Eating carbohydrates in moderation seems to be optimal for health and longevity, suggests new research published in The Lancet Public Health journal. The observational study of more than 15,400 people from the Atherosclerosis Risk in Communities Study (ARIC) in the USA found that diets both low (< 40% energy) and high (>70% energy) in carbohydrates were linked with an increase in mortality, while moderate consumers of carbohydrates (50-55% of energy) had the lowest risk of mortality.
The primary findings, confirmed in a meta-analysis of studies on carbohydrate intake including more than 432,000 people from over 20 countries, also suggest that not all low-carbohydrate diets appear equal eating more animal-based proteins and fats from foods like beef, lamb, pork, chicken and cheese instead of carbohydrate was associated with a greater risk of mortality. Alternatively, eating more plant-based proteins and fats from foods such as vegetables, legumes, and nuts was linked to lower mortality.
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 aging in the long term.
Previous randomised trials have shown low carbohydrate diets are beneficial for short-term weight loss and improve cardiometabolic risk. However, the long-term impact of carbohydrate restriction on mortality is controversial with prospective research so far producing conflicting results. What's more, earlier studies have not addressed the source or quality of proteins and fats consumed in low-carb diets.
To address this uncertainty, researchers began by studying 15,428 adults aged 45-64 years from diverse socioeconomic backgrounds from four US communities (Forsyth County, NC; Jackson, MS; Minneapolis, MN; and Washington County, MD) enrolled in the ARIC cohort between 1987 and 1989. All participants reported consuming 600-4200 kcal per day for men and 500-3600 kcal per 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 6 years later, participants completed a dietary questionnaire on the types of food and beverages they consumed, what portion size and how often, which the researchers used to estimate the cumulative average of calories they derived from carbohydrates, fats, and protein.
The researchers assessed the association between overall carbohydrate intake (categorised by quantiles) and all cause-mortality after adjusting for age, sex, race, total energy intake, education, exercise, income level, smoking, and diabetes. During a median follow-up of 25 years, 6283 people died.
Results showed a U-shape association between overall carbohydrate intake and life expectancy, with low (less than 40% of calories from carbohydrates) and high (more than 70%) intake of carbohydrates associated with a higher risk of mortality compared with moderate intake (50-55% of calories).
The researchers estimated that from age 50, the average life expectancy was an additional 33 years for those with moderate carbohydrate intake -- 4 years longer than those with very low carbohydrate consumption (29 years), and 1 year longer compared to those with high carbohydrate consumption (32 years). However, the authors highlight that since diets were only measured at the start of the trial and 6 years later, dietary patterns could change over 25 years, which might make the reported effect of carbohydrate consumption on lifespan less certain.
In the next step of the study, the authors performed a meta-analysis of data from eight prospective cohorts (including ARIC) involving data from 432,179 people in North American, European, and Asian countries. This revealed similar trends, with participants whose overall diets were high and low in carbohydrates having a shorter life expectancy than those with moderate consumption.
In further analyses examining whether the source of proteins and fats favoured in low-carbohydrate diets plant-based or animal-based was associated with length of life, researchers found that replacing carbohydrates with protein and fat from animal sources was associated with a higher risk of mortality than moderate carbohydrate intake. In contrast, replacing carbohydrates with plant-based foods was linked to a lower risk of mortality.
The findings show observational associations rather than cause and effect. Considering evidence from other studies, the authors speculate that Western-type diets that heavily restrict carbohydrates often result in lower intake of vegetables, fruit, and grains and lead to greater consumption of animal proteins and fats some of which have been implicated in stimulating inflammatory pathways, biological aging, and oxidative stress and could be a contributing factor to the increased risk of mortality. Whilst high carbohydrate diets (common in Asian and less economically advantaged nations) tend to be high in refined carbohydrates such as white rice, may also contribute to a chronically high glycaemic load and worse metabolic outcomes.