A diet based on significantly reducing the amount of carbohydrates on your plate can reduce life expectancy by up to four years, according to a study published in The Lancet Public Health.

It’s the largest study on the subject and plays a key role in understanding the relationship between diet and long-term health, says Dr Scott Solomon, a professor at Harvard Medical School and one of the report’s authors.

Carbohydrates make a meal delicious—whether it’s a baked potato, a crusty loaf of bread or a juicy slice of cantaloupe—but they’re on the no-go list for people with certain medical conditions, such as diabetes, or those with particularly round figures, who are promised slimness by celebrity diets that deprive them of these nutrients.

Carbohydrates, known by various names from glucides to saccharides, are a major component of the diet and the most accessible source of energy for the body. Eliminating carbohydrates from the diet or drastically reducing foods rich in these nutrients can lead to rapid weight loss, which is why diets such as Atkins have become so popular in Europe and the United States.

The latest US study of dietary carbohydrate levels shows that low-carbohydrate diets are associated with higher mortality, but less so for vegetarians. High-carbohydrate diets are also associated with a shorter lifespan, so researchers say moderate carbohydrate intake is the key to a healthy, life-extending diet.

After following a group of 15,400 people for 25 years, the researchers found that diets in which carbohydrates accounted for less than 40% of daily calories and those in which carbohydrates accounted for more than 70% of total daily calories were associated with increased mortality, while moderate carbohydrate consumers (50-55% of daily energy) had the lowest risk of death. The results were compared with seven other observational studies involving more than 430,000 people in over 20 countries.

Dr Sara Seidelmann, the study’s coordinator, says that diets that replace carbohydrates with protein and fat should be discouraged, because research shows that while they reduce both body weight and cardiometabolic risk in the short term, they are not beneficial for long-term health.

However, there is a special mention for vegetarians, says Seidelmann: A low-carbohydrate diet, in which carbohydrates are replaced with plant-based fats and proteins, can promote healthy ageing.

This shows that it’s not so much about the carbohydrates themselves, as epidemiologist Walter Willet points out: “Too much and too little carbohydrate can be harmful but what counts most is the type of fat, protein, and carbohydrate.”

It’s where the nutrients come from that makes the difference, points out Cambridge University professor Nita Forouhi, one of the independent experts who welcomed the study’s findings. The healthiest low-carb diets, the researchers found, were those that replaced starchy carbohydrates with legumes, vegetable oils, and nuts. On the other hand, diets high in animal protein, such as Atkins, increase the risk of heart failure—this is also the conclusion of the Public Health England study, which recommends that 50% of total calorie intake should come from carbohydrates, not from refined carbohydrates, but from those that release sugar slowly into the bloodstream, such as whole foods.

The study, published in The Lancet Public Health, may provide an explanation for the high mortality rate among obese people, who tend to fall into the trap of either a low-carbohydrate, animal protein-based diet or a low-fat, high-carbohydrate diet.

It should be noted, however, that the study shows associations rather than a cause-and-effect relationship because the data reported by the subjects may not have been very accurate, and the diets were measured at the start of the study and six years later, so there is a possibility that dietary patterns may have changed over the subsequent 19 years.

Although the study remains observational, the authors point out that the Western pattern of low-carbohydrate diets most often involves a decrease in the consumption of vegetables, fruits, and grains and an increase in the consumption of animal protein and fats. This dietary pattern is responsible for inflammation, biological ageing and oxidative stress, which may make it a factor in increased mortality risk.

On the other hand, the study’s findings are in line with the advice of health organisations everywhere, points out Catherine Collins, an NHS dietician, who recommends a carbohydrate intake of half the daily calorie requirement. In addition, the data from this research could recalibrate the general popularity of low-carb diets (and that of the doctors who recommend them) for diabetes. The study could be a starting point for re-evaluating this diet, or at least urging caution in the long-term management of this type of diet, Collins says.

Carmen Lăiu is an editor at Signs of the Times Romania and ST Network.