Eating either a low-carb diet or a high-carb diet raises the risk of an early death, according to a major new study which will dismay the many people who have ditched the likes of bread, rice and potatoes for weight loss or health reasons.
Researchers who pooled the results of eight large studies have found that eating a moderate amount of carbohydrates is best for a healthy lifespan. Less than 40% or more than 70% of calories from carbohydrates carried a higher risk of mortality, The Guardian reports.
Not all low-carb diets are equal, however. People who ate a lot of meat and fats instead of carbohydrates, such as lamb, chicken, steak, butter and cheese, had a higher mortality risk than those who got their protein and fats from plant-based foods such as avocados, legumes and nuts. Popular weight loss diets such as Atkins and Dukan include a substantial amount of meat-based foods.
“Low-carb diets that replace carbohydrates with protein or fat are gaining widespread popularity as a health and weight loss strategy,” said Dr Sara Seidelmann, a clinical and research fellow in cardiovascular medicine from Brigham and Women’s Hospital inBoston, who led the research published in the Lancet public health journal.
“However, our data suggests that animal-based low-carbohydrate diets, which are prevalent in North America and Europe, might be associated with shorter overall life span and should be discouraged. Instead, if one chooses to follow a low-carbohydrate diet, then exchanging carbohydrates for more plant-based fats and proteins might actually promote healthy ageing in the long term.”
Seidelmann, who is both a cardiologist and a nutritionist, told the Guardian the team had published a substantial body of work “to thoroughly answer a question and not simply provide just one piece of the picture”.
“Nutrition is high up on everybody’s mind but there is such confusion about what we should eat. One day, a study is coming out telling us high carb is better, another day a study is telling us low carb is better.”
Trials to compare low-carb and high-carb diets directly are not possible, because they have to be carried out over many years and people find it hard to stick to a diet over any length of time. Instead, her team carried out observational research with more than 15,400 people, aged 45 to 64, from diverse socio-economic backgrounds from four US communities who were enrolled in the atherosclerosis risk in communities study.
Those people filled out questionnaires on their eating patterns on two occasions, six years apart. Their health was followed up for 25 years, allowing for factors that might alter the results, such as smoking, income and diabetes.
These results were pooled with seven other observational studies carried out across the world, involving a total of more than 430,000 people.
They found that 50-year-olds eating a moderate carb diet, with half their energy coming from carbohydrates, had a further life expectancy of 33 years, which was four years longer than those on low-carb diets and one year longer than those who ate a high-carb diet.
The authors said they could not prove cause and effect, because of the nature of the studies.
However, they said people who embraced western-type diets that heavily restricted carbohydrates often ate fewer vegetables, fruit, and grains and more animal proteins and fats.
Some of those animal products have been implicated in stimulating inflammatory pathways, biological ageing and oxidative stress, and could be a contributing factor to the increased risk of mortality.
High-carb diets are common in Asian and poorer nations, they said, where people eat a lot of refined carbohydrates such as white rice. Those also contribute to a chronically high glycaemic load and worse metabolic outcomes.
“These findings bring together several strands that have been controversial.
Too much and too little carbohydrate can be harmful but what counts most is the type of fat, protein, and carbohydrate,”said Walter Willett, a professor of epidemiology and nutrition at Harvard T H Chan School of Public Health and the co-author of the study.
Low-carb diets are popular for weight loss because they work quite well in the short term, said Seidelmann, and they are usually meat-based. The study was not set up in a way that would make it possible to compare moderate carb with low-carb plant-based diets but, said Seidelmann, “the more plant-based [the diet was], the lower the mortality”.
“No aspect of nutrition is so hotly contended on social media than the carb versus fat debate, despite the long term evidence on health benefits firmly supporting the higher carb argument,” said Catherine Collins, an NHS dietitian.
The “cult of low carb high fat eating” was based on a lifestyle choice and the flimsiest of evidence, she said. Its devotees were “at odds with advice from WHO and government health bodies globally – including the UK’s Public Health England – that recommend a carb intake to provide around half our daily calorie needs”.
She added that it the findings raise questions about the current hyping of low-carb diets for people with diabetes. “The feting and promotion of GPs promoting often bizarre low carb diets to manage diabetes has gained much media traction,” she said. “If nothing else, this study provides some redress to this one-sided debate, and adds caution to such practice for long term management.”
In a commentary in the journal, Dr Andrew Mente and Dr Salim Yusuf, from McMaster University in Canada, said it was not possible to rule out completely all the factors that might skew the results, but that the findings were that logical and moderate carbohydrate consumption was likely to be better for people than low or high-carb diets.
“Essential nutrients should be consumed above a minimal level to avoid deficiency and below a maximal level to avoid toxicity. This approach maintains physiological processes and health (ie, a so-called sweet spot). Although carbohydrates are technically not an essential nutrient (unlike protein and fats), a certain amount is probably required to meet short-term energy demands during physical activity and to maintain fat and protein intakes within their respective sweet spots,” they wrote.