What to measure before buying any supplement at all

The question everyone asks on reaching a supplement aisle is: “what am I short of?” It turns out that for four of the best-selling products, the answer cannot be guessed: it is measured. And the test costs less than a month of capsules taken for nothing.

Why this page exists

There are two families of supplement, and they are confused constantly.

The shortfall correctors. Iron, vitamin D, B12, iodine. They restore a level that is too low. In someone whose level is adequate they bring nothing — and iron, in excess, is toxic to the liver and the heart. For those, testing is not a precaution: it is the condition.

The direct-effect products. Creatine, protein, psyllium, phytosterols, caffeine. Their effect does not depend on a prior shortfall: they act in everyone, within the limits of what their studies show. For those, no test is needed.

Ferritin: the most frequently forgotten test

It is the most important of the four, and the most neglected. You can be short of iron without being anaemic: haemoglobin stays normal, the standard panel looks fine, and yet the stores are empty. Vaucher’s trial studied exactly that profile — non-anaemic women with low ferritin and unexplained fatigue: supplementation reduced fatigue compared with placebo.

What to ask for: serum ferritin. Ask for a CRP at the same time: ferritin rises with inflammation, and a “normal” ferritin with a high CRP can mask a genuine shortfall.

Who should do it: heavy periods, plant-based diet, regular blood donation, endurance sport, fatigue with breathlessness on exertion or hair loss.

What it changes: everything. Without that figure, taking iron is tossing a coin with a product whose overdose is a major cause of serious poisoning in children.

Vitamin D: the benefit is concentrated in those who are deficient

Martineau’s individual participant data meta-analysis pooled 25 randomised trials and about 11,000 participants: supplementation reduces the risk of acute respiratory infection. But the benefit is very clearly concentrated in people who were deficient to begin with. In those whose status is adequate, the effect fades.

What to ask for: 25-hydroxyvitamin D, written 25(OH)D. Ideally at the end of winter, when the level is lowest: that is when the figure is informative.

What it changes: the goal is to be in range, not at the top. Beyond that there is no additional benefit and hypercalcaemia becomes possible. Re-check three months after starting.

B12: a shortfall that sets in without warning

Pawlak’s review shows a high prevalence of deficiency in vegetarians and vegans, often with no symptoms at first. The trap is how slow it is: liver stores cover several years, which lets you feel fine for a long time — while the nerves take the damage.

What to ask for: serum B12, and if possible homocysteine or methylmalonic acid, which detect the shortfall earlier.

Who should do it: vegan or strictly vegetarian diet, over 60, treatment with metformin or a proton pump inhibitor — both reduce absorption.

The lipid panel: to know whether two products concern you

Two supplements on this site act on cholesterol, through two different and stackable mechanisms: psyllium, where Jovanovski’s meta-analysis finds a reproducible fall in LDL, and phytosterols, which carry a quantified claim authorised by the European Union — 7 to 11 % less LDL for 1.5 to 3 g a day.

But you need to know where you stand. A lipid panel tells you whether those two products are of any interest to you, and above all it gives you the starting point for checking, three months later, whether they did anything.

What to ask for: total cholesterol, LDL, HDL, triglycerides. Omega-3s act on triglycerides, not on LDL: those are two different targets, and many people confuse them.

What order to go about it in

1. Note your symptoms for two weeks. Tired on waking or at the end of the day? Breathless? Hair falling out? That is what your doctor will ask, and it steers which tests are ordered.
2. Book an appointment and ask for the form. Ferritin + CRP, 25(OH)D, B12, lipid panel. All four fit into a single blood draw.
3. Buy nothing in the meantime. That is the hard part, and it is the whole point of the exercise.
4. Read the results with your doctor, not with a forum. A figure outside the range does not mean the same thing depending on your age, your sex and your context.
5. Re-check at three months if you corrected something. If nothing has moved, the problem was not there.

The supplements that need no test at all

For completeness: not everything goes through a blood test. Products whose effect does not depend on a prior shortfall can be tried directly, respecting their dose and their evaluation period.

• Creatine monohydrate — 5 g a day, effect on strength in three to four weeks.
• Protein powder — a convenient food, not a medicine: useful only if your meals do not reach 1.6 g per kilo.
• Psyllium — transit and LDL, built up gradually over ten days.
• Phytosterols — LDL, with meals, 1.5 to 3 g a day.
• Caffeine — immediate effect, and you will know straight away.

And the reminder that applies to this whole page: none of these products replaces sleep, movement and a varied diet. They are additions, or they are useless.