Study suggests that low and high-carb diets could shorten life

  • img Harvey Cawdron
  • POSTED ON 21 Aug 2018


A study has suggested that low-carb diets could shorten life expectancy by up to 4 years. 

Low-carb diets have become an increasingly popular means of losing weight and have shown potential for lowering the risk of some illnesses. 

But a US study over 25 years suggests that moderate carb consumption - or eating plant-based protein and fats instead of meat - is healthier. 

The study was dependent on people remembering the amount of carbohydrates they consumed. 

In the study, published in The Lancet Public Health, 15,400 participants from the US filled out questionnaires on the food and drink they consumed, along with portion sizes. 

From this, scientists estimated the proportion of calories they received from carbohydrates, fats and protein. 

Researchers monitored a group for an average of 25 years, and found that those who got 50-55% of their energy from carbohydrates (the moderate carb group in line with British dietary guidelines) had a slightly lower risk of death when compared with the low and high-carb groups. 

The main sources of carbohydrates are starchy foods such as potatoes, pasta, cereals, rice and breads, but things like sugar, vegetables and fruit can also provide them. 

The NHS Eatwell guide provides information on how to achieve this kind of healthy, balanced diet and reduce the risk of serious illnesses in the long term. 

Researchers estimated that, from the age of 50, people in the moderate carb group were expected to live for another 33 years. 

This was 4 years more than the extra-low-carb group, people who got 30% or less of their energy from carbs. It was 2.3 years more than the low-carb group, people who got 30-40% of their energy from carbs. It was also 1.1 years more than the high-carb group, people who got 65% of their energy from carbs. 

These findings were similar to previous studies the authors compared their work to, which involved over 400,000 people from more than 20 countries. 

After this, the scientists compared low-carb diets rich in animal proteins and fats to those that contained plenty of plant-based protein and fat.

They found that eating more beef, pork, chicken and cheese in place of carbs was linked with a slightly increased risk of death. 

However, replacing carbohydrates with more plant-based proteins and fats, like legumes and nuts, was found to slightly lower the risk of mortality.

The person who led the research, Dr Sara Seidelmann, clinical and research fellow in cardiovascular medicine from Brigham and Women's Hospital in Boston, said 'Low-carb diets that replace carbohydrates with protein or fat are gaining widespread popularity as a health and weight-loss strategy.'

She added 'However, our data suggests that animal-based low carbohydrate diets, which are prevalent in North America and Europe, might be associated with shorter overall life span and should be discouraged. Instead, if one chooses to follow a low carbohydrate diet, then exchanging carbohydrates for more plant-based fats and proteins might actually promote healthy ageing in the long term.' 

The authors have speculated that Western-type diets that restrict carbohydrates often lead to lower intake of vegetables, fruit and grains, resulting in a higher consumption of animal proteins and fats, which have been connected with inflammation and aging in the body. 

Someone not involved in the research, Professor Nita Forouhi, from the MRC epidemiology unit at University of Cambridge, said 'A really important message from this study is that it is not enough to focus on the nutrients, but whether they are derived from animal or plant sources. When carbohydrate intake is reduced in the diet, there are benefits when this is replaced with plant-origin fat and protein food sources, but not when replaced with animal-origin sources such as meats.'

But, there are limitations to the study. 

The results display observational associations instead of cause-and-effect, and what people consumed was based on self-report data, which may have been inaccurate. Moreover, the authors acknowledged that because diets were measured only at the beginning of the study and 6-years later, dietary patterns could have significantly changed over the 19 years that followed. 

Professor Tom Sanders, professor emeritus of nutrition and dietetics at King's College London, said that the use of a food questionnaire in the study resulted in people underestimating the amount of calories they had consumed. He added that 'One explanation for the finding in this and the other US studies is that it may reflect the higher risk of death in the overweight/obese, who may fall into two popular diet camps - those favouring a high-meat/low-carbohydrate diet and those favouring a low-fat/high-carbohydrate diet'. 

Dr Alison Tedstone, chief nutritionist at Public Health England, said 'This provides further evidence that low-carb diets could be incredibly damaging to our long-term health. High-fibre starchy carbohydrates should provide about half of our energy, including fruit and vegetables, while reducing intake of higher fat meat and dairy.'

Walter Willett, Professor of Epidemiology and Nutrition at Harvard T. H. Chan School of Public Health and co-author of the study, said that 'These findings bring together several strands that have been controversial. Too much and too little carbohydrate can be harmful but what counts most is the type of fat, protein, and carbohydrate'.