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Both Low- And High-Carb Diets Were Linked To Shorter Lifespans In This Large New Study

People who got about half their calories from carbohydrates had longer lifespans than those who ate high- or low-carb diets, the study found.

Posted on August 16, 2018, at 7:06 p.m. ET

When it comes to eating carbohydrates, there are a lot of strong opinions.

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Some people choose to eat few or very few carbs and a lot of fat or protein, like you get with the ketogenic diet. Others champion an essentially no-carb diet that requires eating nothing but meat, aka the carnivore diet.

Now a new, large study sheds light on what the long-term health effects might be of those choices, which so far have been a bit murky. The study is not the final word on carbohydrates by any means (no single study ever is). But the results will probably be of interest to anyone who’s wondered what the hell they should eat for lunch.

In the study, Dr. Sara Seidelmann, a cardiologist and a nutritionist at Brigham and Women’s Hospital in Boston, and colleagues looked at 15,400 people aged 45 to 64 who lived in four communities in North Carolina, Mississippi, Minnesota, and Maryland. The participants filled out dietary questionnaires back in the late 1980s, and then again six years later, according the report in the Lancet Public Health.

Over the next 25 years, more than 6,000 of those surveyed died. The researchers found that people who reported eating a low-carb diet (less than 40% of calories from carbohydrates) and those eating a high-carb diet (more than 70% of calories) were slightly more likely to die during the study than people with a carb intake in the 50% to 55% range.

Overall, they estimated that a 50-year-old person who ate a diet in the midrange for carbohydrate intake would live an additional 33 years, compared with 32 years for the high-carb diet and 29 years for a very low-carb diet.

For low-carb diets, eating more plant-based foods seemed to be better than eating meat.

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When the researchers looked at the low-carb group, they found that diets that swapped carbs for meat-derived fat and protein — lamb, beef, pork, and chicken — were linked to a higher mortality risk during the study period. However, if people swapped carbs for plant-based proteins and fat — like vegetables, nuts, peanut butter, and whole grain bread — it was linked to lower mortality.

However, the researchers couldn’t really say whether a low-carb diet with plant-derived fats and protein was equivalent to getting half your calories from carbs, Seidelmann told BuzzFeed News.

"We were unable to make that direct comparison," she said. “I think the overall conclusion is that in terms of healthy aging, the right strategy is likely moderate carbohydrate intake that is rich in plant-based whole foods.”

But there are some things you should know about the study design, and it's ability to conclusively say that carb intake affects mortality.

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The study is observational of what people decided to eat, rather than them being randomly assigned to one diet or another. Therefore, this type of study can’t prove cause and effect, and it should be “interpreted with caution,” according to an editorial by Andrew Mente and Salim Yusuf from McMaster University in Hamilton, Ontario.

However, they also said that "moderate intake of carbohydrate (e.g., roughly 50% of energy) is likely to be more appropriate for the general population than are very low or very high intakes."

Low-carb diets have been shown to help people lose weight.

Seidelmann said she understands the appeal of low-carb diets, which can help with short-term weight loss.

“But I think that we need to reconsider whether what achieves results in the short term is the best thing for us in the long term,” she said, adding that there might be “alternatives in the long term that may be more beneficial for healthy aging."

If you do follow a low-carbohydrate diet, choosing plant-derived substitutions "is a better strategy for long-term health,” she said.

High-carb diets may not be good if many of those calories are coming from refined foods, like white rice or white noodles, Seidelmann said, emphasizing that such carbohydrates are “perhaps closer to pure sugars, that are more easily broken down by the body into sugar, and are void of things like fiber and much of the nutrient content of the food,” she said.

The study does suggest it's probably a good idea to eat plant-based food, a nutritionist said.

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The research does make sense and supports what other studies have found, said Cynthia Sass, a registered dietitian and board-certified specialist in sports dietetics who regularly counsels clients and athletes on weight loss and nutrition.

“In my practice, I find that clients who follow moderate carb diets — about 40% of calories — are able to successfully lose weight and see marked improvements in health, such as levels of cholesterol, blood sugar, and blood pressure, all while experiencing more everyday energy, better mood, and excellent digestive health,” she told BuzzFeed News in an email.

The type of foods you choose to eat on lower-carb diets is key, said Sass, who is based in New York and Los Angeles.

“Plant foods are the foundation of the optimal human diet,” she said. “In addition to hundreds of studies to support this, we see that populations around the globe who live the longest, healthiest lives (without high rates of chronic diseases) have a high consumption of whole plant-based foods.”

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