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Moderate carb intake may be best for health and longevity, study finds

Equivalent to 50-55% of energy intake; more or less linked to heightened risk of death

Caroline White

Friday, 17 August 2018

Eating a moderate amount of carbohydrates may be the best for health and longevity, suggests an observational study* published in The Lancet Public Health journal.

The study of more than 15,400 people from the Atherosclerosis Risk in Communities Study (ARIC) in the USA found that diets both low (less than 40% energy) and high (more than 70% energy) in carbohydrates were linked to a heightened risk of death, while moderate consumption (50–55% of energy) was associated with the lowest risk.

The findings, confirmed in a pooled analysis of data from studies on carbohydrate intake, including more than 432,000 people from over 20 countries, also suggest that not all low-carbohydrate diets are equal.

Substituting carbs for animal-based proteins and fats from foods like beef, lamb, pork, chicken and cheese was associated with a greater risk of death, while eating more plant-based proteins and fats from foods such as vegetables, legumes, and nuts was linked to a lower risk of death.

“We need to look really carefully at what are the healthy compounds in diets that provide protection”, says Dr Sara Seidelmann, clinical and research fellow in cardiovascular medicine from Brigham and Women’s Hospital, Boston, USA who led the research.

“Low-carb diets that replace carbohydrates with protein or fat are gaining widespread popularity as a health and weight loss strategy. 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.”

Previous randomised trials have shown that low carb diets help short-term weight loss and improve cardiometabolic risk. But the long-term impact of carbohydrate restriction on death is hotly contested, with research so far producing contradictory results.

To address this uncertainty, researchers studied 15,428 adults aged 45-64 from diverse socioeconomic backgrounds from four US areas who were enrolled in the ARIC study between 1987 and 1989. All participants said they ate the equivalent of 600-4200 kcal/day for men and 500-3600 kcal/day for women, and participants with extreme (high or low) caloric intake were excluded from the analysis.

At the start of the study and again six years later, participants filled in a questionnaire on what, how much, and how often they ate/drank. Their responses were used to estimate the cumulative average calories they derived from carbs, fats, and protein.

The researchers assessed the association between overall carbohydrate intake and death from all causes after taking account of potentially influential factors, such as age, education, exercise, income level, smoking, and diabetes.

During an average monitoring period of 25 years, 6283 people died. There was a U-shape association between overall carbohydrate intake and life expectancy, with low (less than 40% of calories from carbs) and high (more than 70%) intake of carbohydrates associated with a higher risk of death compared with moderate intake (50-55% of calories).

The researchers estimated that from age 50, average life expectancy was an additional 33 years for those with moderate carbohydrate intake—four years longer than those on a low carb diet (29 years), and one year longer than those on a high carb diet (32 years).

But the researchers highlight that since diets were only measured at the start of the trial and sic years later, dietary patterns might have changed over 25 years, which might have influenced the findings.

In the next stage of the study, the researchers pooled the data from eight prospective cohorts (including ARIC) involving 432,179 people in North America, Europe, and Asia. This revealed similar trends: high and low carb diets were associated with a shorter life expectancy than moderate intake.

“A mid-range of carbohydrate intake might be considered moderate in North America and Europe where average consumption is about 50%, but low in other regions, such as Asia, where the average diet consists of over 60% carbohydrates,” explains Dr Seidelmann.

Further analyses looking at whether the source of proteins and fats favoured in low-carbohydrate diets was associated with lifespan, the researchers found that replacing carbohydrates with protein and fat from animal sources was linked to a higher risk of death than moderate carbohydrate intake. But replacing carbohydrates with plant-based foods was linked to a lower risk.

“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,” says Walter Willett, professor of epidemiology and nutrition at Harvard TH Chan School of Public Health and co-author of the study.

Writing in a linked comment**, Drs Andrew Mente and Salim Yusuf from McMaster University, Hamilton, Canada, say, “Such differences in risk associated with extreme differences in intake of a nutrient are plausible, but observational studies cannot completely exclude residual confounders when the apparent differences are so modest. Based on first principles, a U-shaped association is logical between most essential nutrients versus health outcomes.

“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.”

Catherine Collins, NHS dietitian, said: “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. Yet supporters of the cult of Low Carb High Fat (LCHF) eating, itself based on a lifestyle choice and the flimsiest of evidence supporting benefit, will no doubt disagree with this newest research on the subject.

“Such a stance is 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. This approach reduces our calories from fat to around a third of total calories, with protein making up around 20% of the total, and alcohol calories sneaking in, too, where consumed.”

She added: “Two points are worth noting from this research – first, that weight gain associated with increasing age is independent of the type of diet followed. Following a high carb diet does not lead to a propensity for weight gain, another factoid promoted in the LCHF community.

“Secondly, and more importantly, it raises some timely and important issues about whether people with diabetes should be using a low carb or very low carb approach as long-term treatment. Diabetes of any type is characterised by metabolic abnormalities that increase the risk of cardiovascular disease and other diseases of ageing beyond those of the general population.

“The feting and promotion of GPs promoting often bizarre low carb diets to manage diabetes has gained much media traction from the uncritical support of the LCHF movement that promote over-hyped personal beliefs as ‘fact’, and run counter to the dietary recommendations of Diabetes UK. If nothing else, this study provides some redress to this one-sided debate, and adds caution to such practice for long-term management.”

*Seidelmann SB, Claggett B, Cheng S, et al. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. The Lancet Public Health, August 16 2018, DOI:10.1016/S2468-2667(18)30135-X
**Mente A and Yusuf S. Evolving evidence about diet and health. The Lancet Public Health, 16 August 2018, DOI:10.1016/S2468-2667(18)30160-9

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