference.

How to evaluate supplement evidence

To classify each supplement we use four levels, based on the usual review hierarchy (ISSN, Examine.com, IOC position statements):

• Strong evidence: multiple meta-analyses with consistent effect and a clear mechanism. Recommended.
• Moderate evidence: several clinical trials showing an effect, but with limitations (small samples, heterogeneity).
• Limited evidence: promising but contradictory studies. May help in very specific contexts.
• Insufficient or no evidence: pure marketing. Not worth the money.

Supplements with strong evidence

1. Creatine monohydrate

The most studied supplement on the planet (more than 1,000 published trials). It increases muscle phosphocreatine availability, which translates into better performance in short, explosive efforts (strength sets, sprints, jumps), more lean mass gain, and better recovery between sets. Additional documented benefits: cognitive performance during sleep deprivation and a possible neuroprotective effect.

• Dose: 3-5 g per day, every day, indefinitely. A loading phase (20 g/day for 5-7 days) speeds saturation, but isn't required.
• When to take it: timing barely matters; consistency is what counts.
• Form: monohydrate. 'HCl', 'buffered', or 'premium micronized' versions aren't superior—just more expensive.
• Side effects: 1-2 kg of intramuscular water gain in the first weeks (not fat). Safe long-term in healthy people.

2. Whey protein

Whey isn't 'magic': it's powdered food. Its value is how convenient it is for hitting your daily protein target (1.6-2.2 g/kg for hypertrophy) when you can't get there with real food. It has a good essential amino acid profile, high bioavailability, and fast absorption.

• Whey concentrate: the most cost-effective option. ~75-80% protein, some lactose. For most people it's the best choice.
• Whey isolate: more expensive, ~90% protein, almost no lactose. Useful if you're lactose intolerant or dialing in macros tightly.
• Whey hydrolyzed: hydrolyzed, faster, more expensive. Usually not worth it.
• Casein: slow-digesting protein. Useful before bed if you train late, not essential.

3. Caffeine

The most studied stimulant and the most effective ergogenic after creatine. It improves max strength, power, endurance, and perceived exertion. It blocks adenosine receptors and increases adrenaline release.

• Effective dose: 3-6 mg/kg body weight, about 30-60 min before training.
• For a 75 kg person = 225-450 mg (what you'd get from 2-4 espressos or one 200 mg pill).
• Tolerance: develops in 2-4 weeks; cycling (5 days on, 2 days off) helps maintain the effect.
• Watch out if you train late: up to 6 h half-life. After 4:00 PM you can wreck your sleep.

Supplements with moderate evidence

4. Beta-alanine

It increases muscle carnosine, which buffers acid produced during high-intensity efforts. Evidence is solid for sustained efforts of 1 to 4 minutes: long bench press sets, short intervals, CrossFit, Hyrox. For efforts under 30 seconds or over 10 minutes the effect is marginal.

• Dose: 3.2-6.4 g per day for 4-12 weeks (it's cumulative: takes time to notice).
• Split into 2-4 doses to avoid paresthesia (skin tingling, harmless).
• No acute effect: taking it right before training doesn't work.

5. Citrulline malate

Precursor of arginine and nitric oxide. The strongest evidence is for muscular endurance (more reps in later sets at the same load) and reducing post-workout muscle soreness (DOMS). For 'better pump' or 'more max strength' the evidence is weak.

• Dose: 6-8 g of citrulline malate (not L-citrulline alone), 30-60 min before training.
• Works acutely (same-day effect), no loading required.
• Very bitter taste: mix it with a flavored drink or buy a flavored version.

6. Omega-3 (EPA + DHA)

Fatty fish is the best source. If you eat 2-3 servings a week, you don't need to supplement. If not, marine omega-3s have solid evidence for cardiovascular health, lipid profile, post-workout recovery, and reducing low-grade inflammation. For hypertrophy or fat loss the direct effect is small.

• Dose: 1-3 g/day of combined EPA + DHA (not total oil: read the label).
• Quality: look for IFOS certification or purity testing (some brands have mercury or are rancid).
• Vegetarian/vegan: algae oil (mainly DHA).

7. Vitamin D3

Not an ergogenic per se, but vitamin D deficiency is extremely common in Spain (especially October through March, and in people who train indoors). Deficiency is linked to worse strength, worse recovery, and worse bone health. If your blood work comes back low (<30 ng/ml of 25-OH-vitamin D), supplementing is cost-effective.

• Dose: 1,000-2,000 IU/day for maintenance; 4,000 IU/day if starting from a deficit (consult your doctor).
• Better with vitamin K2 (MK-7) if you're on sustained high doses.
• Take with a meal containing fat to improve absorption.

Supplements with limited evidence

8. Magnesium (bisglycinate)

The general population consumes less magnesium than recommended, and deficiency is linked to worse sleep, worse recovery, and more cramps. If your diet is low in nuts, legumes, and leafy greens, supplementing is reasonable. For sleep and recovery, bisglycinate is the form with the best bioavailability and the least laxative effect.

