A large Canadian study is challenging conventional wisdom that says a low-fat diet is optimal for cardiovascular health and reduces the risk of premature death.
The McMaster University study of more than 135,000 people in 18 countries found that eating a moderate amount of all types of fat is linked to a reduced risk of early mortality compared to the much-touted low-fat diet — while consuming a high-carbohydrate diet is associated with an increased risk of dying early.
"Contrary to popular belief, increased consumption of dietary fats is associated with a lower risk of death," said lead author Mahshid Dehghan, a nutrition epidemiologist at the Hamilton university's Population Health and Research Institute.
"Those with a high-fat intake, about 30 per cent of energy intake, had a 23 per cent lower risk of mortality and an 18 per cent lower risk of stroke, compared to the low-intake group, which had 11 per cent energy from fat," Dehghan said from Barcelona, where she presented the findings Tuesday to the European Society of Cardiology Congress.
"The association with lower mortality was also seen with all major types of fat, by which I mean saturated, monounsaturated and polyunsaturated fatty acids."
Saturated fat is found in meat and dairy products, while monounsaturated fat is contained in nuts, avocados, and vegetable and olive oils. Polyunsaturated fat is found in walnuts, sunflower and flax seeds, fish, corn, soybean and safflower oils.
Current global guidelines recommend that 50 to 65 per cent of daily calories come from carbohydrates, and less than 10 per cent from saturated fats. But Dehghan said that advice is mostly based on evidence from studies in North America and Europe.
Cardiovascular disease is a global epidemic, with 80 per cent of the burden of disease in low- and middle-income countries. Diet is a key modifiable risk factor for cardiovascular disease, experts say.
Dehghan said the healthiest diet would be made up of 50 to 55 per cent carbohydrates and 35 per cent total fat, including both saturated and unsaturated types.
"We found no evidence that below 10 per cent of energy from saturated fat is beneficial — and going below seven per cent is even harmful," she said, adding that a diet containing 10 to 13 per cent of energy from saturated fat was found to be beneficial.
A diet that provides more than 60 per cent of energy from carbohydrates — one common among populations in China and South Asia — was associated with a 28 per cent higher risk of premature death, researchers found.
"The message of our study is moderation as opposed to very low or very high intake in consumption of both fats and carbohydrates."
"We're not advocating an extreme diet," agreed co-author Andrew Mente. "We're not saying that people should go on a low-carb, very high-fat diet because we didn't find any benefit with a very low-carb diet either.
"There's a sweet spot for carbohydrates, which is about 55 per cent of energy intake."
The PURE (Prospective Urban Rural Epidemiology) study was published Tuesday in The Lancet. In a linked commentary in the journal, Drs. Christopher Ramsden and Anthony Domenichiello of the U.S. National Institute on Aging called the research "an impressive undertaking that will contribute to public health for years to come."
"The relationships between diet, cardiovascular disease and death are topics of major public health importance.... Initial PURE findings challenge conventional diet-disease tenets that are largely based on observational associations in European and North American populations, adding to the uncertainty about what constitutes a healthy diet. This uncertainty is likely to prevail until well-designed randomized controlled trials are done."
Mente, also a nutrition epidemiologist at the Population Health and Research Institute, was lead author of a second analysis from the PURE study presented Tuesday at the cardiology meeting.
That paper — one of three from PURE published in The Lancet — found that eating three to four servings of fruit, vegetables and legumes per day reduces the risk of premature death.
"And consuming higher amounts, pretty much you have the same level of risk," Mente said from Barcelona. "There's no added benefit with consuming more than four servings.
"This is important because existing guidelines recommend that people consume at least five servings per day, which is less affordable in the poorer countries because fruits and vegetables — particularly fruits — are more expensive as a proportion of people's incomes."
Lower-income Canadians may also be unable to afford the five to 10 daily servings of fruits and vegetables recommended in the country's Food Guide.
"So what our study shows is you can achieve maximum benefit through fruits and vegetables and legumes, and it's also affordable at the same time."
Mente said the study also showed raw vegetables appear to confer greater health benefits than those that are cooked because of a loss of nutrients from being exposed to heat.
With the federal government in the process of revamping Canada's Food Guide, the research could be a timely addition to consultations on what Canadians should be eating, Mente suggested.
"We would hope that independent thinkers perhaps reconsider the guidelines and look at our data, and perhaps rather than putting limits on total fat and saturated fat, perhaps we should be putting limits on the amount of carbohydrates that people consume."