These stories cite a massive study published in the Lancet last week that began with researchers signing on more than 15,000 people in their middle ages—ranging from the mid-40s to mid-60s—living in four communities in the US to participate. The research team, led by scientists at Harvard’s School of Public Health and Brigham and Women’s Hospital, had the participants answer questionnaires about their eating habits. The team sorted the participants into five groups based on their carb-consumption as a percentage of their diets, and noticed trends across three of them more broadly:
- low-carb (less than 30% of diet are carbohydrates)
- high-carb (more than 65% of diet are carbohydrates)
- medium-carb (the three middle groups between low- and high-carb consumption)
Then, they followed the participants for an average of 25 years.
Eventually, of course, many of them passed away. At the end of the study, the researchers calculated the life expectancy of the participants, and found that, at age 50, those in the low-carb group would have been expected to live to 79 compared to 82 for the high-carb group. Those whose diets were comprised of 50%-55% carbs had the highest life expectancy, at 83. In other words, the data created a U-shaped curve, meaning that eating too few or too many carbs as a percentage of diet were both linked to higher mortality rates.
It’s true that the lowest-carb group had the shortest life expectancy of the group. It’s a blow to the fad diets that restrict carbohydrates, which have been associated with temporary weight loss. But the trends identified in the Lancet study are not necessarily linked to carb consumption alone. It’s the types of carbs and other food groups that matter more.
The surveys also found that those who ate diets low in carbohydrates were often getting more of their remaining calories from meat, rather than plants. Eating animal protein and fats has been linked to health problems like heart disease, kidney disease, and weight gain. Further, within the low-carb group, those who ate more fruits and vegetables had a higher life expectancy than the others.
On the opposite end of the curve, people who eat higher amounts of carbohydrates tend to eat more refined carbs, like sugars and grains without fiber, which are high in calories without the nutritional benefits (paywall).
The authors only asked participants to answer the food-habit survey twice in the 25 years of the study. That seems like it should be a red flag about the value of these data. However, Donald Hensrud, a nutritionist and director of the Mayo Clinic’s Healthy Living Program who was not involved with the research, thinks this makes it all the more impressive that the study spotted a link between carb intake and longevity. If it were a weak trend, it would need more data points to become apparent. Additionally, to validate their findings, the researchers examined other studies into particular diets and longevity, covering some 432,000 individuals globally. They found similar results.
“My takeaway from this, putting it altogether across different populations…is [that] the optimal carb intake is kind of in the middle,” says Hensrud, with around 50% to 55% of calories in the form of carbs, with the rest coming from proteins and fats. And, he adds, “There’s wiggle room for how much of each.”