01
Which deficiencies are actually common.
The supplement market implies that modern life leaves everyone depleted in everything. The clinical reality is narrower and more useful. In people eating a varied diet in a high-income country, a small number of deficiencies account for nearly all real, symptomatic cases: iron, vitamin D, vitamin B12, folate, and iodine. Magnesium and zinc insufficiency occur but cluster in specific groups. Most other micronutrients are genuinely rare deficiencies outside of malabsorption, alcohol dependence, restrictive eating, or bariatric surgery. That matters practically, because broad "full micronutrient panel" testing mostly produces borderline numbers with no clinical meaning - and a shopping list.
- Common and worth checking: iron (ferritin), vitamin D, B12, folate - these have clear tests, clear thresholds, and clear treatments
- Common but poorly measured by blood tests: magnesium and zinc - assess risk and diet rather than relying on a serum level
- Population-level issue: iodine, particularly in pregnancy and in people avoiding dairy, fish, and iodised salt
- Rare without a specific cause: most B vitamins, vitamin A, vitamin E, copper, selenium - usually secondary to malabsorption or very restricted diets
- Symptoms overlap almost completely. Fatigue, brain fog, and hair thinning are not specific to any one nutrient - which is exactly why you test rather than guess
What Isabella tracks
Isabella holds your lab history in one timeline - ferritin, vitamin D, B12, folate - so you can see the trend rather than a single number in isolation, and flags when a result is due for a recheck.
02
Iron - the most common, the most missed, the most misdosed.
Iron deficiency is the most common nutritional deficiency in the world, and it develops long before anaemia appears. Stores deplete first - ferritin falls - then haemoglobin drops. That means you can be tired, breathless on stairs, losing hair, and struggling with exercise while your full blood count still reads normal. Ferritin is the marker that matters, with one important caveat: ferritin is an acute-phase reactant and rises with any inflammation or infection, so a "normal" ferritin alongside a raised CRP can hide genuine depletion. Many clinicians treat below 30 ng/mL even without anaemia, and lower-normal levels are increasingly recognised as symptomatic in menstruating women.
- Test: ferritin plus a full blood count, ideally with CRP so inflammation can be accounted for
- Highest risk: heavy menstrual bleeding, pregnancy, endurance athletes, vegetarian and vegan diets, coeliac disease, IBD, frequent blood donors
- Dosing has changed: alternate-day dosing of a single morning tablet is now commonly preferred - daily and split dosing raise hepcidin and reduce absorption
- Take with vitamin C or orange juice; avoid tea, coffee, calcium, and antacids within a couple of hours - all meaningfully reduce absorption
- Continue for around 3 months after levels normalise to refill stores, then retest - stopping at the first normal reading is why deficiency recurs
- Symptoms usually lag the blood work by several weeks. Feeling unchanged at week three does not mean it isn't working
Important caution
New iron deficiency in an adult man or a postmenopausal woman is not a diet problem until proven otherwise - it needs investigation for gastrointestinal blood loss. Never take iron long-term without a diagnosed deficiency: iron overload causes real organ damage, and in haemochromatosis it is dangerous.
03
Vitamin D - genuinely common, wildly over-claimed.
Vitamin D insufficiency is real and widespread at higher latitudes, where skin synthesis effectively stops through the winter months. Deficiency has unambiguous consequences for bone: rickets in children, osteomalacia and increased fracture risk in adults, particularly alongside low calcium. What has not held up is the broader claim set. Large randomised trials of vitamin D supplementation in largely replete populations have not shown reductions in cardiovascular events, cancer incidence, or all-cause mortality. The honest summary: correcting a genuine deficiency matters, especially for bone and muscle health in older adults; supplementing an already-adequate level does very little.
- Test: serum 25-hydroxyvitamin D. Deficiency is generally below 25–30 nmol/L; insufficiency roughly 30–50 nmol/L
- Higher risk: darker skin at high latitude, limited sun exposure, covered clothing, older adults, obesity, malabsorption, housebound or care-home residents
- Typical maintenance in guidance for at-risk adults is around 400–1000 IU daily through autumn and winter; treatment doses for diagnosed deficiency are higher and time-limited
- Very high intermittent "mega-doses" have been associated with increased falls in older adults in some trials - more is not safer
- Vitamin D is fat-soluble and accumulates. Sustained very high intake can cause hypercalcaemia, kidney stones, and kidney damage
- If you're supplementing for bone health, the calcium and protein in your diet and your resistance training matter at least as much
What Isabella tracks
Isabella tracks your vitamin D results against the season they were taken in - a level of 45 nmol/L in September means something quite different from the same number in March, and a single result without that context is easy to misread.
