Low-carb diets emphasize the consumption of proteins and fatty foods -- such as high-fat dairy and meat -- and the avoidance of low-fat and processed food products rich in starch and sugar. According to several studies, such as the one-year controlled trial published in The New England Journal of Medicine, a low-carb diet is effective at inducing weight loss, especially for people with obesity. Additionally, it improves risk factors for coronary heart disease, such as triglyceride and high-density lipoprotein cholesterol levels.
On the other hand, a high-carb diet advocates getting most (45 to 65 percent) of a person's daily calories from carbohydrate-rich foods, such as whole grains and starchy vegetables. People who follow this diet are also advised to avoid sugary or sweetened foods, along with nuts and seeds, which are low in carbohydrates. In a 12-month study conducted by researchers from the Stanford University School of Medicine, they found that a high-carb diet has the same effect as a low-carb diet in terms of weight loss. They also reported that apart from shedding a few pounds, no other health benefits were associated with either diet.
Despite the reported efficacy of both diets in reducing the weight of diabetics and obese people, recent meta-analyses have linked them to high mortality. For instance, a systematic review of 17 studies involving 272,216 individuals found that all-cause mortality was significantly increased among participants with high low-carbohydrate scores. While the diet was not significantly associated with the risk of cardiovascular disease (CVD), it nonetheless elevated the risk of mortality in the long-term.
A similar result was reported in a large cohort study done by researchers from different countries. The Prospective Urban Rural Epidemiology (PURE) study involved over 135,000 individuals aged 35-70 from 18 different countries across five continents. The researchers reported a total of 5796 deaths and 4784 major CVD events documented during their follow-up. High carbohydrate intake was associated with an increased risk of total mortality, but not with the risk of CVD. Because of their findings, the researchers asked for global dietary guidelines to be reconsidered.
In the present study, the researchers focused on a large prospective cohort involving four American communities with diverse socioeconomic backgrounds. The study had a median follow-up of 25 years. The researchers also contextualized their findings in a meta-analysis by combining their data with those from seven previous studies that involved North American, European, Asian, and multinational cohorts.
The researchers reported a U-shaped relationship between carbohydrate intake and mortality. Not taking into account specific food sources, they found that diets with high (>70 percent) and low (<40 percent) energy intake from carbohydrates were associated with increased mortality. On the other hand, people who gained 50-55 percent of their energy from carbohydrates had the lowest mortality risk.
When food sources were considered in the equation, the researchers found that the replacement of carbohydrates with animal-derived proteins or fat had an associated increase in mortality risk. Meanwhile, replacement with plant-derived macronutrients was associated with a significantly lower risk. These findings were corroborated in their meta-analysis.
Based on their findings, the researchers concluded that high- and low-carb diets decrease the life expectancy of people. A longer lifespan is associated with a 50-55 percent carbohydrate intake -- a level considered to be moderate in North America and Europe but low in other regions like Asia. In addition, animal-based low-carb diets, which are more popular in North America and Europe, need to be discouraged. If carbohydrate restriction is necessary for health reasons, such as reducing weight and cardiometabolic risk, the researchers suggest replacing carbohydrates with predominantly plant-based fats and proteins because they contribute to better long-term health outcomes and healthier aging.