Supplements of vitamins and trace elements are consumed widely in the UK, yet there is little evidence that their ingestion is beneficial to health or wellbeing. This review summarises the evidence for health benefits or harm in the situations where supplements are taken.
In only a few cases does the Department of Health recommend supplements in the otherwise healthy population – folic acid before and in the early stages of pregnancy, as requirements are higher, and vitamin D in infants under 5 for example, as their diet is inadequate and they are unlikely to get enough from exposure to sunlight.
Deficiency of individual micronutrients leading to signs of clinical deficiency is fairly common in clinical practice (e.g. anaemia due to iron deficiency, or folate or vitamin B12 deficiency) and it is clear that the relevant nutrient must be provided to correct the deficiency state. Subclinical deficiency is more common, where the deficit in a nutrient is not sufficiently severe to cause clinical signs, but there may be metabolic or non-specific clinical effects. Benefits of supplements on immune function or cognition in these situations have been difficult to prove. In particular, concern about the adequacy of the antioxidant defences has led to several studies of high-dose antioxidants in an attempt to reduce mortality from cardiovascular disease or cancer. These have been found to lead to an increase, rather than a decrease, in mortality.
There is good evidence that benefit is obtained from ingestion of micronutrients as part of a varied, balanced diet, including at least five portions of fruits and vegetables per day. Purified supplements may be of value in certain at-risk groups who are known to have a poor or inadequate diet. In such cases, supplements should be limited to provision of no more than the Reference Nutrient Intake to minimise the risk of excess.