Low-carb diets could shorten life by four years, warns Lancet study

17 Aug 2018 --- Following a low-carb diet could shorten life expectancy by four years, suggests new research published in The Lancet Public Health. The high levels of animal fats and proteins that often replace carbohydrates in typical low-carb diets appear to be associated with the higher risk of mortality. Eating more plant-based proteins and fats in place of carbohydrates was linked to lower mortality and even reversed the greater mortality risk. 

“Low-carb diets that replace carbohydrates with protein or fat are gaining widespread popularity as a health and weight loss strategy. However, our data suggest that animal-based low carbohydrate diets, which are prevalent in North America and Europe, might be associated with shorter overall lifespan 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 promote healthy aging in the long-term,” says Dr. Sara Seidelmann, Clinical and Research Fellow in Cardiovascular Medicine from Brigham and Women’s Hospital, Boston, who led the research.

The observational study of more than 15,400 US citizens found that that diets both low (< 40 percent energy) and high (>70 percent energy) in carbohydrates were linked with an increase in mortality, while moderate consumers of carbohydrates (50–55 percent of energy) had the lowest risk of mortality.

Researchers estimated that from the age of 50 people in the moderate carb group were on average expected to live for another 33 years. This was four years more than those on extreme low-carb diets, 2.3 years more than the low-carb diet group and 1.1 more years than the high-carb group.

The researchers then compared low-carb diets rich in animal proteins and fats with low-carb diets rich in plant-based protein and fat. 

Participants were followed for six years and completed a dietary questionnaire on the types of food and beverages they consumed, what portion size and how often, which the researchers used to estimate the cumulative average of calories they derived from carbohydrates, fats and protein.

The primary findings, confirmed in a meta-analysis of studies on carbohydrate intake including more than 432,000 people from over 20 countries, suggest that eating more animal-based proteins and fats from foods like beef, lamb, pork, chicken and cheese instead of carbohydrate was associated with a greater risk of mortality. Alternatively, eating more plant-based proteins and fats from foods such as vegetables, legumes and nuts was linked to lower mortality.

Previous randomized trials have shown low carbohydrate diets are beneficial for short-term weight loss and improve cardiometabolic risk. However, the long-term impact of carbohydrate restriction on mortality is controversial with prospective research so far producing conflicting results. Furthermore, earlier studies have not addressed the source or quality of proteins and fats consumed in low-carb diets.

“This work provides the most comprehensive study of carbohydrate intake that has been done to date and helps us better understand the relationship between the specific components of diet and long-term health,” says Dr. Scott Solomon, The Edward D Frohlich Distinguished Chair at Brigham and Women’s Hospital and Professor of Medicine at Harvard Medical School and senior author on the paper. 

“While a randomized trial has not been performed to compare the long-term effects of different types of low carbohydrate diets, these data suggest that shifting towards a more plant-based consumption is likely to help attenuate major morbid disease.”

Seidelmann tells NutritionInsight that the researchers were surprised by the consistency of the results across a wide range of geographic and social distributions.

However, some limitations of the study are that it shows observational associations rather than cause-and-effect and what people ate was based on self-reported data, which might not be accurate.

Animal and plant-based proteins and fat: Whats the difference?
The researchers highlighted that the lack of nutrients in diets heavy in animal protein and fat play a large role in the findings. It is likely, they explain, that different amounts of bioactive dietary components in low carbohydrate versus balanced diets, such as branched-chain amino acids, fatty acids, fiber, phytochemicals, haem iron and vitamins and minerals are involved in the result.

Long-term effects of a low carbohydrate diet with typically low plant and increased animal protein and fat consumption have been hypothesized to stimulate inflammatory pathways, biological aging and oxidative stress.  

On the other end of the spectrum, high carbohydrate diets, which are common in Asian and less economically advantaged nations, tend to be high in refined carbohydrates such as white rice; these types of diets might reflect poor food quality and confer a chronically high glycemic load that can lead to negative metabolic consequences.

Low-carb diet popularity
Low carbohydrate diets which restrict carbohydrate in favor of increased protein or fat intake, or both, are a popular weight-loss strategy and these findings are particularly salient due to their long-standing prevalence.

Keto and paleo have enjoyed immense popularity, with keto reaching number three on the diet trend list according to the “What's Trending in Nutrition” national survey.  

The diets, that have vast numbers of worldwide followers, “piggyback” off the popularity of low-carb diets. Seidelmann warns of the trend towards following low-carb diets, which can be heavy in animal fat and protein. “If one chooses to restrict carbohydrate intake as an approach for short-term weight loss, replacement of carbohydrates with plant-based fats and proteins should be strongly considered as an alternative,” she notes. 

This is not the first study to flag health concerns on low-carb and high-fat diets. NutritionInsight has recently reported on findings published in The Journal of Physiology that found that ketogenic diets may cause an increased risk of Type 2 diabetes in the early stage of the diet. The Zurich-based researchers showed that even though ketogenic diet fed animals appear healthy in the fasted state, they exhibit decreased glucose tolerance to a greater extent than high carbohydrate, high fat, western style diet (HFD) fed animal.

Furthermore, an article in the latest issue of The World of Food Ingredients examined keto and some other trending weight management strategies. 

“Keto is very popular right now. However, the diet does not have proven benefits for general health and wellness and it’s a challenging one to follow,” according to Dietician and Nutritionist, Sharon Palmer. “I am concerned about the elimination of many healthful foods to achieve the strict ratio of protein, carbohydrates, and fat, such as eliminating whole grains, fruits, legumes and vegetables. We know that these foods are important for health, so it would be difficult to achieve the level of nutrients you need (vitamins, minerals, fiber and phytochemicals) on a keto diet for the long term.”

In this way, low-carb diets are known to promote weight loss, but the wider, long-term health ramifications warrant further research. In particular, the elimination of whole food groups may be a cause for concern in both the paleo and keto diets i.e. paleo eliminates dairy while both diets eliminate wholegrains. The British Association of Dieticians (BDA) assert that the limitation of whole food groups in a dietary plan is a sign of “fad” diets that should be avoided if healthy weight loss is the goal, rather people should opt for balanced and nutritious diets. This food group elimination can also lead to both the keto and paleo diets traditionally being heavy on animal protein and fat, which clinical studies increasingly suggest may not promote longevity or heart health, they warn.

On a concluding note, Seidelmann tells NutritionInsight, “I hope that our study helps to raise the discourse regarding nutrition in general. There is nothing more important to our health than what we eat each and every day. I encourage everyone to make healthy food choices based on the best data and evidence available.”

By Laxmi Haigh

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