Micronutrients: Why Small Quantities Carry Large Clinical Weight

A person needs roughly a thousand times more protein each day than vitamin B12, measured by weight. Yet a deficiency of B12 can cause anaemia, irreversible nerve damage and cognitive decline, while the body tolerates wide variation in protein intake. That asymmetry is the essence of micronutrients: the quantities are tiny, the consequences large. For practitioners, micronutrients present a particular challenge. Deficiencies are common and often missed, their assessment is less straightforward than a single blood test suggests, and the enthusiasm for supplements means that excess is also a real clinical problem.

Why small quantities matter

Micronutrients work mainly as cofactors and structural elements rather than as fuel. A single enzyme may depend on a particular mineral at its active site; a metabolic pathway may require several vitamins at different steps. When a cofactor is scarce, the pathways that depend on it slow down, and the effects appear wherever those pathways matter.

• Iron is central to haemoglobin, oxygen transport and mitochondrial energy production.
• Vitamin B12 and folate are required for DNA synthesis and methylation, so deficiency affects rapidly dividing cells such as blood cells and the nervous system.
• Iodine is an essential component of thyroid hormones.
• Zinc is required by hundreds of enzymes and is important for immunity and wound healing.
• Magnesium is involved in energy metabolism, muscle and nerve function and hundreds of enzymatic reactions.
• Vitamin D acts as a hormone regulating calcium and bone metabolism, with receptors in many tissues.
• Selenium is needed for antioxidant enzymes and for converting thyroid hormones.

Because so many micronutrients work in interconnected pathways, deficiencies rarely produce symptoms confined to one system, which is one reason they are often overlooked.

The spectrum of deficiency

Deficiency is not a single state. It progresses through stages.

1. Depletion of stores, with no functional effect yet. Ferritin falls before haemoglobin does.
2. Functional or subclinical deficiency, in which biochemical processes are impaired and nonspecific symptoms such as fatigue may appear, but classical signs are absent.
3. Overt deficiency, with recognisable clinical disease such as anaemia, neuropathy, goitre or rickets.

Conventional practice is well equipped to detect overt deficiency. The earlier stages are where functional practitioners can add value, provided they interpret evidence of subclinical deficiency carefully and do not attribute every symptom to marginal nutrient levels.

Why deficiency happens

A useful framework considers 4 mechanisms.

The medicine category deserves emphasis because it is so often forgotten. Long term metformin use is associated with lower vitamin B12 levels, and periodic monitoring is recommended in many guidelines. Long term proton pump inhibitor use has been associated with B12 deficiency and, less commonly, with low magnesium, which prompted regulatory warnings. A thorough medicine review is part of every micronutrient assessment.

Common deficiencies worth knowing

Global patterns vary, but several deficiencies are common across many populations.

Iron deficiency is the most common nutritional deficiency worldwide and the leading cause of anaemia, particularly affecting women of reproductive age, children and people with chronic blood loss.

Iodine deficiency remains a major cause of preventable thyroid disease and impaired neurological development in regions without effective salt iodisation, and mild deficiency occurs in some high income countries, particularly in pregnancy.

Vitamin B12 deficiency is common in older adults, in people following vegan diets without supplementation, and in people taking certain medicines.

Vitamin D insufficiency is widespread, particularly in people with limited sun exposure, darker skin living at higher latitudes, and those who cover most of their skin.

Folate deficiency is less common where foods are fortified, but remains important in pregnancy.

Zinc deficiency is common in some low income settings and in people with malabsorption.

Why assessment is harder than it looks

A blood level is not always a reliable measure of status. Several complications recur.

• Blood levels may not reflect stores. Most magnesium is inside cells and bone, so serum magnesium can be normal despite depletion.
• Inflammation distorts markers. Ferritin rises and serum zinc and iron fall during inflammation.
• Borderline results need functional markers. For B12, a result in the indeterminate range may be clarified by methylmalonic acid or holotranscobalamin, where available.
• Recent intake affects some measures. Serum levels of some nutrients reflect recent meals or supplements.
• Assays and thresholds vary. Thresholds for sufficiency differ between expert bodies, most notably for vitamin D.

For these reasons, micronutrient assessment combines the history, dietary review, medicine review, clinical signs and targeted testing, rather than relying on a panel of serum levels.

More is not better

The idea that supplementing a nutrient is harmless, even if unnecessary, is contradicted by evidence.

2 large randomised trials of beta carotene supplementation in smokers and people exposed to asbestos, intended to prevent lung cancer, instead found increased rates of lung cancer in the supplemented groups. High dose vitamin B6 taken for long periods can cause sensory neuropathy. Excess vitamin A in pregnancy is teratogenic, and high intake is associated with liver toxicity and reduced bone density. Excess vitamin D can cause hypercalcaemia. High iodine intake can trigger thyroid dysfunction in susceptible people. Selenium supplementation beyond adequate intake has been associated with an increased risk of type 2 diabetes in some trials.

Supplement combinations add a further risk, as many products contain overlapping ingredients. A patient taking a multivitamin, a separate B complex and an energy product may be taking far more B6 than they realise.

Food first, targeted supplementation second

Most people meet their micronutrient needs from a varied diet. Supplementation is most clearly justified in specific situations:

• confirmed deficiency, at a dose appropriate to correct it, with monitoring;
• established needs, such as folic acid before conception and in early pregnancy, and B12 for people following vegan diets;
• conditions or medicines that impair absorption, such as bariatric surgery or long term metformin;
• groups with recognised risk where public health guidance recommends it, such as vitamin D in some populations.

Outside these situations, the case for routine supplementation is usually weak, and the case for improving diet is usually strong.

An illustrative case

A 67 year old man with type 2 diabetes has taken metformin for 9 years and a proton pump inhibitor for reflux for 5. He reports numbness in his feet, poor memory and fatigue, which have been attributed to diabetes and ageing. A micronutrient focused review notes 2 medicines associated with low B12. His B12 is low and his methylmalonic acid is raised; his magnesium is at the lower limit. His doctor starts B12 replacement, reviews whether the proton pump inhibitor is still needed, and considers his neuropathy in light of the result. 3 months later his fatigue has improved; his foot symptoms are partly better. A cause that had been hidden in plain sight has been addressed.

A practical checklist

1. Review diet, including restrictive patterns and food access.
2. Review medicines and supplements, including doses.
3. Consider absorption: digestive symptoms, surgery, coeliac disease.
4. Consider increased needs or losses: pregnancy, menstruation, growth, illness.
5. Test when the result will change the plan, and interpret with inflammation and context in mind.
6. Correct confirmed deficiency, then look for its cause.
7. Avoid unnecessary and overlapping supplements.
8. Recheck to confirm correction.