"Eating a low-fat diet 'increases your risk of dying young by 25%'," is the stark but somewhat misleading report in The Sun. The study the headline is based on mainly looked at people in lower- and middle-income countries, where diets are very different, so the results may not be relevant to the UK.
Many previous studies linking high levels of saturated fat to heart disease and early death were carried out in high-income countries, such as the UK and US, where heart disease and consumption of saturated fats are both relatively high. The resulting recommendations that people avoid a high-fat diet may not be very relevant in countries such as Bangladesh and Zimbabwe, where eating enough may be a more pressing concern than weight gain. That's why this latest study focused on lower- and middle-income countries.
The results of this latest study suggest people who get more than three quarters of their total calories from carbohydrates have a 28% higher risk of death than those who get about half their calories from carbs.
However, people from lower- and middle-income countries are more dependent on refined carbohydrates, such as white rice. These are known to be less healthy than unrefined sources, such as brown rice and wholemeal bread, which are more readily available in the UK.
The researchers say their results suggest global dietary guidelines should be revised. However, their recommendations – that carbohydrates should provide 50 to 55% of energy intake and fat around 35% – are in line with existing UK dietary guidelines.
The whole "fats vs carbs" debate is arguably a bit of a sideshow: the truth is that, based on the latest UK obesity statistics, many of us simply eat too much.
Where did the story come from?
The study was carried out by researchers from universities and research centres in 18 countries around the world: Canada, Sweden and the United Arab Emirates (high-income countries); Argentina, Brazil, China, Chile, Colombia, Iran, Malaysia, occupied Palestinian territory, Poland, South Africa and Turkey (middle-income countries); and Bangladesh, India, Pakistan and Zimbabwe (lower-income countries).
It was funded by many local and national organisations, and by several pharmaceutical companies. The results were presented at the European Society of Cardiology Congress in Barcelona, Spain, and published in the peer-reviewed medical journal The Lancet.
The reporting of this study in the UK media was generally poor. None of the sources made clear the limited relevance of the study to the UK. For example, The Sun reported: "Cutting back on butter, cheese and meat raised the risk of an early death." But people in the study in countries such as India were unlikely to be "cutting back" on cheese and meat – it's more likely they were unable to afford to eat a lot of it, or that their traditional diet did not include much meat or dairy.
The Independent said: "Consuming high levels of all fats cuts early death rates by up to 23%." However, the report does not mention that these "high" levels were around 35% of calorie intake – around the average for the UK.
What kind of research was this?
This was a population-based cohort study using food-frequency questionnaires to sample adults aged 35 to 70 in 18 countries. Researchers wanted to see whether the dietary balance of fat, protein and carbohydrate was linked to people's chances of dying from any cause, or having a major cardiovascular event such as a heart attack, stroke or heart failure.
Cohort studies, like all observational studies, may be affected by confounding factors. This means we can't be sure that one factor (diet) is directly linked to another (death or cardiovascular disease).
What did the research involve?
Researchers recruited adults from 18 countries – 3 high-income, 11 medium-income and 4 lower-income. People filled in questionnaires about their diet, and were assessed for a range of health and lifestyle factors.
They were followed up at three, six and (for those who could be contacted) nine years to see what had happened to them. The groups were then divided into "quintiles", or fifths, from the highest consumption of different nutrients recorded to the lowest.
After adjusting for confounding factors, the researchers looked to see how diet was linked to chance of death or cardiovascular disease.
They recruited 148,723 people, of whom 135,335 remained after excluding those with missing data, a history of cardiovascular disease or who gave implausible answers on their dietary questionnaire.
The questionnaires were designed to be suitable for the country or region being sampled, and all of them mapped back to a method for translating food (potatoes, butter) into food types (carbohydrates, saturated fats).
The researchers adjusted their figures to account for:
- education level
- physical activity
- waist:hip ratio
They also looked at whether the people had diabetes, whether they lived in an urban or rural location, and their total calorie intake.
Asian countries – which had much higher levels of carbohydrate consumption than other countries – were also analysed separately to see if the results held true across different regions.
What were the basic results?
Of the 135,335 people in the study, 1,649 died of cardiovascular disease and 3,809 died from other causes.
The researchers compared the group of people who ate the most carbohydrate (average 77.2% of calories) with those who ate the least (average 46.4% of calories). They found:
- People who ate the most carbohydrate were 28% more likely to have died than those who ate the least (hazard ratio [HR] 1.28, 95% confidence interval [CI] 1.12 to 1.46).
- There was no difference in the risk of having major cardiovascular disease (HR 1.01, 95% CI 0.88 to 1.15).
They compared people who ate the most total fat (35.3%) with those who ate the least (10.6%). They found:
- People who ate the most fat were 23% less likely to have died than those who ate the least (HR 0.77, 95% CI 0.67 to 0.87).
- There was no difference in risk of having major cardiovascular disease (HR 0.95, 95% CI 0.83 to 1.08).
Looking at different types of fat, they found that every type – saturated, polyunsaturated and monounsaturated – revealed a similar pattern. However, when they plotted the results on a graph, it didn't go in a straight line, suggesting that both too much and too little fat could be a problem.
How did the researchers interpret the results?
The researchers said: "We found that high carbohydrate intake (more than about 60% of energy) was associated with an adverse impact on total mortality and non-cardiovascular disease mortality. By contrast, higher fat intake was associated with lower risk of total mortality."
They added: "Individuals with a high carbohydrate intake might benefit from a reduction in carbohydrate intake and increase in consumption of fats."
However, they also warned that the study "does not provide support for very low carbohydrate diets", saying that "a certain amount of carbohydrate is necessary to meet short-term energy demands during physical activity, and so moderate intakes (eg 50 to 55% energy) are likely to be more appropriate than either very high or very low carbohydrate intake".
The results of the study have been presented in the media as if they overturn all current dietary guidelines. In the UK at least, that is completely misleading. The study results support the UK guidelines, having found that people who get around 50% of their calories from carbohydrates and 35% from fat, as recommended by Public Health England, were likely to live the longest.
There are some limitations to the study, not least that observational studies cannot prove cause and effect.
For example, the very low fat and high carbohydrate levels of diets found among some participants in the study might simply represent poverty – rice, flour and sugar tend to be much cheaper than animal products such as butter and meat. It's not a surprise that people living on diets where most of their energy comes from nutrient-poor sources, such as white rice, are likely to live shorter lives. However, this does not apply widely in the UK.
The researchers may have a point that global guidelines for diet need to be revised in the light of these international findings, particularly in parts of the world where under-nutrition is more of a problem than obesity. However, UK guidelines are already in line with the study findings.
For more information about a healthy diet, see the Eatwell Guide