The strongest case can be the least glamorous
Vitamin B12 has an obvious place when dietary supply is unreliable, including a vegan diet without dependable fortified foods or supplementation. Treating B12 deficiency also matters. That is a strong reason for the right person to take it. The same bottle can be important nutritional support for one person and an unconvincing “energy” purchase for another, that yields no result at all.
Iron has a similarly concrete job in confirmed iron-deficiency anemia. Replacing iron addresses an identified problem; taking it because you feel tired is a much less informed decision, and more likely to be pure speculation or an attempt to brute force the problem by trying supplements that promise energy without addressing the causes.
These examples also expose a problem with asking whether vitamins or other supplements “work.” Work for whom? Treating a deficit, preventing a particular problem and enhancing an already healthy person's performance are different questions.
Vitamin D shows why the starting point matters
Vitamin D is another example of a nutrient with an essential job and an overextended sales pitch. Deficiency can cause rickets in children and osteomalacia, a softening of bone, in adults. Preventing or treating those problems is a good reason to ensure adequate vitamin D.
But the question changes when the person taking it has no identified deficiency. The VITAL fracture trial assigned 25,871 middle-aged and older adults to vitamin D or placebo. Participants were not selected for deficiency, osteoporosis or low bone mass. Over a median of 5.3 years, vitamin D did not significantly reduce fractures. That is substantial evidence against buying vitamin D for fracture prevention in that broad setting, while leaving its deficiency-related uses intact. [1]
We would resist both easy conclusions: that vitamin D is useless because this trial was negative, or that everyone should take more because bones require it. The useful question is whether extra vitamin D addresses a problem you might think you have.
A diagnosis can make a supplement worthwhile
Recurrent calcium kidney stones give us a more specific example. When a person forms these stones repeatedly and a urine test shows low citrate, a clinician may offer potassium citrate to help prevent another stone. The American Urological Association guideline[2] recommends that approach for recurrent calcium stones with low or relatively low urinary citrate. The relevant questions are the stone type and urine findings, not whether potassium supplements generally promise better health.
That precision is a strength. A supplement does not need to help every healthy adult to deserve a place in medicine.
Training gives some supplements a clear purpose
For strength training, we would put adequate protein intake ahead of a collection of individual amino-acid products. Protein powder earns its place when it makes that intake easier to achieve, but even this is not universal to all protein powders. The actual composition of the protein powders matters quite a lot, and we've written about it separately in our whey-and-beef mix article.
In a review of 49 trials involving 1,863 adults, extra protein added about 0.30 kg of fat-free mass beyond the comparison groups across training programs lasting at least six weeks. Fat-free mass includes muscle, water and other nonfat tissue; it is not a direct muscle measurement. Training still did most of the work. [3]
Creatine monohydrate also has a very defensible place for repeated intense efforts and strength training. Trials support improvements in strength. But the increase on a body-composition report needs careful wording: lean mass includes water, and studies that directly image muscle find a smaller, less certain growth effect.
Caffeine can improve endurance performance. Whether that earns it a place depends partly on what it costs the person taking it: disrupted sleep, restlessness or palpitations can make a measurable performance advantage a poor trade.
Useful options do not all have equally strong evidence
Psyllium illustrates why we would leave room for a reasonable option without calling it conclusively proven. The American gastroenterology guideline suggests fiber for adults with chronic idiopathic constipation—persistent constipation without an identified underlying cause—and identifies psyllium as the fiber with supportive evidence among those evaluated. The recommendation is conditional and the certainty is low. That is a real clinical use, with an honest qualification.
Psyllium also has evidence for lowering LDL cholesterol. A 2018 review combined 28 trials involving 1,924 participants and found an average reduction of about 0.33 mmol/L—roughly 13 mg/dL. That makes it more interesting than a fiber powder chosen solely on a “gut health” slogan.
We would regard it as a potentially useful addition to a broader cholesterol-management plan.
