What evidence should we use for public health policy?
The rising costs of intervention studies, as well as continued pressure for academics to publish, have led to an increased reliance on observational data to inform public health policy. While it may seem simple to correlate Lifestyle Factor A with Health Outcome B and decide that the risk of disease has increased or lowered by a specific percentage, further evidence steps are needed before rushing to enshrine the findings into advice for the general public. This is because observational studies, and their meta-analyses, are not sufficiently controlled and can end up producing misleading results, mostly due to the effects of confounding – when a different factor may be the underlying cause rather than the one you measured.
One example is the case of vitamin D and heart disease. A compelling meta-analysis of 32 observational studies with thousands of participants found that low vitamin D status was statistically associated with morbidity and mortality from heart disease. Indeed, with every rise in blood vitamin D levels, the risk of a cardiovascular event went down. So far so good; potentially a tabloid headline calling for widespread vitamin D for preventing heart disease. But wait a minute – what do randomised controlled trials say? Well, according to one published in JAMA last month, it would appear that giving high dose vitamin D supplements to older people doesn’t have any significant effect on their risk of cardiovascular disease.
It’s a similar case when you look at the evidence on dairy foods and body weight. Several observational studies over the years have found that higher intakes of dairy foods are statistically associated with lower body weight and fatness. However, an intervention trial published this week in the American Journal of Clinical Nutrition found that boosting dairy intake in overweight teenage girls for a year had no impact on body weight or weight gain compared with the placebo group.
This mismatch between the findings of observational studies, and those produced by randomised controlled trials has been seen time and again. For coffee and cancer. For red meat and blood lipids. For fruit and vegetable consumption and body weight. Given this lack of consistency in evidence, sure we owe it to the public to consider a wide range of high quality studies before jumping to conclusions about optimal dietary advice. This is one of the mains reasons why the output of the Scientific Advisory Committee on Nutrition and the European Food Safety Authority is so valuable. It may not provide a quick tabloid headline but it ensures that public health advice can be trusted.