9th April 2019
A study published in the Annals of Internal Medicine has hit the headlines today with the finding that the use of dietary supplements was not associated with health benefits while nutrients from foods appeared to reduce the risk of mortality.
This was a large prospective study with over 30,000 participants, followed up over about 6 years, who were asked about their usage of dietary supplements. Statistical methods were used to control for the fact that those who take vitamin and mineral supplements are often better educated, wealthier and have healthier lifestyles than those who do not – indeed the authors found that initial associations between lower risk of mortality and supplement use disappeared when these factors were taken into account. However, participants were asked whether they had used any dietary supplements in the last 30 days, which may not reflect habitual use or capture changes in use, and is subject to recall bias.
Overall the study found that use of supplements did not have any benefit in terms of reducing mortality from cancer or cardiovascular disease, while certain nutrients from food were associated with a beneficial effect.
This is consistent with findings in other large studies and from BNF’s recently published Task Force report, Cardiovascular Disease: Diet, Nutrition and Emerging Risk Factors, which reviewed the evidence on vitamin supplements and cardiovascular outcomes. This found that, despite interest in nutrient supplementation, studies haven’t shown any consistent benefit for prevention of heart disease in Western populations, but did find that there were clear benefits with a number of nutrients from foods and from healthier dietary patterns such as the Mediterranean diet.
Nutrient-rich foods often contain a range of essential nutrients, fibre, and naturally occurring bioactive compounds. Studies have shown positive associations between foods/diets and good health but it is often unclear precisely which food component is responsible. This, in part, may explain the findings with supplements in this study.
Research on diet is increasingly looking at the effects of dietary patterns on health, rather than isolated nutrients, and it’s clear that the diet as a whole and not single nutrients in isolation, can have the greatest beneficial impact on health. However, there are some situations where experts recommend specific micronutrient supplements alongside a healthy, varied diet. For example, folic acid is advised before and during the early weeks of pregnancy to protect the developing fetus from neural tube defects. Vitamin D supplements (10 micrograms per day) should also be considered in the winter months because 1 in 5 adults in Britain and almost 40% of teenage girls have low blood levels of vitamin D, which has the potential to adversely affect musculoskeletal health. For those who are housebound or always cover their skin, a supplement is recommended throughout the year.
The finding that high intakes of calcium could increase cancer risk has been seen in previous studies and particularly related to risk of prostate cancer, although this study did not look at specific cancer sites. The World Cancer Research Fund’s (WCRF) most recent report classified the evidence for an association between diets high in calcium and prostate cancer as ‘limited, suggestive’ and so more evidence is needed to examine this relationship. It’s worth noting that the WCRF also found probable evidence for an association between calcium supplements and reduced risk of colorectal cancer; so the relationship between calcium and cancer risk is a complex one.
While the study found that vitamin D supplements of 10 micrograms a day in people with vitamin D status of 50 nmol/l of 25-hydroxy vitamin D (25OHD – the marker used when measuring vitamin D in the blood – 25 nmol is considered the level for adequate status in the UK) was associated with increased all-cause and cancer mortality, there is no suggested mechanism by which this would occur and other studies have reported variable results. This finding needs to be taken in the context of the relatively strong evidence for a beneficial effect of vitamin D on bone health and the fact that around 20% of the UK population has been found to have low vitamin D status (below 25 nmol 25OHD).
In both cases, these findings should be interpreted with caution as this type of study cannot show causation, and there is strong evidence for calcium and vitamin D in supporting bone health and both have approved EU health claims for this relationship. Therefore, people prescribed calcium and vitamin D supplements should not discontinue their use on the back of this study.