Iron is the most self-prescribed supplement in the world — and the most dangerous one to self-prescribe. Fatigue has many causes; low iron is only one of them, and a ferritin test is the only way to know.
Why testing comes first
Iron overload is toxic. Hereditary hemochromatosis affects roughly 1 in 300 people of Northern European descent, and unneeded supplementation accelerates liver and heart damage. Symptoms of too little and too much iron overlap — tiredness, brain fog, weakness — which is why guessing is risky and a simple ferritin + CBC panel settles it.
Who is actually at risk
- Premenopausal women, through menstrual losses; needs rise further in pregnancy.
- Vegetarians and vegans: plant (non-heme) iron absorbs at a fraction of the heme rate.
- Frequent blood donors, and anyone with celiac disease or inflammatory bowel disease.
- Most adult men and postmenopausal women are not deficient — routine supplementation is not for them.
Practical schedule
- Only supplement on a confirmed low ferritin; dose and duration follow the lab, not the label.
- Ferrous sulfate or fumarate are the studied workhorses; take with vitamin C, separate from tea, coffee, calcium, and antacids.
- Retest in 2–3 months; stop when stores normalize.
- Store all iron out of children’s reach — iron overdose is a leading cause of fatal poisoning in young children.
Vitamin D is the most supplemented vitamin in the world and one of the most argued about. Here is a sober reading of the human evidence.
Where the evidence is strongest
Vitamin D maintains normal muscle function and immune system function, and adequate intake helps reduce the risk of falls associated with postural instability in older adults — claims the European Food Safety Authority (EFSA) has reviewed and authorized. Deficiency (<30 nmol/L 25(OH)D) is associated with rickets in children and osteomalacia in adults; this is uncontested.
Where evidence is mixed
Large randomized trials such as VITAL (n=25,871, 2022 reporting) found vitamin D supplementation did not significantly reduce cardiovascular events or cancer incidence in generally replete populations. Observational links between low vitamin D and dozens of conditions are real observations, but they may reflect reverse causation: illness lowers sun exposure and serum levels.
Practical takeaways
- Testing beats guessing: ask for a 25(OH)D blood test if you have little sun exposure, darker skin, or live far from the equator.
- Common supplemental doses (400–2000 IU/day) are considered safe for most adults; the tolerable upper intake level is 4000 IU/day (US IOM).
- Take it with a meal containing fat — vitamin D is fat-soluble.
Most nutrient gaps on plant-based diets can be managed with food choices. Vitamin B12 is the exception — reliable plant sources essentially do not exist.
Why B12 is different
B12 is produced by microorganisms. Fortified foods and supplements are the only dependable sources on a vegan diet. Untreated deficiency can cause megaloblastic anemia and, over time, neurological damage — some of which may not fully reverse.
What the research recommends
The Academy of Nutrition and Dietetics position paper on vegetarian diets advises vegans to consume fortified foods or supplements regularly. Adequate intake helps reduce tiredness and fatigue and supports normal red blood cell formation (EFSA-authorized functions).
Practical schedule
- Cyanocobalamin or methylcobalamin both work; cyanocobalamin is the most studied and stable form.
- Common adult maintenance patterns: 25–100 mcg daily, or 1000 mcg 2–3 times per week.
- If you take metformin or long-term acid reducers, ask your doctor to monitor B12 — both medications lower absorption.