New research is throwing a bomb into the increasingly popular low-carb diet movement, suggesting diets both low and high in carbohydrates can be associated with a higher risk of early mortality. The study claims moderate consumption of carbohydrates is the healthiest long-term proposition but low-carb diets can be improved if proteins and fats are coming from plant and not animal sources.

Several studies from the past 12 months have demonstrated the dangers of high-carbohydrate diets. Carbs have undeniably become the food villain of the 21st century and, in addition to triggering weight loss, more extreme low-carb/high-fat diets have been found to help everything from improving the efficacy of cancer drugs to reducing the frequency of epileptic seizures.

However, the long-term effects of low-carb diets are still a source of much scientific debate. While a convincing animal study revealed last year that a ketogenic-style diet increased the lifespan and healthspan of mice, it has been much harder to validate these results in human subjects. A comprehensive, albeit small, Stanford study earlier this year compared low-fat and low-carb diets, only to come out with inconclusive results, suggesting both diets can be effective for losing weight and maintaining health.

The new research set out to investigate the long-term effects of low-carb eating on overall mortality rates. The study first targeted over 15,000 adults across four locations in the United States, and after 25 years the researchers calculated the association between overall carbohydrate intake and life expectancy. Carbohydrate intake was calculated as an average percentage related to overall caloric consumption.

The results revealed that those subjects eating a lower carbohydrate diet, with carbs accounting for less than 40 percent of an overall caloric intake, suffered from a higher risk of early mortality compared to those eating a moderate carb intake. A U-shaped association was identified in the data suggesting a low-carb intake (less than 40 percent) and a high-carb intake (more than 70 percent) both increased rates of mortality, whereas the sweet spot seemed to be around 50 to 55 percent carb intake.

After confirming these overall results against a broader meta-analysis of data comprising over 400,000 people from North America, Asia and Europe, the researchers set out to identify whether the source of proteins and fats in a low-carb diet made any difference to overall mortality. Here the real crux of the study became prominent. The data showed that low-carb diets incorporating higher amounts of animal fats and proteins resulted in a higher risk of early mortality but low-carb diets with lots of plant-based proteins and fats resulted in lower risks of early mortality.

"These findings bring together several strands that have been controversial," says Walter Willett, co-author on the new study. "Too much and too little carbohydrate can be harmful but what counts most is the type of fat, protein, and carbohydrate."

The research inarguably suffers from some significant limitations in both its methodology and conclusions. Dietary patterns based on self-reported questionnaire data are notoriously problematic, and the data does not grapple with what we more recently would refer to as very-low-calorie, or ketogenic, diets. Specific types of carbohydrate intakes are also left relatively unexamined in the new study, leaving a gap in understanding the difference in value between sugar and starchy carbs compared to unrefined cereals.

While low-carb diets are being increasingly associated with a variety of beneficial health outcomes, from weight loss to diabetes management, this new research is suggesting that a simplistic high/low-fat/carb distinction is overly reductive.

"A really important message from this study is that it is not enough to focus on the nutrients, but whether they are derived from animal or plant sources," explains Nita Forouhi, a scientist from the University of Cambridge, not affiliated with the new study. "When carbohydrate intake is reduced in the diet, there are benefits when this is replaced with plant-origin fat and protein food sources but not when replaced with animal-origin sources such as meats. Many low-carb diet regimes do not make this distinction, but it is important."

Again, it is worth noting that this study does not examine more extreme ketogenic-style diets. Long-term research into the health outcomes for these kinds of eating patterns is still unclear and more causal research is still necessary. However, it still seems that, as with most things in life, the Goldilocks rule certainly applies. A healthy, balanced diet with moderate consumption of lean meats and lots of vegetables, fruits, whole grains, legumes, and nuts, still seems to be the best advice for a long life.

The new research was published in the journal The Lancet Public Health.