The Carbohydrate Conundrum
A recent study has found that diets both low and high in carbohydrates were linked with an increase in mortality, while moderate consumers of carbohydrates had the lowest risk of mortality.
In this 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 (such as the ketogenic diet which is recommends 70%-80% of calories be from animal fat - a modern day twist of the debunked Atkins-like diet) are gaining widespread popularity as a health and weight loss strategy. The information in the above links is interesting (so check them out), and if nothing else very entertaining to see such passion about the opposite ends of the dietary spectrum. That being said, accumulating data does suggest that animal-based low carbohydrate diets are 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 randomized 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 had not addressed the source or quality of proteins and fats consumed in low-carb diets.
To address this uncertainty, researchers in this study began by studying 15,428 adults aged 45-64 years from diverse socioeconomic backgrounds in the US 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 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" shaped 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). The authors did highlight that since diets were only measured at the start of the trial and 6 years later, dietary patterns could have 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 over 430,000 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 favored 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.
These findings bring together several strands that have up until now been controversial. Too much and too little carbohydrate can be harmful but what appears to count most is the "type" of fat, protein, and carbohydrate.
It should be noted that the findings of this study show "observational" associations rather than a true cause-and-effect. But when considering evidence from other studies, a consistent relationship emerges that 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 - that later of which have been implicated in stimulating inflammatory pathways, biological aging, and oxidative stress. This could be a contributing factor to the increased risk of mortality. High carbohydrate diets (typical in vegetarians, in most of 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 and worse metabolic outcomes.
This work provides the most comprehensive study of carbohydrate intake that has been done to date, and helps us better understand the relationship between the specific components of diet and long term health.
Based on first principles, a U-shaped association is logical between most essential nutrients versus health outcomes. We see this particular association for example with vitamin D and E. 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 (i.e., a so-called sweet spot). Although carbohydrates are technically not an essential nutrient (unlike protein and fats), a certain amount is likely required to meet short-term energy demands during physical activity and to maintain fat and protein intakes within their respective sweet spots. On the basis of these principles, moderate intake of carbohydrate (e.g., roughly 50% of calorie intake) is likely to be more appropriate rather than are very low or very high intakes. Interestingly, the quintessential diet that hits this target is the Anti-Inflammatory eating pattern, which in addition focuses on beneficial low glycemic carbohydrates.