A supplement doesn't just get absorbed. Once it's in your gut, it competes with other nutrients for the same doorways, accumulates in tissue if it's fat-soluble, gets converted — or fails to be converted — depending on your genetics, and arrives inside a chemical manufacturing matrix that has nothing to do with nutrition. Here's what actually happens.
Your gut wall uses a small set of shared protein transporters to absorb minerals. They don't discriminate — iron, zinc, copper, calcium, and magnesium all compete for the same gateways. In food, these minerals arrive in balanced ratios inside organic cellular structures that regulate their release slowly. In a concentrated supplement, they arrive as a chemical flash that floods the transporters.
The practical consequences are real and documented. A high-dose zinc supplement actively reduces your absorption of iron and copper at the same time. Long-term high-dose zinc is a recognised clinical cause of copper deficiency. Calcium supplements — particularly the 500–1000 mg doses commonly taken by women over 50 — compete directly with magnesium for absorption, quietly worsening a deficiency that affects blood pressure, sleep, and muscle function. Taking an iron supplement with a glass of milk can reduce iron absorption by up to 60%, because calcium and iron share the same transporter.
The fix is simple if you know about it: space supplements out across the day rather than taking them all at once. One exception works in your favour — taking iron with Vitamin C actively improves absorption, because C converts iron to a more absorbable form. Timing and pairing matter as much as dose.
📄 Full competitive absorption table →There are two types of vitamin. Water-soluble ones — Vitamin C, the B vitamins — dissolve in water and your kidneys excrete whatever you don't use. You can take more than you need and the excess leaves. Fat-soluble vitamins are different. Vitamins A, D, E, and K dissolve in fat and are stored in body fat and liver tissue. They do not get excreted. They build up.
Chronic high-dose Vitamin A supplementation is linked to liver toxicity, bone thinning, and headaches. In pregnancy, excess Vitamin A is associated with birth defects. The tolerable upper intake level is 3,000µg (10,000 IU) — many high-dose supplements sold legally in the UK approach or exceed this. The label doesn't warn you that it accumulates.
A meta-analysis of 19 randomised trials found that Vitamin E supplementation above 400 IU per day was associated with a statistically significant increase in all-cause mortality. The exact mechanism is debated, but the signal was consistent enough across large populations to matter. Dietary Vitamin E from nuts and seeds carries no such signal — only the high-dose isolated supplement.
Vitamin D is the one fat-soluble vitamin the NHS recommends routinely in winter, and rightly so. But "more is better" doesn't apply. Excess Vitamin D drives hypercalcaemia — dangerously elevated blood calcium — causing nausea, kidney stones, and in severe cases heart rhythm problems. Doses above 4,000 IU daily should only be taken under medical supervision following a confirmed blood test.
The counterintuitive truth: the nutrients you most need to be careful about supplementing are the ones the body handles least forgivingly in excess — and they happen to be among the most heavily marketed.
Many supplements are not the active form the body actually uses. They are precursors that need to be converted by enzymes before they become biologically useful. Whether that conversion happens efficiently depends partly on your genetics.
Folic acid vs folate is the clearest example. Folic acid is the synthetic form used in almost all supplements and fortified foods. Folate is the natural form in leafy greens and legumes. They are not the same thing. Around 40% of the population carries a variant of the MTHFR gene that significantly impairs their ability to convert folic acid into the active form the body uses. These people may be getting very little benefit from folic acid supplements while potentially allowing unconverted folic acid to accumulate. The supplement label doesn't mention this. The existence of this distinction is not in the industry's interest to advertise.
Synthetic Vitamin E is another case. Natural Vitamin E from food is a specific molecule (d-alpha-tocopherol). Synthetic Vitamin E is a mixture of eight molecular variants — only one of which matches the natural form. Your liver has a transport protein that strongly prefers the natural version and degrades the synthetic ones. Research shows the body retains roughly twice as much natural Vitamin E compared to the same dose of synthetic. Half a typical supplement's active ingredient is cleared before it reaches its target.
Open any standard supplement tablet and separate the active ingredient from everything else. In many cases the nutrient represents a minority of the physical mass. The rest — often the majority — is excipients: fillers, binders, lubricants, coatings, and flow agents added purely to make industrial manufacturing possible.
Some are genuinely inert. Others are less comfortable reading about.
The most common tablet lubricant. It coats every particle of active ingredient in a thin hydrophobic layer to stop machinery clogging. Inside your gut, that same fat layer resists water, delaying tablet dissolution. Controlled studies confirm it extends disintegration time — meaning the nutrient releases later, at a lower concentration, further down the gut than intended. Your supplement may technically contain the dose on the label while delivering it less effectively than the label implies.
Added purely to make tablets a clean, uniform white, because white reads as "clinical" and "pure" to consumers. In 2021 the European Food Safety Authority concluded titanium dioxide can no longer be considered safe for human consumption, citing evidence that nanoparticles accumulate in tissues and can cause DNA damage. It was banned as a food additive across the EU. It remains legal in UK supplements. Check the ingredients list of anything you currently take.
The supplement industry's central proposition is that more nutrients are better, that supplementation provides an insurance policy against dietary gaps, and that the risk is low. The clinical trial evidence is inconvenient for all three of these claims.
This is not the fringe view. The US Preventive Services Task Force reached the same conclusion in 2022 after reviewing all available evidence: insufficient evidence that multivitamins reduce cardiovascular disease or cancer in healthy adults, with explicit concern about beta-carotene in smokers.
The industry's response — pivoting to "modern soil depletion" and "stress increases requirements" — is difficult to falsify and conveniently justifies continued supplementation regardless of what the trials say. It's worth being aware of that dynamic.
This does not mean all supplementation is worthless. There are specific, evidence-backed situations where it genuinely matters:
Notice what's not on that list: "just in case," "feeling tired," "heard it's good for immunity." These account for the majority of supplement spending in the UK. The evidence doesn't support them.
The most useful question isn't "which supplements should I take?" It's "do I have a genuine gap, and what's causing it?" That requires a blood test for deficiency, or — more accessibly — consistent food logging that shows where your intake actually falls short.
Your Nutriofia weekly report shows exactly which nutrients you are consistently under on through diet. If you are hitting 95% of your iron target from food, you don't need an iron supplement. If you are at 55% of your Vitamin D even in summer, you might. The numbers tell you what the supplement aisle cannot.
The supplement industry is not a scam in its entirety. Some products have genuine, specific, evidence-backed uses. But the default for most people eating a reasonably varied diet should be food first — not because supplements can't help, but because whole food delivers nutrients inside cellular matrices that regulate absorption, contains co-factors that isolated supplements cannot replicate, and builds long-term habits that compound rather than masking short-term gaps one pill at a time.