Heart and lungs

Western diet and heart risk

“Western diets cause a third of heart attack deaths worldwide,” the Daily Mail reports. Western-style diets high in meat, fat, dairy and salt put people at higher risk of heart attack. According to the newspaper, a study has found that poor diet is responsible for 30% of the risk of heart disease.

This story is based on a large study in 52 countries, which looked at the diets of people who had a heart attack, and then worked out the association of diet with heart attack risk. There was a reduction in risk of heart attack with higher fruit and vegetable intake, and an increased risk with diets higher in meat, dairy and salt. The design of the study means that it cannot prove that diet causes heart attack. However, the fact that a diet rich in saturated fat and salt can lead to coronary artery disease comes as no surprise. This type of diet has been previously linked to raised cholesterol, fatty build-up in the arteries and high blood pressure. Eating a balanced healthy diet, being active and avoiding smoking are the best ways to avoid heart disease.

Where did the story come from?

This research was carried out by Dr Romaina Iqbal and colleagues of the INTERHEART study from the Population Health Research Institute McMaster University and Hamilton Health Sciences, Ontario, Canada; The Aga Khan University, Pakistan; University of Zimbabwe; Sultan Qaboos University, Oman; and Hungarian Institute of Cardiology. The INTERHEART study was funded by national bodies of several countries. It was published in the peer-reviewed medical journal, Circulation.

What kind of scientific study was this?

This was a case-control study which aimed to examine the association between diet and heart attack (myocardial infarction/MI) across the globe.

The INTERHEART study enrolled 12,461 patients who had suffered a heart attack, from 262 medical centres in 52 countries. Controls were 14,637 healthy people without heart disease, recruited from the same medical centres (e.g. visitors or relatives) and matched to each subject by age and sex. Information was collected from cases and controls on sociodemographic details, lifestyle factors (including alcohol, smoking, and physical activity), as well as body measurements.

Diet was assessed used using a 19-item food frequency questionnaire (number of times per day/week/month a food was consumed, but no information on portion size), which was designed so that it could be used across countries, with some slight regional variations. Blood samples were taken to assess blood lipid (fat and cholesterol) levels.

The researchers restricted their analyses to 5,761 cases of heart attack and 10,647 controls who did not have angina, diabetes, high blood pressure or high cholesterol, to try to minimise confounding factors from known risk factors for heart disease. They used a complex statistical method of ‘factor loading’ to group foods into patterns of Oriental diet (high use of tofu, soy and other sauces), western (high intake of meat, fried food and salty snacks) and prudent diet (high levels of fruit and vegetables). Then they looked at any links between dietary type and risk of heart attack. They divided the group into four (quartiles) for each dietary type, and those with low intakes were compared with those with higher intakes of each diet.

In a separate analysis, the researchers ‘translated’ the participants’ dietary profiles into a score which depended on the amount of different food consumed. Higher scores were given for foods known to increase risk of heart attack such as meat, salty snacks, fried foods and lower scores for foods known to be protective, such as fruits and vegetables. A higher score overall represented a poorer diet. Using this model, the researchers were able to determine how much of the risk for heart attack could be reduced if the population were to eat known low-risk food. This is known as the population attributable risk (PAR).

What were the results of the study?

The prudent diet pattern reduced risk of heart attack. When compared with the lowest intake group, risk decreased by 22% with the next highest intake, by 34% for the third highest intake, and by 30% for the highest. For the Western diet, the relationship between intake and heart attack was not linear: compared with the lowest intake group, risk of heart attack decreased by 13% with next intake category. There was a borderline significant 12% increase in risk with third intake category, and 35% increased risk of heart attack with the highest intake. There was no relationship between the Oriental diet and heart attack.

Using the dietary risk score, they found that, compared with the group of participants with the lowest risk score (i.e. those with the healthiest diets), people with the highest scores had almost a double increased likelihood of heart attack (1.92 times). Using this risk score, they calculated that 30% of the burden of heart attack in these participants was due to poor diet.

The researchers found associations between the extent to which a particular dietary pattern was followed (i.e. quartiles 1-4) and various factors, including age, sex, BMI, waist-hip ratio, blood pressure, blood sugar, smoking, education and household income. Assessing individual food items rather than dietary patterns, they found significant increased risk of heart attack from salty foods and fried foods, and significantly decreased risk from vegetables (raw, cooked and green leafy) and fruit.

What interpretations did the researchers draw from these results?

The authors conclude that using a simple risk score, they have found that an unhealthy diet increases the risk of heart attack worldwide (contributing about 30% to heart attack burden in this population).

What does the NHS Knowledge Service make of this study?

This is a large and in-depth study, which has attempted, using a simple food questionnaire, to categorise the dietary intake of subjects with and without heart attack across 52 countries, and then examine the associations between dietary pattern and heart attack risk. The results demonstrated a clear reduction in risk of heart attack with higher fruit and vegetable intake; an increased risk of heart attack with diets higher in meat, dairy and salt (although the relationship was weaker); and no relationship between heart attack and an Oriental dietary pattern.

The main limitation of this study is in the method of assessing diet:

  • Food frequency questionnaires always include some degree of error because asking people to estimate how often they consume foods on a daily/weekly/monthly basis, may have limited accuracy.
  • Food quantity, portion size and calorific content of the foods were not assessed, and so grouping the diet into quartiles of intake based on dietary patterns may also be slightly inaccurate.
  • As the ‘case’ group had already suffered from a heart attack, they may have given biased responses about their dietary intake, e.g. if they were trying to find a possible explanation for why the heart attack could have occurred. Alternatively, they may have changed to a healthy diet since the heart attack and be giving responses about their current diet rather than their previous one.
  • Additionally, the authors state that their questionnaire “has not been validated against another dietary measure”.

It is unclear how carefully the controls and the cases were matched for other factors besides age and sex. Other medical and sociodemographic factors may have differed between them, which could mean that they were not entirely representative of the cases. Lastly, the study excluded a large number of cases and controls who already had established risk factors for heart attack. Associations between heart attack and diet may have been different had they been included, in addition to allowing some quantification of the attributable risk of these other factors.

A case control study cannot prove causation, but these findings support those of other studies. The fact that a diet rich in saturated fat and salt can lead to coronary artery disease should come as no surprise, considering how such a diet can increase the risk of known risk factors such as raised cholesterol, fatty build-up in the arteries and high blood pressure. The best ways to decrease the risk of heart disease are to eat a balanced healthy diet, remain active and avoid smoking.

Sir Muir Gray adds...

This strengthens the advice that if you want to lower your risk of heart disease and stroke, eat less saturated fat and salt. Pass the olive oil.

NHS Attribution