Vitamin supplements: too much of a good thing?

Vitamins are vital amines, needed for everyday chemical reactions in our bodies. Deficiencies can be harmful, but that doesn’t mean that taking more than you need is beneficial. In fact, it can be very harmful yet the message that more is better prevails. Does vitamin supplementation help those who are not deficient?

This paper from the 80’s (here) shows that overdosing in children is not only common, but more likely in families that take multiple supplements.

Supplements are meant to address deficiencies, yet by far the majority of people who take supplements take it to prevent disease (here). And with 1 in 4 Australians taking supplements, the chance of overdosing is high.

But before we talk about the harms of overdosing, what is the evidence that supplements help? They are commonly taken, usually for the prevention of conditions including pneumonia, dementia, complications of childbirth, the common cold, and fractures.

A search of the Cochrane library found exactly 100 review articles with the word “vitamin” in the title. Excluding studies of vitamins to treat deficiencies, and studies still not complete, there were 54 reviews looking at the role of vitamins in preventing various conditions. With the exception of vitamin D preventing asthma attacks (but no effect on asthma symptoms) (here) there was no significant clinical benefit of vitamins for any of the conditions addressed in any of the reviews, listed below:

•       Vitamin D alone doesn’t prevent fractures. There may be a slight benefit when combined with calcium, but there are also significant complications (here)
•       No evidence to support the use of B6-Mg as a treatment for autism (here)
•       No evidence for a significant benefit of vitamin D for chronic pain (here)
•       No consistent evidence that folic acid +/- vit B12 helps cognition in older people (here)
•       Use of vit E not supported for pre-term infants (here)
•       Insufficient evidence for Vit B6 in pregnancy (here)
•       Routine use of Vit E supplements in pregnancy not supported (here)
•       No evidence fo Vit A preventing blindness after measles (here)
•       No evidence to support topical vit A for nappy rash (here)
•       More research needed on vit E for tardive dyskinesia (here)
•       Inconsistent evidence of the effect of vitamin and mineral supplements on macular degeneration (here)
•       Vit C not recommended for tetanus (here)
•       Vit B6 no good for cognition (here)
•       No evidence to support vitamins for epilepsy (here)
•       Waiting on more evidence about vit A for neonates in developing countries (here)
•       No evidence that vit E helps with dementia (here)
•       No evidence for vit C in asthma (here)
•       No role for vit A in pregnancy but can help in deficiencies (here)
•       No evidence for vit C in diabetic eye disease (here)
•       No evidence for vit A in cystic fibrosis (here)
•       No evidence for vit A for post partum women (here)
•       No evidence for vit A in retinitis pigemtosa (here)
•       No evidence for vit C in pregnancy (here)
•       No evidence for vit K in pre-term infants (here)
•       No evidence for supplements in macular degeneration (here)
•       Insufficient evidence for vit E in vascular claudication (here)
•       No effect of vit A on pneumonia in kids (here)
•       No effect on fracture rate with Vit D and calcium, despite increasing bone density (here)
•       No effect of vit D on fracture prevention for steroid induced osteoporosis (even if it did improve bone density) (here)
•       Question marks over the effect of vit D in preventing cancer in the elderly (here)
•       No evidence for vit K in preventing cardiovascular disease (here)
•       No evidence for vitamins A, C or E for cataracts (here)
•       No evidence for supplementary antioxidant micronutrients for the prevention of kwashiorkor in pre-school children (here)
•       No evidence for vit B therapy in diabetic kidney disease (here)
•       No evidence for vit A supplementation in infants (here)
•       Failure of vit C to prevent the common cold (here)
•       Little or no effect of vit A on mother-to-baby transmission of HIV (here)
•       No benefit of vti D in chronic kidney disease (here and here)
•       No improved survival in measles with vit A (here)
•       No evidence for vit C in preventing pneumonia (here)
•       No evidence for vit D in cystic fibrosis (here)
•       Insufficient evidence for vit B in peripheral neuropathy (here)
•       No benefit of vit A for respiratory tract infections, except for those with deficiencies (here)
•       Lack of evidence for vit C and E in asthma (here)
•       Lack of evidence for vit D in multiple sclerosis (here)
•       Vit D does not help bone density of children (here)
•       Nor does it prevent infections in children (here)
•       Possible marginal benefit from vit A in low birth weight infants (here)
•       Taking any vitamin supplements prior to pregnancy or in early pregnancy does not prevent women experiencing miscarriage. However, evidence showed that women receiving multivitamins plus iron and folic acid had reduced risk for stillbirth (here)
•       Unclear evidence for vit D in pregnancy (adding calcium is harmful) (here)
•       Insufficient evidence for vit B12 for improving cognition (here)
•       Further evidence needed for the effect of vit D on mortality (here)
•       No evidence for vit K in cystic fibrosis (here)
•       Vit E supplementation improved the vit E levels in patients with cyctic fibrosis, but didn’t do anything to help them (here)

What about the harms of taking more than you need? Overdosing of minerals and vitamins can cause conditions such as diarrhoea, dizziness, impaired immunity, heart problems, anaemia, gastrointestinal upset, nausea, fatigue, joint pain, hair loss, nerve damage, birth defects, and central nervous system, liver, bone and skin disorders.

I was alerted to this problem by a colleague who is a kidney specialist. He has seen many patients with complete kidney failure (needing dialysis) from an overdose of vitamin C. (One case of fatal vitamin C overdose is reported here).

The bottom line
Vitamins are essential, so deficiencies can cause illness. But taking too much can also be harmful, and there is abundant evidence that taking vitamin supplements (for those who are healthy and not deficient) does not improve health and can cause significant harm.