Quick answer: the five checks that reveal supplement quality fastest are: (1) active ingredient forms — is the mineral bisglycinate or oxide? is the vitamin D3 or D2?; (2) elemental vs compound dose — “500mg magnesium citrate” and “80mg elemental magnesium” are the same thing, stated differently; (3) proprietary blends — if individual doses are hidden, assume underdosing; (4) capsule material — HPMC vs gelatine; (5) what the claim is actually permitted to say vs what the marketing says.
1. The form of each active ingredient
Different forms of the same nutrient have radically different bioavailability. The form is stated in parentheses after the ingredient name on the Supplement Facts panel.
- Magnesium oxide (~4% bioavailability) vs magnesium glycinate/citrate (~25–60% bioavailability). A 2003 study in Magnesium Research found citrate produced 4x higher serum magnesium than oxide at equal elemental doses. Most cheap multivitamins use oxide.
- Vitamin D2 (ergocalciferol) vs D3 (cholecalciferol). D3 raises serum 25-OH vitamin D approximately 87% more effectively per IU than D2 (Trang et al., 1998, AJCN). D2 is sometimes used in cheaper fortified products.
- Cyanocobalamin B12 vs methylcobalamin. Cyanocobalamin requires conversion to active coenzyme forms — conversion may be impaired in people with MTHFR gene variants (~40% of UK population). Methylcobalamin is already active.
- Zinc oxide (least absorbed) vs zinc gluconate/citrate/bisglycinate (significantly better absorbed). A 2014 EJCN study (Wegmüller et al.) confirmed zinc citrate was absorbed equivalently to gluconate and significantly better than oxide.
- Folic acid vs methylfolate (5-MTHF). Same MTHFR conversion issue as B12.
How to check: look at the bracketed form name after each ingredient. If it says just “magnesium” without specifying the form, contact the manufacturer or assume the cheapest form (oxide).
2. Elemental dose vs compound weight
Minerals are bound to a carrier compound. The label may state the total compound weight or the elemental mineral content — these are very different numbers:
- “500mg magnesium citrate” = approximately 80mg elemental magnesium (magnesium is ~16% of the compound by weight)
- “70mg zinc gluconate” = approximately 10mg elemental zinc (zinc is ~14% of gluconate)
- “12.5mg zinc oxide” = approximately 10mg elemental zinc (zinc is ~80% of oxide)
A supplement listing “70mg zinc gluconate” and one listing “12.5mg zinc oxide” may both provide 10mg elemental zinc. Comparing by compound weight is meaningless. Only the elemental content tells you the actual dose. Quality supplements declare elemental content separately and prominently.
3. Proprietary blends: why hidden doses are a red flag
A proprietary blend lists multiple ingredients under a single total weight (e.g. “Cognitive Complex 800mg: lion’s mane, bacopa, ginkgo, ashwagandha”) without disclosing individual doses. This means you cannot know if any ingredient is at a clinically relevant dose or a token amount included for label appeal.
Example: effective bacopa monnieri requires 300mg standardised extract. Lion’s mane requires 750mg+ whole fruiting body. A blend containing both at “800mg total” cannot provide effective doses of either. If individual amounts are not disclosed, assume underdosing unless there is specific evidence otherwise.
UK food law requires ingredients to be listed in descending order by weight. In a blend, ingredients at the bottom of the sub-list are present in the smallest amounts. An “immune blend” where zinc is listed last contains less zinc than the other ingredients in that blend — likely a token amount.
4. Capsule material: the most commonly missed label check
The capsule shell is listed in the “other ingredients” or “inactive ingredients” section, not with the actives. Look for:
- HPMC (hydroxypropyl methylcellulose) or “plant cellulose capsule” — plant-derived, vegan, halal
- Pullulan — also plant-derived (tapioca starch), vegan, halal
- Gelatine (or “gelatin”) without source specification — almost certainly porcine in UK supplements, not halal, not vegan
- Bovine gelatine — not vegan; only halal if specifically certified from halal-slaughtered cattle
A supplement with only halal active ingredients in a porcine gelatine capsule is not halal. The capsule is part of what you ingest. Many UK brands never specify capsule source — if it says “gelatine” without further detail, contact the manufacturer.