Beta-alanine has a narrower sports case than creatine. A review of 40 studies found a small overall exercise benefit. Results were more favorable in tests of continuing until exhaustion than in tests of completing a set task, such as a fixed distance, faster. The case concerns short, intense efforts; we would keep it an optional, uncertain choice for improving competitive results. It needs regular use over weeks and can cause tingling.
Cranberry illustrates a better question than “Does it work?”
For cranberry, the worthwhile question is whether it helps prevent urinary tract infections in someone prone to them. A 2023 Cochrane review found that, among women with recurrent infections, cranberry products probably reduced symptomatic, culture-confirmed infections. That analysis included eight studies with 1,555 women; its central estimate was about a 26% lower risk than placebo or no cranberry treatment, with uncertainty ranging from a substantial reduction to only a small one. Absolute benefit depends on how often infections would otherwise occur.
The same review did not find convincing benefit in several other groups, including pregnant women and institutionalized older adults. Products and their active-ingredient content varied, so the evidence does not identify one dependable dose across every juice and capsule. We would describe cranberry as a reasonable preventive option for some women with recurrent infections.
Melatonin earns a narrower claim than “better sleep”
Timed melatonin can help with jet lag because it can shift the body's sleep–wake clock. The CDC's travel guidance emphasizes that timing depends on the direction of travel and the traveler's body clock. Taken at the wrong time, melatonin can worsen the misalignment. Light exposure and the sleep schedule also matter. [4]
That is a useful purpose with an important practical condition. It is a stronger argument than treating melatonin as a universal solution for poor sleep.
Evidence belongs to a preparation, not just a familiar name
“Omega-3 works” can conceal a large change of product. For example, REDUCE-IT tested a prescription preparation of purified EPA, one of the omega-3 fatty acids, in patients taking cholesterol-lowering statins. They had raised triglycerides, a type of blood fat, and substantial cardiovascular risk. Treatment reduced a combined measure covering cardiovascular death, heart attack, stroke, coronary procedures and hospitalization for unstable angina. That result does not automatically apply to an ordinary fish-oil capsule with a different formulation and amount, taken by a healthy person. [5]
We would want any recommendation based on that trial to say exactly what was tested and who was treated. The same standard applies to cranberry preparations and amino-acid blends: a shared ingredient name is a starting point for comparison, not proof that the products with similar ingredients or different doses will produce the same result.
This also changes how we view a long ingredient list. If the evidence concerns a complete formulation, credit belongs to that formulation. If it concerns one ingredient, adding a trace amount to a blend does not establish the blend's effectiveness. More ingredients can make the scientific question harder to answer rather than making the product more useful, and many supplement manufacturers abuse this principle to claim benefits based on trace amounts of some ingredient that some study found that it occasionally provides.
A useful benefit still has a cost
The size of a benefit also has to justify the cost and inconvenience. A small performance improvement may matter greatly to a competitive athlete and very little to someone exercising for general health.
Our skepticism also needs evidence
A claim that a supplement is useless needs scrutiny too. While we do our best to cover as much of the research as we can, we may overlook a relevant clinical trial or count several papers from the same participants as independent support. We may miss some studies or just not have access to them, and we do our best to correct and expand our article when we find new information that changes the outlook on some supplement becomes available to us.
Conversely, finding another positive paper is not enough to settle the question. We would ask whether it tests the product and use under discussion, how it compares with contrary findings, and whether an independent group has reproduced the result.
That is why we favor a list of supported uses over a permanent list of virtuous supplements. “Useful for this purpose in these circumstances” is a claim we can examine and update. “Good for you” is often too vague to survive the same scrutiny, or too individual for us to make any statements about.
What we would buy—and what would change our minds
We would change our view when good trials showed a meaningful benefit in the population and outcome at issue—or when better studies weakened a benefit we had accepted. Repetition by independent groups, appropriate comparisons and results that matter to daily life would carry more weight than another plausible mechanism.
Our standard is demanding, but it leaves plenty of room for useful supplements: know what you are considering a product for, and measure whether it gives you the result you were looking for.
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