Dietary supplements are natural health products used to supplement the diet, such as vitamins, minerals, amino acids, enzymes and plant extracts. Although these products are more complementary medicines, they are usually referred to as dietary supplement by consumers whose intention is to augment their diet rather than treat disease.
One reason for the persistent popularity of vitamins and minerals is the perception that they are harmless. There are many potential harms or potential adverse effects but, unlike conventional medicines, manufacturers of vitamins and minerals are not required to present extensive documentation about safety.
Marketing of vitamins and minerals is generally based on their claimed benefits with little, if any, mention of their potential harms. Consumer information leaflets are not provided, and few dietary supplements carry warnings of potential adverse effects on their packaging. Nonetheless, there are well‑recognised harms from the ingredients of dietary supplements, especially when taken in high doses.
Some of the commonly used vitamins and minerals and their
potential adverse effects.
Vitamin A (Retinol)
Ingestion of >300,000 IU may result in
acute toxicity. Chronic toxicity (hypervitaminosis) is associated with doses
>10,000 IU/day. Symptoms of chronic hypervitaminosis A include skin
desquamation, liver impairment, loss of vision and severe intracranial
hypertension.
Vitamin B3 (Niacin or Nicotinic acid)
Taking high doses of vitamin B3 are commonly associated with peripheral
vasodilation (expansion of blood vessels) causing skin flushing, burning
sensation, pruritus and hypotension. Vasodilation may also occur in the eye
resulting in reversible toxic cystoid macular oedema.
Vitamin B6 (Pyridoxine)
At doses ≥200
mg/day of vitamin B6 have been associated with severe
sensory peripheral neuropathies. Risk often arises from multiple products being
taken all containing pyridoxine.
Vitamin C (Ascorbic acid)
At very high doses, Vitamin C is associated
with precipitation of cysteine, urate or oxalate kidney stones, especially in
people with a predisposition for kidney stones. Vitamin C may reduce
effectiveness of antineoplastic drugs such as vincristine, doxorubicin,
methotrexate, cisplatin and imatinib in cancer patients.
Vitamin D (Colecalciferol)
Very high doses of Vitamin D may cause
hypercalcaemia (increase quantity od calcium), with symptoms from thirst and
polyuria to seizures, coma and death. High intermittent doses of vitamin D have
been associated with increased risk of falls and fracture in the elderly.
Vitamin E (Alpha-tocopherol)
Very high doses of Vitamin E shows
antiplatelet effect and increased risk of haemorrhagic stroke reported.
Calcium
Carbonate salt of calcium can cause gastric reflux and constipation. High‑dose
calcium may induce vascular and soft tissue calcification, hypercalciuria,
kidney stones and secondary hypoparathyroidism. Interferes with absorption of
magnesium, iron and zinc if taken simultaneously, and can reduce absorption of
many other drugs e.g. levothyroxine, tetracyclines.
Magnesium
High doses often result in diarrhoea, nausea and abdominal cramping due to the
osmotic effect. Like other divalent cations, magnesium may chelate and reduce
absorption of other minerals or medicines such as tetracyclines.
Zinc
Zinc is often associated with altered or impaired taste and smell. Intranasal
zinc can cause anosmia. Doses ≥80 mg/day in clinical trials were associated with adverse prostate
effects.
Selenium
Associated with acute and chronic toxicity. Signs of chronic high‑dose
‘selenosis’ are hair and nail loss or brittleness, lesions of the skin and
nervous system, nausea, diarrhoea, skin rashes, mottled teeth, fatigue and mood
irritability.
Vitamins and
minerals are generally used safely when prescribed in medical settings for the
treatment or prevention of deficiency states and other appropriate conditions.
For example, vitamin B3 is used
for hyperlipidaemias and folic acid is used in pregnancy to prevent birth
defects (e.g. anencephaly, spina bifida). The key to the safety of vitamins and
minerals is the prescribed dose, which is usually derived from research
demonstrating that the benefits outweigh the harms. This is often not the case
when consumers are self‑medicating with products purchased on the open market,
as consideration is rarely given to the effective or safe dose.