• Dose: 200-400 mg of elemental magnesium at night.
• Form: bisglycinate or citrate. Avoid oxide (poor absorption) unless you want a laxative effect.
• Not a fat burner or a miracle. It's safe and cheap; if you sleep poorly, try it for 4 weeks.

9. Ashwagandha (KSM-66)

An adaptogen with moderate-to-emerging evidence for reducing cortisol, improving sleep quality, and, to a lesser extent, improving strength and body composition. KSM-66 is the standardized form used in most studies.

• Dose: 300-600 mg/day of KSM-66.
• Best if you have a lot of chronic stress or sleep poorly. If life is calm and you sleep well, the effect is small.
• Don't mix with sedatives or anxiolytics without supervision.

Overhyped supplements or weak evidence

L-carnitine

Probably the perfect example of a famous supplement people buy expecting to lose fat. The reality: L-carnitine does play a role in transporting fatty acids to the mitochondria, but your muscle levels are already nearly saturated from diet (meat, fish). Oral supplementation raises muscle concentrations only slightly because absorption is low.

• Recent meta-analyses (Pooyandjoo 2016, Talenezhad 2020) show modest effects on weight loss (~1 kg in 8-12 weeks), but only with high doses (2-4 g/day) and prolonged supplementation.
• The effect on aerobic/anaerobic performance in healthy subjects is practically nil.
• Possible utility in people over 60 (reduces fatigue) and strict vegetarians (lower baseline levels).
• Form: L-carnitine tartrate or acetyl-L-carnitine (the latter crosses the blood-brain barrier; better evidence for cognitive function than fat loss).

BCAAs (branched-chain amino acids)

If you hit your daily protein target (1.6-2.2 g/kg), BCAAs add nothing extra: you're already getting plenty of branched-chain amino acids from whey, chicken, or eggs. Their hype comes from the 90s, when most people ate little protein. Today they're an unnecessary expense for 95% of people.

Very specific exception: prolonged fasted training in advanced athletes trying to minimize catabolism. For the average reader, skip this.

Glutamine, thermogenic fat burners, fat burners

Glutamine has no effect on body composition or hypertrophy in healthy people. Commercial fat burners (caffeine + various extracts) owe almost 100% of their effect to the caffeine they contain. Save your money.

How to choose a quality supplement

1. Clear label: dose per serving and exact form (e.g. 'creatine monohydrate Creapure®', not 'patented matrix').
2. Purity certification: Informed Sport, NSF Certified for Sport, or Cologne List if you compete and may be tested.
3. Cost per effective dose, not per tub: a 1 kg tub at €30 can cost more than a 500 g tub at €18 if doses don't match.
4. Established brand: Myprotein, Bulk, NOW Foods, Optimum Nutrition, Foodspring, BiotechUSA are usually reliable. Brands that appear and disappear every 6 months—skip them.
5. Short ingredient list. The longer the list of 'patented complexes', the more suspicious.

Recommended starter stack (if you're training for strength)

If you have to pick just a few, this would be the minimum viable stack with the best cost/benefit ratio for an average gym-goer:

Monthly total: €30-50 covers everything that actually moves the needle. Anything sold above that stack—demand specific studies before paying.

Frequently asked questions

Do I need supplements if I'm a beginner?

For a beginner, the absolute priority is building a solid foundation of progressive training, adequate nutrition, and rest. Supplements offer a marginal benefit compared to these pillars. It's better to invest first in a good coach, quality food, or gym equipment.

Do supplements have side effects?

Supplements with strong and moderate evidence, like creatine, whey protein, or caffeine, are safe for most healthy people at recommended doses. However, some can cause mild digestive discomfort (creatine, magnesium), tingling (beta-alanine), or affect sleep (caffeine if taken late). Always consult a professional if you have doubts or pre-existing conditions.

Can I combine several supplements from the list?

Yes, supplements with solid evidence can be combined without issue. In fact, the 'starter stack' combines creatine, protein, and caffeine. Make sure to respect individual doses and timing to maximize effectiveness and avoid negative interactions, especially with stimulants.

How do I know if a supplement is high quality?

Look for transparent brands with clear labels (ingredients, doses, active form). Third-party purity certifications like Informed Sport, NSF Certified for Sport, or Cologne List are crucial, especially if you're an athlete subject to anti-doping tests. Avoid products with 'proprietary blends' or unclear ingredients.

Conclusion

90% of products on the supplement aisle are nicely packaged air. The 10% that actually works boils down to creatine, protein powder (if you can't hit it with food), caffeine, omega-3, and vitamin D if you're deficient. Beta-alanine, citrulline, magnesium, and ashwagandha can add value in specific contexts. Everything else (BCAAs, glutamine, fat burners, blends with patented names) is usually money wasted.

The useful rule: never add a supplement unless you're clear on what specific problem it solves and what evidence backs it. And never ask a tub for what only your diet, sleep, and training can give you.