Start Here: How to Decide Which Supplements to Consider
Most adults need at most one or two supplements, not a shelf of them. Vitamin D is the one nutrient we look at first. See the population data for the numbers.
Everything else depends on your diet, age and health, so ask your clinician. Most other popular categories rest on evidence that ranges from solid to thin.
55% of the products in our own catalog carry no third-party certification at all.
Informational summary of population health data and published clinical trials — not medical advice. Ask your clinician before you supplement for any health reason.
Want the full grade on a specific category? Jump to the default stack or what you can probably skip →
Start from the population, not the marketing
Supplement marketing starts from what sells. This page starts from a different question: what does blood-test data actually show is common in US adults? That data comes from NHANES, the CDC's national health survey, and it's blunt about which gaps are real and which are rare.
| Item | Value (%) |
|---|---|
| Vitamin D | 22.1% |
| Iron | 3.5% |
| Vitamin B12 | 3.6% |
| Magnesium | 2.5% |
| Folate | 0.2% |
Read that chart plainly: for one nutrient, being low is common no matter who you are. For the rest, being low is uncommon overall and concentrated in specific groups. That distinction is the whole first filter.
| Nutrient | % of US adults low | Who's actually at risk | Worth testing? |
|---|---|---|---|
| Vitamin D | 22.1% overall; 32.6% at 18-30 | Adults of all groups; rates are higher in younger adults | Yes — the 25(OH)D test |
| Iron | 3.5% overall; 6.2% in women | Menstruating women, pregnancy, endurance athletes | Yes if at-risk — ferritin test before you supplement |
| Vitamin B12 | 3.6% overall; 3.7% at 60+ | Adults 65+ and vegans/vegetarians; some medicines can lower B12 | Yes if at-risk — MMA test catches what serum misses |
| Magnesium | 2.5% deficient by serum | Varies; ask your clinician | Uncertain — serum poorly reflects body stores, so testing has real limits |
| Folate | 0.2% overall | Essentially nobody, per this data — folate fortification of US grain products works | No, outside pregnancy planning |
Source: CDC NHANES 2017-March 2020 Pre-Pandemic, survey-weighted; B12 and magnesium from published NHANES analyses. See our vitamin D, iron, B12, and magnesium deficiency pages for the full breakdowns.
Why this ranking, and what it cannot tell you
Ordering nutrients by measured population prevalence is the opposite of how supplements are usually sold, and it is the reason this page recommends so little. The underlying numbers come from NHANES, the CDC's continuous national survey, which draws blood from a sample designed to represent the US population — not from people who already suspected a problem. A consolidated analysis of twelve nutrient biomarkers across NHANES 1999–2023 sets out how deficiency, insufficiency and excess have moved over that period and which subgroups carry them (PMID 42163863).
The magnesium row deserves its asterisk. Serum magnesium is a poor indicator of whether you have enough: less than 1% of the body's magnesium is in blood, and the body defends that concentration by pulling from bone and tissue, so a normal reading can sit on top of a real shortfall. Reviews of magnesium biochemistry make the case directly (PMID 42647115), and there is a standing argument that the reference interval itself needs revisiting (PMID 28140318). Read magnesium's low measured prevalence as a limit of the test rather than proof the gap is rare — it is the one nutrient here where a blood number should not settle the question.
Method: prevalence figures are the published NHANES estimates for each biomarker, taken at the deficiency cut-off each survey uses, and compared here on a single axis so the nutrients can be ranked against each other rather than read one at a time. What this cannot tell you: whether you are deficient — a population rate is not a personal probability, and the subgroup rates on this chart (women for iron, adults 65+ for B12, 18–30 for vitamin D) matter far more than the headline figure if you are in one of them. It also cannot rank nutrients with no reliable blood marker, which is why magnesium sits where it does and why the chart is a starting point rather than a verdict.
The second filter: are you actually in a risk group?
Population averages hide a lot. The nutrients above matter far more for some people than others, and three things can change your needs: what you eat, what you take, and how old you are.
- Diet pattern. If you're vegan, B12 stops being optional — it comes almost entirely from animal foods, and one cohort found roughly half of vegan men were deficient versus about 7% of vegetarians. See B12 deficiency signs and who's at risk.
- Medicines. Some medicines can lower certain nutrients. Ask your clinician or pharmacist before you add a supplement.
- Age. B12 absorption declines with age even though the RDA doesn't rise. In NHANES 2007–2018 the serum-deficiency rate at 60 and older (3.7%) was almost the same as for all adults (3.6%), so age is a reason to test when there's a cause, not proof of a gap. See B12 dosing for seniors.
- Pregnancy. Nutrient needs change. Ask your clinician.
If none of these apply to you, you've likely just ruled out most of the reasons to take anything beyond the population-level default below.
The third filter: what has actually been studied?
Once you're past deficiency and risk group, the next question is whether a supplement's marketed claim matches what the trials measured. This site reviews that outcome by outcome:
- Ashwagandha — studied for stress and cortisol measures; hormone markers are preliminary.
