'Avoid fads and stick to diet guidelines,' say US heart experts

"Avoid celebrity fad diets and just eat your greens to stay healthy," the Daily Mirror reports.

A review carried out by US cardiologists found little evidence fads like juicing and coconut oil prevent heart disease and other cardiovascular diseases.

But they did find evidence extra virgin olive oil, blueberries and strawberries, leafy green vegetables and controlled portions of nuts are beneficial.

The researchers also stated the alleged benefits of gluten-free diets for people who don't have disorders like coeliac disease are "unsubstantiated".

The authors don't report any of the methods they used as part of their research, so we don't know how they found and selected the studies they included.

It's possible not all of the available evidence was considered, and this may have influenced the findings. The findings should therefore primarily be considered to be the authors' opinion following their review of the evidence.

A systematic review of high-quality studies is needed before we can draw any firm conclusions.

Still, the recommendation that following a Mediterranean-style diet, with plenty of vegetables, fruit, nuts, beans, cereal grains, olive oil and fish, is beneficial is supported by independent research, including a major study we looked at in 2016.

Read more about the benefits of a Mediterranean-style diet.

Where did the story come from?

This review was carried out by researchers from a large number of institutions, including the National Jewish Health in Colorado, the University of South Carolina, George Washington University School of Medicine, and the Instituto de Salud Carlos III in Madrid.

It was published in the peer-reviewed Journal of the American College of Cardiology and is available to read for free online.

The aim of the review was to look at some of the commonly promoted foods and dietary patterns – or "fads" – said to improve cardiovascular health, and establish whether sufficient evidence can back up these claims. 

The researchers haven't reported any sources of funding for this study, but many have received grants or have links to related institutions, such as the California Walnut Commission, Pressed Juicery, Avocado Nutrition Sciences, Seafood Nutrition Partnership, McDonald's Global Advisory Council, Canola Oil Council, McCormick Spice Institute, National Cattlemen's Beef Association, Healthways and TerraVia. Dr O'Keefe has a financial interest in Cardiotabs, a nutritional supplement company.

The UK media covered this research accurately and quoted the study team, including pointing out some limitations of their research.

Professor Freedman was quoted in The Mirror as saying: "Some nutrition studies tend to be based on surveys that rely on people's memories of what they ate, which isn't always reliable."

And the Daily Mail pointed out that, "A juice diet can make you fat by concentrating the calories and making it easier to overconsume." 

What evidence did they find?

The researchers reviewed the evidence on a number of food groups to form conclusions about their cardiovascular benefits.

They didn't provide any information about the methods they used for their review, such as search terms, study inclusion criteria, the method of selection and quality appraisal.

This means the study can't be considered to be a systematic review on this topic.

The main findings for each food group are described below.

Eggs and dietary cholesterol

The 2015 Dietary Guidelines Advisory Committee report suggested there was no available evidence to link the consumption of dietary cholesterol with cholesterol levels in the body.

However, they discussed the general link seen, where high-cholesterol foods increase blood cholesterol, as shown by a recent meta-analysis. The authors of this review therefore advise limiting the intake of dietary cholesterol from eggs or any high-cholesterol foods.

Vegetable oils

Solid fats, such as coconut oil and palm oil, are said to have harmful effects on cardiovascular risk. It's been claimed some tropical oils might have health benefits, but the researchers didn't find any evidence to support this.

But they did find a beneficial link between liquid vegetable oils and lower levels of blood cholesterol and other fats. The clearest benefit was seen for olive oil, which reduced cardiovascular risk.

Antioxidants

Evidence suggests fruit and vegetables are the best source of antioxidants for reducing cardiovascular risk. The evidence didn't find a benefit from high-dose antioxidant dietary supplements.

Nuts

Nuts may be included as part of a balanced diet to reduce cardiovascular risk. But it's important to control portion size, as nuts are very calorific and can easily result in consuming too many calories.

Leafy green vegetables

A diet rich in leafy green vegetables has significant beneficial effects on cardiovascular risk factors and is associated with a reduced risk.

Juicing

Evidence for juicing suggests it's harmful when the pulp is removed, but is otherwise inconclusive.

The main concern was the increase in calorie intake and addition of sugars, as it's easy to consume too many calories.

Overall, the researchers felt until better data is available, the consumption of whole foods is considered preferable.

Plant-based diet

The evidence suggests a mainly plant-based diet could improve cardiovascular risk factors and reduce the progression of coronary heart disease.

Gluten

For patients with gluten-related disorders, a gluten-free diet rich in fruit and vegetables, lean protein sources, nuts and seeds is important for managing symptoms. No health benefits were found for people who didn't have gluten-related disorders.

Conclusion

This review looked into food groups often linked to cardiovascular risk, some of which may be overstated or based on poor evidence.

Overall, the researchers reported there being evidence solid fats are harmful. Examples include coconut and palm oil, eggs, fruit and veg juicing with pulp removal, and "[US] Southern diets" that include added fats, fried and processed foods and sugar-sweetened drinks.

There's also evidence extra virgin olive oil, blueberries and strawberries, leafy green vegetables and controlled portions of nuts are beneficial for cardiovascular health.

Investigating whether the health claims made about certain foods and drinks are based on solid evidence is important.

But this study has limitations – the main one being the researchers' methods weren't outlined in the review, so we don't know how they found and selected the studies they included. It's possible not all the available evidence was considered, and this may have influenced the findings.

This study should therefore primarily be considered to be an opinion piece following the authors' review of the evidence.

Dietary studies often have inherent limitations. Observational studies usually rely on people recalling what they've eaten, which is subject to bias.

It's also possible any links found were influenced by other dietary, health and lifestyle factors (confounders). It's particularly difficult to isolate the effect of a single food item with any certainty.

Randomised controlled trials looking at diet can be difficult to conduct, especially on topics like heart disease, which can take many years to see results.

It can also be hard to control what people are eating, or recruit enough people and follow them up for long enough to get reliable evidence.

Many of this review's conclusions do seem plausible, however. But a systematic review clearly describing the study inclusion criteria and methodology would give greater confidence in the overall message about the state of the evidence.

The advice to avoid fad diets, which are often hyped by the media, seems sensible. As we have mentioned many times before, the idea there is a single superfood that's the key to preventing chronic diseases is wishful thinking.

It may be unexciting, and not newsworthy, but the current evidence we have continues to support the tried and tested diet guidelines to prevent cardiovascular disease: that is, a balanced diet with lots of fruit and veg, but not too many calories, saturated fats, sugar and salt.