A new comprehensive study from Brigham and Women's Hospital in Boston has found that diets both low and very high in carbohydrates were linked with an increase in mortality, while people who had a moderate carbohydrate intake (50 to 55% of daily calories) had the lowest risk of mortality.
The study also revealed that all low carb diets are not equal.
Low-carb diets that replaces carbohydrates with plant-based proteins and fats were associated with a lower risk of early death compared to diets that replace carbohydrates with animal proteins and fats.
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, this new 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.
About the study
Previous randomized trials have found that low carbohydrate diets are beneficial for short-term weight loss and do improve cardiometabolic risk. However, the long-term impact of carbohydrate restriction on mortality is controversial with prospective research 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).
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 and all cause-mortality after accounting for age, gender, race, total calorie 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).
There were limitations, however. Diets were measured only twice – 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.
As well, the study was observational in nature and does not prove cause and effect.
In the next step of the study, the authors performed a meta-analysis of data from eight prospective cohorts 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.
Plant vs. animal proteins, fats
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.
How a low carb diet may increase mortality
While too much and too little carbohydrate can be harmful, what counts most is the type of fat, protein, and carbohydrate consumed.
The authors speculate that Western-style diets that heavily restrict carbohydrates often result in a lower intake of vegetables, fruit, and whole grains and lead to greater consumption of animal proteins and fats. That results in less fibre, protective phytochemicals, antioxidants and certain vitamins and minerals, compounds that reduce inflammation in the body and fend off harmful free radicals.
Both inflammation and free radical damage to cells are thought to promote biological aging.
High carbohydrate diets (common in Asian and less economically advantaged nations) tend to be high in refined carbohydrates such as white rice and may also contribute to a chronically high glycemic load.
All research on this web site is the property of Leslie Beck Nutrition Consulting Inc. and is protected by copyright. Keep in mind that research on these matters continues daily and is subject to change. The information presented is not intended as a substitute for medical treatment. It is intended to provide ongoing support of your healthy lifestyle practices.