A diet that significantly reduces carbohydrate intake may shorten life by up to four years, according to a study published in The Lancet Public Health.

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

Carbohydrates, whether in the form of a baked potato, a crusty piece of bread, or a juicy slice of watermelon, delight our taste buds. However, they are on the forbidden list for people with certain medical conditions, such as diabetes, or those striving for a leaner physique, as famous diets promise weight loss by cutting out these nutrients.

Also known as sugars, carbs are a major component of our diet, providing the most readily available source of energy for our bodies. Eliminating carbohydrates from one’s diet or significantly reducing foods rich in these nutrients can lead to rapid weight loss, which is why diets like Atkins have become so popular in Europe and the United States.

The most recent American study on the percentage of carbohydrates in our diet shows that low-carb diets are associated with higher mortality rates, except for vegetarians. Conversely, high-carb diets are also linked to reduced lifespans, which is why researchers suggest that a moderate intake of carbohydrates could be the key to a healthy diet that extends one’s life.

After monitoring a cohort of 15,400 individuals for 25 years, researchers have found that diets where carbohydrates accounted for less than 40% of daily calories, as well as those where carbohydrates made up over 70% of total daily calories, were associated with increased mortality. On the other hand, individuals with moderate carbohydrate consumption (50-55% of daily energy) had the lowest risk of mortality. These findings were compared with seven other observational studies, involving over 430,000 people from more than 20 countries.

Dr. Sara Seidelmann, the study’s lead author, highlights that diets replacing carbohydrates with proteins and fats should be discouraged. Research indicates that although such diets may reduce body weight and cardiometabolic risk in the short term, they are not beneficial for long-term health.

However, there’s a special note for vegetarians, Seidelmann explains: a low-carb diet where carbohydrates are replaced with plant-based fats and proteins can promote healthy ageing.

This proves that it’s not so much about carbohydrates themselves, as epidemiologist Walter Willet emphasises: “Too much and too few carbohydrates can be harmful, but what matters most is the type of fats, proteins, and carbohydrates.”

It’s the source of these nutrients that makes the difference, says Nita Forouhi, a professor at the University of Cambridge and one of the independent experts who welcomed the study’s results. The healthiest low-carb diets, according to the researchers, are those where starchy carbohydrates are replaced with vegetables, vegetable oils, lentils, and beans. On the other hand, diets rich in animal proteins, like the Atkins diet, increase the risk of heart failure. This is also the conclusion of a study by Public Health England, which recommends that 50% of total caloric intake should come from carbohydrates, not refined ones but rather those that slowly release sugar into the bloodstream, such as whole foods.

The study published in The Lancet Public Health may provide an explanation for the high mortality rates among obese individuals who tend to fall into the trap of either a low-carb diet based on animal proteins or of low-fat diets rich in carbohydrates.

It’s important to mention, however, that the study reveals more associations than cause-and-effect relationships, as the data reported by the subjects may not have been very accurate, and, on the other hand, the diets were measured at the beginning of the study and six years later, so there’s a possibility that dietary patterns changed over the following 19 years.

Although, by its nature, the study remains observational, the authors have emphasised that the Western pattern of low-carb diets often involves reducing the consumption of vegetables, fruits, and grains and increasing the consumption of animal proteins and fats. This dietary pattern is responsible for inflammation, biological ageing, and oxidative stress, which can turn it into a factor increasing the risk of mortality.

On the other hand, the study’s findings align with advice from health organisations worldwide, says Catherine Collins, a dietitian with the NHS (the UK’s National Health Service), which recommends a carbohydrate intake covering half of daily caloric needs. Moreover, the data obtained from this investigation could recalibrate the media’s popularity surrounding low-carb diets (and the doctors who recommend them) in the case of diabetes. The study might serve as a starting point for reevaluating this diet or, at the very least, could encourage a more cautious approach to the long-term management of this type of diet, according to Collins. 

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