Data regarding the dietary habits of tens of thousands of adults in the United States suggest that those who follow healthful low fat and low carb diets have a lower overall death risk.

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A large new study has examined the link between diet type and total mortality.

“Diet plays an important role in […] public health, and suboptimal diet is estimated as the first leading cause of death and the third leading cause of disability-adjusted life-years lost in the [U.S.],” write Dr. Zhilei Shan — from the Harvard T. H. Chan School of Public Health in Boston, MA — and colleagues in their new study paper.

Their findings now appear in the journal JAMA Internal Medicine.

In their research, Dr. Shan and team analyzed the data of more than 37,000 people to determine whether or not there were any associations between different diet types and mortality.

More specifically, the investigators wanted to find out if different types of low fat and low carb diet are associated with total mortality.

Although there have been several studies into the links between diet and death risk, the researchers note that — to their knowledge — none have looked specifically at how low carb and low fat diets of different qualities may fit into the equation.

“Consumption of carbohydrates from refined grains and added sugars has been adversely associated with health outcomes, whereas consumption of carbohydrates from whole grains, nonstarchy vegetables, and whole fruits appears to be beneficial,” explain the study authors.

“Likewise,” they say, “replacing saturated fat with unsaturated fat was associated with lower risk of heart disease and mortality.”

For these reasons, the investigators thought it was important to identify and emphasize any associations between different diet types, different quality diets, and mortality risk.

For their study, the researchers analyzed the data of 37,233 U.S. adults with a mean age of 49.7 years. The data came from eight cycles of the National Health and Nutrition Examination Survey (NHANES), from 1999 to 2014.

In total, the follow-up period accounted for 297,768 person years. This refers to the amount of follow-up time for all the participants included in the NHANES surveys.

During this time, the researchers recorded a total of 4,866 deaths, of which 849 were related to heart disease and 1,068 were related to cancer.

Using the NHANES reports regarding people’s macronutrient consumption, the researchers were able to infer different diet quality types.

The team found no association between overall low carb and low fat diet scores and total mortality risk.

However, they did find an association between unhealthful low carb and low fat diet scores — indicating adherence to poor quality low fat and low carb diets — and a higher total mortality risk.

On the other hand, better quality low fat and low carb diets were associated with a lower total mortality risk.

The researchers also report that “participants with a higher overall [low carb dirt score] score” — indicative of a poorer quality diet — “were more likely to be older and non-Hispanic white, to have higher [body mass index], educational level, income level, and cholesterol intake, and to have lower total energy intake.”

As for those with higher low fat diet scores, the investigators point out that they tended to be part of an ethnic minority group, to not smoke, to have lower weights, and to have lower cholesterol levels.

When trying to find an explanation for the association between different diet quality types and mortality risk, Dr. Shan and colleagues point out that many biological mechanisms could play a role.

They write, “Fat provides more than twice as much energy as carbohydrates and protein by weight. A high saturated fat diet is highly palatable and may […] [lead] to overconsumption and obesity.”

“Low quality carbohydrates, such as refined grains and added sugars, provide limited nutritional value, and their high glycemic load could be associated with high postprandial glucose and insulin, inflammation, insulin resistance, and dyslipidemia.”

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Although the researchers emphasize that the main strength of their study was the sheer size of the population sample — as well as the lengthy follow-up time for data collection — they also caution that the research had a number of limitations.

These include the fact that they could not identify which specific versions of the various low carb and low fat diets each participant had followed.

This means, in the study authors’ words, that “the results could not be directly translated to the assessment of health benefit or risk associated with the popular versions of the diets.”

They also note that the participants self-reported the data regarding their diets, which means that they may not be entirely accurate. The team was also unable to verify whether or not the relationship between diet quality and mortality risk was of a causal nature.

The funding for this study came from the Young Scientists Fund of the National Natural Science Foundation of China, the Major International (Regional) Joint Research Project of the National Natural Science Foundation of China, and the National Institutes of Health (NIH).

It is also worth noting that some of the researchers disclosed possible conflicts of interests, as some received personal fees and grants — for separate work — from private sponsors including the California Walnut Commission and the Standard Process, Metagenics, and Diet Quality Photo Navigation program.