Do diets and supplements have heart benefits?
A study has found no heart benefit from most diets and supplements, although some people eating less salt were less likely to die early. We look behind the headlines.
10 July 2019
The Daily Mail reported that the Mediterranean diet makes no difference to heart health or the risk of an early death. News reports also warned that nutritional supplements might be doing people more harm than good, but that most have no effect. The Independent was the only paper to highlight that much of the evidence used in the study was deemed ‘suboptimal quality’, making it difficult to draw firm conclusions.
Researchers at West Virginia University analysed results from 277 clinical trials involving nearly one million people. Only four of these studies were new; the majority of the evidence they relied on has already been included in previous reviews.
For each of the diets and supplements, the researchers assessed the quality of the evidence available. In many cases they concluded that existing evidence was ‘suboptimal quality’, meaning that they could not be certain about their findings.
In many cases they concluded that existing evidence was ‘suboptimal quality’, meaning that they could not be certain about their findings.
They found ‘moderate-certainty’ evidence to show that eating less salt lowers the risk of dying prematurely in people with normal blood pressure, and lowers the risk of death from heart and circulatory disease in people with high blood pressure.
The researchers didn’t find any significant effect on heart health or death when they looked at other diets, including reduced-fat and Mediterranean diets. These results challenge official nutrition advice which recommends the Mediterranean diet to reduce the risk of type 2 diabetes, high blood pressure and raised cholesterol, which are all risk factors for heart disease.
A typical Mediterranean diet includes lots of vegetables, fruits, beans and cereal products, for example wholegrain bread, pasta and brown rice. It also uses unsaturated fats such as olive oil instead of saturated fats such as butter.
Other results from the study supported conclusions from previous research that has found no significant benefits from taking most nutritional supplements.
How good was the research?
A strength of the research is that it included results from many studies involving lots of people. However this also meant it was comparing information from studies which had been conducted in many different ways, and the researchers admitted that their findings were limited by the ‘suboptimal quality’ of the evidence they were relying on.
The studies being analysed were all randomised controlled trials (RCTs), where two or more groups of people are asked to take different treatments or follow different lifestyle advice, and their outcomes are compared.
Earlier this year, a separate review of evidence on the Mediterranean diet found it was associated with a lower risk of coronary heart disease and stroke.
RCTs are often seen as the gold standard for medical testing, and are helpful in assessing whether nutritional supplements have any benefits or harms. However when it comes to lifestyle interventions – such as asking people to follow the Mediterranean diet - RCTs can be difficult to conduct in a fully controlled way. People might struggle to follow the diet for the full duration of the study, making it harder to detect any genuine benefits of the diet.
In response to the new analysis, UK experts have highlighted that the evidence supporting the protective effects of certain diets on heart health is not just limited to RCTs. Earlier this year, a separate review of evidence on the Mediterranean diet found it was associated with a lower risk of coronary heart disease and stroke.
Research has previously demonstrated that diets such as the Mediterranean diet can help people to lose weight, which in turn reduces their risk of heart and circulatory disease. These conflicting conclusions highlight the difficulty in analysing information from different sources, and the reason why healthy eating guidelines are based on a consensus reached by independent experts reviewing a range of evidence.
The BHF view
BHF Senior Dietician Victoria Taylor said:
"This is a review of randomised controlled trials and meta-analyses of randomised controlled trials (RCTs) looking at the effects of nutritional supplements or dietary interventions on death or events such as a heart attack or stroke.
"It can be difficult to get a clear answer about a direct relationship between dietary interventions and death or events. Relying on RCTs alone may not give us the full picture for a dietary intervention like the Mediterranean diet. Unlike drug trials, studies on dietary approaches are very difficult to conduct and may vary widely in their approaches and definitions of the interventions. It would also be all but impossible to carry out a research trial where you carefully controlled the diets of thousands of people over many years.
"In the UK, our dietary guidelines are based on independent review of the full evidence base. This encompasses a range of types of study, not just RCTs, in order to offer the best approach to eating to prevent ill health."