Moderate carbohydrate intake may result in good health

Turns out, eating carbohydrates in moderation can be optimal for health and longevity.

Low-carb diets that replace carbohydrates with proteins and fats from plant sources are associated with lower risk of mortality compared to those that replace carbohydrates with proteins and fat from animal sources.

The observational study of more than 15,400 people 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) have 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 suggested 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 were 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, the data suggested that animal-based low carbohydrate diets might be associated with a shorter overall lifespan 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.

To address this uncertainty, researchers began by studying 15,428 adults aged 45-64 years from diverse socioeconomic backgrounds. 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.

The researchers also assessed the association between overall carbohydrate intake (categorized by quantiles) and all cause-mortality after adjusting for age, sex, race, total energy intake, education, exercise, income level, smoking, and diabetes.

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. 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 showed 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 ageing, 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.

The authors noted some limitations including that dietary patterns were based on self-reported data, which might not accurately represent participants' food consumption; and that their conclusions about animal-based sources of fat and protein might have less generalisability to populations which tend to have diets high in carbohydrates, but often consume fish rather than meat.

On the basis of these principles, moderate intake of carbohydrate (eg, roughly 50% of energy) is likely to be more appropriate for the general population that are very low or very high intakes.

The findings appeared in the Journal of The Lancet Public Health.