New low carb link to life expectancy should be interpreted with caution

The media is reporting on a study which suggests a low carb diet could shorten life expectancy, but there are a number of limitations to the study and the findings should be interpreted with caution.
The study, conducted by Brigham and Women’s Hospital in Bosto, concluded that diets which are high or lower in carbohydrate were associated with greater mortality, an increased risk of early death.
Specifically, those who received 50-55% of their energy from carbohydrates (moderate carb consumption) had a slightly lower risk of death compared to low and high carb groups.
The way the study was run should pose a number of questions before the results are assessed.
The study involved a total of 15,400 people from the US filling out questionnaires on the food and drink they consumed, as well as portion sizes. Questionnaires were conducted at the start of the 25-year trial and six years later.
The questionnaires relied upon people remembering what they ate, and it is this information that scientists used to estimate the proportion of calories they received from carbohydrates, fats and protein. There are often accuracy issues with food questionnaires and it is not possible to verify the accuracy of the answers in the study.
Furthermore, with diet composition only reported at the start of the trial and six years later, the researchers acknowledge that dietary patterns could have changed over the subsequent 19 years of follow-up.
Another point to consider is that the definition of low carb diets used by the researchers was not as low as the low carb diets some people follow for health reasons. Therefore, the results of the study may not be relevant to people who are following stricter low carb or ketogenic diets.
What can be concluded from the study, published in The Lancet, is that people who tried lower carb diets or high carbohydrate diets were more likely to die earlier. It’s hard to confirm more than this with any reliability.
All in all, the study does not provide robust evidence to be able to assess the long-term health outcomes of low carb diets.
Diabetes.co.uk has long advocated a low carb diet, as outlined in our Low Carb Program which can be recommended on the NHS, and the evidence base is growing over low carb’s benefits. Last year, the international PURE study showed how total mortality rates were significantly higher in people who ate a diet higher in carbohydrate and/or lower in fat compared with a low carb diet.