5. What EU-authorised claims actually mean
In the UK (retained EU law post-Brexit), supplements may only use pre-approved health claims from the EU claims register. These claims are specifically worded and only permitted on products providing a minimum declared amount of the relevant nutrient per serving.
Examples of what is permitted:
- “Vitamin D contributes to the normal function of the immune system” — permitted at ≥15% of the NRV (1.5μg/day) per serving
- “Magnesium contributes to the reduction of tiredness and fatigue” — permitted at ≥15% NRV (≥56.25mg elemental) per serving
- “Vitamin C contributes to normal collagen formation for the normal function of skin” — permitted at ≥15% NRV (12mg/day) per serving
What is not permitted: any claim that a supplement prevents, treats or cures any disease or infection. If a supplement says it “fights colds” or “prevents infections” it is making an unlawful medicinal claim. This is a red flag for the product’s regulatory compliance overall.
The gap between EU-authorised claim wording and marketing copy is often where supplement companies overstate. “Contributes to normal immune function” (authorised, defensible) is different from “boosts your immune system” (not authorised, misleading). Learning this distinction helps you evaluate any supplement more accurately.
Bonus check: expiry and storage
Supplement potency degrades over time — the rate depends on the nutrient. Probiotics (live bacteria) degrade fastest and require refrigeration or enteric coating to maintain viable counts. Fat-soluble vitamins degrade slower but are sensitive to light and heat. Water-soluble vitamins (B vitamins, C) are relatively stable. If a probiotic product does not state CFU guaranteed at end of shelf life (only at manufacture), the count at the time you take it may be significantly lower than labelled. Look for “guaranteed at expiry” or “guaranteed potency through best before date.”
Full guide on supplement quality verification: are third-party tested supplements better. Browse the Halal Vitamins UK collection for products with declared elemental doses, named mineral forms and HPMC capsules throughout.
What is the difference between elemental magnesium and magnesium compound weight?
Magnesium in supplements is always bound to a carrier compound — citrate, glycinate, oxide, malate etc. The compound weight includes both the magnesium and the carrier. The elemental weight is just the magnesium itself. “500mg magnesium citrate” provides approximately 80mg elemental magnesium (magnesium is ~16% of the citrate compound by weight). “400mg magnesium oxide” provides approximately 240mg elemental magnesium (magnesium is ~60% of oxide). Always compare products on elemental magnesium content, not compound weight — they are completely different numbers that make direct comparison meaningless.
What does EU authorised claim mean on a supplement label?
It means the claim has been assessed by the European Food Safety Authority (EFSA) and meets the scientific standard for the specific wording to be used on food and supplement products. Only pre-approved claim wording from the EU claims register may be used. The claim must relate to a specific nutrient present at ≥15% of the NRV per declared serving. No supplement can legally claim to prevent, treat or cure any disease or condition — any product making such claims is non-compliant. EU authorised claims are a meaningful quality signal — they indicate the manufacturer is working within the regulatory framework rather than making unsubstantiated claims.
How do I know if a supplement has proprietary blends?
On the Supplement Facts panel, a proprietary blend appears as a named combination (e.g. “Immune Blend”, “Cognitive Complex”) with a single total weight, followed by the individual ingredients listed without amounts. If you see ingredients listed under a parent name without individual doses, that is a proprietary blend. UK food law requires the ingredients to be listed in descending order within the blend — so the first ingredient is the largest by weight and the last is the smallest. But without individual amounts, you cannot assess whether any ingredient is at an effective dose. As a general rule, avoid proprietary blends if transparency and evidence-based dosing matter to you.
