Middle-aged people who get roughly half their daily calories from carbohydrates live several years longer on average than those with meat-heavy low-carb diets, researchers reported Friday.
The findings, published in The Lancet medical journal, challenge a trend in Europe and North America toward so-called Paleo diets that shun carbohydrates in favour of animal protein and fat.
Proponents of these "Stone Age" diets argue that the rapid shift 10,000 years ago -- with the advent of agriculture -- to grains, dairy and legumes has not allowed the human body enough time to adapt to these high-carb foods.
For the study, receiving less than 40 percent of total energy intake from carbohydrates qualified as a low-carb regimen, though many such diets reduce the share to 20 percent or less.
At the other extreme, a 70 percent or higher share of carbohydrates -- such as pasta, rice, cakes, sugary drinks -- can also reduce longevity, but by far less, the scientists found.
"Low-carb diets that replace carbohydrates with protein or fat are gaining widespread popularity as a health and weight loss strategy," said lead author Sara Seidelmann, a researcher at Brigham and Women's Hospital in Boston.
"However, our data suggests that animal-based low carbohydrate diets might be associated with shorter overall lifespan and should be discouraged."
Replacing meat with plant-based fats (such as avocados and nuts) and proteins (such as soy products and lentils) reduces the risk of mortality, Seidelmann and her team found.
The optimal balance of food groups for longevity remains hotly debated.
Many studies have concluded that eating carbohydrates in moderation -- 45 to 55 percent of total calorie intake -- is best, but others report improved short-term, cardio-metabolic health with high-protein, high-fat diets.
Measures of metabolic health include blood pressure, good and bad cholesterol, and blood sugar levels.
- Plant vs animal protein -
"Low carbohydrate dietary patterns favouring animal-derived protein and fat sources, from sources such as lamb, beef, pork, and chicken, were associated with higher mortality," the study said.
"Those that favoured plant-derived protein and fat intake, from sources such as vegetables, nuts, peanut butter, and whole-grain breads, were associated with lower mortality," it said, adding that this suggested "the source of food notably modifies the association between carbohydrate intake and mortality."
Seidelmann and colleagues poured over the medical histories of nearly 15,500 men and women who were 45-64 when they enrolled -- between 1987 and 1989 -- in a health survey spread across four locations in the United States.
Participants filled out detailed questionnaires about their dietary habits -- what foods, how much, how often, etc.
Over a 25-year follow up period, more than 6,000 of the men and women died.
People who got 50-55 percent of their calories from carbohydrates outlived those with very low-carb diets, on average, by four years, and those with high-carb diets by one year.
A review of medical records for an additional 432,000 people from earlier studies confirmed the results, which are also in line with World Health Organization (WHO) recommendations.
"There is nothing to be gained from long-term adherence to low-carbohydrate diets rich in fats and proteins from animal origins," said Ian Johnson, a nutrition researcher at Quadram Institute Bioscience in Norwich, England, commenting on the research, in which he did not take part.
But carb quality, not just quantity, is crucial he added.
"Most should come from plant foods rich in dietary fibre and intact grains, rather than from sugary beverages or manufactured foods high in added sugar."
Fibres also help maintain a healthy gut flora, now considered to be a major factor in health and disease.
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