Are Your Supplements Working?

1. Which supplements can a blood test actually see?

Only some. Start by sorting your shelf into three piles.

“Most people should get all the nutrients they need by having a varied and balanced diet, although some people may need to take extra supplements.”

Clear signal. These show up well in a blood test:

• Vitamin D, read as 25-hydroxyvitamin D, the stored form in your blood.
• Vitamin B12, the vitamin your nerves and red blood cells rely on.
• Folate, the B vitamin in methylfolate and folic acid.
• Ferritin, the protein that holds iron in reserve, so it shows how much iron you have banked.

Partial signal. Magnesium. Only about 1% of your body’s magnesium sits in your blood. So a normal serum magnesium result rules out a big shortfall but can miss a small one.

No direct signal. Omega-3, zinc, collagen, ashwagandha and most herbal products. Our panels do not measure them. For these, the blood test is a safety check, not a scorecard.

2. How long should you wait before you retest?

About three months for most nutrients. Test sooner and you catch the level halfway up.

• Vitamin D: NHS guidance in South West London asks for 3 to 6 months before a repeat test.
• Iron: NICE says haemoglobin should rise by at least 10 g/L within two weeks, but stores need about three more months to refill.
• B12 and folate: there is no fixed NHS schedule, so three months keeps the comparison fair.

Keep the conditions the same both times. Test at the same time of day, ideally in the same season. Skip that morning’s iron tablet, because iron jumps after a dose. Stop high-dose biotin a few days before, as it can skew some results. Our guide to supplements that affect blood tests lists the rest.

3. How do you read the change?

Ask one question per marker. Did it move into its healthy range, and stay there?

Take vitamin D. UK labs call below 25 nmol/L deficient and 25 to 50 nmol/L possibly too low. Above 50 is enough for most people. A move from 30 to 70 is a supplement doing its job. A move from 30 to 34 after three months says the dose, the timing or the product needs a second look. Our low vitamin D despite supplements guide covers the usual reasons.

Small moves are often noise. Every result wobbles a little from day to day and lab to lab. So treat a change of a few units as no change, and look for a clear shift in one direction.

A level that was already normal is the other common finding. If your B12 was healthy before and is healthy now, the tablet may not be adding much. That is worth knowing, and worth raising with a pharmacist before you keep buying it. For ferritin that will not budge, see iron tablets not working.

4. Is a big stack safe for your liver and kidneys?

Check, rather than guess. Your liver and kidneys clear what you swallow, and some herbal products can strain the liver.

Two markers cover the basics. ALT is a liver enzyme that leaks into the blood when liver cells are irritated. eGFR is an estimate of how well your kidneys filter, worked out from creatinine. Creatine powder can raise creatinine on its own, so say if you take it. More on this in herbal supplements and the liver.

The Helvy Vitamins & Minerals panel (£159) reports vitamin D, B12, folate, ferritin, iron and magnesium, so one kit covers the before and the after. The Thyroid & Vital Organs panel (£159) reports ALT, creatinine and eGFR for the safety check. For the wider picture of everyday markers, see our full blood count guide.

A blood test measures. It does not diagnose. A qualified clinician reads your results together, and anything well outside range is one to take to your GP.

Frequently asked questions