04
B12 and folate - where the stakes are neurological.
B12 deficiency deserves more attention than it gets, because untreated it causes neurological damage that can become permanent - peripheral neuropathy, balance problems, and cognitive change - and because those effects can appear before anaemia does. It is not only a vegan issue. Absorption depends on stomach acid and intrinsic factor, so it declines with age, with pernicious anaemia, after gastric surgery, with long-term proton pump inhibitor use, and with long-term metformin. Folate matters most in pregnancy, where adequate intake before conception substantially reduces neural tube defects. The two interact in a way that carries a specific warning: giving folate to someone with undiagnosed B12 deficiency can correct the anaemia while the nerve damage quietly progresses.
- Test: serum B12 and serum folate. If symptoms persist with a borderline B12, active B12 or methylmalonic acid can clarify a true deficiency
- Higher risk for B12: vegan and strict vegetarian diets, adults over 60, long-term PPI or metformin use, pernicious anaemia, Crohn's, gastric or bariatric surgery
- Vegans and strict vegetarians need a reliable B12 source - supplement or fortified foods. This one is not optional and not negotiable in pregnancy
- Pregnancy: folic acid before conception and through the first trimester is one of the best-evidenced interventions in medicine - higher doses apply in some situations, so ask
- Neurological symptoms - numbness, pins and needles, unsteadiness, memory change - warrant prompt assessment, not a supplement trial
- Never treat unexplained macrocytic anaemia with folate alone until B12 has been checked
Important caution
B12 deficiency with neurological symptoms is time-sensitive. Delay can leave permanent deficit. If you have numbness, tingling, balance problems, or new cognitive change, see a clinician rather than starting a supplement and waiting to see what happens.
05
Magnesium - real, common, and badly measured.
Magnesium is where popular health content and clinical medicine diverge most. Intakes below recommended levels are genuinely common, and magnesium is involved in hundreds of enzymatic reactions including muscle and nerve function. But serum magnesium is a poor test: less than one percent of the body's magnesium is in the blood, and levels are tightly defended by the kidneys and bone, so you can be depleted with a perfectly normal result. Because there's no good routine test, the practical approach is to assess risk and dietary intake rather than chase a number. Evidence for supplementation is strongest for constipation and for migraine prophylaxis; the sleep and anxiety claims are popular but considerably weaker than the marketing suggests.
- Dietary sources first: nuts, seeds, legumes, whole grains, leafy greens, dark chocolate - most people with low intake are simply short on these
- Higher risk: type 2 diabetes, alcohol dependence, chronic diarrhoea or IBD, long-term PPI use, loop and thiazide diuretics, older adults
- Form matters for tolerability: citrate and glycinate are better absorbed than oxide, which is largely a laxative
- Best-supported uses: constipation and migraine prevention. Sleep and anxiety benefits are plausible but weakly evidenced
- Excess oral magnesium usually causes diarrhoea first - but in chronic kidney disease it can accumulate dangerously. Check before supplementing if your kidney function is reduced
- Don't diagnose yourself from a normal serum magnesium either way - the test simply doesn't answer the question
What Isabella tracks
Since magnesium status can't be settled by a blood test, Isabella tracks what can be measured: your logged symptoms - cramps, constipation, migraine frequency, sleep quality - before and after a supplement trial, so you find out whether it is doing anything for you.
06
Iodine, zinc and selenium - small amounts, narrow margins.
These three share a feature that makes them awkward: the gap between too little and too much is narrower than for most nutrients. Iodine is essential for thyroid hormone production, and requirements rise substantially in pregnancy, where deficiency affects fetal brain development - it has re-emerged as a concern in people avoiding dairy, fish, and iodised salt, including many plant-based diets. Zinc supports immune function and wound healing, and deficiency is plausible in vegetarian diets high in phytates, in older adults, and in chronic diarrhoea. Selenium deficiency is rare in most regions. For all three, supplementing without a reason is the more likely harm.