- Berberine — studied for blood sugar and weight measures; meta-analyses disagree on the weight results.
- Tongkat ali — small trials in men who start with low hormone levels; nothing shown for men with normal levels.
- Curcumin — studied for knee pain, mainly in bioavailable forms; plain turmeric is a different product.
- Collagen — studied for skin; the hair and nail research is thin.
Notice the pattern: almost nothing on this list is a flat yes or no. Studies measure specific outcomes, at specific doses, in specific groups.
The fourth filter: is the product even verified?
A supplement is only as good as the bottle in front of you. The FDA doesn't test supplements before they reach shelves. Independent certification is one way to check that a label matches what's inside.
Across 1367 products in our own catalog spanning 187 categories, 55% carry no third-party certification at all — no USP Verified, NSF, or IFOS seal.
That's the last filter, not the first. A certified product that treats a gap you don't have is still a waste of money. But once you know what you actually need, certification separates a verified purchase from a guess. See the full certification-gap data and what "third-party tested" actually means.
A short starting list to discuss with your clinician
Run a healthy adult through all four filters, and here's what is left to discuss:
Vitamin D, lowest cost per day here NatureWise Vitamin D3 2000 IU in Olive Oil (360 mini softgels) $0.04/day (price reviewed October 20, 2026)
It is on NSF/ANSI 173, a list that checks the bottle holds what the label says. Ask your clinician whether vitamin D fits you; the dosage guide covers the research.
Check price →Past that, everything is conditional. See the guides for vitamin B12, iron and magnesium, and ask your clinician what fits your diet, age and health.
Not ten supplements. Not a multivitamin plus five "just in case" bottles.
What you can probably skip
This is the part worth sitting with, because it's the opposite of what most supplement content tells you. Specifically, and by name:
- A daily multivitamin, if you eat a varied diet. It can't target the vitamin D gap that's actually common, and it bundles ingredients you likely don't need alongside the one or two you might. See do you need a multivitamin? for the full evidence, including where it does help (65+, pregnancy, restrictive diets).
- Folate or folic acid, unless your clinician advises it. At 0.2% deficient, it's the lowest gap in this site's dataset, thanks to US grain fortification.
- Ashwagandha specifically for hormone levels. The hormone research is a handful of small, often manufacturer-funded trials that a dedicated meta-analysis calls inconclusive. See the full review.
- Berberine specifically for weight loss. Meta-analyses disagree on whether there is a weight effect at all. See the full review.
- Tongkat ali. The trials were small. See the full review.
- Plain turmeric capsules. Turmeric is only 1-6% curcuminoids by weight, and the curcumin studies used enhanced formulations. See the full review.
- Collagen for hair or nail growth specifically. The skin research is larger; the hair and nail research is thin. See the full review.
- Weighing certification. With 55% of this site's own catalog listing no independent testing, certification is one factor to compare alongside cost per day.
Frequently asked questions
Is there one supplement most people need?
No single supplement fits everyone. Vitamin D is the one nutrient in our population data where low blood levels are common across the board (22.1% of US adults), rather than concentrated in one group. Ask your clinician whether a vitamin D blood test makes sense for you.
Do I need a multivitamin?
Probably not, if you eat a varied diet. A multivitamin cannot target a specific gap the way a single nutrient can, and it bundles several ingredients you may not need. Ask your clinician if you are unsure.
Which popular supplements have the thinnest research behind them?
Weight changes from berberine, hormone changes from ashwagandha or tongkat ali in men with already-normal hormones, and hair or nail growth from collagen. In each case the marketing claim and the trial data do not line up closely.
How do I know if a supplement brand is actually independently tested?
Look for a USP Verified, NSF Certified for Sport, or IFOS (for fish oil) seal on the label — those are the only three programs that independently confirm a bottle contains what it claims. The FDA does not test supplements before they reach shelves. About 55% of the products in our own catalog carry none of these seals, so the label alone is not enough to assume verification.
Should I get blood tests before starting supplements?
Ask your clinician. Blood tests such as a 25(OH)D test, an MMA test or a ferritin test can show more than symptoms can, and iron in particular is best discussed with a clinician before you take it.
Related guides
- Vitamin D Deficiency: Signs & Who's at Risk — the 22.1% figure, by age
- B12 Deficiency: Signs & Who's at Risk — vegans and seniors
- Best Iron for Women — iron intake and testing
- 55% of Supplements Have No Third-Party Certification
- Do You Need a Multivitamin? — the 5.5-million-participant evidence review
- Our Methodology — how we grade evidence and verify products
Sources
This page synthesizes data and citations already published on the pages linked throughout — deficiency prevalence from CDC NHANES (see the vitamin D, iron, B12, and magnesium pages for full citations), evidence verdicts from the ashwagandha, berberine, tongkat ali, curcumin, and collagen verdict pages, and certification data from our supplement certification gap research.