How does diet quality contribute to the risk of death?

Data on the dietary habits of tens of thousands of adults in the US suggest that those who follow healthy low fat and low carb diets have a lower overall risk of death.

A major new study examined the link between diet type and total mortality
A major new study examined the link between diet type and total mortality

“Diet plays an important role in[…] public health, and suboptimal diet is reported to be the first leading cause of death and the third leading cause of disability-adjusted life-years lost in[ the United States],” writes Dr. Zhilei Shan — from the Harvard T. H. Chan School of Public Health in Boston, MA — and colleagues in their new paper on research.

Their findings are now published in JAMA Internal Medicine.

Dr. Shan and team analyzed the data from more than 37,000 people in their research to determine whether or not there were correlations between different diet styles and mortality.

In particular, the researchers wanted to find out if different types of low-fat and low-carb diets are correlated with overall mortality.

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

“The intake of carbohydrates from refined grains and added sugars has been adversely associated with health outcomes, while the consumption of whole-grain carbohydrates, non-starchy vegetables and whole-fruit appears to be beneficial,” the authors of the study clarify.

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

For these reasons, the investigators felt it was important to recognize and highlight any associations between different types of diets, different diets of quality and mortality risk.

Low fat, low carb, and mortality

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

The follow-up period amounted to a total of 297,768 years for people. It applies to the sum of follow-up time used in the NHANES surveys for all the participants.

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

Using the NHANES reports about people’s consumption of macronutrients, the researchers were able to infer different types of diet quality.

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

However, they found an association between low-carbon unhealthy and low-fat diet scores— indicating adherence to low-fat and low-carb diets of poor quality— and a higher risk of total mortality.

On the other hand, low fat and low carb diets of better quality have been linked with a lower risk of overall mortality.

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

The investigators point out that those with higher low fat diet ratings appear to be part of an ethnic minority group, not to smoke, have lower weights, and have lower cholesterol rates.

Dr. Shan and colleagues point out that many biological mechanisms can play a role when trying to find an explanation for the correlation between different types of diet quality and mortality risk.

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

“Low-quality carbohydrates, such as refined grains and added sugars, have little nutritional value and their high glycemic load could be associated with high post-prandial glucose and insulin, inflammation, insulin resistance and dyslipidemia.”

While the researchers emphasize that their study’s main strength was the sheer size of the population sample — as well as the lengthy follow-up period for data collection— they also note that the analysis had a number of limitations.

These include the fact that they have not been able to determine which specific versions of the different low carb and low fat diets each participant had followed.

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

They also note that the participants self-reported their diet data, meaning they may not be entirely accurate. The team also failed to verify whether the relationship between diet quality and mortality risk was causal in nature or not.

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

It should also be noted that some of the researchers reported potential conflicts of interest, as some received personal fees and grants— for separate work— from private sponsors including the California Walnut Commission and the Standard Method, Metagenics and Diet Quality Photo Navigation programme.

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