- Iodine: dairy, white fish, seaweed and iodised salt are the main sources - plant-based diets without dairy or iodised salt are the main gap, and pregnancy raises requirements
- Iodine excess also causes thyroid dysfunction - high-dose kelp supplements are a genuine and recurring cause of both hyper- and hypothyroidism
- Zinc: useful in diagnosed deficiency; long-term high-dose zinc induces copper deficiency, which causes anaemia and neurological problems
- Zinc lozenges may shorten cold duration modestly; that is a short-term use, not a reason for months of high-dose supplementation
- Selenium: deficiency is uncommon in most regions and excess causes hair loss, nail changes, and neuropathy. Brazil nuts are enough - a couple a day, not a handful
- Serum zinc and selenium are unreliable in the presence of inflammation and are not routine tests without a specific clinical reason
Important caution
These are the nutrients where over-supplementation causes documented harm: iodine-induced thyroid disease, copper deficiency from long-term high-dose zinc, and selenium toxicity. If you're taking a high-dose single-mineral supplement without a diagnosed deficiency, that's worth reviewing.
07
Who is actually at risk - diet, medication, and absorption.
Deficiency almost always has a mechanism behind it: not enough coming in, not enough being absorbed, or too much being lost. Identifying which one applies is more useful than the supplement decision itself, because it tells you whether this is a one-off correction or something that will recur every year until the cause is addressed. Heavy periods are the single biggest driver of iron deficiency in women of reproductive age and are frequently normalised rather than treated. Medications are an underrated cause: long-term acid suppression impairs B12 and iron absorption, metformin lowers B12, and diuretics deplete magnesium. Coeliac disease is a common and frequently undiagnosed reason that multiple nutrients are low at once.
- Heavy menstrual bleeding: the leading cause of iron deficiency in reproductive-age women - treatable, and worth raising even if you've been told it's normal
- Plant-based diets: plan for B12 always, and check iron, iodine, zinc, and omega-3 - a well-planned vegan diet is fine, an unplanned one often isn't
- Medications: PPIs (B12, iron, magnesium), metformin (B12), diuretics (magnesium, potassium), methotrexate (folate), some anticonvulsants (vitamin D, folate)
- Malabsorption: coeliac disease, Crohn's, ulcerative colitis, pancreatic insufficiency, and bariatric surgery - multiple deficiencies together should always prompt this question
- Older adults: reduced stomach acid, lower intake, less sun exposure and more medication - B12 and vitamin D are the usual culprits
- Pregnancy and breastfeeding: requirements rise for folate, iron, iodine, and vitamin D - this is the one situation where structured supplementation is standard advice
What Isabella tracks
Isabella holds your medication list, diet pattern, and cycle data alongside your labs - so when ferritin falls again it can connect it to heavy periods or a new PPI prescription rather than treating each low result as a fresh mystery.
08
Testing well - and the over-supplementation trap.
Good micronutrient testing is narrow and repeated. Bad testing is broad and single-shot. A focused panel driven by your symptoms and risk factors - typically ferritin with CRP, full blood count, vitamin D, B12 and folate, plus thyroid function since it mimics all of them - answers most questions. A 40-marker direct-to-consumer panel mostly returns borderline values that carry no clinical meaning but do generate a supplement recommendation, which is often the point. The step that actually matters is the one most often skipped: retesting after treatment. A deficiency you corrected but never re-checked is a deficiency you're guessing about, and a supplement you kept taking indefinitely because nobody told you to stop.
- A sensible first panel for unexplained fatigue: full blood count, ferritin, CRP, vitamin D, B12, folate, thyroid function, and coeliac serology if there are any gut symptoms
- Retest after treating: iron at around 3 months, vitamin D after 3–4 months, B12 as advised - then stop, continue, or investigate on the basis of the result
- Reference ranges are population spreads, not personal targets - a low-normal ferritin can still be symptomatic, and that's a clinical conversation
- Fat-soluble vitamins (A, D, E, K) accumulate - these are the ones where sustained high-dose supplementation causes real toxicity
- Tell your clinician everything you're taking. High-dose vitamin K interacts with warfarin; iron and zinc reduce absorption of several medications; St John's Wort interacts with many drugs
- If a test result comes with a supplement recommendation from the same company, treat the recommendation with appropriate scepticism
What Isabella tracks
Isabella tracks the retest you were meant to have - flagging when an iron or vitamin D recheck is due, comparing it against the previous result, and showing your symptom scores over the same window so you can see whether treatment changed how you actually feel.
